'

BALLOON-BORNE THREE-METER TELESCOPE

FOR

FAR-INFRARED AND SUBMILLIMETER ASTRONOMY

Grant NAGW-509

Final Report

For the period 1 September 1983 through 31 March 1988

Principal Investigator

Giovanni G. Fazio

June 1988

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Prepared for

lional Aeronautics and Space Administration

Washington, DC 20546

Smithsonian Institution

Astrophysical Observatory

Cambridge, Massachusetts 02138

The Smithsonian Astrophysical Observatory

is a member of the

Harvard-Smithsonian Center for Astrophysics

The NASA Technical Officer for this grant is Dr. Frederic C. Gillett,

Code EZ-7, National Aeronautics and Space Administration, Washington, D.C.

20546
BALLOON-BORNE THREE-METER TELESCOPE

FOR

FAR-INFRARED AND SUBMILLIMETER ASTRONOMY

Grant NAGW-509

Final Report

For the period 1 September 1983 through 31 March 1988

Principal Investigator '

Dr. Giovanni G. Fazio

Co-Investigators

Dr. William F. Hoffmann

Steward Observatory

University of Arizona

Dr. Doyal A. Harper

Yerkes Observatory

University of Chicago

June 1988

Smithsonian Institution

Astrophysical Observatory

Cambridge, Massachusetts 02138

The Smithsonian Astrophysical Observatory

is a member of the

Harvard-Smithsonian Center for Astrophysics

The NASA Technical Officer for this grant is Dr. Frederic C. Gillett,

Code EZ-7, National Aeronautics and Space Administration, Washington, D.C

20546
- -INAL PAGE IS

OF POOR

Model of Proposed Balloon-Borne Two-Meter Telescope for Far-Infrared and

Submillimeter Astronomy

ii
Abstract

This final report presents the scientific objectives, engineering

analysis and design, results of technology development, and focal-plane

instrumentation for a two-meter balloon-borne telescope for far-infrared

and submillimeter astronomy. Next to observing from space, balloon-borne

platforms offer the best opportunity to escape the absorption and emissivity of the earth's atmosphere, permitting high photometric sensitivity and

spectroscopy of atomic and molecular lines normally blocked by the earth's

atmosphere. Quite apart from duplicating the capabilities of an airborne

telescope, a balloon-borne telescope permits study of several astrophysically important atomic and molecular lines that will otherwise have to

await study from such proposed facilities as ISO, SIRTF, and, in many cases

LDR. However, even when observable by ISO or SIRTF, the larger aperture of

a two-meter balloon-borne telescope will mean that these lines can be'

studied with a spatial resolution 2 to 3 times greater than from these

spacecraft. For photometry, the two-meter balloon telescope would be able

to simultaneously provide better angular resolution at far infrared wavelengths, larger reference-beam spacings, and larger scanned areas than is

practical with airborne or ground-based measurements. Thus the balloon

telescope will fill a crucial niche between facilities like SOFIA and IOmeter class ground-based telescopes which can provide good data on smaller

structures and IRAS sky-maps which provide excellent data on structures

larger than a few arcminutes at wavelengths less than 120 microns. It is

impossible to completely understand the the complex relationships between

molecular clouds, ambient stars and low-mass protostars without thorough

coverage of this range of spatial scalelengths. The balloon telescope can

also play a crucial role in extragalactic astronomy by its ability to image

large areas at a variety of scale-sizes, wavelengths, and angular and

spectral resolutions. It will be particularly valuable for examining the

nearest galaxies, which are inconveniently large for airborne and groundbased instruments but which offer the best opportunities for examining the

detailed structure of spiral arms, nuclear regions, bars, and individual

star-forming complexes.

This is the Final Report under Grant NAGW-509 for a "Balloon-Borne

Three-Meter Telescope for Far-Infrared and Submillimeter Astronomy". Work

under this grant initially focused on the development of a preliminary

design for a three-meter telescope and gondola. That work was essentially

complete in August 1985 and reported on in Semiannual Report #4, (dated

October 1985). Subsequently, additional engineering work simplified the

design of the telescope gimbal system and downsized the telescope and

gondola to a two meter primary. Our goals were to reduce the costs, to

meet the weight constraints for launch at the National Scientific Balloon

Facility in Palestine Texas and to provide easy transportability for rapid

recovery and for operations from remote launch sites. Under related NASA

Grant Ol-NGR-03-002371 to the University of Arizona, several light weight

mirror panels were tested and techniques developed for overcoating and

polishing these mirrors in a continuing development effort. Semiannual

Report #5 describes the simplif1ud gimbal system. This report describes

iii
the unique scientific objectives a two-meter telescope could attain, the

downsized two-meter telescope, and the gondola and initial instruments. It

reports recent advances in lightweight mirror technology and presents a

preliminary plan for carrying out the project.

iv
ORIGINAL PAGE IS

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CONTENTS

Page

Abstract iii

1.0 INTRODUCTION 1

1.1 Unique Capabilities of Balloon-Borne Observations 1

1.2 Program Summary - Development of the Two-Meter Design ... 3

1.3 The Two-Meter Balloon-Borne Infrared and Submillimeter Telescope 4

1.4 Relationship of Three-Meter/Two-Meter Telescope to the Large

Deployable Reflector (LDR) 5

2.0 SCIENTIFIC OBJECTIVES 6

2.1 Introduction 6

2.2 Submillimeter Spectroscopy 10

2.2.1 Introduction 10

2.2.2 Far-Infrared and Submillimeter Lines 10

2.2.3 Models of Atmospheric Transmission 15

2.2.4 Unique Balloon Altitude Science Programs 17

2.3 Photometry 29

2.3.1 Star Formation and the Interstellar Medium 29

2.3.2 Galactic Structure 30

2.4 Cometary Science with a Two-Meter Balloon-Borne Telescope . 31

2.5 Scientific Performance 32

3.0 GONDOLA DESIGN 34

3.1 Telescope Design 34

3.1.1 Telescope Requirements 34

3.1.2 Description of the Cassegrain Telescope 37

3.1.3 Optics Design Trade-Offs 41

3.1.4 Telescope Mechanical Design 44

3.1.5 Telescope Thermal Design 46

3.1.6 Chopping Technique 47

3.2 Support Structure Design 48

3.2.1 Mechanical Concept 48

3.2.2 Launch, Landing, and Recovery Considerations 50

3.3 Pointing System 52

::.3.1 Pointing Requirements 52

3.3.2 Operating Modes 52

3.3.3 Gimbal Design 53

3.3.4 Azimuth Servo 55

3.3.5 Elevation and Cross-Elevation Servos 56

3.3.6 Aspect System 60
CONTENTS (Cont.)

Page

3.3.7 Estimate of Total Pointing Performance 61

3.4 Electrical Systems Design 62

3.4.1 Command and Telemetry System 62

3.4.2 Gondola Electronics System 64

3.4.3 Gondola Power Requirements 64

3.4.4 Electronics Packaging 64

4.0 MIRROR DEVELOPMENT PROGRAM 66

4 .1 Approach and Scope 66

4.2 Conclusions from Mirror Development Program 68

4.2.1 The Dornier Mirrors 68

4.2.2 The Hexcel/JPL Mirrors . . . 68

4.2.3 The Pyrex Face Sheet Composite Mirror 68

4.2.4 The United Technology Graphite/Glass Mirrors 72

4.2.5 Foamed Aluminum Core Mirror 72

4.2.6 Optical Polishing Composite Facesheets 72

4.2.7 Silicon Monoxide Over Coating and Polishing 72

5.0 EXPERIMENT ACCOMMODATIONS 74

5.1 Focal Plane 74

5.2 Photometric Infrared Camera 74

5.2.1 Instrument Description 74

5.2.2 Limits to Sensitivity 75

5.3 Heterodyne Submillimeter Receiver 75

6.0 GROUND SUPPORT EQUIPMENT REQUIREMENTS 82

6.1 Optical GSE 82

6.2 Mechanical GSE 83

6.3 Electrical GSE 83

7.0 NSBF DESIGN DRIVERS AND PAYLOAD SUPPORT REQUIREMENTS 86

7.1 Payload Support Requirements 86

7.1.1 Staging 86

7.1.2 Command and Telemetry Trailer 86

7.2 Launch Requirements 88

7.3 Ascent and Float Environment 90

7.4 Termination, Landing and Recovery 91

8.0 IMPLEMENTATION PHASE SUMMARY DEVELOPMENT PLAN . 91

8.1 Introduction 91

8.2 Statement of Work 92

8.3 Schedule 93

8.4 Implementation Phase Budgetary Cost Estimate 95

8.4.1 Introduction 95

8.4.2 Program Plan 95

8.4.3 Cost Estimate Assumptions 102

9.0 COMPARISON OF THREE-METER AND TWO-METER GONDOLA CONCEPTS . . . 102

References 104

VI
CONTENTS (Coiit.)

FIGURES

Page

FIGURE 1.1-1 Atmospheric transmission 2

2.1-1 Typical Sizes of Objects to be Studied by the TwoMeter Balloon-Borne Telescope 9

2.2-1 Atmospheric transmission at 14 and 32 km in the vicinity of the [01)63.184/im line 22

2.2-2 Atmospheric transmission at 14 and 32 km in the vicinity of the [01 J145.526/im line 23

2.2-3 Atmospheric transmission at 14 and 32 km in the vicinity of the [N H]l21.889/ira line 24

2.2-4 Atmospheric transmission at 14 and 32 km in the vicinity of the ground-state transition of para-HaO ... 25

2.2-5 Atmospheric transmission at 14 and 32 km in the vicinity of the Q'2 487 GHz line 26

2.2-6 Atmospheric transmission at 14 and 32 km in the vicinity of the [SI) 56.322/im line 27

2.2-7 Total cooling per H^ molecule as a function of H-^

density 28

2.5-1 Broadband Sensitivity at 100/^m vs. the DiffractionLimited Beam Size 33

3.1-1 Optical Design of Two-Meter Balloon Telescope ... . 36

3.1-2 Image Blur vs. Field Position and Chopper Angle ... 38

3.2-1 Three Views of Gondola 49

3.3-1 Gimbal Assembly 54

3.3-2 Fine-Pointing Control System Response to a Sinusoidal

Command Signal 58

3.3-3 Telescope Point Response to a SxlCT3 Nms Torque Pulse 59

3.4-1 On-Board Command and Telemetry System for the BalloonBorne Three-Meter Infrared Telescope 63

3.4-2 Gondola Electronics Systems 65

4.2-1 Dornier Quad 25 As Replicated Surface Figure Error

Contour Level 2/im 69

4.2-2 Quad 25 Peak-to-Valley Change in Figure over 82°C

Temp. Change 70

4.2-3 Hexcel 4 Comparison of JPL Model 71

4.2-4 Silicon Monoxide CFRP Mirror Coating Chamber ... . 73

5.3-1 Comparison of actual receiver performances in terms of

number of noise photons at the input 77

5.3-2 Bowtie and Bowtie-Array SIS Junctions 79

5.3-3 Laboratory SIS results at 345 GHz 80

5.3-4 Block Diagram of Proposed Heterodyne Submillimeter

Receiver 81

6.3-1 Gondola Electrical Ground Support Equipment 85

7.1-1 Gondola Supported Within the NSBF High Bay Experiment

Staging Building 87

7.2-1 Gondola Suspended fr m Launch Vehicle 89

8.3-1 Program Schedule 94

VII
CONTENTS (Cont.)

TABLES

Page

TABLE 2.2-1 Important Submillimeter Transitions - Atomic Ions . . 11

2.2-2 Important Submillimeter Transitions - Neutrals ... 11

2.2-3 Important Submillimeter Transitions - Hydrides ... 12

2.2-4 Important Submillimeter Transitions - Heavy Molecules 13

2.2-5 Parameters Used in Three-Layer Atmospheric Approximation 16

2.2-6 VLSR'S of Selected Galactic HII Regions and Planetary

Nebulae . 18

2.2-7 VLSR'S of Nearby, Gas-Rich Star Forming Galaxies . . 19

2.2-8 Some Important Transitions Observable from a Balloon,

but not an Airplane, 20

2.2-9 Some Important Transitions Observable from a Balloon,

but not an Airplane 21

3.1-1 Telescope and Gondola Specifications 35

3.1-2 Telescope Optics Design and Performance 40

3.1-3 Telescope Optics Alignment Tolerances 41

3.1-4 Two-Meter Telescope Subassembly Weight Summary ... 45

3.2-1 Gondola Weight Summary 51

3.2-2 Gondola and Telescope Products and Moments of Inertia 52

3.3-1 System Error Performance for Two Star Tracker Configurations 62

3.4-1 Estimated Power Requirements 66

8.4-1 Total Cost Estimate 96

8.4-2 Equipment, Services and Subcontract Cost Summary . . 97

8.4-3 Engineering Cost Estimate 99

8.4-4 Heterodyne Submillimeter Receiver Estimated Costs . . 100

8.4-5 Two-Meter Balloon-Borne Telescope - Cost Summary . . 101

9-1 Comparison of Two-Meter and Three-Meter Gondola Specifications 103

viii
ORIGINAL PAGE IS

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1.0 INTRODUCTION

1.1 Unique Capabilities of Balloon-Borne Observations

At far-infrared and submillimeter wavelengths, the dominant observational obstacle is atmospheric opacity due mainly to lines of HzO, 03, and

02', the lower the altitude, the greater is the pressure broadening of

these lines, and the more severe is the overall atmospheric attenuation.

In spite of this general problem, a couple of low (10-20%) transmission

"windows" at 350 and 450/jm and a few somewhat higher (50-80%) transmission

"windows" between 700/^m and 1mm permit glimpses into the submillimeter from

mountaintop altitudes (see Figure l.l-l). Higher altitude airborne platforms allow for significantly improved access to the entire range of farinfrared and submillimeter wavelengths, although even at an altitude of

12km interference from atmospheric features can prevent observations of

many important atomic and molecular species from galactic and extragalactic

sources. Next to observing from space, balloon-borne platforms offer the

best opportunity to escape the effects of the earth's atmosphere. It will

be shown below that quite apart from duplicating the capabilities of an

airborne telescope, a balloon-borne telescope permits study of several

astrophysically important species that will otherwise have to await study

from such proposed facilities as ISO, SIRTF, and, in many cases, LDR.

However, even when observable to ISO or SIRTF, the larger aperture of a

two-meter balloon-borne telescope will mean that these lines can be studied

with a spatial resolution 2 to 3 times greater than from these spacecraft.

For photometry, the background radiation for a balloon-borne telescope

is primarily from the telescope mirror and not the sky, since at these

altitudes the atmospheric emissivity is less than 1%. Also at balloon

altitudes the ambient temperature of the mirror is lower, typically 230 K.

Hence, if the mirror emissivity can be made <10% the photometric sensitivity can be considerably improved over observations from an airborne platform. The balloon platform also offers a more stable environment in terms

of fluctuations in the background radiation. As a result, the balloonborne telescope can simultaneously provide at far-infrared and submillimeter wavelengths better angular resolution, larger reference-beam spacings,

and larger scanned areas than is practical with airborne or ground-based

measurements.

In terms of combined photometric and spectroscopic capabilities a twometer balloon-borne telescope can fill a crucial niche between facilities

such as SOFIA and a 10-meter class ground-based telescope and space telescopes such as IRAS, ISO, and SIRTF.
Page 2

1000 500

1

300

I

Wavelength (yum)

200 150 125 100

32 km

c

o

'en

en

en

c

o

cu

Q.

in

O

4.2 km

300 600 900 1200 1500 1800 2100 2400 2700 3000

Frequency (GHz)

Figure 1.J.-1. Atmospheric transmission at mountaintop (4.2km), airplane

(14km), and balloon (32 km) altitudes for 1mm, T-O^tm and 0.32/wn of precipitable water vapor, respectively, along a 45° elevation line-of-sight.
Page 3

1.2 Program Summary - Development of the Two-Meter Design

The primary focus of this grant has been the definition of a balloonborne three-meter telescope for infrared and submillimeter astronomy and

the identification of the wide ranging scientific program this exciting new

facility could support.

This work was essentially completed in August 1985 and reported on in

Semiannual Report #4 under this grant (NAGW-509) released in October 1985

and subsequently re-released as an independent study report: "BalloonBorne Three-Meter Telescope for Far-Infrared and Submillimeter Astronomy

Preliminary Design Report" also dated October 1985 (the "yellow cover

report"). References in this report to "the PDR" are to either of these

documents.

In releasing these reports, we felt we had carried the design process

as far as we could under the funding constraints of the grant and were

prepared to move into a four-year program to complete the engineering

design of the gondola, and to build, test and fly it. Had we been able to

proceed, we would be today in the final stages of assembly and test of the

gondola and looking forward to field operations next year.

Unfortunately., NASA funding constraints precluded beginning the hardware program at once and our attention was directed towards the exploration

of potential international collaborative efforts to develop the gondola.

Although no collaborations resulted from these initiatives, the discussions

led us to conclude that many of the scientific objectives of the original

program could be met with a two-meter telescope and futhermore, that the

newly burgeoning field of sub-millimeter astronomy could make even more

effective use of such a facility than we originally believed.

Accordingly, the small balance of funds remaining after the completion

of the three-meter telescope report was directed towards rescoping the

design of the telescope downward to accommodate a primary mirror two meters

in diameter. The result of this "mini study" has been the conceptual

design of the two-meter telescope described in this report. The new design

is smaller, lighter, easier to transport and far less costly than the

three-meter design. Furthermore, it could fly from Palestine Texas, the

most convenient and efficient launch site, under the new, more stringent,

flight rules.

Additional development and evaluation of lightweight mirror technology

and telescope design has been carried out under NASA Grant Ol-NGR-03-

002371, Dr. William F. Hoffmann, University of Arizona, Principal Investigator. Dr. Hoffmann is also a Co-Investigator on this grant.

Contributors to this report include Giovanni Fazio, George Nystrom,

William Hoffmann, Doyle Harper, Robert Martin, Paul Ho, Gary Melnick, John

Black, Christopher Walker, and Richard S. Taylor. It was compiled and

edited by Richard S. Taylor.
Page 4

1.3 The Two-Meter Balloon-Borne Infrared and Submillimeter Telescope

As shown in the photograph of the gondola model in the frontispiece,

the telescope is of Cassegrain design. It uses a two-meter primary with a

focal ratio of 1.0 and a 20 cm secondary. The telescope is mounted at or

near its center of gravity in a yoke frame which provides both telescope

support and three axis pointing.

Coarse azimuthal pointing is achieved by driving the yoke against the

balloon attachment bearing in the yoke. Fine azimuthal pointing utilizes

the telescopes limited range (±3°) cross-elevation bearings. Coarse and

fine elevation pointing are achieved with the single elevation bearing set.

Active closed loop pointing systems control motion in all three axis

with a stability approximately 1 arcsecond rms. A three axis gyro, updated

by a focal plane star tracker, is the fine pointing co-ordinate reference.

For launch and landing, the telescope is held vertically by means of a

capture lock just below the balloon attachment bearing.

The total weight of the two-meter gondola is 1,731 kg (3,809 Ibs.)

including all NSBF required equipment. The overall height is 5.8 meter (19

feet). The overall diameter is 2.4 meters (8 feet), or just less than the

maximum limits for normal highway loads, making hauling, shipping and

recovery operations possible without disassembly of the gondola structure.

We believe that careful design can bring the weight of the gondola below

the 3500 Ib. Palestine Texas launch limit.

The two-meter design baselines the CFRP aluminum honey comb-core

primary mirror because of its very low weight availability and demonstrated

performances. In addition, we are continuing to evaluate other lightweight

mirror approaches. Section 4.0 summarizes the status of the University of

Arizona mirror development program.

The two-meter mirror size also opened up the possibility of seriously

re-considering focal plane guiding because of the superior optical finish

we expect to be able to achieve on the smaller mirror. Focal plane guiding

removes a series of structural instability terms from the pointing system

stability equations. We have baselined focal plane guiding, using either

the infrared imaging photometer experiment or a separate optical imaging

camera and expect to be able to achieve sub-arcsecond rms pointing stability (Section 3.3).

Additional work has also been done on the development of experiment

concepts. We report on the design of a new submillimeter spectrometer for

the gondola (Section 5.3) and on the status of the infrared imaging photometer experiment which is nearing completion at the University of Chicago

(Section 5.2).

Gondola costs reported in Section 8, although based on the ground-up,

bolt by bolt estimate developed for the three-meter design, have some

uncertainty due to the repeated application of inflation factors and postfacto adjustments to the rate of exchange of the DM. We estimate the total
Page 5

price of this design to be $6,830K without instruments. Approximate costs

for the submillimeter spectrometer are $1,071K and the IR Photometer are

$475K, bringing the total program cost to $8,377K including the first

flight. Alternative design approaches and the use of more modern technology may allow us to reduce this cost somewhat. The primary costs drivers

are the lightweight mirror, the lightweight telescope structure and the

pointing system. Lightweight mirror technology has developed significantly

since the original quotations were obtained from Dornier and MAN and considerable savings may now be possible in that area. One verbal quote we

received recently from ERG, Inc., Oakland, CA, indicates that a suitable 2M

mirror might be made for as little as $250K rather than the $500K estimated

by MAN. A local Boston firm, Nuclear Metals Inc., is also developing a

sintered casting process for large lightweight mirrors which may allow

similar savings. Similarly, a new evaluation of the telescope structure

design may allow cost savings in that area. It may also be possible to

take a sequential approach to the development of the pointing system allowing refinement of the design and achievement of the final specifications

over the first few flights.

1.4 Relationship of Three-Meter/Two-Meter Telescope to the Large Deployable Reflector (LDR)

There are three areas where the balloon-borne telescope can contribute

to LDR development: motivating and focusing light weight submillimeter and

far infrared mirrors and structures; providing a test bed for submillimeter heterodyne receivers for space; and increasing the body of scientific

observations needed to effectively plan LDR scientific programs. In each

of these areas, the balloon-borne telescope has a unique potential contribution.

Already, the balloon mirror development program has identified carbon

fiber reinforced plastic (CFRP sandwich panel mirrors) as a promising

approach for the LDR mirror. Our collaborative development program with

Dornier System has provided characterization of the thermal distortion of

state-of-the-art CFRP mirrors in the temperature region of LDR and has

resulted in a factor of 25 improvement in test mirror thermal distortion.

This work is continuing in a collaboration with JPL and Hexcel Corporation.

We are currently experimenting with overcoating and polishing CFRP mirrors

to provide visible wavelength imaging capability, of importance to both the

balloon telescope and LDR for alignment and pointing.

A major issue the LDR for photometric observations is whether the

sensitivity will be limited by the fundamental shot noise in the background

photon flux, which is on the order of a millionth of the background flux,

or whether it will be limited by variations in the background flux due to

varying thermal loads on the telescope. Also of importance for both telescope is the stability of the point spread function of the telescope for

"super resolution" imaging analysis and for detecting faint companions and

structures near bright sources.

Since the background for the balloon telescope, like LDR, is primarily

from the telescope, not the sky, ant" since both telescopes have a similar
Page 6

radiation environment, experience in understanding and dealing with these

problems with the balloon telescope will be directly relevant to LDR. In

addition, it will be possible to test particular reimaging and beam switching techniques for LDR on the balloon telescope. As an example, the secondary chopper approach to sky chopping, which has dominated over other

approaches of infrared astronomy, has achieved this position through experience. No other technique has worked as well in practice. Beam switching

will be essential for photometric observations with LDR. It would be

desirable to demonstrate any innovative techniques with astronomical observations before committing the LDR design.

The second area is as a test bed for LDR instruments, in particular

newly developed submillimeter heterodyne receivers. Only very limited

observations are possible from the ground because of the poor submillimeter

atmospheric windows. In addition, these instruments are complex and warrant gaining experience with remote operation in a space-like environment.

Balloon-borne systems have provided a test bed for instruments developed

for space UV, X-ray, gamma ray, and cosmic background astronomy. One

reason cited for the lack of success of submillimeter proposals in the

recent Explorer competition was limited experience with the required heterodyne receivers. The balloon-borne telescope offers a motivation and text

bed for such development.

The third area concerns the body of scientific knowledge available to

effectively plan LDR scientific programs. In this area, the balloon- borne

telescope has a special capability for carrying out exploratory observations of important astrophysical lines which cannot be observed from the

ground or from aircraft altitudes because of strong, broad atmospheric

absorptions lines at the same wavelengths. Two such examples are observation of the ground state transitions of ortho and para water and low level

transitions of molecular oxygen. Even at balloon altitudes, the atmospheric absorption prevents observation of these lines in galactic sources.

However, for extragalactic objects with radial velocities of 300 km/sec or

more, the lines are Doppler shifted sufficiently out of the narrow atmospheric absorption lines at balloon altitudes to be potentially observable.

This is discussed in more detail in Section 2.0.

2.0 SCIENTIFIC OBJECTIVES

2.1 Introduction

Far-infrared and submillimeter observations are essential for attacking a number of very fundamental astronomical questions:

(1) Structure and evolution of galaxies and the nature of energy

sources in active galactic nuclei.

(2) Understanding the large scale structure of our galaxy.

(3) Star formation and evolution processes in molecular clouds.

(4) Properties of the interstellar medium.
Page 7

(5) Properties of the sun and solar system objects.

Specific balloon-borne far-infrared and submillimeter astronomical

observations that can contribute to the solution of these scientific problems include:

(1) High resolution far-infrared and submillimeter spectroscopy of

galactic, extragalactic, and solar system objects.

(2) Large area surveys of the galactic plane in spectral lines, e.g.,

01 (146 microns), C II (157 microns), etc.

(3) High angular resolution large area mapping of star formation regions, galaxies, etc., at submillimeter wavelengths.

Until LDR is flown, no presently planned infrared satellite experiment

can perform these observations. The potential for balloon-borne infrared

astronomy is great, and to date it has not been determined what its limitations are .

The balloon-borne telescope permits observations of several ionic,

atomic, and molecular transitions from galactic sources that are inaccessible from airborne telescopes and offers particularly impressive advantages

for the study of these lines in other galaxies.

Far-infrared atomic fine-structure lines, unlike their optical counterparts, are essentially unaffected by reddening or self-absorption. Farinfrared fine-structure lines are also very insensitive to electron temperature. Thus, they can provide very direct information on electron density,

relative ionic and atomic abundances, cloud masses, and spectral types of

ionizing stars. In addition, lines like those from 0 I at 63 ^tm and 146 /im

and C II at 157 /jm, which are major coolants for neutral gas at temperatures of 100-1,000°K, provide essential data on the heating of interface

regions between ionic and molecular clouds and on shock heating of the

interstellar medium. N II lines at 122 /^m and 204 /im can be combined with

the N III line at 57 pm to study electron density, nitrogen abundances, and

ionization structure in H II regions.

Molecular lines at far-infrared and submillimeter wavelengths are

important for two reasons. First, they represent dominant coolants and

important diagnostic indicators for gas with temperatures of a few hundred

d"grees. Observations of emission from many different levels of CO, for

•xample, can yield accurate estimates of molecular hydrogen density, temperature, and total CO column density in warm, shocked gas in active star

fi.o-uiation regions. Second, far-infrared observations may allow detection

of molecular species not accessible at other wavelengths (e.g., many of the

most abundant hydrides). This could provide important clues to both abundances and chemical processes in the clouds. Other important molecules

include OH, CH, and NH3.

Perhaps the greatest contribution of a balloon-borne telescope would

be to significantly expand the ability to observe important molecular

coolants, such as H20 and 0%, in extragalactic sources. These molecules

compete with CO as the most important coolants of the denser portions of

ORIGINAD PAGE IS

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Page 8

molecular clouds.

In Figure 2.1-1 the typical sizes of objects this Two-Meter BalloonBorne Telescope will study are indicated, as well as the distances at which

they are likely to lie . Three diagonal lines indicate angular resolutions

of 30 arcseconds, about the best achievable at 100pm wavelength with onemeter-class telescopes, such as the Kuiper Airborne Observatory (KAO),

other balloon-borne telescopes, and the Space Infrared Telescope Facility

(SIRTF) ; 12 arcseconds, the resolution achievable with the Two-Meter

Balloon-Borne Telescope; and 1 arcsecond, the resolution achievable with

the LDR. ,From the figure it can be seen that the two-meter telescope will

allow us to resolve and study in detail such objects as collapsing protostellar condensations in our own galaxy, clusters of protostars in the

Magellanic Clouds, giant molecular clouds in nearby galaxies, and spiral

arms in distant galaxies.
Page 9

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'/ /

/ /

CMC'S IN

OTHER GALAXIES

/

' 1 !

VIRGO EDGE OF

CLUSTER THE

UN'VERSE

s-2_

ID

10

 J

 -

to

r^ _

10"

FRAGMENTATION, |

COLLAPSE i

PRESTELLAR DISKS

J_ -J_.

-\

I0 I0 I0 10^ 10"

DISTANCE I

I0 I0 I0 I09 I010

Figure 2.1-1. Typical Sizes of Objects to be Studied by the Two-Meter

Balloon-Borne Telescope
Page 10

2.2 Submillimeter Spectroscopy

2.2.1 Introduction

The discussion that follows is divided into three parts: (l) a review

of the most important transitions between 50/im and 1mm, (2) a brief discussion of the method used to model the atmospheric transmission, and (3) the

unique science that can be conducted by a balloon-borne telescope.

2.2.2 Far-Infrared and Submillimeter Lines

The spectroscopic features in the far-infrared and Submillimeter are

expected to be dominated by molecular rotational transitions and by atomic

fine-structure lines. From the results of millimeterwave spectroscopy, not

only is it known that the interstellar medium contains many molecular

species, but also from observations of these various molecules it is clear

that the temperature of the gas is typically between 10 and 100 K. Generally, then, it is expected that most of the energy emitted by the medium

will be in the range Imm-lOO/xm.

Most of the lines occurring in the far-infrared and Submillimeter may

be placed into one of three categories: (1) ground-state, fine-structure

transitions in atoms or atomic ions, (2) low-lying rotational transitions

in light molecules, such as hydrides, and (3) mid-level rotational transitions in heavy molecules. These transitions are summarized in Tables 2.2-1

- 2.2.4.

(a) Atoms and Ions

Atoms and atomic ions may possess a ground state with a net orbital

electronic angular momentum so that the state may be split by spin-orbit

effects. Several of the light atoms and ions have a ground state fine

structure of this kind which provides transitions in the far-infrared and

Submillimeter. With increasing atomic weight the transitions move rapidly

into the near-infrared and optical.

(b) Light Molecules

These molecules contain hydrogen, incorporated into the structure in

such a way that the lowest rotational modes are dominated by the hydrogen

mass so that the lowest frequency appears in the Submillimeter. For most

of the simple hydride molecules this is the case, and they can only easily

be observed in the Submillimeter.

(c) Heavy Molecules

Although heavy molecules possess transitions which can be observed in

the millimeter band, it is most helpful to an understanding of the physical

conditions giving rise to the emission to have available the higher energy

transitions for comparison.
Page 11

Species

N+

C+

N+

0++

N++

0++

Transition

3

Pj -

 3

P0

2rp . 2p

3/2 ~*

 rl/2

3p2 _ 3pj

3

Pl -»

 3P0

2

Pa/2 -*

 2Pi/2

3p2 _+ 3pt

Frequency

(GHz)

1470.3

1900.536

2459.553

3393.045

5230.428

5785.712

Wavelength

(Mm)

203.9

157.741

121.889

88.355

57.317

51.816

Table 2.2-1. Important Submillimeter Transitions - Atomic Ions

Species

C°

C°

O°

Si°

Si°

0°

s°

Transition

3

Pi

3

P2

3

Po

3

Pi

3

P2

3

Pi

3

Po

-

 3Po

-

 3

P!

->

 3Pi

-

 3Po

-

 3Pl

-

 3P2

-

 3Pl

Frequency

(GHz)

492.162

809.350

2060.061

2311.751

4378.194

4744.775

5322.830

Wavelength

(urn)

609.134

370.411

145.526

129.682

68.474

63.184

56.322

Table 2.2-2. Important Submillimeter Transitions - Neutrals
Page 12

Species

H2D+

A1H

LiH

H2S

CH

H20

NH3

SiH

HC1

SiH

NH

H30+

H2O

NH3

H2D+

SH

CH

OH

CH

CH2

OH

HD

OH

Transition

7KpKo =

7 =

7 =

7KP K0 =

2

n FI -> F2 : 7

7

lio

JK =

2

n FI : 7 =

7 =

2

n FI : 7 =

3

S- , N = 1 -

JK =

In

7A =

7KpK0 = 2

n3/2 7 =

2

n FI : 7 =

7 =

2

n1/2 7 =

7 =

2

n F2 : 7 =

7 =

7 = 1- l^A',

2

n3/2 7 =

J =

J =

2ni/2 _ 3/2 : 7

7

ho -»• hi

1 -> 0

1 -* 0

hi -> GOO

= 3/2+ -* 1/2-

= 3/2- -» 1/2+

- loi

lo -» GO

3/2- - 1/2+

1 - 0

3/2+ — 1/2-

-0, 7 = 2- 1

"0 * 0

-* GOO

20 - lo

loi —> OQO

5/2 -> 3/2

5/2~ - 3/2+

5/2+ — 3/2-

3/2" -> 1/2+

3/2+ -* 1/2"

3/2~ - 1/2+

5/2+ — 3/2-

,K'C = 111 -» OQO

5/2+ -^ 3/2"

5/2" — 3/2+

1 -»• 0

= 5/2+ -* 3/2-

= 5/2" — 3/2+

Frequency

(GHz)

372.421

377.59

443.953

452.390

532.726

536.761

556.936

572.498

624.925

625.919

627.690

974.63

984.656

1113.342

1214.859

1370.204

1382.928

1383.257

1656.964

1661.105

1834.760

1837.853

2006.797

2010.809

2344.697

2509.963

2514.320

2675.023

3786.132

3789.216

Wavelength

(/xm)

804.982

793.96

675.280

662.685

562.751

558.521

538.289

523.657

479.726

478.964

477.612

307.60

304.464

269.273

246.771

218.794

216.781

216.729

180.929

180.478

163.396

163.121

149.388

149.091

127.860

119.441

119.234

112.071

79.182

79.117

Table 2.2-3. Important Submillimeter Transitions - Hydrides
Page 13

Species

02

cs

CO

HCN

CS

02

13CO

CS

HCN

CO

02

CS

HCN

CS

13CO

CO

CS

HCN

CS

13CO

CS

CO

HCN

13CO

HCN

CO

13CO

HCN

CO

HCN

13CO

Transition

Nj =

J =

J =

J =

J =

Nj =

J =

J =

J =

J =

Nj =

J =

J =

J =

J =

J =

J =

J =

J =

J =

J =

J =

J =

,7 =

J =

J =

J =

J =

J =

J -

J =

li

7

3

4

8

32

4

9

5

4

33

10

6

11

5

5

12

7

13

6

14

6

8

7

9

7

8

10

8

11

9

-> lo

-> 6

-» 2

— 3

-» 7

- 12

-> 3

-» 8

-* 4

-> 3

-• la

— 9

-> 5

-> 10

-» 4

— 4

-> 11

-- 6

-* 12

-> 5

-H. 13

-* 5

-^ 7

-> 6

-» 8

-» 6

-> 7

-+ 9

— * 7

-* 10

— 8

Frequency

(GHz)

118.750

342.883

345.796

354.505

391.847

424.763

440.764

440.803

443.123

461.042

487.249

489.751

531.712

538.689

550.926

576.267

587.616

620.301

636.532

661.066

685.435

691.474

708.889

771.183

797.433

806.652

881.273

885.977

921.799

974.475

991.330

Wavelength

(/«n)

2524.568

874.329

866.964

845.666

765.075

705.788

680.166

680.105

676.544

650.250

615.276

612.132

563.825

556.522

544.161

520.232

510.184

483.302

470.978

453.498

437.376

433.555

422.905

388.744

375.947

371.650

340.181

338.375

325.225

307.645

302.414

Table 2.2-4. Important Submiilimeter Transitions - Heavy Molecules
Page 14

Species

CO

HCN

13CO

HCN

CO

HCN

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

J

Transition

= 9 -»

= 12 -»

= 10 -f

= 13 -»

= 10 -»

= 14 -»

= 11 ->

= 12 —

= 13 -*

= 14 -»

= 15 ->

= 16 -»

= 17 -»

= 18 —

= 19 ->

= 20 ->•

= 21 ->

= 22 -»

= 23 --

= 24 ->

= 25 -»

= 26 ->

= 27 ->

= 28 -H.

= 29 -f

= 30 -^

= 31 —

= 32 -

= 33 ->

= 34 —

= 35 -»

8

11

9

12

9

13

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

Frequency

(GHz)

1036.913

1062.974

1101.351

1151.443

1151.985

1239.882

1267.016

1381.995

1496.924

1611.795

1726.604

1841.346

1956.020

2070.615

2185.136

2299.570

2413.917

2528.174

2642.332

2756.388

2870.342

2984.182

3097.911

3211.521

3325.007

3438.3GS

3551.593

3664.687

3777.640

3890.446

4003.105

Wavelength

(/im)

289.120

282.032

272.204

260.362

260.240

241.791

236.613

216.927

200.272

185.999

173.631

162.812

153.267

144.784

137.196

130.369

124.193

118.581

113.458

108.763

104.445

100.461

96.772

93.349

90.163

87.190

84.411

81.806

79.360

77.059

74.890

Table 2.2-4 Important Subraillimeter Transitions - Heavy Molecules

(Continued)
Page 15

2.2.3 Models of Atmospheric Transmission

Use is made of the Air Force Geophysical Laboratory Atmospheric (AFGL)

Absorption Line Parameters Compilation (Rothman et al. 1983) atmospheric

absorption-line parameter database to obtain the wavenumber, line strength,

pressure broadening coefficient, and energy level of the lower state for

over 109,000 known transitions of H20, 03, 02, C02, CO, N20, HC1, OH, and

CH4 between about 0.76/im and 3.26mm. At the wavelengths of interest here,

50/zm to 1mm, essentially all of the atmospheric interference is due to

lines of H20, On, and Q?. Table 2.2-5 summarizes the molecular abundances,

pressures, and temperatures employed in the calculation of atmospheric

transparency at 14 and 32 km.

The H20 abundance at 14 km is an average over many KAO flights (Meyer

1988), while the 32 km value corresponds to an average value measured

during balloon flights over Palestine, Texas, during the spring and fall

(Chance and Traub 1988) . The 02 abundance is taken from Allen (1973) . The

vertical distribution of 0;i is taken from the U.S. Standard Atmosphere

Supplements. The value for CO is given by Farmer (1974) as 0.06 ± 0.01

ppmv at latitude 30°N and altitudes greater than 10 km. The OH concentrations were obtained from Solomon (1986) and the HC1 concentrations are from

Farmer et al. (1988).

The model atmosphere used is a three-layer, Curtis-Godson approximation (Goody 1964) with the parameters as given in Table 2.2-5. The effective pressures in Table 2.2-5 are a mean for each layer and are used in the

calculation of collision line-broadening. The effective temperatures are

taken to be those which actually occur at each effective pressure using the

U.S. Standard Atmosphere (1966).

According to Goody (1964), the Curtis-Godson approximation is quite

accurate for the weak and strong line strength limits at each point in the

absorption line profile. For intermediate line strengths, the transmission

error appears to be at worst about 5% at the line core, and much smaller

elsewhere. Inspection of the comparison calculations reported in Goody's

book suggests that for species that are well-mixed, the error in average

transmission across a line should be well under 1%. Some further support

for the Curtis-Godson approximation can be derived from experiences in

fitting theoretical line profiles to high resolution observations of the

terrestrial 02 near 7635 Angstrom (Carleton and Traub 1972) and 1.27/im

(Noxon et al. 1976) and H20 lines near 8197 Angstrom (Traub and Carleton

1974); in all cases the experimental and calculated profiles were identical to within the noise level of the data, which was typically less than 5%

of the continuum.

Validation of the calculated atmospheric transmission profiles at

balloon altitudes in the far-infrared have been obtained more recently by

Chance and Traub (1987), Robbins et al. (1988), and Murcray et al. (1988).

Very good qualitative agreement between calculated and observed atmospheric

transmission profiles is also seen in the far-infrared at airplane altitudes; however, the absence of a reliable line-of-sight water vapor content limits our ability to quote an exact measure of agreement.
Page 16

Parameter Layer 1 Layer 2 Layer 3

14km

Effective altitudes (km)

Effective number density (cm~3)

Column densities (cm~2)

Effective pressures. (mbar)

Effective temperatures (K)

H2O concentration '

O2 concentration

O3 concentration

CO concentration

OH concentration

HC1 concentration

15.16

3.9 x 1018

1.4 x 1024

118.08

217

5.5 x 10~6

2.1 x 1Q-1

8.0 x 10~8

6.0 x 10-8

1.0 x 10-13

2.6 x ID"9

18.42

2.4 x 1018

1.4 x 1024

70.85

217

5.5 x 10-6

2.1 x ID"1

3.0 x 10-7

6.0 x 10~8

4.0 x 10-13

2.6 x 10-9

25.50

7.6 x 1017

1.4 x 1024

23.61

226

5.5 x 10~6

2.1 x ID"1

7.0 x 10~6

6.0 x 10~8

4.0 x 10-11

2.6 x 10~9

32 km

Effective altitudes (km)

Effective number density (cm~3)

Column densities (cm~2)

Effective pressures (mbar)

Effective temperatures (K)

H2O concentration *

O2 concentration

Oa concentration

CO concentration

OH concentration

HC1 concentration

33.24

2.3 x 1017

9.0 x 1022

7.41

232

4.0 x 10~6

2.1 x 10-1

6.0 x 10~6

6.0 x ID"8

1.0 x 10-11

2.6 x 10-9

36.82

1.3 x 1017

9.0 x 1022

4.45

242

4.0 x 10~6

2.1 x 10-1

8.0 x 10~6

6.0 x 10-8

5.0 x ID"11

2.6 x 10-9

45.05

4.2 x 1016

9.0 x 1022

1.48

258

4.0 x 10~6

2.1 x 10-1

6.0 x 10-6

6.0 x ID"8

2.0 x ID'10

2.6 x 10-9

' Corresponds to 7.0 /im at 14 km and 0.32 /zm at 32 km of precipitable H2O

along a 45° elevation line-of-sight.

Table 2.2-5. Parameters Used in Three-Layer Atmospheric Approximation
Page 17

The calculating grid is chosen to have points spaced by 0.0002cm~1 at

both 14 and 32 km levels. The pressure-broadened full-width (FWHM) expected for a typical molecule at 14 and 32 km levels is 0.007 and

0.0008cm"1, respectively; thus these calculations should accurately represent the true width of the atmospheric features.

2.2.4 Unique Balloon Altitude Science Programs

To determine which of the transitions given in Tables 2.2-1 - 2.2-4

are accessible only from a balloon-borne platform, and not from an airborne

platform, the atmospheric transmission within about ±1000 km s"1 of each of

the listed transitions was examined. This range of velocities was selected

to include all bright galactic H II regions and planetary nebulae (see

Table 2.2-6) as well as many of the brightest nearby star forming galaxies

(see Table 2.2-7). For simplicity, the results discussed below are divided

into two categories: (l) the transmission of the atmosphere within 100 km

s"1 of the line center, which includes the VLSR'S of most galactic sources,

and (2) the atmospheric transmission greater than +100 km s"1 and less than

-100 km s"1 from the line center, which corresponds to the VI.SK ' s of most of

the nearby, infrared bright extragalactic sources. These results are

summarized in Tables 2.2-8 and 2.2-9. Examples of a few cases are shown in

Figures 2.2-1 - 2.2-6.

The main result is that a balloon-borne telescope permits observations

of several ionic, atomic, and molecular transitions from galactic sources

that are inaccessible from an airborne telescope. Because of the narrowing

of all atmospheric features at 32 km compared to 14 km combined with the

relatively large Doppler shifts of many extragalactic sources, a balloonborne telescope offers particularly impressive advantages for the study of

other galaxies.

The greatest advantage to galactic spectroscopy of a balloon-borne

telescope is the significantly increased observability of the [01]

145.526/wn fine-structure line (Figure 2.2-3). Measured in conjunction with

the widely observed [01] 63.184^m fine-structure line, the ratio of these

line strengths can be used to determine the neutral gas temperature. Further, when the [C II] 157.741/im line is also measured, the ratio of these

three line strengths can be used to tightly constrain both temperature and

density (cf. Watson 1985). Likewise, the [N II] 121.889/im fine-structure

line (Figure 2.2-4), which is only observable at or above an altitude of 32

km, could be combined with the [N II] 203.9/101 and the [N Hi] 57.317/un

fine-structure lines to study the electron density, nitrogen abundance, and

ionization structure of galactic H II regions.

In recent years, airborne spectrometers have achieved the sensitivity

needed to study galactic molecular emission other than CO, such as OH, CH,

and NHa. In principle, accounting for the line strengths of two or more

transitions from the same molecule is a powerful means of limiting the

range of density, temperature, and, in some cases, the radiation field

strength in the emitting region (cf. Melnick et al. 1987). Whereas OH, CH,

and NH3 have several transitions ,/hich occur at far-infrared and submillimeter wavelengths, not all of the lowest transitions are free from atmo-
Page 18

Source *LSR

(km sNGC 7538 - 60

G 333.6-0.2 -48

W3 -40

IRC+10216 -26

Eta Carina - 18

BD +30 3639 - 13

NGC 6357 -4

NGC 6720 (Ring Neb.) 0

DR21 +1

M8 +3

W49 +9

NGC 2024 + 8

Orion-KL +10

NGC 2071 +10

M17 +18

NGC 7027 +24

Sgr A +50

W51 +58

Sgr B2 + 66

Table 2.2-6. VLSR' S of Selected Galactic HII Regions and Planetary Nebulae
Page 19

Source

 V*<SR

(kms-1)

1C 342 + 30

NGC 6946 + 55

M82 (NGC 3034) + 220

NGC 253 +250

M83 (NGC 5236) +490

Cen A (NGC 5128) + 500

NGC 2146 +865

NGC 1068 (M77) +1120

M51 (NGC 5194) + 1470

Table 2.2-7. VLSR'S of Nearb>, Gas-Rich Star Forming Galaxies
Page 20

Species

CO

NH3

CH

O°

N+

OH

Transition

J =

JK =

2

n Fj : J =

3

Po

3

P2

2Hi/2 -* 3/2 J

J

10 -» 9

2o -» IQ

5/2-. -i. 3/2+

-

 3Pj

-

 3P>

= 5/2+ -» 3/2-

= 5/2- -> 3/2+

Frequency

(GHz)

1151.985

1214.859

1656.964

2060

2459.553

3786.132

3789.216

Wavelength

(/mi)

260.240

246.771

180.929

145.526

121.889

79.182

79.117

Table 2.2-8. Some Important Transitions Observable from a Balloon, but not

an Airplane, for +100 < VLSR < -100km s'1.

spheric interference at 14 km. At balloon altitudes, the full spectrum of

lower transitions from these molecules is observable.

Perhaps the greatest contribution of a balloon-borne telescope would

be to significantly expand the ability to observe important molecular cloud

coolants in extragalactic sources such as H20 and 02- As is shown in Figures 2.2-5 and 2.2-6, at an altitude of 32 km the terrestrial counterparts

to these ground-state H20 and 02 lines become sufficiently narrow that any

source with a VLSR greater than ±300 km s"1 will be clear of any atmospheric interference. This would also apply to potentially interesting

higher-lying H20 lines not indicated in Table 2.2-9. As shown in Figure

2.2-8, H20 and 02 compete with CO as the most important coolants of the

denser portions of molecular clouds.

Most of the energy emitted by neutral atomic gas with temperatures in

the range 10-5000 K and densities below about 108 cm"3 is radiated in the

[01] 63.184pm and [C II] 157.741pm fine-structure transitions, making them

the brightest lines in the infrared spectrum of gas-rich galaxies (cf.

Crawford et al. 1985; Lugten et al. 1986). At an altitude of 14 km, the

[01] 63.184pm line cannot be observed from sources with VLSR's much greater

than +200 km s'1 (Figure 2.2-1). At an altitude of 32 km, this line can be

observed from most of the sources in Table 2.2-7.
Page 21

Species

02

02

02

H2O

H20

CO

SH

N +

CII2

N +

0°

s°

Transition

(1,1)

(3,2)

(3,3)

lio

In

J =

2

n3/2 J =

3Pi

in

3P2

3P,

3

Po

- (1,0)

- (1,2)

- (1,2)

- loi

-» 000

10 -» 9

5/2 -» 3/2

-

 3Po

— OQO

-

 3Pi

-

 3P2

-

 3Pi

Frequency

(GHz)

118.750

424.763

487.249

556.936

1113.342

1151.985

1382.928

1383.257

1470.3

2344.697

2459.553

4744.775

5322.830

Wavelength

(/mi)

2524.568

705.788

615.276

538.289

269.273

260.240

216.781

216.729

203.9

127.860

121.889

63.184

56.322

Table 2.2-9. Some Important Transitions Observable from a Balloon, but not

an Airplane, for -100km s"1 > VLSR and VI.SR > +100km s'1.

Because sulfur has an ionization energy of 10.36 eV, the [Si] 56.322/zm

fine-structure will arise almost exclusively from well shielded neutral gas

regions - regions not entirely probed by the [C II] 157.741^m line which

exists within a photon energy range of 11.26 and 24.38 eV. As such, this

line, along with its companion fine-structure line at 25.246/im and the

fine-structure lines of [CIj, can be used to study deeper (i.e. higher Av)

portions of extragalactic photodissociation regions (cf. Tielens and Hollenbach J985)(Figure 2.2-7).

Thus. ranch as ground-based radio astronomy has spent the past three

decades successfully studying a large variety of galactic and extragalactic

phenomena in the lines of a few important atomic and molecular species (eg.

CO, HI, OH. CS, NH.i) , opening the skies to regular study in the lines

discussed above will complement the scientific contributions from the KAO

•ind SOFIA and wil l fil l a unique niche in far-infrared and submillimeter

astronomy until I-DK is a reality.

ORIGINAL PAGE IS

OF POOR QUALITY
fc-S

o

o° .8

c

o

'in

V!

'§

(fi

c.

D

SL

D.

en

O

_E

<

.6

.2

Page 22

Relative Velocity (km sec~ )

500 0 - 500

[01]

 3P, -

 3F

32 km

t i I I I ! I I t I I ) I \ I I I I I 1 I !

157.6 157.8 155 156.2 155.4 155.6 158.6

Wovenumber (cm )

Relative Velocity (km sec )

+ 500 0 - 500

i i i i i i i i i i i I i i i I I 1—1 1—1—1 1

157.6 157.8 158 158.2 158.4 158.6 158.8

Wavenumber (cm~ )

Figure 2.2-1. Atmospheric transmission at 14 and 32 km in the vicinity of

the (Ol]63.184Mm line
-1% Relative Velocity (km sec" )

+ 400 0 - 400

Page 23

1

o

2 -8

X

^^s

§

'8 -

6

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Page 25

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Page 26

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Page 27

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2.3 Photometry

2.3.1 Star Formation and the Interstellar Medium

Far-infrared observations provide a unique set of tools for studying

star formation: first, because stars form in dense clouds that heavily

attenuate radiation at shorter wavelengths and, second, because many important processes occur in gas and dust clouds with temperatures between a few

tens and a few hundreds of degrees Kelvin. Most of the cooling for such

clouds is provided by far-infrared continuum and line emission.

Perhaps the most fundamental quantity provided by far-infrared measurements is the total luminosity of dust-embedded energy sources. Very

little dust is required to completely absorb stellar ultraviolet and optical emission, and this energy is subsequently re-emitted in the far infrared.

Most far-infrared investigations of active regions of star formation

have been limited not by instrumental sensitivity but by source confusion.

The mean projected separation of stars in nearby open clusters (for which

membership is fairly complete) is of the order of 0.1 pc. Separations

within the most compact parts of the youngest clusters (e.g., the Trapezium

and KL clusters in the Orion Nebula) may be smaller by a factor of 3-10.

Other phenomena relevant to the process of star formation (e.g., Jeans'

lengths for gravitational collapse in dense molecular clouds, the sizes of

the high-velocity molecular flows seen in Orion and other sources, and the

projected thickness of shock and ionization fronts) also have scale sizes

in the range of . Ol-.l pc. Since the nearest star-forming regions are

typically a few hundred pc away, only the closest sources and the largest

scales can be adequately resolved by current far-infrared telescopes (Figure 2.1-1).

In addition to extending our knowledge of regions such as the

Orion/BN/KL complex, the two-meter telescope should provide a powerful tool

for studying lower-mass protostars and their relationships and interactions

with their natal clouds. The ability to image structures with angular

scales of 10 arcseconds to 10 arcminutes at wavelengths between 30 and 300

microns is crucial to an understanding of the nearest dark-cloud complexes

(150-200 pc). Recent observations with the University of Chicago 32-

channel far infrared and submillimeter cameras on the 0.9-meter telescope

of the Kuiper Airborne Observatory and the three-meter telescope of the

NASA Infrared Telescope Facility have begun to probe these regions with

beam sizes of 45" and reference-beam (chopper) spacings of 5'. At this

beam size, it is possible to detect clouds which are externally heated by

the ambient interstellar radiation field. Imbedded sources of the order of

on solar luminosity can just be clearly distinguished against this background surface brightness. This is also the level at which it should be

possible to study regions which have been mechanically heated by gaseous

outflows from active protostars.

Another area of star formation that can be best studied at submillimeter wavelengths by a two-meter balloon-borne telescope is large "infrared

quiet" molecular clouds. Observations of the J = 1 -» 0 rotational transi-
Page 30

tion of CO near the galactic plane in Monoceros (l = 216 degrees) reveal a

molecular cloud with unusually low peak CO temperatures (T < 2 K) but wide

lines (7 km/s) typical of much warmer clouds (Maddalena and Thaddeus,

1985). At the assumed distance of 3 kpc, the cloud is large (250 x 100

pc), has a mass of 7 - 11 x 105 M0, and is well removed from the galactic

plane (130 pc). Except for a possible H II region, all the signs of star

formation usually shown by clouds of comparable mass are missing. The

cloud was not detected in the IRAS survey, and unlike cloud complexes of

similar size, it is a single, continuous object that apparently has not

been torn apart by star formation. Clouds with such properties are rare in

the Galaxy; only two other similar objects have been found. Large area

(3x 4 degrees) mapping at submillimeter wavelengths could determine if the

cloud is young and has not yet formed stars, but will evolve into a typical

cloud complex once star formation begins.

These studies are currently limited to a small class of sources which

are sufficiently small, isolated, and geometrically simple that the 5'

reference-beam spacing does not seriously affect the interpretation of the

far infrared and submillimeter data. The two-meter balloon telescope would

be able to simultaneously provide better angular resolution at far infrared

wavelengths, larger reference-beam spacings, and larger scanned areas than

is practical with airborne or ground-based measurements. Thus the balloon

telescope will fill a crucial niche between facilities like SOFIA and IOmeter class ground-based telescope which can provide good data on smaller

structures and IRAS sky-maps which provide excellent data on structures

larger than a few arcminutes at wavelengths less than 120 microns. It is

impossible to completely understand the complex relationships between

molecular clouds, ambient stars and low-mass protostars without thorough

coverage of this range of spatial scalelengths.

2.3.2 Galactic Structure

The major unsolved problems of galactic structure are how galaxies

form, how they evolve through successive generations of star formation, and

the nature and significance of energetic events in their nuclei. A fullscale attack on these problems in the far infrared will require the very

large collecting area and high angular resolution of space-based instruments such as LDR -- particularly for attempts to observe galaxies at very

high redshifts. However, the Two-Meter Balloon-Borne Telescope could

contribute substantially to our understanding of processes occurring in the

nearest galaxies, thereby laying the groundwork for later work with larger

instruments.

At 100/zm the Two-Meter Balloon-Borne Telescope can resolve an object

about 180 pc in size at a distance of approximately 3 Mpc. This is a

sufficiently large distance to include the nearest field galaxies and

groups beyond the Local Group. One-to-two pc is comparable to the scale

sizes of individual giant molecular clouds, to the thicknesses of the disks

of spiral galaxies, and to the scale of "circum-nuclear" structure in our

own galaxy (e.g., to the major features in far-infrared maps made with

angular resolutions of a few arcminutes). This is the scale required, for

example, to distinguish low-lumin sity "nuclear" sources from nearby,
Page 31

luminous molecular cloud complexes. It is also the scale at which one can

begin to test hypotheses concerning star formation, in spiral arms and

circum-nuclear "rings" (Figure 2.1-1).

As in the studies of star formation described in Section 2.3.1, the

balloon telescope can play a crucial role by virtue of its ability to image

large areas at a variety of scale sizes, wavelengths, and angular and

spectral resolutions. It will be particularly valuable for examining the

nearest galaxies, which are inconveniently large for airborne and groundbased instruments but which offer the best opportunities for examining the

detailed structure of spiral arms, nuclear regions, bars, and individual

star-forming complexes.

2.4 Cometary Science with a Two-Meter Balloon-Borne Telescope

Comets are of interest because they are the least modified bodies

remaining from the formative phase of our planetary system. The compositions and physical structure of cometary nuclei are central data for interpreting the chemical and physical processes which accompanied their formation. A few comets may be visited in future by spacecraft for in-depth

investigation, but the vast majority can only be studied remotely, and it

is certain that the taxonomy of cometary classes can only be established by

characterizing many of them. Several decades of observations at ultraviolet, visual, and radio wavelengths have provided a wealth of information on

the dissociation products of the primary molecules, and even detection of a

few trace constituents (e.g. S2, CO, HCN). However, the general detection

of the dominant species has only become possible with the development of

the theory of solar infrared fluorescence (cf. Weaver and Mumma 1984) and

the extension of infrared spectroscopic instrumentation to the cometary

problem, particularly at airborne and spacecraft altitudes. Recent spectroscopic measurements of HaO and CH^ from the KAO and of H20, CC>2, the

organic grain feature, and possibly HjCO from the Vega spacecraft have

established the feasibility of characterizing both the volatile and solid

phase fractions of the cometary nucleus at infrared wavelengths (Mumma et

al. 1986, Weaver et al. 1987, Larson et al. 1988, Combes et al. 1988).

Theoretical modelling of the far infrared spectrum of water demonstrates

that the strongest rotational transitions become optically thick at

~1 x 105 km from the nucleus, thus those lines can be used to probe the

kinetic temperature throughout the coma, if their line shapes are measured.

While many important investigations can only be carried out from

airborne telescopes, and a few with ground-based instrumentation, there are

several which require a balloon-borne telescope. First is the detection of

COj, near 4.3 microns. This is not possible from 12 km altitude, owing to

opacity of the terrestrial atmosphere. However, the opacity is reduced

enough at 30 km to permit its detection when the geocentric doppler shift

is favorable, similar to the present situation for HjO from airborne telescopes. Second is obtaining a complete inventory of water lines in the 2.7

micron band, part of which is obscured by the 101-000 band of C02 at aircraft altitudes. This will permit a highly accurate measurement of the

ortho-para ratio, which may be a cosmogonic invariant in comets, preserving

a record of the temperature of formation of the primordial water molecules.
Page 32

Third is the measurement of the far-infrared spectra of comets, which

promises to be exceedingly rich for detection of new species, and for

probing the physics of the cometary coma through fully resolved spectra of

the water lines (e.g. 110-101) .

The ability to measure the 3.0-2.5 micron region, free of the obscuring .effects of CC>2 and H20 should permit complete characterization of the

competing forms of the OH stretch found or suspected to be present in

Halley's comet (prompt emission and solar fluorescence from gas-phase OH,

nu-3 and hot-band stretch in H20, OH in hydrated minerals, and possibly OH

stretch in water cluster molecules). In addition, complete freedom from

obscuration in the other bands will provide similar gains for other molecular species, and will assist in separating molecular and solid phase contributors to the 3.4 micron feature, for example.

2.5 Scientific Performance

The infrared performance of the Two-Meter Balloon-Borne Telescope is

determined by the aperture, effective telescope emissivity, and the telescope temperature, the same items that determine the LDR performance. With

the exception of the aperture, the values for these quantities on the

gondola should be similar to those of the LDR. This telescope will operate

at an ambient temperature of about 230°K and should have an emittance near

10%. A lower value of the mirror emittance may be achievable. Figure 2.5-

1 gives the broadband sensitivity at 100/zm vs. the diffraction-limited beam

size for existing and planned instruments. The highest sensitivity is at

the top. Figure 2.5-1 shows clearly the effectiveness of the Two-Meter

Balloon-Borne Telescope for follow-through of 100-/«n sources at the limit

of IRAS sensitivity, a task of which the KAO is not capable. The Two-Meter

Balloon-Borne Telescope has substantially less sensitivity than SIRTF for

broadband photometry because of its higher temperature. It is superior to

SIRTF only in angular resolution and in high spectral resolution observations limited by detector noise.

The sensitivity of the two focal-plane instruments proposed for the 2-

meter balloon-borne telescope, the photometric infrared camera and the

heterodyne submillimeter receiver, are discussed in Sections 5.2 and 5.3.
Page 3.-.

FAGS IS

EDOR QUALITY

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l!i-;aai Size. For the pointed telescopes, the limiting flux is given for a

signal -to -noise of LOcr, bandwidth AA/A = 0.5, beam size of 1.22 A/D,

photoconductor detective quantum efficiency of »; = 0.5, and integration

'tiDif.- of 30 minutes with half of the time on the source. For the Two-Meter

Balloori-Borne Telescope and LDR, the telescope emisslvity and instrument

efficiency are each taken to be 0.1 and the telescope temperature 220°K.

For the KAO and SOFIA, the telescope ercissivity, sky emissivity, and trans

mission are taken to be 0.2, 0.2, and 0.8 respectively. The IRAS survey

limiting flux is taken from IRAS ..esults.
Page 34

3 .0 GONDOLA DESIGN

3.1 Telescope Design

3.1.1 Telescope Requirements

Four important scientific requirements of the telescope which dictate

the optics specifications are: l) to cover the spectral range not observable or only poorly observable from the ground from 30/zm to 1 mm; 2) to

provide a collecting area much larger than now available from 1-meter size

balloon-borne and airborne telescopes for high resolution far-infrared and

submillimeter spectroscopy; 3) to provide a major advance in far-infrared

spatial resolution; and 4) to take advantage of the potential sensitivity

obtainable from the low thermal background at balloon altitudes. The

optics specifications resulting from these requirements are given in Table 3.1-1. The resulting preliminary design is shown in Figure 3.1-1 and

in the photograph in the frontispiece.

The two-meter aperture provides the a large step in collecting area

and spatial resolution, and is compatible with current U.S. balloon launch

facilities. The spectral range and angular resolution requirements imply

diffraction-limited optical performance at a wavelength of 30/zm. This

means a half-power full-width of the central diffraction fringe of 3.9

arcseconds, which determines the acceptable image blur from optical design

aberrations, misalignment, and fabrication errors. Conventionally, with

optical (visible light) telescopes, this is taken to mean total effective

rms surface errors of 1/26 of the wavelength. This yields a Strehl ratio

(the ratio of the peak intensity of a point source to that produced by an

ideal diffraction pattern) of 0.8. For radio telescope design, the rule of

thumb is to require a surface half as accurate yielding a Strehl ratio of

0.5. Thus we interpret a 30-//m diffraction-limited performance to mean an

image diameter (HPFW) of 3.9 arcseconds and a total surface error of 1- to

2-fim rms. Since much of the most important science done on this telescope

will come from observations at 50 and 100/jm and beyond, the 2-/zm rms surface accuracy appears adequate.

The specification for operation at visible wavelengths is determined

by the desire to use the main telescope to image stars for guiding purposes. This places additional constraints on the character of the optics

surface error and polish. We intend to make every effort to meet this

constraint and will relax it only if it cannot be achieved at reasonable

primary mirror cost.

The unvignetted infrared field of view of 6 arcminutes is chosen to

provide 1H full Airy disk diameters (diameter to the first diffraction

zero) at the longest wavelength of operation (l mm).

Achieving maximum possible sensitivity requires an effective noisefree mechanism for subtracting the thermal background due to the telescope

and sky from the observations. The most successful technique to date for

doing this is beam switching by oscillating the secondary mirror through an

angle either with a square wave or a linear scan. While the use of arrays

of detectors in the far infrared may provide an alternative background
Page 35

Optical Design

Aperture

Spectral Range

Optics Design

Focal Ratio

Back Focus

Interoptic Spacing

Angular Resolution

Infrared Field of View

Optical Field of View

Secondary Chopper

Chop Rates

Deadtime

Position settability

Position repeatability

Position drift

Chop axis rotation settability

Focus settability

Pointing Stability

Inertia! Mode

Magnetometer Mode

Source Offset

Slew Rate

Thermal Stability

Telescope

Mirror Stabilization Time

Command and Telemetry

Number of ERIG User Channels

Useful Range

IRIG VCO Channels Available

PCM Bit Rate

Video Bandwidth

Gondola Characteristics

Total Weight

Overall Height

Overall Width

Overall Depth

Experiment Accommodations

Power

Weight

2 Meters

Visible to millimeter

Cassegrain

f/13.5

93 cm from primary mirror vertex

52 cm from instrument mounting plate

180 cm

Diffraction limited to 30nm (Raleigh Criterion 3.9")

6' diameter unvignetted with ± 3' chop and diffraction spillover at 1 mm

15' vignetted only by the primary

16 Hz up to 1' throw

2 Hz at 6' throw

20% (6ms) at 16 Hz

3% of total throw

0.25"

< I'Vhour

1°

10% of depth of field

1" rms

± 10' azimuth, ± 5' elevation

± 1/2° (Magnetometer Mode)

ioys

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< 1 Hour

To limit of standard NSBF telemetry

1/3/5/7/8/9/10/11/12/B/E/H/HH

81 KBpS; expandable to 256 KBpS

4.5 MHz

1731 kg (3809 Ibs.)

5.8 Meters (19 Feet)

2.9 Meters (9 Feet 6 Inches)

2.5 Meters (8 Feet 2 Inches)

140 W typical; 425 W maximum

125 kg at focal plane

Table 3.1-1. Telescope and Gondola Specifications
Page 36

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Page 37

subtraction technique for some observations, the secondary chopper still

appears necessary for this telescope. Maintaining quality images while

tilting the secondary places major demands on the optical design.

A second aspect of the high sensitivity requirement is minimizing the

thermal emission of the telescope by minimizing obscuration from the secondary mirror and the secondary mirror support structure and maintaining

very low emissive surfaces on the optics. Both the chopper and the low

obscuration requirements dictate a small secondary mirror.

3.1.2 Description of the Cassegrain Telescope

a) Optics Design

Figure 3.1-1 illustrates the design of the telescope optics and specifications. The optical design is based on third order analytic aberration

theory as presented by Gascoigne (1973). The telescope is a classical

Cassegrain with a paraboloidal primary and hyperboloidal secondary. The

secondary is undersized relative to the primary so that it provides the

optical stop of the system. The undersizing of the secondary and the hole

in the primary are selected so that for the full field size, the maximum

chop amplitude, and the full diffraction width (diameter of Airy disk first

zero) at the maximum design wavelength (l mm) the field of view of a detector at the focal plane will "see" only the secondary, the cold sky, and the

primary, but not the warm periphery of the primary nor the edge of the hole

in the primary. The rays from the focal plane which would be reflected

from the center of the secondary into the hole in the primary are deflected

by a button on the secondary to the primary and hence to the sky (very much

out of focus).

The minimum wavelength (30 microns) and associated diffraction HPFW

determines the diffraction-limited performance of the telescope. The

maximum HPFW is defined at the longest wavelength expected to be used (l

mm). The diffraction size of the beam at this wavelength contributes to

the required secondary undersize. The secondary undersize ratio is set to

assure that it is the optical stop of the telescope for all normal use.

The optical design parameters are chosen to provide no spherical

aberration or other third order aberrations in the image at the nominal

focal plane on axis. Figure 3.1-2 shows how the coma, astigmatism, and

field curvature affect the image size as a function of field radius. Figure 3.1-2 also gives the miscollimation coma length due to secondary chopper tilt as a function of image displacement in the focal plane. The chop

angle (in terms of focal-plane displacement), back focus, and field radius

for which the image blur (or coma length) is 1.8 arcsecond in size are also

given in Table 3.1-1. The greatest image degradation comes from the chopper tilt. This is a basic problem for this method of sky subtraction with

a fast primary Cassegrain telescope. Shown on the coma length curve in

Figure 3.1-2 are points for a Strehl ratio of 0.9 at 30pm wavelength and

for 3.0 arcminutes chopper amplitude. When coma is the only optical aberration, the Strehl ratio (ratio of peak intensity of a point source to that

produced by an ideal diffraction pattern) is .9 when the coma length equals
Page 38

STREHL RATIO = 0.9 AT 63

CHOPPER INDUCED

MISCOLLIHATION

COMA LENGTH

STREHL RATIO = 0.9 AT 30

30 u DIFFRACTION HALF POWER WIDTH

FIELD OFF-AXIS BLUR

.5 1 1.5 2 2.5 3

FIELD RADIUS / CHOPPER HALF ANGLE (ARC-MIN)

Figure 3.1-2. Image Blur vs. Fi<_id Position and Chopper Angle
Page 39

the Airy disk full diameter. For this telescope the coma length equals the

30/im Airy disk diameter of 7.8 arcseconds at a chopper angle of 1.4 arcminutes. The coma length equals the 63/im Airy disk diameter of 16.6 arcseconds at a chopper angle of 3.0 arcminutes. At a chopper field amplitude of

3.0 arcminutes, the Strehl ratio is 0.9 at 63/zm. Thus, for a Strehl ratio

of 0.9, at all wavelengths the maximum focal-plane displacement due to

secondary chopper tilt is 22 resolution elements (1.22 A/D). The coma

length is nearly independent of telescope aperture for a given chopper

field angle, primary focal ratio, Cassegrain focal ratio, and back focus.

The coma length varies inversely as the square of the primary focal ratio

for a given chopper angle, Cassegrain focal ratio, and back focus.

b) Alignment Tolerances

The design in Table 3.1-2 and the desire for a diffraction limited

(3.8-arcsecond) beamwidth determine the acceptable alignment tolerances for

the secondary tilt, decenter, and axial motion. This includes both initial

alignment when the gondola reaches float altitude and the stability of

alignment during observing. Initial alignment has far greater tolerances

than alignment stability because of the adjustments which can be made in

secondary axial motion (focus) and secondary tilt after the gondola reaches

float.

Meinel has pointed out that precision centering of the secondary of a

Cassegrain telescope is not essential. The miscollimation coma created by

a secondary decenter can be corrected for by a compensating secondary tilt.

For this reason this telescope requires only a remote controlled secondary

tilt adjustment, not a centering adjustment, for adjusting the collimation

of the telescope after it reaches float altitude. This adjustment can best

be made by observing the symmetry of the aperture illumination with an outof-focus image of the bright star utilizing the focal-plane tracking camera .

c) Initial Alignment

With the exception of decenter, the driver for the optical stability

tolerance requirements on reaching float altitude is the necessity of

insuring rapid and efficient alignment of the telescope at the beginning of

observation. It is necessary to have a star image appear in the field of

the main focal-plane camera when it is centered on the boresighted acquisition guide telescope and to have the telescope focus close enough to correct the result in a detectable star image. The decenter tolerance is

limited by the amount of decenter that can be compensated by secondary tilt

before the resulting tilt of the focal plane blurs images at the edge of

the field of view unacceptably.

With regard to decenter, we have assumed that the initial defocus

tolerance results in an image four times the full diameter of the 30-/«n

central diffraction fringe, that is, 30 arcseconds; and tilt and decenter

result in a coma of 15 arcseconds. Also, we have assumed an angular offset

due to initial tilt and decenter equal to the focal-plane infrared camera

field radius, 3.0 arcminutes. Finally, the initial decenter is limited by

requiring an image defocus blur of less than 1.8 arcsecond at the edge of

the field due to focal-plane til ^ after secondary tilt compensation. The
Page 40

Table 3.1-2. Telescope Optics Design and Performance

Aperture (cm)

Cassegrain focal ratio (f/)

Primary secondary separation (cm)

Primary focal-plane separation (cm)

Infrared field radius (')

Chop field amplitude (')

Minimum wavelength

Maximum wavelength

Visible field radius (')

Diffraction HPFW min (")

Diffraction HPFW max (')

Secondary undersize

Cassegrain focal length (cm)

Focal-plane scale ("/mm)

Field diameter (cm)

Primary focal length (cm)

Primary focal ratio (f/)

Primary sagitta (cm)

Primary departure from sphere (cm)

Primary hole in IR min (cm)

Primary hole in IR max (cm)

Primary hole visible min (cm)

Secondary diameter (cm)

Secondary focal length (cm)

Secondary conic constant (-e2)

Secondary sagitta (cm)

Secondary departure from sphere (cm)

Secondary button min (cm)

Secondary button max (cm)

Secondary axial magnification

Secondary angular magnification

Range for 1" blur of 1.8"

Chop (•)

Back focus (cm)

Field coma (')

Field astigmatism (') quadratic

Field curvature (') quadratic

Total field aberrations (')

200

13.5

180

93

3

3

30

1000

7.5

3.9

2.1

.040

2590

7.96

4.52

201.2

1.00

12.4

.20

12.04

12.09

14.34

20.22

-22.99

-1.365

1.10

.013

2.44

2.99

-167

.210

.33

9.5

26.8

17.4

4.8

4.7

resulting initial tolerances are given in Table 3.1-3.

If there is a predictable and repeatable focus shift from ground to

float altitude, it can be removed by the initial focus setting. For example, a carbon-fiber-reinforced plastic secondary truss support with a

coefficient of thermal expansion of 2 x 10"6°C will result in a secondary
Page 41

defocus motion of 0.23 mm due to a temperature change of 65°C from ground

to float altitude. The tolerances in Table 3.1-3 cover the remaining

mechanical and thermal uncertainties of such a correction. They should be

treated as maximum tolerances. Smaller errors will result in faster telescope realignment during the flight.

Table 3.1-3. Telescope Optics Alignment Tolerances

Alignment for blur of 1.8"

Secondary decenter (mm) -.095

Secondary tilt (') -1.55

Secondary axial motion (mm) .018

Alignment for motion 1.8"

Secondary decenter (mm) -.019

Secondary tilt (') .14

Initial Alignment

Secondary defocus blur of 36" (mm) .35

Secondary decenter for coma of 18" (mm) .95

Secondary decenter corr. blur of 1.8" (mm) 2.05

Secondary decenter for motion of 3' (mm) 2.86

Secondary tilt for motion of 3' (') 21.3

Secondary tilt for coma of 18" (') 15.5

d) Operating Alignment Tolerance

The operating tolerances for secondary tilt, decenter, and axial

motion each defocusing the image by 1.8 arcsecond are also given in Table 3-1.3. These tolerances are for telescope stability between realignment on a reference star, an activity which should take place as little as

possible during the flight. The defocus blur is minimized by refocusing on

a bright star. The decenter blur can be corrected by examining an out-offocus star image and tilting the secondary to restore collimation. The

decenter and tilt shift tolerance affect the stability of the bore sighting

with a tracking telescope. If the gyros are updated by images in the main

telescope these tolerances can be substantially relaxed.

3.1.3 Optics Design Trade-Offs

a) Tradeoff Approach

The telescope design is a compromise between the largest aperture

compatible with the National Scientific Balloon Facility and a size and

shape providing affordable construction, ease of transport, and minimum

launch constraints. We have chosen an aperture of two meters which meets

the requirements and is at a breakpoint in the available equipment and cost

of fabricating light weight carbon fiber reinforced plastic mirrors. In

the design study we have chosen as the variable parameters the Cassegrain

focal ratio, primary-secondary separation, and primary-to-focal-plane
Page 42

distance. We have considered the required field radius, chopper field

amplitude, and maximum operating wavelength to be fixed. The tradeoff

studies investigate the impact of varying the input parameters on the

focal-plane scale, primary focal ratio, secondary diameter, back focus

range, field aberrations, chopper-induced aberrations, and alignment tolerances. These impacts were examined for five different values of each of

the three parameters while the other two were held fixed at the chosen

design values. The calculations were based on the third order analytic

aberration referred to in Section 3.1.2. The tradeoff approach and explanations of the items in Tables 3.1-2 and 3.1-3 are given in PDR Appendix A.

b) Interpretation of Trade-offs

The advantage of reducing the primary to the focal plane distance is

to simplify the telescope structure design and reduce its weight. The back

focus distance is constrained both by instrument requirements and the

required depth of the primary mirror backup structure.

The significance of the items investigated is as follows:

Focal-Plane Scale. This along with the field size determines the minimum

possible size of the instrument dewar window and the scale of the instrument cold optics and is especially important at the longest wavelengths

where even a small number of elements of an imaging array cover the full

six-minute-diameter field. For a small field diameter, the focal plane

scale should be large.

Primary Focal Ratio. Optically ground and polished, paraboloidal primary

mirrors of large size have been made only to f/2. New methods utilizing a

computer controlled flexible lap promise to extend this to much shorter

focal ratios. Replicated primary mirrors have been fabricated with focal

ratios as short as f/.35. In general, the larger the primary focal ratio,

the easier the fabrication and testing of the primary is, although this is

not considered a major constraint for this tradeoff.

Secondary Diameter. The secondary obscuration which impacts the effective

emissivity of the telescope increases as the square of the diameter. The

secondary moment of inertia that dominates the secondary chopper performance increases as the fourth power of the secondary diameter. A major

driver for the telescope is to keep the secondary as small as possible.

Back Focus Range. Since instruments are to be designed for operation at

the design focus, the back focus range is not a significant consideration.

Field Angle for One-Arcsecond Blur. The field aberrations are well within

the specifications for image blur at the specified 6-arcminute infrared

field diameter for all designs examined. The field aberrations are dominated by the field curvature.

Chop Amplitude. Chopper-induced coma is by far the largest optical aberration of the telescope as shown in Figure 3.2-2. Hence, the chopper-induced

image blur is a critical item to minimize over the tradeoff range.
Page 43

Secondary Axial Motion for 1.8-Arcsecond Blur and Secondary Decenter for

1.8-Arcsecond Blur. These two items indicate the tolerances the telescope

must maintain during observations without refocusing or adjusting the

secondary tilt to compensate for decenter. The 1.8-arcsecond blur increases the image half power width by about 10% over the 30 micron diffracted image. While it appears from the mechanical design analysis that

these tolerances are acceptable and can be met, clearly trade-offs that'

increase the tolerances are preferable.

c) Conclusions

The most critical goals of the design tradeoff are in descending order

of priority:

1. Minimize the secondary diameter.

2. Minimize the chopper induced coma length.

3. Minimize the size of the 6-arcminute field diameter at the

Cassegrain focus.

These impact the free parameters as follows:

1. Cassegrain focal ratio. This should be small for maximum chopper

amplitude and minimum field size. It should be large for minimizing the secondary diameter. A focal ratio of f/13.5 is a compromise choice. It is also identical to the NARO 13 meter millimeter

telescope at Kitt Peak for which a number of high frequency heterodyne receivers have been built.

2. Overall length (secondary vertex-focal plane). Reducing this

reduces the secondary diameter and the gondola size but requires a

faster primary and increases the chopper induced coma. For a

given chopper angle the coma length is inversely proportional to

the square of the primary focal ratio. The overall length has no

effect on the field diameter. The chosen design favors low weight

and ease of transportation at the expense of a fast primary and

chopper induced coma. Chopper induced coma can be reduced at the

expense of producing field coma by altering the secondary figure.

3. Back focus (primary vertex-focal plane). Reducing the back focus

improves the chopper amplitude and the field diameter and has no

effect on the secondary diameter. The smallest back focus is best

for the optical performance. The design value is chosen to accommodate expected instrument configurations.
Page 44

3.1.4 Telescope Mechanical Design

The optical design characteristics and optical positioning tolerances

were given in Tables 3.1-2 and 3.1-3. The telescope mechanical design is

aimed at achieving these optical positioning requirements in an efficient

manner. The design for the two-meter telescope is based directly on the

three-meter telescope design presented in the PDR. Our goal in the redesign was to produce a gondola of minimum practical weight. The reduction

in size and the selection of a faster (f=~1.0) primary significantly reduces the telescope's weight, inertia and length. Positional requirements

for the secondary mirror are tightened by a factor of two, however, due to

the change in optical design. Fortunately, the reduction in truss length

offsets this tighter positional tolerance.

Primary Mirror and Cell. The primary mirror cell serves as a pedestal from

which to erect the telescope including its aspect cameras, stabilization

components, mirror protection cover and experiments. Table 3.1-4 lists the

major telescope components along with their expected weights. The primary

cell will be a monocoque structure fabricated from graphite-epoxy composite

in a fashion similar to that proposed for the primary mirror. This affords

maximum structural stiffness per unit of weight while minimizing mirror

distortions created by temperature-induced differential expansions between

the mirror, its mounting system, and the cell. The cell is directly attached to the cross-elevation gimbal via a set of centrally aligned flexpivots. This interface is insensitive to temperature effects between cell

and gimbal. The mirror cell also provides mirror mounting and protection

while serving as a base for the secondary truss. The primary mirror mounting technique will most likely follow techniques developed by DornierSystem GmbH for mounting large-diameter graphite-epoxy millimeter wavelength radio astronomy telescope primary reflectors.

Mirror protection is accomplished in two ways. First, the mounting

cell has a forward skirt that protrudes beyond the mirror, thereby placing

the mirror in a protective cavity. This skirt can be removed to provide

access for mirror mounting. Second, a retractable mirror cover, presently

conceived as a rolled curtain of strong plastic woven mesh, is mounted on

the skirt. The cover will be used during testing and during recovery to .

prevent tools, telescope components, tree branches, and the like from

striking the mirror.

The quadrupod secondary mirror truss mounts at four points just forward of the primary mirror. This is a departure from the standard infrared

telescope design of a circular head ring with four thin secondary vanes and

was chosen to minimize the overall gondola weight while guaranteeing the

positional accuracy required by the optical design. The telescope truss is

designed to place the natural vibrational frequencies of the truss well

beyond the pointing and stabilization servomechanism systems' bandpass to

minimize coupling sensitivity to secondary chopper motion and other vibrational inputs.

The secondary mirror assembly truss mounting must allow positioning of

the secondary to <.002cm and support re-entry loads without failure. Secondary mounting designs of similar characteristics have been developed for
Page 45

Subsystem Unit Weight Total Weight Total Weight

(pounds) (pounds) (kilograms)

I. Secondary Mirror Assembly

Mirror 10

Focus Drive 22

Chopper 25

Tilt System 5

Truss and Attachments 125

187 85

II. Primary Mirror Assembly

Mirror 140

Mirror Cell 250

Mirror Cover 30

5° FOV Aspect TV 15

1°FOV Camera 25

Focal Plane Camera 40

Cross-Azimuth Servo 20

Offset Pointer 20

Miscellaneous 20

560 255

III. Cross-elevation axis

Cross-Elevation Drive System 60

Elevation Drive System 40

Gimbal Frame 85

Cross-elevation Reaction Wheel 25

Elevation Reaction Wheel 25

Electronic Interface 10

Wire Harness 10

Miscellaneous 25

280 127

IV. Experiment Package

Tailpiece 50

Submillimclcr Spectrometer 125

IR Camera 50

Experiment. Selector 12

Combined Electronics 100

Trim Weights 5.

342 155

TOTAL TELESCOPE WEIGHT 1369 Ibs. 622kg.

Table 3.1-4. Two-Meter Telescope Subassembly Weight Summary
Page 46

ground-based telescopes and should not pose any technical problems for this

program.

The telescope is capable of making observations from 10 degrees below

the horizon to an elevation of 65 degrees. The truss must therefore maintain the position of the secondary mirror over this observational range.

The truss structural characteristics are calculated to be:

Natural frequency, gravity axis >210cps

Natural frequency, chopped axis >234 cps

Natural frequency, individual member >137 cps

1 g deflection (zenith to horizon) 2X10"4 in

(>5xl(r4 cm)

Gravity deflection Hr'1 at 5xl(r5 in

fixed pointing (fsl.2xlO~4 cm)

The truss is also subjected to a large thermal excursion from launch

to altitude with a small fluctuation occurring at altitude. A temperature

difference of 54"C (130°F) and ±5.6°C (±10°F) fluctuation was used to determine the material characteristics required to minimize the need to

actively refocus the telescope during an observation. This requirement

mainly influenced the selection of graphite-epoxy as the truss material,

although its strength-to-weight ratio and natural damping qualities are

also highly desirable. The temperature defocus was calculated to be

0.0094 cm over this range which implies an in-flight defocus equal to

0.008 cm. These values are attained by using a coefficient of thermal

expansion of 1.0xlO~6/F° which is conservative for this material.

3.1.5 Telescope Thermal Design

Minimizing telescope distortion induced by the adverse thermal environment at float altitude is the major criterion for thermal design of the

Three-Meter Balloon-Borne Telescope. Active control of telescope optics

and structure at the original launch ambient temperature during flight is

impractical because of the complexity of doing so and the very large heater

power requirements. Fast stabilization of the telescope at a quasi-equilibrium temperature (at which heat exchange with the environment is minimized) and maintenance of this condition throughout the observation period

is the primary goal of this design. Thermal gradients across the mirror

face and through its thickness must be minimal, focal length must be within

tolerance, and quadrupod temperatures must be equal and stable.

The Two-Meter Balloon-Borne Telescope will utilize the enhanced convection environment of the ascent phase and the low air temperatures of the

tropopause to cool critical elements to the desired temperatures, about -

40°C. Although little investigation of convection conditions in a balloon

environment has been reported, our own work (summarized in Section 2.2.4 of
Page 47

the PDR) shows that convection coefficients on exposed surfaces of order

10 W/m2K or greater can be expected during a large portion of ascent, and

about 5 W/m'2K at float altitude.

It is readily calculated that a lightweight glass mirror of density

70 kg/m2, with both surfaces exposed to such a convective environment of

temperature 20°C below itself, will reach -40° during ascent. Such a calculation is conservative with respect to both convection and air temperature, so there is adequate reserve cooling capability; enough, in fact,

that a provision to prevent overcooling may be needed. We have also verified that radiative heat transfer in the cellular construction of a lightweight Pyrex mirror permits the mid-plane to stabilize with a time constant

of less than a half hour. A composite mirror would stabilize even more

rapidly because of higher thermal conductivity as demonstrated in the

cooled test chamber.

During observation an exposed primary mirror would radiate to space at

10-15 W/m2, depending on the emittance of its polished surface. However, a

protective tube surrounding the optical path will reduce the solid angle of

exposure to space, reducing the net radiation to 2-3 W/m2. This radiation

will be balanced by convection to the front surface from the air near the

surface and by conduction and radiation from the rear surface to the front.

This second mode would result in a temperature difference between front and

back which would distort the figure, and must therefore be kept to a minimum by radiative and convective isolation of the back surface (coatings,

insulation, or both), allowing convection at the front to dominate. It is

not yet clear what the convective environment will be inside the telescope

tube (partially shielded from relative motion between the gondola and

surrounding air) and a scaled-down experiment is planned for the near

future to help determine this.

Structural elements such as the quadruped legs will respond to the

ascent convective environment with a time constant of a few minutes. Overcooling in the tropopause will occur, and response will slow as the experiment nears float, but these elements will be within a few degrees of final

temperature within a half hour of achieving altitude. At stable altitude,

and therefore stable air temperature, the quadruped will be stable as well

except for changing radiative environment. Controlled-emittance coatings

will be used on critical structural elements; detail design will determine

whether coatings of high emittance (giving faster response but greater

excursion) or low emittance (yielding the opposite) are best.

3.1.6 Chopping Technique

Telescope specifications call for a secondary chopper with a beam

switching angle referred to the sky of 6 arcminutes at a frequency of 16 Hz

and a dead time of 20%. The secondary mirror diameter is 20.2 cm. It will

be lightweighted. Several options exist such as beryllium, glass, composite, and foamed silicon carbide.

High performance servo-controlled secondary choppers have been developed by Steward Observatory for the Multiple Mirror Telescope, by Kitt Peak
Page 48

National Observatory (KPNO), and by several balloon and aircraft infrared

groups. The Two-Meter Balloon-Borne Telescope secondary design will take

advantage of these developments. It will have a reactionless drive with

high permeability core "loud speaker" type drives pushing against a reaction plate. The drive will permit remote control of the chopper throw and

frequency.

Properties of the secondary chopper are given in Table 3.1-1.

3.2 Support Structure Design

3.2.1 Mechanical Concept

The Two-Meter Balloon-Borne Telescope design concept is shown in

Figure 3.2-1. Two major assemblies, the telescope and support structure,

form the overall gondola.

The support structure is a yoke frame that serves five major functions :

• Telescope structural attachment to the balloon.

• Telescope azimuthal orientation and coarse stabilization.

• Telescope elevation reference frame.

• Packaging for all electronics and gondola support systems.

• Launch, landing and recovery protection.

A yoke frame has been selected as the best method of telescope support

given NSBF size and weight restrictions. The telescope weighs 623 kg

(1,734 Ibs.) and measures 3.24 meters along the optical axis (including

experiments), 2.9 meters in width, and 2.5 meters in depth. The yoke frame

requires only 342 kg (755 Ibs.) to meet the telescope's structural loadcarrying requirements while only increasing the overall gondola size to

5.8 meters in height and 3.9 meters in width with no change in depth.

Deployable crash rings are needed to protect the telescope on landing.

These add to the gondola's width and depth but not sufficiently to cause a

handling problem with the NSBF launch vehicle or staging building.

A momentum transfer device is used at the point of balloon attachment.

It provides 360 degrees of angular rotation for azimuthal orientation and

is decoupled except for the friction required to transfer momentum for

azimuthal stabilization.

The gondola yoke frame will align itself with the earth gravity vector. The elevation servo system uses this gravity alignment for determining the telescope elevation angle and for stabilization purposes.

The telescope will be locked 'n place during launch. Locking is

necessary to ensure maximum telescope and experiment protection for all
Page 49

a

r-4

O

O

GO

HI

•H

0)

0)

H

CO

60

•H

U.
Page 50

times when it is exposed to impact loads. The locking device will be

thoroughly tested with redundant schemes for activation so that the highest

probability exists for achieving the locked configuration. Since telescope

locking can not be absolutely guaranteed, the structural members were sized

to carry the full loads of an unlocked telescope. Stress levels were

allowed to rise to within 80% of the materials' yield strength, thereby

yielding a minimum factor of safety equal to 1.5 for structural failure.

The diagonal and side column members were designed as thin-walled,

deep-section aluminum beams.

3.2.2 Launch, Landing, and Recovery Considerations

Launch requires that the gondola be suspended from the launch vehicle

for transportation to the launch area and subsequent launch. The gondola

will be exposed to low level mechanical vibrations during this activity.

Experience indicates these levels to be less than the vibration levels

experienced during nominal handling in test and experiment integration

phases. Therefore, we will design the gondola for no detectable performance degradation when exposed to nominal handling. This is specified as

not to exceed 3 g's rms over the spectral range of 20-2,000 Hz. The NSBF

requires five feet minimum for ground clearance and various radial clearances at elevations below the launch arms. This relatively light payload

can be supported well in front of the vehicle allowing adequate ground and

vehicle clearance. The crash rings have been stationed at the secondary to

allow proper radial clearance with the launch arms. All sensitive equipment and experiments are fully protected during launch.

Damage is most likely during landings. The gondola will descend in

its normal (upright) flight attitude. On impact, one or both side columns

will contact the ground, absorbing energy via crushable pads and structural

stressing. The structure is designed to sustain these impact loads and

will roll over either naturally or be pulled over by the parachute pulling

on it. The crash rings are designed to absorb this rollover energy by

distortion. The crash rings should maintain a reasonable ground clearance,

even if nominal (50') parachute dragging occurs, to protect the telescope

and experiments. All electronics are completely protected because they are

mounted inside the frame.

Recovery is difficult at best from the various terrains into which

balloon payloads descend. Therefore, the yoke frame as well as the telescope are modular designs. This allows field disassembly into manageable

components if necessary. The gondola can be transported on a standard

flat-bed trailer for transport back to the NSBF without the need for special roadway routing and/or variances.
Page 51

Subsystem Unit Weight Total Weight Total Weight

(pounds) (pounds) (kilograms)

I. Telescope Assembly

(See Table 2.2-3) 1374 625

II. Structure and Subsystems

Structure (Yoke Frame)

Upper 175

Telescope Mounting Interface 50

Side Columns 400

Stabilizer Beams 50

Miscellaneous Structure 80

755 343

Stabilization and Pointing*

Momentum Transfer Unit 50

M-T Reaction Wheel 35

Magnetic and Gravity Sensors 20

Elevation Drive 35

140 64

Electronics

Command 25

Control 50

Batteries 120

Cables and Connectors 65

Enclosures 120

Miscellaneous 30

410 186

Re-entry Devices

Telescope Latch 30

Crash Rings 110

Crash Pads 40

180 82

III. NSBF Equipment

Ballast 400

Electronics 55

120-Ft. Parachute 430

Suspension Ladder 65

25Q 432

TOTAL GONDOLA WEIGHT 3809 Ibs. 1731 kg.

*Remaining items not attached to telescope

Table 3.2-1. Gondola Weight Summary
Page 52

Table 3.2-2. Gondola and Telescope Products and Moments of Inertia

Gondola Inertial Properties about Its Origin (kg-mxm)

Axis

X

Y

Z

3532.

0.

0.

0.

2690,

-116,

0.

-116.

2679.

Telescope Inertial Properties about Its Origin (kg-m*m)

Axis X Y Z

X

Y

Z

461.

0.

0.

0.

478.

-25.

0.

-25.

342.

3.3 Pointing System

3.3.1 Pointing Requirements

The pointing and structural specifications shown in Table 3.1-1 are

dominated by the 30-/xm wavelength beam size, the slewing and scanning

requirements, the maximum allowable gondola weight, the thermal environment, and the operational requirements of balloon flights. The pointing

stability specification of 1-arcsecond rms is set so as not to compromise

the imaging quality of the optics.

The telescope is suspended in gimbals within the gondola frame and is

oriented in two orthogonal control axes, elevation and cross-elevation.

Telescope motion in the cross-elevation axis is limited to about ±3°.

Coarse azimuthal control is achieved by rotating the entire gondola at its

attachment point to the balloon.

3.3.2 Operating Modes

Sources are acquired initially in the Magnetometer Mode which provides

coarse telescope positioning. The gondola is first rotated about the

azimuthal axis, using the Earth's magnetic field as reference, to a positional accuracy of about 30 arcminutes. Then, telescope rotation about the

elevation axis, using the gondola gravitational alignment as reference, to

a positional accuracy of about 10 arcminutes brings the desired source

direction within the range of motica of the cross-elevation axis.
Page 53

Once the source is acquired in the Magnetometer Mode, the gondola is

switched to the Inertial Mode, the fine pointing mode, for science data

acquisition. Pointing stability in the Inertial Mode will be better than 1

arcsecond rms . In addition to fixed pointing, which stabilizes the telescope on a given celestial target', the inertial mode can also be used to

step or scan the telescope with respect to a celestial object. For mapping

of extended sources a raster pattern can be generated by scanning in crosselevation and stepping in elevation at the end of each scan line. The

angular size of such rasters and the scan rates can be varied on command.

In addition to the commanded scan rates and directions, a joystick control

at the ground station can be employed for telescope positioning.

3.3.3 Gimbal Design

Two types of gimbal designs were evaluated during the program. The

first with the baseline flex-pivot design described in the PDR; the second, the active-gimbal ball bearing concept described in Semiannual Report

#6.

The advantages of the flex-pivot designs were detailed in the PDR.

Further examination of the concept revealed several design weaknesses all

of which relate to the limited rotation range of the flex-pivot. A long

period of undisrupted tracking requires a large stiff flex-pivot which is

difficult and expensive to fabricate. The flex-pivot design also requires

a secondary actuation system to unwind the flex-pivots accumulated rotation

due to tracking and to move the telescope through large angles when acquiring a new source. Recent experience on an SAO balloon borne spectrometer

also revealed that a flex-pivot based design would require precise alignment across the telescope frame at a level difficult to maintain in the

field.

The active gimbal ball-bearing design provides unlimited angulation

without the need for a secondary drive system. The graininess and slapstick which they exhibit in slow-speed servos is compensated for by using a

feed-forward path in the torque motor servo loop driven by bearing torque

measurements. The torque sensor, which has an appropriate torsional spring

rate, permits telescope rotation while the ball-bearings are locked by

graininess or slipstick friction. While this design does have certain

inherent non-linearities which the baseline design avoided, the feed forward servo incorporated in the design can compensate for them more than

well enough to achieve the desired pointing accuracy and stability.

The mechanical design of the ball-bearing gimbal is shown in Figure

3.3-1. A backplate acts as a pedestal on which the servo components are

mounted. The torque sensor is rigidly attached to the backplate on one end

and is machined on the other to mount a matched pair of angular contact

bearings. These bearings provide the required rotation via a spindle which

has the servo motor stator and position resolver armature mounted to it.

It also provides the bolt pattern for attachment to either the telescope or

X-elevation frame. The backplate also has provisions for mounting the

servo motor armature and position resolver stator.
Page 54

DHSGSNAE P&GS IS

OF; POOR QUALITY

-—ADJUSTMENT SPACER

-ELECTRICAL CONNECTOR

;-.'.': &3LA OK X-ELEV. FRAME

HAND -VO. 6/6 -2 , 5OO OZ IN TQRQUE.

&EARlN6rKAYDOti COF?F? NO. KBO25 ARC

--HOLE THRU (TO &K/N6 OUT CABLES FROM TELESCOPED.

X-E-EV. FRAME OR TELESCOPE

ELECTRONICS CABLE RET

-POSITION RESOLVER

WRENCH ACCESS

SECT A-A

FIXED SIDE

Figure 3.3-1. Gimbal Assembly
Page 55

The torque sensor is a commercial device which has the appropriate

structural load capacity for all normal operational situations. During

severe load applications (parachute deployment and crash landing for example) the torque sensor will deflect until the diameters of the servomotor

mount and bearing spindle come into contact. This contact will then carry

all such severe loads with a factor of safety >3 insuring that rupture

and/or separation between the telescope and gondola cannot occur.

This design allows complete assembly and functional checkout of the

gimbal to occur at the sub-assembly level. Two completed gimbals are then

installed between the Telescope X-elevation frame and gondola X-elevation

frame (one per side) to form the telescope elevation mount. This installation is easily accomplished by centering the telescope in the frame and

attaching the gimbal assemblies to each by using the appropriate hardware.

All hardware required is accessible from the outside to facilitate this

attachment. The same gimbal design is used in the cross-elevation servo.

3.3.4 Azimuth Servo

The Azimuth Servo is the outermost control loop. It is a coarse

pointing loop which contains the cross-elevation, or fine loop, within it.

In Magnetometer Mode, the Azimuth Servo is used for the initial acquisition of a targeted celestial object, and points the gondola to within

±30 arcminutes of the commanded location. This is close enough to allow

recognition of the star field in the aspect TV camera. Azimuth reference

in this mode is the earth's magnetic field, as sensed by a Schonstedt fluxgate magnetometer. The inner (cross elevation) gimbal is driven to its

center position and locked while the target is being acquired.

In Inertial Tracking Mode, the error input which drives the Azimuth

Servo is switched from the magnetometer to the inner gimbal position sensor

and the inner gimbal is unlocked, allowing the cross-elevation fine servo

to become active. The Azimuth Servo then maintains coarse orientation of

the gondola so that the target object remains well within the limited range

of the cross-elevation gimbal. A threshold, or dead-banding, feature will

be included so that the outer loop becomes active only when the inner

gimbal moves away from its center position more than some minimum angle.

This will reduce the amount of activity in the Azimuth servo, and result in

fewer disturbance torques for the pointing control to cope with.

The coarse Azimuth servo also serves as a means of dissipating momentum built up in the cross-elevation reaction wheel. When this momentum is

dumped (by torquing the cross-elevation gimbal motor) a component of it,

proportional to the cosine of the elevation angle, is coupled into the

gondola azimuth axis. The azimuth reaction wheel responds by taking on a

velocity and this energy is removed by the azimuth loop twisting against

the suspension lines.

A full explanation of the overall system concept with descriptions of

the subsystem elements is contained in the PDR.
Page 56

3.3.5 Elevation and Cross-Elevation Servos

The elevation and cross-elevation servos are the positioning systems

used only in the inertial pointing mode for precise (l arcsecond) telescope

pointing.

(a) Elevation Servo System

During inertial tracking the elevation servo control loop consists of

four elements*:

1) The prime attitude control loop,

2) The gyro update arm,

3) The momentum management loop, and

4) The active bearing assembly.

The elevation servo takes into account telescope inertia, reaction

wheel behavior, and gyroscope behavior using designed-in control laws to

stabilize the telescope. A simple proportional plus derivative controller

implemented in software was selected for the inertial loop. Its projected

bandwidth is 2 Hz with a 0.7 damping ratio. Integral feedback is not

required as steady state errors are implicitly corrected within the momentum management loop. Gyroscopes are used as the primary inertial reference

and are drift corrected by a coaligned startracker. These positional

updates take place every 1 to 5 seconds, too slowly to use this information

directly within the fine control loops, however, more than fast enough for

updating the gyros.

Space Telescope wheels were selected for the reaction wheels. The

reaction wheel momentum is transferred from the telescope structure to the

gondola by the gimbals' brushless torque motor. The reaction wheel velocity is monitored and is used to generate a proportional torque from the

torque motor in the same direction. This causes the telescope to angulate.

The change in position is noted by the prime inertial loop which, while

compensating for this motion, causes the reaction wheel to slow down. The

operation of the momentum management system is mathematically similar to

integral feedback and therefore acts to correct steady state errors.

Finally, the active bearing assembly operates as a torque-free gimbal

for the telescope. The gimbal consists of a ball-bearing, torque sensor,

and a brushless torque motor (the same one used in the momentum management

system). As the telescope elevates, the bearings generate friction torque,

which is measured and compensating torques fed into the gimbal by the

torque motor. This is not a closed loop system and the torque motor reac-

* See also Section 2.4 of the PDR for a complete block diagram

and analysis of the elevation servo.
Page 57

tion time is important. By keeping this time as short as possible the

effects of bearing "graininess" or slip-stick behavior are minimized and

the chances of an interaction between the telescope attitude control loop

and the active bearing are avoided. In order to successfully cancel the

torque generated by the bearing the torque motor constants must be well

understood. The motor will be calibrated at all orientations and conditions of expected operation. If necessary, a look-up table of torque

constants will be used in demanding torque from the motor.

Each gimbal assembly cancels the torque created by its own bearing.

This prevents the bearing torque from distorting the mirror cell.

Which gimbal assembly to select for momentum management is still an

open question. Both are equally capable of dealing with the momentum

transfer, but whether to split the load equally between the gimbals assemblies or give the duty to one of them alone has not yet been determined.

(b) Cross-Elevation Servo System

The cross elevation servo system is virtually identical to the elevation servo, however its range of travel and interaction with the gondola

and coarse loop are different. The cross elevation axis has less than ±3°

of motion. The gimbal is kept centered within this range by occasional

activation of the azimuth coarse loop. The cross-elevation gimbal, as it

moves, changes orientation with respect to the gondola, since it is the

inner gimbal. Therefore, when cross elevation momentum is dumped into the

gondola it will excite a complex gondola motion which is not necessarily

around an axis parallel to the cross elevation gimbal. Passive methods for

damping this gondola momentum will be required.

(c) Selected Components

Components to implement this system including the gimbal motors and

reaction wheels are readily available. Semiannual Report #6 describes

candidate components for this gondola.

(d) System Response

A full analysis of this system is presented in Semiannual Report #6.

System behavior was simulated using the computer program "TF" written at

Stanford University. This program provided a Bode plot (see Figure 3.3-2)

and an impulse response (Figure 3.3-3) for the modelled system. The impulse assumed for the sake of the analyses was 5xlO~3 Nms as mentioned

above, applied at T=0, it resulted in a maximum predicted excursion of the

telescope from its commanded position of .075 arcsec. At such a small

angle the system behavior begins to be quantized by the digital-to-analpg

converters and cannot be considered strictly linear, so the response shown

in Figure 3.3-3 may not be a realistic prediction of system response.

However, we do see that the "snapping" of the ball-bearing has little

effect on the telescope pointing.

The rest of the model behavior is nearly identical to the system

described in the PDR. The new gimbal model predicts that the telescope is

completely isolated from the gondola. This is different from the earlier
Magnitude and Phase Response of Closed Loop Servo

Page 58

0)

•D

•**

°c

o>

(0

I ff

Iff

-100

w

ra

CL

--200

-300

10 100 1000

Frequency (radians/second)

Figure 3.3-2. Fine-Pointing Control System Response to a Sinusoidal Command

Signal
2M Telescope System Response

Page 59

0.08

o

(0

c

oCL

in

0>

DC

0.06-

0.04-

0.02-

0.00-

-0.02 -

-0.04

Time (Seconds)

Figure 3.3-3. Telescope Point Response to a 5xlO~3 Nms Torque Pulse
Page 60

system model in which the telescope was explicitly coupled to the gondola

behavior by the flex-pivot. In practice, the gondola will excite some

telescope motion through the nonlinearities in the gimbal.

3.3.6 Aspect System

Pointing of the telescope at a desired astronomical object is achieved

through the use of three intensified-CCD television cameras. One camera is

used for acquisition, one for fine guiding, and one can be used for both if

necessary. The primary acquisition camera, with a 5° field of view (FOV)

and a limiting stellar magnitude of approximately 8, is used to identify

the initial star field. The primary tracking function is performed by a

focal plane camera, with approximately the same field of view as the main

telescope. A backup camera having a 1° FOV is also used.

The focal plane camera will be used to compute the rotation angle

(after determination of a constant offset) with sufficient accuracy to

permit single-star tracking. Two-star tracking and scans over large angular fields would also be possible. Gyro drift rates should be small enough

to permit tracking for time scales of minutes without updating from the

star sensors, if required.

The sensor being considered at this time for each of these cameras is

the Fairchild intensified CCD camera Model CCD 30001. This camera has a

pixel format of 488 lines by 380 columns. The wafer-type intensifier used

increases the camera sensitivity by a factor of 1,000. The camera has two

types of outputs. One is a standard composite video signal; the second

provides the timing waveforms and video level for each pixel. The latter

output, when coupled to a computer can be used by centroiding, to determine

the image coordinates of the guide star to at least 1/10 pixel accuracy

(0.2" position accuracy). The intensified CCD camera offers the advantages

of fixed pixel structure, high sensitivity, insensitivity to magnetic

fields, lack, of high voltage requirements, and small size, all of which

make it ideal for this application. Such a camera is presently being

evaluated in the OIR Division of the Smithsonian Astrophysical Observatory.

Advances in technology (lower readout noise) may also make it possible

to use unintensified CCD cameras in this application at a later date. This

approach is currently being investigated at the University of Chicago on

ground-based telescopes. The ASTROS CCD tracker, developed by Jet Propulsion Laboratory (JPL) for operation from the Space Shuttle bay on the

Instrument Pointing System (IPS), provides extremely precise measurements

of star image coordinates as inputs to the Image Motion Compensation system

used to stabilize the science instrument focal planes. ASTROS can determine star image position to an accuracy of 0.2" over a field of view of 292

x 295. This requires consistent star image position determination to an

accuracy of 1/100 pixel. The ASTROS tracker will be used in early 1986 on

the first Astro mission of the Space Shuttle to observe Halley's Comet and

a variety of other astronomical targets. If it is possible to approach

this performance, the 1° camera could track as accurately as the focal

plane camera.
Page 61

3.3.7 Estimate of Total Pointing Performance

An error analysis has been performed for two different focal plane

camera configurations:

1) Optical beamsplitter camera at the focal plane;

2) Direct IR focal plane measurement by a high resolution imaging

instrument.

These calculations are based on error magnitudes that have been taken

from the PDR or, if unavailable there, assigned on the basis of best engineering judgement. All calculations assume that the system is boresighted

prior to each observation. The contribution of image blur to the image

position error was reviewed, found to be minor and thus not included. The

two errors resulting from each source are first added together algebraically before root-square-summing these values with errors resulting from

other .uncorrelated sources. The precisions assigned to the secondary

mechanisms and the primary mirror to housing motion are estimates.

Following the methods presented in Semiannual Report #6, we project

total rms stabilities of 1.0 arcseconds for an optical camera beamsplitter

fed at the focal plane and 0.64 arcseconds for a direct fed infrared image

instrument. (See Table 3.3-1.)
Page 62

ORIGINAL FAGS IS

OF POOR QUALITY;

Using

Resolving

Instrument

Fixed Errors

Control System

Star Camera

Focal Plane CCD

Unremovable Dcccntcr

FocalPlanc Knowledge

Total Fixed Error (RSS)

Operational Errors

Star Tracker Misalignment

SUIT Tracker Ccntroiding

Control System

Total Operational Error (RSS)

TOTAL ERROR (RSS)

0.10

0.10

0.20

0.60

0.63

0.64

Using

Optical

0.60

0.20

0.20

0.10

0.10

0.68 arcsec.

0.40

0.20

0.60

0.75 arcsec.

1.01 arcsec.

Note: All values arc for apparent image motion at the focal plane

Table 3.3-1. System Error Performance for Two Star Tracker Configurations

3.4 Electrical Systems Design

3.4.1 Command and Telemetry System

The command and telemetry system will be very similar to the system

already in use at NSBF and compatible with it. The heart of the telemetry

system is the NSBF Consolidated Instrument Package (CIP II) (see Figure 3.4-1). It consists of subcarrier oscillators used to FM/FM modulate

the UHF L BAND telemetry transmitters for efficient use of the authorized

bandwidth. Standard IRIG VCO channels of 1, 3, 5, 7, 8, 9, 10, 11, 12, B,

E, H, or HH are used. The channel HH (referred to as wide band H or WBH)

is used on most flights for PCM data rates of up to 80 kilobits of NRZ or

40 kilobits biphase. There are six of the IRIG channels available for the

user.

The main PCM command system provides for 80 discrete functions and a

16-bit parallel data word with strobes. These would be used for control of

the gondola. A second command decoder can is provided with a different

address. This provides 48 discrete commands and a 16-bit parallel data

word with strobes for the experiments.
Page 63

FROM

GONDOLA

AND

EXPERIMENT

IRIG DATA

DIGITAL

DATA

FROM

GONDOLA

ELECTRONICS

•48 DISCRETE CMOS

16 BIT DAT A WORD

TO

EXPERIMENTS

AUDIO OUT - SECOND ADDRESS

VIDEO

TRANSMITTERS

COMMAND

AMD

TELEMETRY

PACKAGE

/\

DATA

DOWNLINK

•80 DISCRETE CMOS

I 6 BIT DATA WORD

TO

GONDOLA

COMMAND

UPLINK

Figure 3.4-1. On-Board Command and Telemetry System for the Balloon-Borne

Three-Meter Infrared Telescope
Page 64

The PCM data encoder format is controlled by an EPROM. There are 128

inputs available, which can be programmed for either analog single ended,

analog differential input, parallel digital input or serial digital input.

The bit rate is 80.92-kilobits under internal control and 256-kilobit under

external control (planned for the future). The gondola would require about

50 digital inputs (5 at 10 bits each) and 30 analog inputs. The remaining

48 inputs would be available for the experimenter.

3.4.2 Gondola Electronics System

Conceptual design of the most critical of these systems, the pointing

and stabilization systems, is complete and discussed in detail in Section

3.3 of this report. The balance of the systems are straightforward extensions of existing designs in use on the Smithsonian One-Meter Balloon

Infrared Telescope. For flexibility, the designs of the primary experiment

controller will be microprocessor based. Low power technology will be used

throughout the gondola electronics systems. A block diagram of these

systems is shown in Figure 3.4-2.

3.4.3 Gondola Power Requirements

Power requirements are summarized in Table 3.4-1. For the first

flights the primary power will come from lithium/sulfur dioxide cells.

Power Conversion, Inc., produces a lithium pack rated at 36 volts open

circuit and 30 volts under rated load (2a) and at rated capacity (35 ampere-hours) . The gondola would require seven packs in parallel. The

battery complement for each mission will depend on the experiments flown.

When the Two-Meter Balloon-Borne Telescope becomes fully operational, a

silver-zinc battery set is planned which would greatly reduce the cost per

flight without a significant weight increase.

3-4.4 Electronics Packaging

The yoke frame has been sized to provide an electronics cavity on each

gide of the gondola that can be thermally controlled, maintain atmospheric

pressure and utilize standard 19-inch electronic racks. These enclosures

begin just below the telescope elevation drive at a level easily accessible

for ground maintenance and checkout. The volume available 0.34m3 (12 ft3)

is more than sufficient to hold all system electronics, including any NSBF

batteries which must be flown. The left side will be used for gondola

equipment while the right side will be dedicated to experiments/hardware.
Page 65

BALLOON

COARSE AZIMUTHAL

SYSTEM

EXPERIMENT

ELECTRONICS

BATTERY BATTERY

INPUTS FROM

ALL SYSTEMS

COMMAND

and

TELEMETRY

ELECTRONICS

TELEVISION

TRANSMITTERS

A A A

ANTENNAS

Figure 3.4-2. Gondola Electronics Systems
Page 66

Table 3.4-1. Estimated Power Requirements

Average Steady-State

Assembly Power (Watts)

Azimuth Stabilization Drives 168

Elevation Stabilization Drives 56

Pointing and Guidance 84

Command System 28

Experiments 140*

Other Systems 84

Chopping Secondary

Focus Drive

Heaters

Miscellaneous (Power Supply,

Losses , etc.)

Total Steady-State Power 560 watts

Estimated Power per Flight

Preflight Checkout 1 hour = 560 watt-hours

Ascent Power 1.5 hours = 252 watt-hours

Experiment Power 11 hours = 6160 watt-hours

Total Energy Required 6972 watt-hours at 28 volts

(249 ampere-hours)

*Will vary with experiment packages; quoted number is for dual photometer

instrument, but can be as high as 425 watts for a heterodyne spectrometer.

4.0 MIRROR DEVELOPMENT PROGRAM

4.1 Approach and Scope

A number of unique requirements for the Two-Meter Balloon-Borne Telescope primary mirror have caused it to be identified as one of the critical

items for attention during the definition study. It must:

1. be very lightweight. Preferably approximately 31 kg (10 kg per

square meter).

2. maintain its figure at room temperature for testing and at an

operating temperature of -45°C.
Page 67

3. come to rapid thermal equilibrium during a two hour ascent from

high summer temperatures through the tropopause to the operating

temperature at 30-km altitude.

4. survive the high gravity loading during launch and at the time of

parachute opening on descent and survive the landing impact shock.

An additional unique feature of this telescope is its 30-/im

diffraction-limited performance requirement which places it in a domain

between radio reflectors of considerably lower accuracy and optical telescopes of considerably higher accuracy, a domain which has had little

attention for either ground-based or space applications. Because of the

very similar reflector requirements for the Large Deployable Reflector

(LDR), the balloon telescope development and test effort has potential

impact on that future space facility.

The thrust of the mirror development program has been to measure the

low temperature performance of state-of-the-art lightweight mirror materials and to work with manufacturers to further develop the technology. In

addition we have pursued improvements both in the surface figure as replicated and in techniques for optically figuring composite mirrors for further figure improvements. Our activities include:

1. Designing and constructing a thermally controlled test chamber for

optical evaluation of very lightweight mirrors at low temperatures. This is described in detail in the PDR, Section 3.3.

2. Testing a series of nine 0.5-m replicated Carbon Fiber Reinforced

Plastic (CFRP) 6-cm-thick aluminum honeycomb core sandwich mirror

panels made by Cornier Systems of West Germany. The panels are

replicated from a convex Pyrex template with a 10-m radius of

curvature. They were tested both for low temperature distortion

and for deformation under mechanical load. These were variously

obtained by procurement by this project, procurement by JPL in a

complementary development effort, and by loan as part of a cooperative project with Dornier.

3. Testing a series of five lightweight replicated CFRP panels made

by Hexcel Corp. in a collaborative JPL/Hexcel/University of Arizona (UA) program.

4. Testing a CFRP sandwich panel with additional pyrex facesheets

made by Mitsubishi Corp.

5. Collaborating with United Technology Research Center in the development and testing of two very lightweight "egg crate" mirror

panels made of carbon fiber reinforced glass (TSC).

6. Procuring for testing an aluminum facesheet, foamed aluminum core

mirror made by Energy Resources Group.

7. Experimenting with optically polishing the composite facesheet as

fabricated.
Page 68

8. Experimenting with vacuum deposition of silicon monoxide on the

composite for subsequent optical polishing.

4.2 Conclusions from Mirror Development Program

4.2.1 The Dornier Mirrors

The Dornier 0.5 meter octagonal mirror showed spectacular improvement

during the course of the nine mirror development program. The surface

replication accuracy has improved by a factor of two and the thermal stability by a factor of twenty-five. Figure 4.2-1 shows the figure as replicated of Dornier Quad 25, which was tested March 11, 1987. The surface

figure departs from a best fit sphere by 2.5/zm rms. With the astigmatism

which is the dominant figure error, removed, the residual is 0. 8fj,m rms.

Figure 4.2-2 shows the change in focus, XY astigmatism, spherical aberration and rms residual (calculated after subtracting these Zernike terms

plus coma) from room temperature to -60°C. The numbers are the peak-tovalley surface change for the particular Zernike terms. The total change

over a temperature change of 82°C is . 99jim rms.

The Dornier replication accuracy of the 0.5 meter mirror comes close

to meeting our required accuracy (l-2/im rms). The thermal stability equals

our requirements. Dornier and other manufacturers have made many CFRP twometer panels for submillimeter radio astronomy. The remaining uncertainty

is how the thermal performance scales to this size. At present the CFRP

sandwich panel reflector is the baseline mirror approach.

4.2.2 The Hexcel/JPL Mirrors

The five Hexcel mirrors ranging in size from 15 to 40 cm square have

had relatively low replication accuracy. The significant achievement of

the JPL/Hexcel/UA development program is the validity of the JPL modelling

for predicting the UA measured panel performances. Figure 4.2-3 shows the

predicted and measured thermal changes in focus, astigmatism, spherical

aberration, and coma for the 0.4 meter square mirror Hexcel-4.

4.2.3 The Pyrex Face Sheet Composite Mirror

The Pyrex/CFRP composite demonstrated excessive low temperature deformation and is not a useful balloon-borne telescope mirror candidate.
Page 69

Filename: ave. E' 16:52:48 03-0S-87

Quad £5 Figure II 19.5*C Pixel P.Error=.809

Wavelengths per band= 0.378

Fraction of band printed- 0.500

Number of half wavelength shifts= 0.000

Coe f f i c i ent s removed: P i st on Tilt Focus

Q

Q

L-LL

MMM.-M-••

MM-•• •

*Q •

S RR

R

0--RR-RR R

•-BR-'RR .RD.....R.

Figure 4.2-1. Dornier Quad 25 As Replicated Surface Figure Error Contour

Level 2/im
Page 70

co

cn

.22

3§

o

o

o

CJ

o>

<D

2 £ i p

"5

CM f>

I

JOJJ3

Figure 4.2-2. Quad 25 Peak-to-Valley Change in Figure over 82°C Temp.

Change
Page 71

w tn

3 C

u a

a c.

«n

Figure 4.2-3. Hexcel 4 Comparison of JPL Model
Page 72

4.2.4 The United Technology Graphite/Glass Mirrors

Graphite glass has considerable promise for fabricating optics because

of its high strength-to-weight, chemical inertness, freedom from moisture

absorption, and low coefficient of thermal expansion of the laminate

(~.5xlO~6/°C) . However, the techniques for fabrication of the laminates

and assembling these into an egg-crate structures are in their infancy,

resulting in deformations due to material non-uniformity. The UTRC 27cm

diameter mirror NPM-2 distorted by 0.74j«n rms over a 82°C temperature

changing largely from focus change. With the focus term removed, the

distortion was . 41/jm rms. While graphite/glass is promising, it is premature to incorporate it into the balloon telescope design.

4.2.5 Foamed Aluminum Core Mirror

A 0.5 meter lightweight mirror has been fabricated by Energy Resources

Group, Inc. (ERG) by dip-brazing aluminum facesheets to a foamed aluminum

core forming a lightweight homogeneous aluminum structure. The mirror has

an areal density of approximately 20 kg/m2. The mirror is presently being

diamond turned in preparation for figure and thermal stability tests.

4.2.6 Optical Polishing Composite Facesheets

We have successfully improved the figure of a 0.5 meter Dornier CFRP

mirror by optically polishing an epoxy rich front surface. This was done

using an aluminum tool with a pitch face covered with a Pellan pad. The

polishing was done with diamond dust suspended in oil. The only limitation

to this technique appears to be breakthrough of the thin epoxy surface to

the underlying fibers.

4.2.7 Silicon Monoxide Over Coating and Polishing

We have investigated evaporation coating of a CFRP mirror with silicon

monoxide and polishing it to improve the mirror. The initial experiments

successfully achieved a 6.5/zm polishable layer with excellent adherence and

stability. We are currently carrying out a systematic effort to develop

the technique for thicker (25/un) coatings without overheating the mirror

with a goal of achieving optical image quality by polishing this coating.

For this purpose we have rehabilitated a vacuum system for evaporative

coating and are constructing a chamber large enough for a one-half meter

square mirror. The chamber is shown in Figure 4.2-4.
Page 73

23.a1>

t_

D

CO

O

O

u

u

c

e

3

MIRRO R

HEAT

COOLING TUBES

ELECTRODES

PLASMA RING

CO

O

m

S i O EVAPORATIO N

SYSTEM - NEW CHAMBER

Patrick Moid* 7-16-87

I I I I I I I I I I I

Seal* In Inch**

Figure 4.2-4. Silicon Monoxide CFRP Mirror Coating Chamber
Page 74

5.0 EXPERIMENT ACCOMMODATIONS

5.1 Focal Plane

A rotating dichroic beamsplitter will permit switching of the

IR/submillimeter between the IR photometer and the submillimeter spectrometer. Simultaneously, continuous viewing of the sky will be provided an

intensified CCD camera for closed loop bore-sight pointing and offset

pointing.

5.2 Photometric Infrared Camera

5.2.1 Instrument Description

The proposed far-infrared imager will be based on a 60-channel spatial

array of He3-cooled silicon bolometers (an 8x8, rectangular, close-packed

array with unused corner positions). The system will be similar to a 60-

channel camera now under construction for the Kuiper Airborne Observatory

and to 32-channel systems which have been used extensively on the Kuiper

Airborne Observatory at submillimeter wavelengths at the NASA Infrared

Telescope Facility and the Caltech Submillimeter Observatory at Mauna Kea.

Interchangeable re-imaging lenses will permit discrete changes of the

apparent pixel size over a range of ~15"-60".

The spectral sensitivity of the camera will be set by He4-cooled

interference filters mounted in a multi-position turret. Additional spectral blocking will be provided by fixed, low-pass, interference filters,

crystalline re-imaging lenses, and a polyethylene pressure window coated

with diamond dust. The system will be provided with a standard set of

filters with A/AA~0.3 covering the range 40-400//m. Additional (e.g.,

narrowband) filters may be added for special observing projects. It will

be possible to change both filters and re-imaging lenses during flight.

Cooling for the detectors will be provided by a two-stage He3 refrigerator with stages operating at approximately 0.25 and 0.4K. Each detector

will be mounted in an integrating cavity behind a 0.25K Winston light

concentrator. Other optics and band-limiting filters will be cooled to He4

temperature.

Detector signals will be amplified by N2-cooled JFET source-followers

and warm preamplifiers, filtered to remove high frequencies and demodulated

with a digital sampling system consisting of a commutating analog-todigital converter, variable-g^in amplifier, and controlling microprocessor.

Required telemetry bandwidth will be <20 Kbaud.
Page 75

5.2.2 Limits to Sensitivity

The system should be limited by thermal background noise. The photonnoise-limited NEP for a system for which the total efficiency of detector

plus all cold optics and filters is r\ , for which the warm telescope and

atmosphere have about the same temperature T and a combined emissivity e ,

and for which 6 is the diameter of the field of view of each detector is

approximately equal to

(NEP)p * (l+er/M)1/2(8E_14) [ (AA/AjTfje]1/

2 (6/00) [ A/(pm) }~1 W/Hz1

/

2

where 00 = 1-22 A/D, D is the telescope diameter, and M= (e^A7-!)-1 . For

T=230 K, A=100/*m, fj=0.25, £=0.1 (AA/A)=0.3, and

[NEP]p (a 1.25 E-15 W/Hz1/

2

The corresponding NEFD (in Jy/Hz1/

2) is

[NEFD]p ^ 21/

2(3.3 Ell)[NEP]p [ A/(/im) ]/[ijTA(AA/A) )

where A is the telescope collecting area, TJT is the total transmission

efficiency of the combination of telescope, detector system, and atmosphere, and where we have assumed the usual sort of double-beam chopping

and beam-switching. For the 2-m balloon telescope, A=3m2. Thus, for the

parameters quoted above and r=0.7,

[NEFDjp * 0.7 Jy/Hz1/

2

For a point source, only half of the source energy falls with OQ, so

(NEFD)p fa 1.4 Jy/Hz1/

2 (point source)

Except for the quantum statistical factor (l + e f/M)1/

2, which is close to

unity for the parameters and wavelengths of interest here, the NEFD at a

constant 8/60 and constant AA/A should be independent of wavelength.

[NEFD]p «

5.3 Heterodyne Submillimeter Receiver

The photometric infrared camera will be a powerful instrument for

studying continuum radiation from astronomical objects. We also propose to

develop and fly a heterodyne receiver having the high spectral resolution

necessary to study line radiation; with this complementary capability, a

significant new class of scientific problems may be addressed.

The astrophysically richest area of molecular line emission happens to

lie in the least-studied part of the electromagnetic spectrum, the farinfrared/submillimeter. There have been three main impediments to advances

in this field: (l) the atmospheric transmission is poor and is limited to
Page 76

a few wavelength windows even at the best high, dry observing sites; (2)

sensitive submillimeter detector systems involve difficult and recent

technology; and (3) suitable telescopes are only now being built. The

atmospheric limitation is largely overcome by working at balloon altitudes,

while submillimeter receiver technology has now reached a stage of refinement sufficient to make balloon-borne heterodyne instruments desirable. We

expect that our ongoing research programs in receiver instrumentation for

this project and others will further advance the state of the art in submillimeter detection, and a balloon-borne telescope in particular will

provide an invaluable test-bed for the mission concepts needed for future

space-borne submillimeter instruments.

Two distinct technological approaches to heterodyne submillimeter

receivers are currently feasible: they are SIS and Schottky-diode systems,

and take their names from the devices employed to perform the first mixing

of local oscillator and signal radiation. SIS receivers take advantage of

the behavior of Superconducting-Insulating-Superconducting tunnel junctions

when illuminated by microwave radiation, and of the superior mixing properties of the sharp nonlinearity in the SIS device I/V characteristic curve

near the onset of quantum tunneling.

SIS receivers have already emerged as the dominant technology at

millimeter wavelengths, and are progressing rapidly to higher frequencies.

In addition to superior sensitivity, an SIS instrument has a number of

operational advantages over Schottky-diode-based alternatives. Chief among

these advantages is the relatively low local oscillator power required by

SIS junctions (microwatts rather than milliwatts), permitting the use of

low-voltage, high-reliability solid-state harmonic generators utilizing

Gunn diodes as the fundamental source. Schottky receivers at short submillimeter wavelengths have required complex and unwieldy molecular lasers,

optically pumped by C02 lasers, as local oscillator sources. Remote (eg.

balloon-borne) operations in particular dictate the use of a receiver based

on SIS junctions and solid-state local oscillators. To date, harmonic

generators have been built with output powers of about 0.1 milliwatts at

almost 500 GHz (in the sixth harmonic) utilizing Gunn oscillators as the

fundamental source (Zimmermann 1988, personal communication; Millitech

1988, personal communication). Such local oscillator sources are suitable

for current SIS junctions, and we can expect that the frequency range will

expand in the next few years.

Theoretical analyses of SIS detectors have shown them to be capable of

quantum-limited noise performance in both incoherent and coherent detection

modes. Experimental tests performed on SIS junctions have demonstrated

this performance at frequencies near 36 GHz (Shen et al. 1980, Richards et

al. 1980). Currently, SIS heterodyne (coherent mode) receivers are being

built and used consistently at 115 GHz and 230 GHz (Sutton 1984) . These

receivers, although not yet operating at the fundamental quantum noise

limit, outperform Schottky-diode receivers operating at the same frequencies; Figure 5.3-1 shows a comparison of receiver sensitivities reported

for the competing technologies. This sensitivity advantage enjoyed by SIS

increases both the quantity and quality of science that can be done.

Work on SIS receivers is proceeding at a rapid pace in a number of

laboratories around the world, including Caltech/Bell Labs (Ellison and
Page 77

200

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I-a SIS Bowtie

o SIS Waveguide

Schottky Corner Cubj

Schottky Waveguide

- *

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0

100 1000

Frequency, GHz

Figure 5.3-1. Comparison of actual receiver performances in terms of number

of noise photons at the input. The dashed curve represents the best

achieved result of a 10% quantum efficiency, of 10 photons of noise.

Miller 1987), NRAO (D'Addario 1985; Pan et al. 1987), IRAM (Gundlach et

al. 1982), Chalmers University (Rudner and Claeson 1979), NASA-Goddard

Institute (Pan et al. 1983), and Berkeley (McGrath et al. 1985).

The traditional approach to constructing SIS receivers at lower frequencies has been to place the junction in a resonant waveguide structure

for maximum coupling to local oscillator and signal radiation. At frequencies beyond about 350 GHz, fabrication of such small tuned structures

becomes extremely difficult, and the use of open, "quasi-optical" structures becomes more attractive (Wengler et al. 1985).

Instead of using waveguide components, such a receiver employs a

system of lenses to focus the radiation collected by the telescope onto an

antenna connected directly to the SIS junction, and fabricated on the same

substrate. The antenna pattern used by Wengler et al. is referred to as a

"bowtie", after its shape, and provides a good impedance match to the

junction up to ~500 GHz. Hence, the same SIS detector and front-end block

can be operated from ~2 mm to ~600/*m. The frequency of observation is

determined by the local oscillator frequency, while the receiver's bandwidth is limited only by the intermediate frequency (IF) amplifier. Other

quasi-optical structures are being tested by various groups, including

spirals, log-periodic structures, and end-fired dielectric horns. (Compton

et al. 1987).
Page 78

The frequency flexibility of a quasi-optical SIS receiver is a particularly attractive feature for flights on the two-meter balloon facility, as

observations will be conducted from altitudes at which much of the submillimeter spectral band is accessible. Since waveguide receivers are inherently narrow-band, a separate receiver would have to be built for each

spectral region of the band. If multiple transitions of a particular

molecule (eg. CO) are to be observed on the same flight, then a number of

receivers that can be rapidly switched in and out of operation would be

required. The cost and complexity of this type of system might prove to be

prohibitive. However, since a single open-structure SIS receiver can be

used from 2mm to 600/im, the cost and difficulty of switching between different parts of the submillimeter band are dramatically reduced.

For these reasons (sensitivity and broad frequency coverage), openstructure SIS heterodyne receivers are now being developed at UA's Steward

Observatory and at SAO. Bowtie antenna/junction structures fabricated by

R. E. Miller of Bell Laboratories for the current Steward Observatory SIS

receiver are shown in Figure 5.3-2. These are essentially the same as

those used by Wengler et al. at Caltech (who also used junctions fabricated

by Miller) except that the size has been scaled down to provide optimum

performance at 345 GHz.

We have tested a single-element version of the receiver both in the

lab and on the NRAO 12 meter telescope. The lab results indicate that at

345 GHz the receiver has a double-sideband noise temperature of ~350 K. A

plot of the DC and local-oscillator-pumped I/V curve of an SIS junction

operating at 345 GHz at SAO is shown in Figure 5.3-3. The sensitivity of

the system can be computed by comparing the IF power output obtained when

hot and cold loads are placed in the beam of the receiver; these IF powers

are also plotted in Figure 5.3-3. As expected, the junction is most sensitive in the vicinity of the "knee", or non-linear region, of the I/V curve.

The 345 GHz tests on the 12 meter telescope indicate that the bowtie and

optics produced a main beam with a half-power size < the diameter of Jupiter (45" during our run). Poor weather precluded measuring the beam on a

smaller source. Further tests of this system will be made on the Steward

Observatory 1.5 meter telescope in the spring of 1988.

In the near future, we will begin testing the performance of firstgeneration receivers using 2- and 4-element SIS/bowtie arrays. Such array

receivers are needed to make the most efficient use of flight time. We are

also investigating the broad-band characteristics of other types of antenna

structures that may provide better performance than the bowtie, including

log-periodic, spiral, and V-shaped designs. We propose to combine the

results of these studies to construct an SIS array receiver for the twometer balloon-borne telescope.

A block diagram of the proposed heterodyne receiver system is shown in

Figure 5.3-4. Beams from the telescope and from the local oscillator are

combined in a diplexer of the Martin-Puplett design, using wire-grid elements as the polarizing mirrors. Local oscillator power at each observing

frequency will be provided by a Gunn diode multiplied in a harmonic generator; phase stability is provided by a phase-locked loop using a multiplied

crystal oscillator as the reference. The combined signal and local oscillator beams enter a cryogenic dewar housing the SIS mixer, and far-infra-
Page 79

a) SIS Bowtie Junction

SIS Junction

dia.)

Pb-Su

Quartz Substrate

External Leads

b) SIS Bowtie Array

Figure 5.3-2. Bowtie and Bowtie-Array SIS Junctions

red-transmissive optical components couple the optical beam to the junction. For ground-based work we have developed a hybrid design which utilizes a 4-liter liquid-helium reservoir and CTI-22 cold head for the

shields; the hold time is about 10 days. For a balloon-borne receiver we

will construct a simpler batch-fill liquid-helium and -nitrogen cryostat,

since cycle times can be shorter, and simplicity and total weight are

important considerations. A superconducting magnet is provided to apply a

modest field to the junction in order to suppress undesirable Josephson

effects. The first IF amplifier will be a HEMT mounted directly on the

cold work surface of the dewar.

Following additional processing outside the dewar, the signal is

eventually fed into a backend spectrometer. This may be a filter bank, a

digital autocorrelation spectrometer, or an acousto-optic spectrometer

(AOS); we currently favor the AOS, as it will be cheaper, lighter, and

more flexible than the alternatives. We expect the AOS to have 2048 spectral channels and 1 GHz total bandwidth (1.5 GHz may be possible on the

time scale of this project, but 1 GHz is current technology). Spectral

resolution will be 0.5 km/s at A = 1mm and 0.3 km/s at A=600 fua.. Total

bandwidth will be 1000 km/s or 600 km/s respectively. The signals from the

spectrometer will be read at a relatively modest data rate and passed to
Page 80

100

40

zo

Figure 5.3-3. Laboratory SIS results at 345 GHz. The lower two curves are

the I-V curves of the SIS junction with and without local oscillator power.

The upper curves are the IF power output versus bias voltage when the

detector sees a cold or hot load (vertical scale on the right). The system

sensitivity is given by the ratio of the hot/cold IF curves.

the gondola telemetry electronics for relay to the ground control station.

A preliminary estimate for the total weight of the heterodyne submillimeter receiver, backend, and associated interface electronics is 140

pounds. The total power consumption will not exceed 140 Watts.
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The receiver development efforts proposed here will benefit from other

submillimeter heterodyne projects now being pursued at Steward Observatory

and at SAO. In particular, our Steward Observatory collaborators are

involved in the construction and instrumentation of the SMT, a 10-meter

dedicated submillimeter. telescope on Mt. Graham. Work on SIS heterodyne

receivers of the type needed for the two-meter telescope has been going on

for a few years now. SAO is developing heterodyne SIS receivers for a

proposed submillimeter telescope array. Establishment of the SAO receiver

lab is progressing at a rapid pace, and an operating base budget of

$402,000 per year from the Smithsonian Institution ensures a state-of-theart effort.

It should be emphasized that the development of a balloon-borne submillimeter heterodyne system will contribute in turn to the technology base

for submillimeter ground-based instrumentation.

6.0 GROUND SUPPORT EQUIPMENT REQUIREMENTS

The Two-Meter Balloon-Borne Telescope gondola is a complex and versatile experiment and requires certain specialized equipment throughout the

program to support assembly, development testing, subsystem testing and

finally full flight level system testing including experiment testing and

calibration. This section identifies, in general terms, the GSE required

to support these assembly and test activities.

6.1 Optical GSE

Primary and secondary mirror handling, transport and alignment fixtures will be required. These will be supplied by the mirror contractor.

Once assembled, the telescope will require the following specialized equipment for shipping and its alignment and installation into the gondola:

o Shipping container

o Telescope optical alignment fixture

o Star tracker and aspect TV co-alignment fixtures

o Alignment telescope with accessories

o Laser alignment system

o Miscellaneous optical accessories (mirrors, beamsplitters, corner

cubes, etc.)

o Automatic auto-collimator for stability testing

Experiment optical alignment and calibration with the telescope will

require the following optical/mechanical GSE:
Page 83

o Light sources

o IR sources

o Experiment FOV beam mapping device

o Experiment installation/handling fixture

NOTE: The telescope alignment fixtures can also serve to optically align

experiments.

6.2 Mechanical GSE

Mechanical GSE is required for assembly, handling, and recovery of the

gondola. Gondola assembly will employ several fixtures for handling such

large components as the telescope, structural members, crash rings and

experiments. Gondola assembly a,nd test will also require portable staging

for access to the telescope and both servo-driven gimbal axes.

Handling fixtures are required for positioning and/or transporting

major assemblies to facilitate independent testing or repair. The gondola,

for example, will require a large base frame with soft (balloon tires)

wheels to maintain it in an upright position when not attached to ^he

crane.

Recovery will require several shipping containers for critical items,

such as: Experiments, star tracker, aspect TV's, secondary mirror assembly, servo components, etc. Also a standard, 36-foot flat-bed trailer with

air suspension and crane is desirable. This will facilitate gondola disassembly and placement on the trailer for transport back to the NSBF.

6.3 Electrical GSE

The Two-Meter Balloon-Borne Telescope program will have an independent, self-contained telemetry ground station except for the use of the

NSBF receiver, discriminators, and directional antennas. Being independent

of the NSBF ground station as much as possible permits economical full-time

use of the equipment for, testing and flight preparation. A permanent

facility that doesn't have to be moved in and out of the NSBF control room,

or respond to the needs of other payloads, is needed to support the proposed schedule of flights. Only a coax connection is needed to receive the

PCM data. The PCH data is patched out to several areas in the setup bay.

The serial PCM data is decoded by a D-Pad 3 PCM decommutator and sent to a

central minicomputer (e.g., a VAX 11/750) that will provide real-time

output of the scientific data and housekeeping information, on strip

charts, video consoles, and a printer. We assume NSBF will continue to be

responsible for recording the data on digital and analog tapes. The minicomputer will also be used to send commands, and provide star field information for the graphics overlay system. The graphics controller mixes the

incoming vidjeo signal with a graphics overlay generated by the computer and
Page 84

displays the result on a monitor. This is very helpful in rapid identification of the desired area of the sky. Both the acquisition and focalplane television information will be displayed and recorded on tape.

A block diagram of the ground support equipment and facilities is

illustrated in Figure 6.3-1.
Page 85

N S B F Control Room Y Y Y

TELEMETRY

RECEIVERS a DISCRIMINATORS

VIDEO ASPECT A

TERMINAL )

Figure 6.3-1. Gondola Electrical Ground Support Equipment
Page 86

7.0 NSBF DESIGN DRIVERS AND PAYLOAD SUPPORT REQUIREMENTS

7.1 Payload Support Requirements

7.1.1 Staging

Figure 7.1-1 shows the gondola supported within the NSBF high bay

experiment staging building.

The NSBF high bay experiment staging building is ideally suited for

this program. It has a 3-ton traveling crane with a lift height of 30 ft.

and experiment exit doors 27 ft. in height by 14.5 ft. in width. This

combination allows gondola suspension for pointing and stabilization testing with access to evening skies and/or within the bay area. The bay's 24-

ft. width is more than adequate and if about half the bay's length (30 ft.)

is available then this floor area (720 sq. ft.) is adequate for this

testing of all operations planned at NSBF. All other staging area facilities are adequate such as lights, power, heat, air conditioning, office

space, and so forth.

The gondola is designed for convenient transport to sites away from

Palestine, Texas, such as a winter launch site in New Mexico and transport

from the recovery site. The former is particularly important because of

the increasing emphasis of the NSBF on remote launches to overcome constraints of balloon ascents over areas of rapidly growing population in

East Texas.

Space must also be provided within the high bay building for staging

and use of the optical, mechanical and electrical GSE associated with these

missions. GSE requirements are identified in Section 5.0.

7.1.2 Command and Telemetry Trailer

The gondola also has been designed to operate within the existing NSBF

command and telemetry system limitations. This system is adequate for our

projected needs at its present 81-kilobit data rate; however, the planned

expansion to a 256-kilobit data rate is highly desirable to accommodate

efficiently the high bit rates associated with the larger IR imaging arrays

now under development.

The construction and outfitting of a complete command and telemetry

ground station trailer in support of this program is highly desirable.

Tracking antennas and an appropriate area for NSBF equipment and personnel

would be included. This ground station trailer would greatly enhance the

test phase by allowing testing to be conducted in a flight configuration

thereby eliminating the NSBF interface and other specialized test equipment

to support operational testing. It could also be moved to experimenter's

facilities for the same purpose. Further, it removes conflicts normally

associated with operating from the NSBF control room where priorities, set

up time, and communications during testing are sometimes difficult.
Page 87

NSBF STAGING BUILDING

MAX. OPENING

Figure 7.1-1. Gondola Supported Within the NSBF High Bay Experiment Staging

Building.
Page 88

The trailer would also be a valuable asset to support long duration

flights, or for launch times when float winds are high. Also, launches

from remote sites, such as Fort Sumner could be accomplished more easily.

7.2 Launch Requirements

This experiment requires no special and/or different launch requirements than presently employed by the NSBF. The experiment is shown suspended from the launch vehicle in Figure 7.2-1. It requires a suspension

height of 29 ft. which is located 9 ft. ahead of the vehicle. This position is easily achievable because of our light weight. All launch vehicle

radial clearances with the launch arms are as requested by the NSBF.

Launch requirements relative to science aspects will be similar to

those associated with our present One-Meter Balloon Infrared Telescope

program. These have been mentioned previously and are summarized below:

Flight Altitude: 28-30 km

Flight Duration: 6-10 hours at altitude

Launch Time: Local sunset ± 1 hour

Termination Time: After sunrise at altitude

The current NASA balloon flight policy concerning the Palestine, Texas

launch site is:

• Experiments weighing less than 3500 pounds below balloon can fly

any time throughout the year. This weight includes approximately

1050 pounds of NSBF equipment.

• Experiment weighting less than 5000 pounds, below balloon, can fly

without a waiver, when the trajectory is in a westerly direction.

This occurs approximately between May through September. The

remainder of the year a waiver would be required.

The two-meter gondola weighs 3,809 pounds below the balloon. We

believe this can be brought down below 3500 pounds by careful design and

materials selection and expect to qualify the gondola for year round

flights from Palestine.
Page 89

6'

19'

GONDOLA HEIGHT

5'

GROUND CLEARANCE

BALLOON-BORNE

TWO-METER

INFRARED TELESCOPE

I

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TELEMETRY ANTENNAS

9'

LAUNCH ARM

NSBF LAUNCH

VEHICLE

"TtNY TIM"

Figure 7.2-1. Gondola Suspended from Launch Vehicle
Page 90

7.3 Ascent and Float Environment

The gondola will be subjected to a temperature change of 130°F between

launch and float altitude with an expected float temperature minimum of

-50°F. These environments are well understood and have been successfully

managed in numerous balloon payloads including our own One-Meter Balloon

Infrared Telescope.

Thermal design does not represent an area of technical concern or

development, although stabilization of the mirror temperature is a new

design area, which requires diagnostic .measurements and testing.

The gondola must also operate in reduced atmospheric pressure (approximately 10 Torr). Operation of balloon payloads in this pressure range is

also well understood and therefore is not an area of concern.

Present far-infrared balloon-borne telescopes operate at altitudes of

approximately 30^2 km. This altitude is high enough to be above most

(>99%) of the water vapor in the earth's atmosphere, yet not so high that

it taxes the size of the balloon required. This reduces cost, handling

complexity and risk.

The balloon volume required is calculable once the altitude and the

suspended weight are specified. For a suspended gondola weight of

3,809 Ibs. and a flight altitude of 29-31 km, a 11.6 million cubic ft.

(MCF) balloon is required. This is a "small" balloon by NSBF standards

and, therefore, should have the highest probability for a flight success.

In the atmospheric region of interest wind velocity also increases

with altitude, limiting the time in which the payload will remain within

the telemetry range of the base station. Four months a year the wind

velocity at 29-31 km is < 30 knots. Telemetry range at this altitude

is ~300 miles. Therefore, flights of 8-10 hours can be routinely conducted throughout this period. This takes advantage of the full night time

period.

The gondola has been designed to operate without performance degradation at vibration levels in the 3-5 g rms range, which is well above the 1-

2 g rms expected during ground testing, transportation to the launch pad,

launch and ascent.

The number of flights that can occur per year depends primarily on the

manpower and funds available. A typical time between flights for a recovered and refurbished payload of this complexity is about three weeks assuming adequate spare parts and minimal damage. During the first year we

believe that one flight is a reasonable goal with the new gondola. During

the operational phase as many as five flights could be accommodated each

year.

During long duration flights, the increased flight time produces

demands on the gondola and ground operations, including the additional

weight of ballast, additional thermal problems resulting from both day and

night operations, downrange telemetry stations, remote launch operations,
Page 91

additional battery packs, etc. The initial flights are limited to the

night, about eight to ten hours at the float altitude, and launching the

balloon from the NSBF base site. The next step would be to increase the

flight time to observing one night and daytime (approximately 23 hours) ,

followed by eventual cross-country flights of two to three days. Such

cross-country flights have already been performed by the NSBF.

7.4 Termination, Landing and Recovery

The task of recovering, loading and transporting gondolas back from

remote areas is difficult at best. This size gondola requires that careful

attention be paid to recovery operations to assure a safe and straightforward return of the gondola. The two-meter gondola can fit fully assembled

into an enclosed air cushion truck for non-abusive transport back to the

launch site without concern for special routing and/or the need for local

road variances.

Modular design will minimize the damage usually associated with recovery and will keep recovery time to a minimum and allow critical components

to be removed following landing for separate transportation back to the

launch site.

The NSBF requires that a balloon experiment not experience a structural failure resulting in an article or payload free falling from the

suspension train. The structural loads are specified as 10 g's (static)

along the suspension axis or 5 g's at 45° about a 360° cone located at the

attachment point to the NSBF equipment, whichever is limiting. The proposed Two-Meter Balloon-Borne Telescope design meets all these requirements .

8.0 IMPLEMENTATION PHASE SUMMARY DEVELOPMENT PLAN

8.1 Introduction

A plan for designing, building, and flying the Two-Meter Balloon-Borne

IR Telescope has been developed based upon the assumption that the telescope subassembly and optics would be built by industry, the gondola would

be built by the Smithsonian Astrophysical Observatory (SAO), the photometer

built by the University of Chicago, and the Heterodyne receiver built by

the University of Arizona. The resulting cost estimate therefore covers

the development and initial flight of the entire payload.
Page 92

8.2 Statement of Work

The Smithsonian Astrophysical Observatory and its team members -- the

University of Arizona (UA) and the University of Chicago -- will design,

build, and operate the Two-Meter Balloon-Borne Telescope in accordance with

this program plan and NASA requirements. One flight is planned during the

first year of field operations.

Specifically, the Smithsonian Astrophysical Observatory will:

1. Hold overall responsibility for systems design, fabrication, test

and integration including integration of the mirrors, secondary

mechanisms and primary mirror cover subassembly with the telescope .

2. Hold overall responsibility for program management.

3. Define instrument and telescope interfaces.

4. Be responsible for configuration management, reliability, and

quality assurance.

5. Be responsible for field operations, with the support of the University of Arizona and the University of Chicago, including instrument and telescope integration and test.

6. Be responsible for data acquisition and reduction.

7. Perform scientific data analysis and publication of results.

8. Provide the secondary mechanisms and cover mechanisms to the University of Arizona for telescope integration.

The University of Arizona will:

1. Be responsible for optical design, mirror fabrication and test.

2. Provide the telescope assembly to SAO by means of a subcontract to

industry.

3. Perform scientific data analysis and publication of results.

4. Provide the heterodyne receiver.

5. Support field operations and test.

The University of Chicago will:

1. Define instrument accommodation requirements.

2. Provide the IR photometer for the first mission.

3. Support field operations and test.
Page 93

4. Perform scientific data analysis and publication of results.

8.3 Schedule

The Implementation Phase is planned as a four-year program with the

six months devoted to field operations (see Figure 8.3-1). Completion of

the gondola in forty months is an ambitious effort, but the schedule is

realistic. It will require a heavy front-end loading of engineering personnel to allow the many design tasks to be carried out in parallel. The

present SAO Central Engineering organization is ideally suited for working

in this manner because it allows us to transfer people into and out of the

program as required; only a core staff of engineers will be retained

throughout the program for the purpose of providing continuity to the

effort.
Page 94

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8.4.3 Cost Estimate Assumptions

The following assumptions were made as a basis for this estimate

The gyro package is assumed to be provided as GFE.

All shipments are by.government bills of lading (GBL's).

Gondola buildup and test will be done in a rented high bay area at

Hanscom Air Force Base, Bedford, MA.

"WAE" employee is servo system analyst.

Travel is ROM estimate based on experience with similar programs.

Engineering salaries are FY88 actuals with 5% average increase per

year assumed, full-time employees listed at 88% productivity.

No command and telemetry trailer cpsts are included.

9.0 COMPARISON OF THREE-METER AND TWO-METER GONDOLA CONCEPTS

In Table 9-1 the telescope and gondola specifications are given for

both a two-meter and three-meter mirror in the telescope. The three-meter

mirror offers the advantages of higher angular resolution and greater

collection area, and as a result has greater sensitivity for detecting and

resolving astronomical sources.

The three-meter system was our first choice and has been studied

extensively. However, there are difficulties in constructing, testing,

flying, and recovering such a system when compared to a two-meter system.

Obviously the three-meter system i^ also more expensive. The first concern

was the fabrication of a three-meter carbon-fiber epoxy mirror which would

be diffraction limited at 30 microns. We have shown that such a mirror

could be constructed, but with difficulty and it would be expensive. A

two-meter mirror can more readily be constructed using presently existing

facilities, and obviously would cost less. The two-meter system could

probably be launched from NSBF, Palestine, TX where extensive support

facilities exist, whereas the three-meter system would have to be launched

from a remote site.

A smaller sized less expensive balloon would also be required for the

two-meter gondola after flight and its recovery would be considerably

easier, using standard trucking facilities.

It is not easy to relinquish scientific capability, but the two-meter

system will still be a major step forward over our present capability, a

one-meter system which is now 18 years old.

PRECEDING PAGE BLANK NOT FILMED

95"- -f <0 - /O |
Table 9-1 Comparison of Two-meter and Three-meter Gondola Specifications

Page 103

Optical Characteristics

Aperture

Spectral Range

Optics Design

Focal Ratio

Back Focus

Angular Resolution

Infrared Field of View

Optical Field of View

2 Meter Design

2 Meters

Visible to millimeter

Cassegrain

f/13.5

93 cm from primary mirror vertex

Diffraction limited to SO^m

(Raleigh Criterion 3.9")

6' diameter unvignetted with ± 3' chop

and diffraction spillover at 1 mm

15' vignetted only by the primary

3 Meter Design

3 Meters

Visible to millimeter

Cassegrain

f/13.5

140 cm from primary mirror vertex

Diffraction limited to SOjim

(Raleigh Criterion 2.5")

5' diameter unvignetted with ± 2.5' chop

and diffraction spillover at 1 mm

15' vignetted only by the primary

Pointing Stability

Inertial Mode

Magnetometer Mode

Source Offset

Slew Rate

1" rms

±10' azimuth, ± 5' elevation

± 1/2° (Magnetometer Mode)

10VS

1" rms

± 10' azimuth, ± 5' elevation

± 1/2° (Magnetometer Mode)

10VS

Thermal Stability

Telescope -40°±10°C

Mirror Stabilization Time < 1 Hour

-40° ± 10° C

< 1 Hour

Command and Telemetry

Number of IRIG User Channels

Useful Range

IRIG VCO Channels Available

PCM Bit Rate

Video Bandwidth

To limit of standard NSBF telemetry

1/3/5/7/8/9/1 0/11/12/B/E/H/HH

81 KBpS; expandable to 256 KBpS

4.5 MHz

To limit of standard NSBF telemetry

1/3/5/7/8/9/10/11/12/B/E/H/H H

81 KBpS; expandable to 256 KBpS

4.5 MHz

Gondola Characteristics

Total Weight

Overall Height

Overall Width

Overall Depth

1731 kg (3809 Ibs.)

5.8 Meters (19 Feet)

2.9 Meters (9 Feet 6 Inches)

2.5 Meters (8 Feet 2 Inches)

2,168 kg (4,780 Ibs.)

8.0 Meters (26 Feet 2 Inches)

5.49 Meters (18 Feet)

4.0 Meters (13 Feet 3 Inches)

Experiment Accommodations

Power

Weight

140 W typical; 425 W maximum

125 kg at focal plane

140 W typical; 425 W maximum

125 kg at focal plane
Page 104

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