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Produced by the NASA Center for Aerospace Information (CASI)
1 

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NASA CR- I 7·5313 

PRINCETON UNIVERSITY OBSERVATORY 

Princeton, New Jersey 08544 

'~ASA-~tl-1761.614) 1~'l&:.fS'rEL1Alo l'H:.tlUtt NOL-!4UJI 

Jd:!:.OlIPTILN Plil-I" 1Lt. !J~H.lhL(jttAfll II/UPS) 

rln.sl. Rl'~oLt, ~1 Auq. 1S~O - 30 Jun. 19t15 

( 1-' r 1. Dell ten U II i v., N. J. ) 1 I ~ J. u,' c 1 ci ~ 

He AObll"IF AJI C5~~ 14lJ G3/19 JJ011 

FINAL PROJECT REPORT 

for NASA Contract 

NAS5-26268 

Interstellar Medium Absorption Profile Spectrograph 

(I MAPS) 

Coverina the Period 

Auaust 21,1980 to ~une 30, 1985 

Submitted to: 

GODDARD SPACE FLIGHT CENTER 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

Greenbelt, MD 20171, 

By. ~~fS,~~ 

August 27. 1985 

Edward B. Jenkins 

Principal Investig 

Princeton University Observatory 

Princeton. N. J. 08544 

Tel: 609-452-3826 

• , t, I 
TABLE OF CONTENTS 

Table of Contents ........................................................................................ i 

1. BACKGROU~ID .......................................................................................... 1 

2. HISTORICAL SUMMARY or THE PROGRAM .............................................. 3 

3. INSTRUMENT OVERVIEW ......................................................................... 5 

4. GENERAL SCIENTIFIC RATIONALE ............................................................ 8 

4.1. Wavelength Coverage ............................................................................ 8 

4.2. WaveleIlt th Resolving Power ............................................................... 11 

5. SPECIFIC RESEARCH GOALS .................................................................. 13 

5.1. Physical State of H I Regions ............................................................. 13 

5.2. Investigations of H II Gas .................................................................... 16 

5.3. Coronal Gas and Transi lion Regions ................................................... 18 

5.4. Dynamics of Interstellar Shocks ........................................................ 20 

5.5. The Local Interstellar Medium ............................................................ 21 

6. PAYLOAD DESIGN AND PERFORMANCE .................................................. 23 

6.1. Mechanical Collimator ........................................................................ 23 

6.2. Echelle Grating ................................................................................... 24 

6.3. Cross-disperser Grating ..................................................................... 24 

6.4. Detector ............................................................................................. 26 

6.5. Signal Transmission ............................................................................ 29 

6.6. Compensation for Small Pointing Errors ............................................ 29 

7. TECHNICAL HIGHLIGHTS ........................................................................ 30 

7.1. Problems with CCD Performance ....................................................... 30 

7.2. First Flight of IMAPS on a Sounding Rocket (27.071UG) ..................... 41 

7.3. Second Flight of IMAPS on a Sounding Rocket (27.082UG) ................. 56 

7.3.1. Subsystem Performance and Useful Engineering Data ................... 57 

7.3.1.1. Rocket Vehicle Performance ........................................................ 57 

7.3.1.2. Time Profile of the Payload's Skin Temperature .......................... 57 

7.3.1.3. Attitude Control System (ACS) Performance ............................... 60 

-;C.-
, 

• 

, , 

. , , 

1.3.1.3.1. Target Acquisition ..................................................................... 60 

1.3.1.3.2. Stability .................................................................................... 60 

1.3.1.4. Telemetry Signal Quality .............................................................. 66 

1.3.1.5. Event Timing ................................................................................. 61 

1.3.1.6. Performance of the ICCD Detector ............................................... 12 

1.3.1.6.1 Distribution of Event Charge Amplitudes ................................... 12 

'7.3.1..6.2. Lateral Spread of Charge in Each Event .................................... 15 

1.3.1.6.3. General Appearance of a Frame ................................................ 16 

1.3.1.6.4. Rate of Ion Events ...................................................................... 80 

1.3.1.1. SensiU·. ity of the Entire Instrument.. ........................................... 80 

1.3.1.B. Background Light ......................................................................... 82 

1.3.1.8.1. Di1Tuse La Background ............................................................... 82 

1.3.1.8.2. Scattered Waht from the Gratings ............................................ 83 

1.3.1. 9. Spectral Resolution .................................................... , ................. 83 

1.3.2. General Character of the Scientific Data ........................................ 84 

8. FINANCIAL STATUS OF THE PROJECT ..................................................... 90 

9. KEY PERSONNEL ON THE PROJECT ........................................................ 91 

APPENDIX A: Subcontractor Selection ...................................................... 92 

APPENDIX B: Calculation of Temperature Rise ......................................... 98 

APPENDIX C: IMAPS Reflectivity Measurements ...................................... 100 
t. BACKGROUND 

In 1976 NASA released an announcement of opportunity (AO) which requested 

r~search groups to submit proposals for experiments to be ftown on future Spacelab missions. In response to that AO, Princeton University and the Ball 

A~rospace Systems Division (BASD at Boulder, Colorado) teamed up 1 to submit a 

proposal in November of that year to fty a high-resolution spectrograph for observing hot stars in the ultraviolet. 

The payload instrument, called the Interstellar Medium Absorption Profile 

Spectrograph (IMAPS) , was an ultraviolet echelle spectrograph which could record 

stellar spectra from 965 to l165A with a wavelength resolvingl'0wer )../t:,.).. = 2x 105 

(i.e., < o.olA ).2 

There was a two-fold motivation for building and ftying IMAPS, one scientific 

and the other technical. The scientific purpose, outlined in detail in §§ 4 and 5, was 

to establish an observing program with an instrument which could reach beyond 

the capabilities of the telescope-spectrometer built by,Princeton for the Copernicus satellite. This new initiative was to build upon the earlier pioneering conclusions on the nature of the diffuse interstellar medium which were derived from the 

many years of operation of Copernicus. Specifically, we felt a strong need for having an instrument which could record stellar spectra in the astrophysically important wavelength rcll1~e extending from the Lyman limit (912A ) to the vicinity of 

1100 to 1200A, but with Significantly higher wavelength resolving power than t.hat 

provided by Copflrnicus ()..It:,.).. = 20,000). 

lThe initial d~cision to respond to the NASA AO was made by the scientific investilation team at 

Princeton. Far in advance of writil1l the proposal. Princeton decided that it would be beneficial 

to enlist the help of ar aerospace instrument development contractor to create the initial instrument concept and prepare the proposal. Appendix A of thls report lives a detailed account 

of how we competitively solicited a potential partner and how BASD was selected. 

2Tms description applies to [MAPS in its present form. The original proposal called for a broader 

wavelenath coverale (925-13151) and a lower resolution (1.3)(1011) • 

., ,... , .--

-~----~----_"'--_#_I _____ --... -· .~~'-____ _ -.. 
- 2 -

The main techmcal objective was to have a prorram which made use of a new 

photon-counlina ultraviolet detector developed rinceton. The 1n1lial exploralicn on t.he feasib1lity and operaliIli charactenstics of the device was supported by 

a ~ASA SR&T arant to Princeton (~ SG-761B) . TIllS detector. a windowless. 

lntensifted CCO camera (ICCD) described in f8.4. represented a very promislna 

candidate concept for a two-dimensional imaae sensor to be used on future space 

astronomy enterprises which must function at wavelenalhs covered by lMAPS 

(such as the proposed Lyma.n [FVSE~ Explorer rrussion).3 By 197 8. we had reached 

a point in the detector proaram where ftiaht instrument development was an ideal 

way to brina forth a more mature desian. Also. we knew that ultimately we would 

need to prove that the detect'Jr could function well h an actual ftiaht environment 

before it would be an acceptable candidate for ultraviolet instrumentation in general. 

On the basis of a Cate.ory III ralina by the peer review of our proposal. NASA 

turned down our request for a Spacelab mission assiar.rnenl. While the proposal 

was judaed to have excellent scientific merit, some apprehens10n was expressed 

about the technical risk in tryina to fty a detector which was not proven. On the 

basis of this evaluation, we felt it was proper to resubmit our proposal within the 

framework of a soundina rocket researr , proarbm. The appropriateness of this 

strateay was underscored by the basic phllosophy of the NASA soundina rocket 

research effort, namely, to allow invesliaators to develop new instrument concepts 

at low cost, with a aenerous ~· xibility in sched\..:e, and without the risk that 

II Many other detector co~uration. IL table for far-uv wavelenattw ur<t a microchannel plate 

with a photocathode deposited on the fro:.t lurface. Photocathod .. opera~ "lI on the nlI,ed terrain of an YCP have two drawbackl, of as yet undetermined severity, when cc.'1'lpared with monolIthic photocathodel on a lolld substrate employed by the Princeton [CCD. The dlacontinuoWl 

structure and complicated t aelds of the MCP photocathodel result In 1) a I,wer overall quantum et!lclency and 2) spatially periodic modulationa In responae caWied by the MCP pore structurel which can prochlce undellrable et!ects If the Ima, .. have small-scale details. The latter 

ef!ect hu yet. to be measured directly. but lUI ext.tence leems very plausible. 

....... "':--....... -~ . ~----- ----

nn, l::-:L:::
- 3 -

technical dUTiculties or initial ftiaht failures would jeopardize the large investment 

of a complex ftiaht program. 

The initial submission of a proposal for sounding rocket research USing the 

lMAPS instrument was made in July 1979. This proposal was followed by amended 

budgets and schedules in November 1979 and March 1980. On August 21,1980, 

NASA awarded (throuah the Goddard Space Flight Center) contract NAS5-26268 to 

Princeton University to embark upon the proposed program. In line with our original commitment to BASD in our proposal writiIli agreement, we enaaged them 

under subcontract to build the payload and support testina and flight operations. 

The effort we outlined called for buildina IMAPS to fly on Black Brant rockets, with 

the ultimate intent of converting it to operate on Spartan missions in accord with 

the NASA's planned transition from suborbital to orbital transportation of small 

astrl')nomical payloads. 

2. mSTORlCAL SUMMARY O~" THE PROGRAM 

The design, procurement of parts and assembly operations at BASD advanced 

in an orderly fashion from late 1980 to mid-l&82. To a good approximation, we 

were able to maintain the program within the projected costs and schedule 

throUjhout these phases. After the basic payload fabrication had been completed 

in late 1982, NASA support was shifted from the contract discussed in this report 

to a grant (NAG5-e16) since the ren:ainina effort was to be primarily mission 

operations, payload refurbishment and scientific research, rather than the initial 

procurement of a complicated instrument from a subcontractor. 

In preparing the payload for flight, we experienced a delay of about one year 

after we discovered the sensitivity of IMAPS was at least one or two orders of magnitude loo low. Ultimately, we traced the difficulty to problems with the imaging 

detector (see §7.1). Specifically, the design of the on-chip amplifier for the CeD 

l ____ ."_.~._ .. ____ ._ .... ~ __________ ... #J ........ ______ --'.L .. ___ ._._. __ . 

. ~ t" 
, 

~ 

- 4 -

that we purchased iritially was not aood enouah to aive us the necessary charge 

s~nsilivity and hiah-frequency resvonse to detect individual photoelectrons. Also, 

its charae transfer efficiency was not adequate. In early 1964 we obtained from 

RCA a CCD with a newer desian. This CCO showed excellent operalil'li charactenstics, and as a result. the earlier difficulties with our detector were surmounted 

very effectively. 

The first launch of the IMAPS instrument on a Black Brant rocket occurred on 

October 5, 1964. Unfortunately, on this ttiaht there was a catastrophiC inrush of 

air intl' the payload. The severely degraded vacuum caused the detector's high 

voltage system to bl'eak down and as a result we were unable to obtain any data. 

An analysis of the probable cause of this failure is discussed in detail in §7.2. 

Shortly after the first fiight of lMAPS. BASD began refurbishment of the payload for a second flight. Concurrently, we started an investigation on what caused 

the vacuum failure and what steps we would take to minimize the chances that it 

could occur again. After we had satisfied ourselves that we had worked on the 

vacuum problem as thoroughly as possible. we embarked on final preparations for 

the second flight. 

The second fiight of lMAPS occurred on April 20, 1965. All of the payload systems functioned perfectly, and excellent recordings of the data were obtained. We 

were not only pleased with the quality and quantity of scientific information available from this fiight. but also we gathered valuable indications on how well the 

ICCD image sensor can perform in a space environment. A discussion of the preliminary conclusions from the fiight data is contained in §7.3. 

In response to a solicitation from NASA Heaciquarters to existing sounding 

rocket experimenters. we submitted a proposal in June 1984 to convert IMAPS for 

operation on Spartan and fiy it in early 1988. In early 1985 we learned that NASA 

accepted our proposal. Currently. the payload is being reconfigured so that it will 

-.. , l ", 
- 5 -

t" 

operate on Spartan. ., 

3. INSTRUIIENT OVERVIEW 

The payload we new on sound ina rockets and Spartan was a stilless, objective 

gratina spectrograpt. which recorded ultraviolet stellar spectra at a resolution of 

approximately o.oo4A (profile FWHM) and with a sample interval (pixel width) of 

o.oo2A over the very difficult wavelength ranae 965 < A ( 1165;\, a region of 

extreme scientific importance. A key feature of the optical design was its extreme 

simpliCity; aside from a mechanical collimator which restricts the field of view to a 

one degree circle to eliminate confusion from nearby slars, the incident starlight 

interacts with only two optical elements before it is Imaged. First, the parallel 

beam hits an echelle graling. Diffracted light from the echelle then goes to a parabolic croJs-disperser ~ratilli which imagtl'!S the spectrum on a 1.5 cm diameter 

photocathode in e. windowless, pholon-counting Intensified CCD deteclor (ICCD). 

The system is illustraled in Figure 1. 

ICCD DeTECTOR 

I!CHELLI! 

OFF-A)(II PARASOLIC 

CROIS DISP'I!RSI!R 

Figure 1 

The high wavelength dispersion of this instrument was achieved by having the 

.t:_-_________ ........ _·11_' _____ ...... '-~:./:. ___ _ 
I 

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echelle set at a larae anale 01 incidence (83°) and employins a very lona focal 

1enath lor the cross-disperser (1800 mm), While the area of the entrance aperture 

was only 250 cm2, the Haht collectina power 01 the instrument was very aood for a 

device workina in this wavelenath interval because there were relatively few 

re1\~clions and the KBr photocathode, beina opaque and on a soUd substrate. had 11 

hiah quantum efficiency. This payload was built to tlt within the standard Black 

Brant soundina rocket envelope (17-inch diameter cylinder). 

The principal research moUve was to build upon the pioneerina investiaations 

from the uv spectrometer and telescope on the Copemicus satellite, but with a 

quant-am leap forward in wavelenath resolvina power (a factor 01 10). As imp~.~d by 

the name of our instrument, Int'TSteUa.r Med.iu.m. Absorytion. Profile Spectrograph. 

(IMAPS) , the areatest emphasis was to obtain very accurate line profiles from 

absorption features which are produced by constituents in the interstellar 

medium, both atomic and molecular. 

There are, of course, other areas of astrophysical research which also will 

benetlt enormously from observations by IMAPS. Much can be learned about the 

composition, dynamiCS and overall structure of hot stellar atmospheres, and likewise for the chromospheres around relatively cool stars. II special mstrumental 

accommodations are made, IMAPS can even make unique observations of solar system objects, since its large dispersive power would, in all likelihood, enable us to 

record simultaneously the images of various emission features without overlap. 

Nevertheless, while these other research areas are important, the main emphasis 

01 the research supported by this contract centered on contributions IMAPS could 

make in furthering our knowledge of interstellar matter, since this is where the 

particular strengths of IMAPS gave the greatest additional leverage. Also, this 

topic was the chief area of interest to the Principal Investigator. 
A

-7-

Over the quarter-century span of NASA's existence, we have witnessed an

impressive growth in the power of uv space astronomy instruments to gather information. In certain respects, IMAPS represents a continuation of this trend, even

within the shadow of far large-r and more costly ventures which operate at longer

(and easier) wavelengths. The diagram in Figure 2 illustrates this point.

8

A

V

3

1985 - [2)

61MAPS 1986 - (5]

d

5 (on[t;5f)

C *Copernicus 1972-81 (5] *BUSS 1979 - (3j

4 Y.O rI s 1,M.-72 •lUE 1978 - [S]

y

ggm

(2,

*Recker n 871 (2] 

d

1

g 6arnith 1969 (2J

*Carruthers 1968 - (2]

C

F /0A0-2 1968-74 (5J

_

OTD-1 1972-74 (4]

ey o odes. J

> g d

•iI^^1F^an 4^2 ( 1 J

(11 - unpointed rocket

3

v

r+0 1

Chubb dd !Hytam ^1^83 1

lio`2ess 1964 fl^

[2] - pointed rocket

(3'1 - pointed balloon

[4) • unpointad satellite

5Pointed satellite

n I I1 I I I 0 1 2 8 4 5 0 7

Log (effective area X multiplex factor)

Figure 2

On logarithmic scales we see a comparison of two important attributes for some

selected past and present astronomical missions (and one future one). A very fundamental measure of a uv instrument's usefulness is its wavelength resolving

power, expressed here in dimensionless form X/ AX. The logarithm of this quantity

is represented by the y axis. On the x axis is plotted the logarithm of each

instrument's effective aperture area, in em2 (i.e. the product of collecting area,

instrument efficiency and detective quantum efficiency4), multiplied by the

instrument's capability of multiplexing (i.e., simultaneously recording) many

different wavelength elements. The latter quantity measures how rapidly one can

record all of the elements of a spectrum to a given level of accuracy for a star of a

4Instruments using photographic emulsions directly were assumed to have a DQE = 3%.

Wo

_..^..-,...7

li

G
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particular briahtness. (Of course, if onf!'S objective is to study only one or twu 

speciftc lines in a spectrum. larae multiplexing powers offer no real advantaae.) 

The location of the point for IMAPS in this diaaram reveals the forward-look1na 

character of trJs milSton. within the framework of NASA's overall proaram In uv 

space astronomy. 

4. GENERAL SCIEN11nC RATIONAIZ 

4.1. Wa.elencLb CGferqe 

For astrophysical spectroscopy. wavelenaths extendina from !lpproximately 

1150A down to the Lyman limit at 912A are of particular interest and present pi 

special challenge to · .. ~tronomers. Observations at these wavelenaths must occur 

from space vehicles and must be performed by instruments which contain no 

transmission optical elements nor detectors which have front windows. In desianina and buildina the telescope and spectrometer aboard the Copernicus satellite, 

extra effort was spent to incorporate all-reftective. LiF-overcoated optics and 

open-face photomultipliers so that the spectral coveraje could extend below 1150A 

. With Copernicus out of operation, IMAPS now provides the only opportunity to 

observe spectra at hiah resolution from celestial sources at these wavelenaths. 

The spectroaraphs aboard IUE and those proposed for lhe Hubble Space TlllScope 

incorporate detectors with MaF 2 or LiF faceplates to support a photocathode, and 

these instruments have virtually no sensitivity at lhe shorter wavelengths. 

A topic which merits some discussion is the scientiftc reason for placing a special emphasis on the wavelenath ranae covered by IMAPS. which is situated within 

a very much laraer span of the spectrum to be studied from space vehicles. We 

believe these shortest wavelengths have a unique importance, and to understand 

why, we may look back upon some of the research achievements of the Copernirus 

rt4' ;~ -.. 
·9· 

satellite. Over it.. eiaht. years of operation. this instrument. hal allowed us to 

exploit. t.}'!: ultraviolet spectra of hot stars for their plentiful and sc:ientiftcally 

lucrative absorption features. 

For the interstellar medium. Copemicu. has allowed UI to obtain for the nut 

time an ext.ensive overview of the pat.t.erns of element abundances. measure t.he 

distrlbution and rot.ational excit.ation of molecular hydroaen. wit.ness tremendous 

contrasts in temperature and ionization. and reailt.er the widelpread presence 01 

rapidly rr.ovina parcels of aal which have been intluenced by eneraetic phenomena. 

Many of t.he mOlt. intereBUna conclusions were derived from Lines which only 

appear below 11651: 

[1] All of the electronic transitiona from the around vibrational state of H2 occur 

below about 114OA. For one star (~ Pup). the equivalent widths of well over a 

hundred lines from ditrerent. rot.ationallevels were measured. 

[2] The amount. of deuterium produced in the early development of the untvertte is 

best indicated by interst.ellar cz.tomic abundances of 0 because deuterated 

molecules are modifted by chemi~al fractionation. Hiah order members of the 

Lyman serles must be observed since the La (and frequently LI) lines from 

interstellar hydroaen are broad enouah t.o swamp the accompanyina deuterium lines. 

[3] Principa! lines of the Lyman system of HD start at 110el and &0 shortward. 

When compared wit.h the abundance of H2 and evaluations of t.he interstellar 

ratio of dt.OmiC D and H. measurements of HD inform us about. t.he rat.es of 

ion-mol cule reactions in clouds. which in turn are aoverned by the rateoil of 

hydroaen ionization by low-eneray cosmic rays and x-rays. 

[4] The widespread presence 01 a very hot phase (T » lotSK) of the interstellar 

medium was signaled by absorptions from a VI at 1032 and 1038A. Other 

highly ionized atoms which have absorptions at longer wavelengths. such as N 

[I9IJI .~.".. =-- ... ,~ 

~",, ________ ...::""::;;...#~I ____ -"""L- __ '_ -'-____ _ 
f 

. ," 

.. 

, 

t 

~ 

~ 

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- 10-

v, C IV, and Si lV, are hard to interpret because they can be produced either 

by collisional ionization in a hot gas or photoionization from very hot stars 0 

VI is by far the nlost conspicuoul tracer of caron'll gas in our galaxy because 

its cosmic abundance is fairly high, and its peak fractional abundance from 

collisional equilibrium occurs at the prevaiHna high temperatures. 

[5J There are other astrophysically important atoms or ions with tranaiUons from 

the ground state producing ~~sorpUon features onlll below 1150A; to name a 

few, there are C III (9771), N (J (1084A), N 1lJ (988A , ollen dUficult to separate 

from a nearby feature of S1 II with the resolution of eopem;cus), F I (952, 955A 

), P lV (951A), S IV (10631), S '¥1 (933, 944A), Cl II (1064, 1071;\), Cl III (1005, 

1009, 1015 A ), Cl IV (973A), A I (1048, 106S;\), and Fe III (1122A , allen there's 

interference from nearby C I lines). 

Cl MO 

D 14 • 

~ '10"'1 

Fiaure 3 

1 

I 

~ 

I 

I 

-1 

I 

An alternate, though Simplistic, approach to grasping the relative importance 

of cert.ain wavelenaths is l'leTdly to count the numbe .. of likely interstellar absorption lines which could Appear in a given spectrum. 15 Figure (3) shows the 

15Atomic Une. and lheir approximate f-value. were taken from a comprehenlive compilalion by 

Kurucz and Peytremann (SAO Specie! Report 382). For a line to be counted, il had to be slrorlj 

enouah t.o live a au delection for a !'iCO I expOlure of a 15th mall'Jtude BO slar wilh a color excess 

E(B-V) • 0.10. A column denlity of 15)( 1020 cm,a Ume. an ftlemenl's lolar abundance relative to H 

.. ~ _r.J ___ '-..·~_~ _____ ..:. •• .;.;...._"'_1 ___ " -_"-==== ... _~;-,:_. __ _ 
- 11 -

distribution of such lines with wavelenath. for the atomic lines, only those from 

the around state were included; many atoms will show additional absorptions from 

coUisionally excl\.ed fine-structure levels. The larie number of H2 lines gives an 

impressive advanta"e to the score for),. < 1100)\, but even if we discount this fact 

we find the density of atomic lines surpasses that of the lonier wavelenalhs. 

4.2. Wavelenalh ResolviDa Power 

A substantial increase in spectral resolvirlj power is by far the most impol tant 

contribution that IMAPS will offer for progress in research on interstellar .naUer. 

As illustrated in Fiaure (2), the i'nticipated velocity r",solution of 1.3 km s-l for 

IMAPS will surpass that of all iJdSt mi~sions, as well as those currently planned, to 

record uv stellar spectra. 

There are a 'lumber of important benefits which arise when spectra are 

recorded at increased resolution. Weak and narrow interstellar lines are more 

easily localized in velocity, and fewer nearby continuum photons, which add to the 

noise, are blended into the profile (for perfectly smooth stellar continua at a given 

intensity, the sensitivity to weak lines is inversely proportional to the square root 

of the velocity interval over which the line must be measured). Also, the better 

re~olution permits us to distinguish narrow interstellar lines with greater ease 

when stellar features are not greatly broadened by rotation. 

A fundamental problem with spectra of only modest resolution is that, in most 

instances, equivalent widths and average velocities are the only attributes of the 

lines which can be measured with much reliability. As a consequence, when lines 

was assumed. The strerlltru. of H2 transitions were computed for a combination of two populations, one with NJ• o = 3xI013cm-2 and Trot = 1000 K and the other with NJ• o = 1011lcm-2 and Trot = 

100 K. There wu no verification that lines did not interfere with each other by chance, so possibly in a few crowded areas the actual numbers of useful lines will be slightly les9 than those 

shown. 
,. 

.. 

, 

~ 

J 

I 

- 12 -

become saturated one must resort to curve-of-growth techniques Lo obtain column 

densities. Uncertainties in the real behavior of these curves was the most important factor which limited the precision in interpreting Copernicus data. at times 

aiving at least an order of magnitude uncertainty in abundances. 

When the resI)lvilli power is high enouah Lo show the exact shape of a profile. 

one may derive optical depths T(V) = -In[lllco",,,,,uu", J at every point and 

integrate them over over all velocities to obtain an unambiguous answer for the 

column density. using a single absorption line with no reliance on a curve of growth 

whatsoever. The only requirements for the success of this method are that a.) the 

zero intensity level is known accurately. b) the finest detail in the velocity structl1re is not much smaller than the resolution. and c) the profile is not too strongly 

saturated (for instance. central transmission ~ 570 or maximum T' 3). 

From the above arguments. we see that improvements in resolution are 

beneficial for the study of both weak and strong lines. In effect, both ends of the 

dynamic range are extended for detecting and interpreting lines of differing 

strength. This leads to a substantial improvement in the availability and usefulness of absorptions from both common and rather scarce atoms, ions and 

molecules. Beyond this, however, there also should be a many-fold increase in our 

chances of untangling the superposition of lines from different regions along a line 

of sight If we have the velocity resolution to identify components which are slightly 

displaced from each other. To fully grasp how frequently complicated arrays of 

narrow components are Steen bunched together over narrow velocity ranges. we 

need only to look at scans of interstellar absorption lines recorded at visible 

wavelengths with a resolution of .... 1 km s-l, such as those published over many 

years by L. Hobb!l and his associates. Finally, as the following section outlines, the 

increased resolution is valuable for solving some specific scientific problems in 

interstt!llar matter research. 

b.b _' ____ ~_ .... _.' _'-._-_-._--_'-~_'_' ._.". __ ~_-;_-_..-- __ -_ ..._ .. ¥ __ .. - ....... _. ~ ,~.~:~'~:~' .~:~ _____ ' 

,~ ., 
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- 13· 

5. SPECmC RESEARCH COAl..c:J 

5.1. Physical Stale of H I Helions 

OBJECTIVE: to unravel the widely varying regimes 01 pressure, temperature, radiation density, composition and chemistry within neutral clouds. 

Most of the gas in our galaxy resides within regions which predominately contain neutral atoms. Recently, we have become more keenly aware of the heterogeneity of these H I regions, which are characterized by substantial differences in 

temperature, density, rates of molecular synthesis and reactions, element abundances and starlight radiation densities. However, the stratification of these attributes within individu'\l clouds and their general statistical properties are not well 

known, primarily becau.se we have been hampered by either inadequate velocity 

resolution (spectra recoT'ded by IUE or Copernicus), uncertain or very broad coverage of space (radio studies of emission and absorption lines), or a poor selection 

of observable species (absorption lines in the visible). Further progress in. understancing the structure of H I regions will be greatly facilitated by experimental 

methods which overcome these limitations . 

For any specific observation, spectra recorded by lMAPS will delineate parcels 

of gas at well-defined radial velocities which probably reside within limited volumes 

of space, yet at the same time this instrument retains a great breadth in sampling 

conslituentg which respond in very different ways to the physical emironmenl. 

For instance, the abundance of singly ionized carbon (C II) in any part of an H I 

region is apprOximately proportional to the local density of hydrogen nH times the 

gas-phase abundance ratio of C to H. The density of neutral carbon (C 1) is proportional to that of C II times the electron density divided by the density of Ionizing 

radiation. The amount of this C I in either of the two excited fine-structure levels 

.Ip, 
,. 

N 

il 

- 14 • 

is governed by local H I (and H2) densities and the kinetic temperature. Doubly 

ionized carbon (C III) is not expected to be abundant inside H I regions, but it may 

be prer.ent near the cloud boundaries where it is sustained by ionizing radiation or 

shock heatini. Observations of just these four form£ of carbon (3 stages of ionization plus the CI excited levels) will enable us to better understand the density and 

temperature regimes inside t he clouds. Neutrals and ions of other elements (e.g .. 

Si, S, and CI) can be used to check the conclusions derived from C. 

In H I clouds which can be observed with IMAPS, the abundances of various 

simple molecules are very responsive to physical conditions. Molecular hydrogen 

is formed by the interaction of H atoms with grains and destroyed primarily by 

ultraviolet radiation. We can monitor the intensity of the dissociating radiation by 

examining the optically pumped populations of high-J rotational levels, determine 

nH from intermediate-J ratios, and, if n(H2) InH is appreciable, measure the kinetic 

temperature by the rotation temperature between j=O and 1. HD molecules are 

formed by an exchange of H2 with deuterium ions; the concentration of ions is in 

turn governed by t.he rate of cosmic ray and x-ray ionizations. Thus, measurements of HD allow us to sample another important environmp.ntal influence in neutral clouds. Other molecules, such as CO and OH, are also believed to result from 

such exchange reactions. The rotational excitation of CO will indicate the temperatures and densities in the regions where such molecular activity is high 

(Coperni~u.s was not able to resolye individual lines which contributed to the CO 

band rotation branches), ::\nd these results can be confirmed independently from 

the relative populations of rotationally excited HD. 

Absorption lines of CH+ are observed in the visible spectra of some stars. 

Since CH+ is rapidly destroyed by an exchange reaction with H2, we have evidence 

for dense regions which do not contain appreciable amounts of H2. perhaps on the 

outer edges of the clouds or in recently shocked material. IMAPS, with its 

L -'---.~ '" 

t- ,..till 
- 15 " 

excellent selectivity for velocities. can let us study CH+-bearini reaions with less 

interference from other dense portions of a cloud. 

While most of the aas within H I rea ions is at a temperature T ~ 100 K. there is 

considerable evidence from 21-cm absorption and emission. as well as from COPiTnicus data, that some neut.ral aas is at substantially hiiher temperatures (1000 K 

~ T ~ 8000 K). In many instances, the wavelenath resolving power of IMAPS will 

be sutficient to measure the thermal broadenina of interstellar lines directly. 

instead of relyina on indirect determinations with the curves of arowth. We can 

differentiate between turbulent and thermal broadening by studying atoms of 

different mass, since the square of the observed velocity broadening is equal to the 

sum of Ie T / m and the squares of the rms tw'bulent and instrumental profile 

widths. To give a quantitative illustration uf how this may be done, we can consider 

B~s with T = 100 K and b = 1 km s"l from mechanical motions - turbulence this 

small is often seen for profiles which show up in 21-cm absorption. Here, the weak. 

lines of D I, N I and A I would have apparent full widths at hali maximum (FWHM) of. 

respectively, 4.1. 3.6 and 3.4 pixels. Under these conditions the temperature 

broadening is just discernible. On the other hand, when T reaches 1000 K the 

widths increase to 8.4, 5.3 and 4.2 pixels, and the differentiation with mass is easily 

measurable. If weak members of the Lyman series of H I can be measured. the 

contrast is even greater (FWHM = 11.4 pixels at 1000 K with b = 1 km 5"1). If b were 

as large as 10 km 5"1 in the warm H I regions. emperatures on the order of 1000 K 

would still render D I lines which are wider than those of the heavif r lines by 1 

pixel. 

The origin and distribution of the warm. H I gas are poorly known. Is there a 

continuous distribution of material which pervades most of the space between the 

cool clouds? Are there discrete packets of warm gas. as with cooler material? Or. 

as a third alternative. is most of the warm H I situated in a transition zone where 

. ii.~ -__________ - _ ••.• -.. """ ~ .. ________ --i/lri ---'-----' """" ____ " '_'''' __ 
- 16-

partial ioruzation 1S occurrina between a cool cloud surface and a much hotter 

(and fully ionized) environment? We can obtain a great deal of if'.Jiiht on these 

possibilities. inasmuch as observations with IMAPS will enable us to identlfy isolated velocity components of H I at appreciable temperatures. assess their number 

per unit vC'UJ:"le of space, and recognize any correlations of their velociti~s with 

those of the cooler clouds. The association of hot and cool H I gas may explain the 

apparent intermediate temperatures (where thermal instabilities should occur) 

deduced from comparisons of 21-cm absorption and emission. 

5.2. Inv_tJaaUou of HOC .. 

OBJECTIVE: to isolate absorption lines from fully ionized regions and use 

them to reveal the pervasiveness and character 01 ionizing processes. 

By observina ions which could not be produced by photoionizalions less energetic than those which ionize hydrogen. we can identify absorption components 

which arise from gases which are almost fully ionized. The most useful of these 

lines are those which are weak; a great preponderance of such features occur at 

the very short wavelengths covered by IMAPS. The velocities of these profiles permit u.s to identify which components of neutral or weakly ionized atoms arise from 

H I regions and which come from H II regions. Only on rare occasions when the 

velocity separations were particularly favorable was this done successfully with the 

Copern,icu.s observations. IMAPS should have sufficient resolution to make such 

detailed component identifications commonplace. 

Interesting conclusions about the topological relationships of H I and H II 

regions should arise from global comparisons of their respective velocity components. If H II regions are isolated domains restricted to the neighborhoods of 

hot stars. we would expect to find an H I component associated with each H II 

'M, 

..:1.01. \lP..,..:~·- -.. "- .: ........ -... -- - - - - ~ ~ ---
- 17 -

component. but with a vast number of H [ clouds which are not accompanied by 

ionized gas. On the other hand. if the neutral gases are strongly clumped, thp. H I 

clouds will be embedded in a low density medium which can be ionized by stars. 

since the Lyman-limit photons (and soft x-ray background) will have a long mean 

free path. Then. nearly all of the H [ clouds would be bathed in ionizing radiation 

and moderately dense H II zones would surround them (and share their velocity). 

As with H [ regions. temperatures and velocity dispersions can be measured 

independently from the line widths of individual H II components. Ions in excited 

tine-structure levels preferentially reside within the denser clumps of gas. which 

allows us to determine how frequently such condensations occur and how their 

t.emperatures differ from the low density material. The radiation fields from stars 

which are isolated from others6 can be determined by studyilli the ionization 

ratios Fe IIIFe III. PiliP III and 5 II 15 III. since their lines are weak and unsaturated. On some occasions ratios of stronger lines will yield reliable ratios. e.g .. 

5i 1I/5i III. C Ille III and N II/N III. Also, we shall be able to measure the relative 

depletion of elements of H II regions and compare them with the patterns from 

element to element (Le .. refractories 1)5. volatiles) found in H I regions. Of the 

doubly ionized species just discussed. the HRS coverage does include 5 III. P III 

and 5i Ill. However, the latter is frequently strongly saturated, or inside the core 

of a strong stellar 5i III feature, and the former two can be blended with nearby 

strong interstellar resonance lines. Finally, we can draw together our results on 

heat inputs (from stellar radiation fields)and cooling rates (which arc governed by 

the abundances and densities), and derive self-consistent mod~ls to explain the 

observed temperatW'es. 

aA survey by [MAPS will tend to emphasize such stars. since neiehbaring stars af comparable 

mqnltude must be more than 1° away. 

~",-____ .,,_-_-_.~_ .. -_-._._. -_,---_*_1 ___ '--"-' ,_ .. ,_ ...._-......... _. =_ .~. __ 

J.

.. 1;)/ • __ 
.. 

, 

~ 

~ 1 ~. ~ t.,. e 

Oi': J 

• 

• 18 -

5.3. Coronal Gu and TranmUon ReaioDi 

OBJECTIVE: to test a nu.m.ber 0/ theoretical alternatives on ths origin 0/ 

gases with T .... 105 - 108 K by mapping the detailed. structure 0/ 0 VI 

absorption profile s. 

A major surprise from the initia.l surveys of steUar spectra by Copernicus was 

the discovery of ubiquitous 0 VI absorption features (at 1032 and 103aA ), which 

indicated the widespread presence of coronal gas in space, with T .... 4)( lOOK, 

Observations of dinuse, soft x-ray emission indicate the eXIstence of material at 

even higher temperatures (T > 108 K). While the Cop"rn.icu.s and x-ray emission 

data have yielded extensive inlormation on the general properties of the coronal 

gas, such as average density, temperature, regional distribution, and so fort.h. an 

important atlribute which he.:.i eluded us is the fine-scale distribution of thil>; gas. 

For instance, is 0 VI evenly distributed over extensive voids between cooler concent.rations of material, or do t.he observed profiles arise from confined regions, 

perhaps many in nwnber, whlch move about. at random and create composite 

profiles over t.he many hundreds of pc of typical lines of sight? We are still uncertain even about what fraction of all spa.ce is fUled with coronal gas . 

As with the early studies of cool mat.t.er from the visible absorption and radio 

emission lines, t.he key t.o understanding the morphology and origins of 0 VI 

regions is founded upon detailed st.udies of velocity structures. lMAPS will, for the 

first. time, perJ'!'.H such investigations to be performed wit.hout having to correct 

for instrumental smoot.hing. We will search for 0 VI components which seem to be 

correlated with ordinary H I (or H II) velocity peaks. Tentative indications that 

these very different phases have a kinematical link have been derived from data 

recorded by Copem.icus. Persuasive theoretical arguments indicate that 0 VI lines 

may arise from evaporative interfaces between the envelopes of cool clouds and a 

~ 

I ..... _. _____ . ___ = __ =_.-............ _____ . ____ -_-_--'-....... -:--.~--. __ .__ .__ '__ • tiW' __ ' .. ~ _____ T 

. ~i ~t/""'; -.• -.... -.. 
.. 

~ 

l , 

! . 

. 

!'. 

10: 

~ 

~ 

- 19 -

much holler medium. 8y carefully measurini '~h\~ velocity displacements and 

column densities of these correlated 0 VI peaks (if they exist), we can compare our 

results with theoretical models for velocity gradients across evaporative fiows, and 

in turn use this inf\Jrmation to derive the ('verall eneriY and mass exchanQe budgets between the cool clouds and the surroundini very hot medium. The total 

enerQY fiow can, in turn, be checked !or consistency with our estimates of overall 

eneq~y inputs from supernovae and 0-8 associations to ascertain the importance 

of this phenomenon on the collective states of gas III the ialactic plane. 

More precise constraints on the distribution of temperatures within isolF.1ted 0 

VI components may i.e deduced from exact measurements of line shapes, wh!ch, 

when combined with data from other ions such as S IV, S VI and N Y?, may give an 

insight on the dominant regimes where heatlng and cooling take place. The sound 

speed in a general medium at the 0 VI-producing temperatures is about 80 km s·l, 

and in a truly int ercloud environment the turbulence from shocks refiectirli ott of 

clouds may stir up the material and make it harder to detect. This may explain 

why there seems to be too little 0 VI for the amount of soft x-ray and EUV emission 

seen in the local medium, if we wish to rule out the existence of both extraordinarily high pressures (p I Ie » 104cm-3K) and small ruling factors for the hol gas . 

Thus, an important goal for IMAPS is the chance of discovering an additional 

source of 0 VI-bearing gas with an extraordinarily high velocity dispersion which 

may have been overlooked in the Copernic'J.S survey because of the limited range 

of A scanned for each line . 

Finally, still another possibility is that for very hot and luminous stars, lMAPS 

may reveal some components which may be linked with conductive zones between 

expanding, radiatively cooled circumstellar shells and the shocks produced by 

'The N V doublet at 1240A is within the wavelerijlth coveraie of HRS. 

l 

.. ~ -----------.. _~_' -----....... -.-:.- .;. .... _._- 2JIt. t~' .;.1. -. . -- ~~ ---
- 20-

strona stellar mass-loss winds. as described in a theory on "interstellar bubbles". 

5.4. Dynamics ollnlerlt.cUar Shocb 

OBJECTIVE: to per/arm detailed. mapping 0/ hilJh-speed com.ponents so that 

we can better understand the praperties 0/ viDlently disturbed gas and th2 

structure 0/ recombining and cooling gas behind. shock /ronts. 

Recent observatlons and theories have emphasized the importance of shocks 

from supernovae. and the secondary disturbances which follow. as a source of 

motion and heatiIli for the inl.erstellar mediwn. The not.ion that. thesE. shocks can 

propa/iate great distances with relative ease seems t.heoretically plausible and is 

support.ed by observations of extensive systems of hiah velocity aas in the general 

viCinity of known supernova remnants and o-a associations (e.g .. the Vela remnant. Orion associat.ion. 11 Carinae). Original blast. waves in a low density medium 

may appear in t.he form of weak. hiah velocity (300 . 500 km s·l) 0 VI features in 

spectra recorded by IMAPS. and if the aas is not preionized. shifted L~ absorption 

from trace amount.s of neutral hydrogen may be also present. Only very marginal 

indications of such features were derived by the Copernicu.s investigators. since 

they were hampered by the need to invest a large amount of observina tlme to 

scan a broad velocity interval with high photometric accuracy (the simultaneous 

imagery of many spectral elements by lMAPS greatly improves our efficiency for 

locating spectral features at unknown velocities). 

As the high speed shocks encounter dense interstellar clouds. they slow down 

and the temperature of the post-shock material is reduced to a range (T I::j 106 K) 

where radiative cooling is important. At this stage many elements. such as C. N. S. 

Si and Fe. will have a Significant fraction of doubly ionized atoms and will be very 

conspicuous in the spectral coverage of IMAPS. IMAPS will be able to distinguish 

, i. I, i 

[ . 

.. 

\ 

- 22· 

larae, hot reaion may have been created by a supernova which went off not too far 

away some 150,000 years aao. 

As a result of the arosl pressure mismatch between the local H I aas and a 

newly created hot enVironment. there is probably an isothermal shock eatma lts 

way toward us from the cloud's periphery and travelina at a velocity equal to 

[poul .... IP, .... j" Urnes the local sound speed of 10 - 15 km s·t. To observe this 

shock over all (or most) directions in the sky with IMAPS would not only confirm 

the picture just described, but would also allow UI to probe one particular example 

of a cool-cloud Ihot-medium interface and learn more about the physical processes 

which occur therein. Such inlormation would be of tremendous value to alobal 

theories on the creation and mutual interactions of various phases ollhe mterstellar medium, such as that proposed t"y dcKee and Ostriker l.n 1976. 

Finally. Copernicus scanned the L-a emission profiles from the chromo~pheres 

of nearby, late-type stars. Absorption features from interstellar atomic hydroa en 

and deuterium are superposed almost on top of the peaks of these prorues. Unlortunately, at the resolution of Copernicus it was fairly ditTicult to know precisely 

what the shape of the emission profile would be without the interstellar absorption. 

s'" the inferred Hand 0 column densities were rather controver:,ial. The factor of 

10 better resolution of IMAPS should help enormously to unravel the details of how 

the observed emission and absorption protues really behave. These observations 

could be done startina at the L-(1 emisSion line, and the weaker, hiiher order 

Lyman lines could be used to check the conclusions. 
I , 

·23 -

e. PAYLOAD DESIGN AND PERFORIIANCE 

e. t. Mechanical CoWmator 

The mechanical collimator il composed of a stack of precisely coreaistered 

electroformed andai, with each arid containing an array of square apertures (watTle 

patter n). The 10 wide (FWHM) triangular response in both dlrecUons on the sky. 

shown in Figure 4, prevents bClth the;:. :;ro-order and dUfracted light of all but the 

very clolest interferin& stars from contaminat.ing t.he spect.rum of t.he deSired target. st.ar. The collimat.or apert.ure 15 approxlnmtely elliptical in shape. w1th semlaxel of 7.4 and 9.6 cm. 

Direct. light. transmitted by the collimator has been measured to be 62% of the 

inCident. nux. Some of the energy loss can be at.t.ribut.ed t.o the ineVlt.able blockage 

of t.he beam by t.he opaque parts of t.he grids; diffraction and t.he result.ant. Wldeanale scat.tering of light from t.he main bet..m account. for t.he remainder. Undes1rable efTect.s from t.he cross-shaped array of diffraction spikes in t.he image format 

are reduced by having pseudorandom offset.s in t.he spacini bet.wel!n t.he vanes (to 

eliminat.e coherent. addj~ion of phases which t.end t.o make t.he patt.ern more 

"spiky"). The bright.est. (innermost.) diffraction 

peaks each have an int.ensit.y of only 0.2b;'';; t.hat. of 

t.he main beam. To ke'Jp t.hese spikes ofT of adjacent. 

portion.'1 of a pa.rtil!war echelle order, the axes of 

thp collimator's irid pat.tern are oriented 450 wit.h 

respect. to t.he dispersion direction. 

/ 

J. 

. "30 I' JO 

F', ~ 4 - M .... w~.,j A .. ~"I .. , RuponM 0: 

the IMAPS t.4c.""""Q.' C.I', .... to~ 

l ..... _-;.;;-"';;;;' ===='"=;=,nZI!=!!!!::I:,:=m :::: ..... ::.:: ... =-===:::. ==::..-. :.a.',. __ ._ ._ .. _._. __ .1 ';'-:
- 24 -

The echelle aratlna il a replica of an exllt!na B&L aratina ruled over a 200 )( 

• 

400 nun area at 79 iroovel mm-' with a blaze set at tan e ::: 2. MeaSuNments at 

vIsible wavelenaths show that thi. particular irattna hal very low scattered Haht 

and aholtl. An Important feature of our deli~n il that the lncidence and 

diffraction analel are equal. with the two beams beina offset on either side of the 

plane perpendicular lo the ruHnas by a small anale (3.5°). This conftjuralion 

insures that movement of the spectrum caused by either attitude chanaes (errors 

in ACS pOUltina) or thermal bendina d',tortions of the payload ("hot-doaaina") are 

track,.d precisely by a broad-band reference lm8&e of the taraet star on the imaae 

format. Both the echelle artllina and the cross-disperser (described below) were 

coaled WIth Aluminum and a special overcoat of LiF desijned to alve iood 

renectivities at the desired wavpl.enaths covered by lMAPS. The frf.!e spectral 

ranae of the echelle orders on the detector's imaae plane is about 4 times lhe 

width of the CCO. Thus. ,,0 oblain complete wavelenath coveraae. we provided a 

capability to tip the echelle in 4 different positions and make separate expo!lures 

in each case. 

8.3. CroII.-cliJperlier GraUq 

The cross-disperser aratiIli wu specially ruled in 8.5 partitions on an off-axis 

paraboloid blank for the ex1st1na (soundina rocket) version oflMAPS by Hypertine. 

Inc. This arati.nj works in a Wadsworth conflauration and focuses an imaae of its 

nrst order of ditrraction on the detector at a distance of 1600 mm. 

For soundina. rocket niahts we depend on thermal inertia 6Jld the short duration !If the mission to insW'e that temperature chanaes will not appreciably alter 

the focus adjustment of the cross disperser. 8 For Spartan missions we will 

IIActually. we intentionally let the :,.':us to be correct for lome approximately elev,~ed temperaturft. In anticipation that lome of the aerodynamic heatilli from the hot outer skin would be 

transferred to the meterina lupport Itructure of the instrument _. lee the dllcUIIlon In 01.3. 

r~· _., ________ ....;: .. ~_~_I _____ -d:~'__' .... ~~ .. _- --At'" r'I' ;~ -r". 
• 

- 25-

circumvent the possibility that thermal excursions will throw our instrument out 

of focus by inst.alling an invar meterina rod between the cross-disperser and the 

detect.or and allowina the dete-ctor to move on a special mount (t.he dist.ance 

bet.ween these two elements is t.he only critical dimension in our payload, light. 

beams elsewhere in t.he payload are all parallel). Ordinarily, lat.eral movements of 

an imaae, caused by di1ferential ftexure of optical supports, can be a problem for 

standard spectrometers because they cause imaar. smearing or result in an image 

driftiIli off a slit. For IMAPS, such motions !ore of no consequence because, as discussed earlier, the resultina errors look exactly like pointina errors which are 

compensated out by an on-board data processina system. In aeneral, we feel t.hat. 

our optical bench can t.olerat.e a t.hermal environment similar t.o that. specified for 

the SAO invar spectromet.er aboard Spartan 2, or perhaps even worse: first.-order 

calculations seem to indicate t.hat. an overall temperat.ure excursion of 40° C would 

be needed to throw our f /10 beam appreciably out of focus. 

At. the center of t.he det.ect.or format. t.he geomet.ric images formed by the 

cross-disperser should theoretically be perfect, but. t.here will be some ir.lage blur 

from coma at t.he field corners. Ray-tra.:e analysis indicat.es t.hat t.he mazim:u.m 

dimension of the blurred image in the dispersion direction will range from 171J. t.o 

221J. (perpendicular to t.he dispersion, the blur may reach 301J. at. t.he very worst). 

Our estimate that the FWHM of the instrumental profile will be around 301J. includes 

this effect plus a peSSimistic allowance for possible aberrations induced by mount.-

ing stresses on the grat.ings or defocusing cllused by any residual lack of t.emperature compensation. 
·26· 

8.4. Detector 

As emphasized in §4.1, short-waveleflith photons must be allowed to reach lhe 

photosensitive medium without havina to penetrate any sort of window, since bulk 

pieces of even the best optical substance (LiF) are virtually opaque to such radiation. Indeed, it is for just this re ,son that such aeneral-purpose, uv observing 

facilities as IUE and the (first generation) spectroaraphs aboard the Hu.bble Space 

Telescope are unable to operate over nearly all of the very profitable wavelength 

ranee covered by IMAPS. 

The IMAPS spectrograph casts its image onto a photocathode which consists of 

a layer of KBr on a solid substrate exposed to the vacuum environment. The quantum efficiency of an opaque photocathode of this type, approximately 7770 near the 

middle of our spectral coverage, is markedly superior to that of the conventional 

semitransparent variety. Quite simply, this gain results from our being able to 

avoid the usual dilemma of making a semitransparent photocathode thin enough 

to allow the electron'~ to escape, while at the same time insuring that it is thick 

enough to interact with most of the arriving photons. 

ION TRAP 

SmCo PERMANENT 

MAGNET 

~"~::~;§~~~~~f~~ELECTROSTATlC AS!5EJotBLY 

CCD WIlli 

ELECTRONIC 

COMPONENTS 

ELECTRONICS 

MOUNT[ NG BOARD 

TO VIDEO PROCESSING 

AND DRIVE CIRCUITS 

Figure fj 

-18 

KV 

K Br PHOTOCAlliO~E 

ELECTRONS 

ELECTRON 

BOMBARDAB 1.E CCD 

~ ..-411 _'. ~"""~:h -.. -~-~.-. * Jit-...... ' . ... '_-______ ........ ----.-.1 ..."-- --:. .. ----- ---.- -
- 27-

Inside the IMAPS ICCO image section. electrons emitted ott the front face of 

the photocathode are accelerated by a 19 kV potential toward an RCA (SID-50 1) 

CCO with 320 x 512 separate charge collecting wells. each with a square dimension 

of 30~. As illustrated in Figure 5. a magnetic focusing field is inclined to the voltage gradient by a small angle (20°) to allow the electrons to diverge from the optical beam and hit the CCO. From laboratory tests using a patterned photocathode 

(shadow evaporated throUjh a standard Air Force bar-chart test pattern), we have 

verified that the oblique focus arrangement for the electrons produces as sharp an 

image as the CCD is able to show us. Theoretical calculations of the fQcusing properties of the oblique configuration give a 6~ diameter fc:,r the electron spot emitted by a single point on the photocathode. 

So that it can respond to direct electron bombardment. the CCD has its normal supporting layer of cover glass etched away. Upon impact. each high-energy 

electron produces 3 to 4 thousand secondary charges in thG ceo's silicon rayer. 

The entire field of the CCO is read out 15 times a second, and a display of the 

frame clearly shows photoevents as large spikes, spread over one, sometimes lwo, 

and very occasionally three or four CCO wells (depending on whether or not the 

incident photoelectron landed near a well boundary). It is not necessary to cool 

the CCD, because not very much dark current is accumulated over the 1/15 

second duration of an exposure. The ratio of an event's charge to the noise in a 

single well is about 35 to 1. With this large a Signal margin, we are very confident 

that we can resolve the pOSition of a hit to within half of a 30~ CCD well, by analyzing the charge centroid. (This is a conservative estime.te for positional accuracy; 

theoretically we should be able to da evan better.) The resulting sample interval of 

15~ is fine enough to avoid aliasing problems with the spectral informaiLion (unless 

the imagf! quality is much better than we predicted). 

--

, & II'., '
- 28-

If ions enter the ICeD image section, or are created therein, they are 

accelerated toward the photocathode. When they hit, they each produce a bunch 

of electrons (about to or 20 in number) which will, in turn, register as a great big 

event on the ceo. As long 0.5 t.hese ions events are not so numerous that the 

detector field becomes much too busy, we can make th(~m have virtually no 

influence on our observations because they are easiiy r~cognizable (because of 

their large amplitude) and can be rejected by the event-detection processing by i:l 

computer after flight. In our laboratory vacuum tests of the IMAPS ICeD, we found 

that the ion event rate was very low (only a few events per tl15 second frame), 

and there was vp.ry little increase when we deliberately let the pressure degrade to 

2)( 1O-4torr. Likewise, on the successful sounding rocket flight, the ion rate was 

very low (see §7.3~1.6). 

The low ion rate we experience with the detector is probably attributable to a 

number of precautions that we have buill inlo the design. First, the optical system 

and the detector are enclosed in a cylindrical vacuum vessel which is always 

pumped down for a long period of time before flight. This pwnpdown virtually 

eliminates outgassing from adsorbed volatiles or trapped gases (within small voids) 

inside the payload. As a second pree,;autivn, we installed an ion trap in front of the 

detector and put the mechanical coillinator (the only opening to the outside) at a 

slightly positive potential with respect to the outer shell of the payload. Finally, 

we have fOiJnd from our laboratory testing that the IT~ost important parameter for 

keeping the ion rate low is cleanliness of the payload components, especially those 

associated with the image section. In recognition of this, we thoroughly cleaned 

and vacuum baked the ICeD assembly. 

~ -..-, ...... IioL ...., --_-.._---_ ......... ---'-' -----.... , ..~,-'-'-;..: .. - "1IIf".n, <1-' ~ -.. 
r 

r 

~ 

- 29-

8.5. Signal Tnmlmiaion 

For soundini-rocket ftiihts. we amplify the raw video signal from the CCD. 

packaae the information into the standard broadcast TV signal and synchronization protocol (but with each CCD row expressed in 4 TV lines and 2 TV frames per 

CCD frame), and then telemeter the signal to iround for recording on a VCR tape, 

After tUiht, the video recordinis are digitized and photoevents are subsequently 

recognized and centroided by computer processing, While this extremely simple 

approach has been suitable for observations of short duration on a vehicle which 

can transmit wide-band telemetry. we will need to adopt a more elaborate 

approach for our Spartan tUght.s, None of the t.ape recordprs proposed for Spart.an 

have the capacit.y t.o st.ore a st.andard TV broadcast for a whole flight; t.hus we vlill 

need to create an on-board processing syst.em to carry out the event analysis in 

realtime and thereby consolidate the information into many fewer bits, 

8.6. Compensation for Small Point.i.ng Errors 

On rocket flights, there is no active system to stabHize the wandering of a 

spectrum while the rocket's attitude mov~s wit.hin the ACS limit cycle, Instead, a 

small mirror9 ( ..... 1 cm2 in area) attached to the echelle grating directs a broadband image of the target. star to the cross disperser which in turn focuses it near 

one edge of the ICCD picture format. As the spectrum moves in x or y, this bright, 

point image moves in precisely the same manner (see §8.2), and we use it as a 

position reference in each recorded frame, As long as the rate of angular motion 

is less than about 5" s-l, the smearing of spectral detail in a single 1/15 second 

frame will be completely negligible (a 15~ sample is equivalent t.o 1. 72" in the sky), 

9rhere are actually 5 such mirrors installed on the echelle housilli, one for each of the 4 tip position.~ and an extra one on an opposite side of the echelle to serve alS a focus monitor in one of the 

positions, 

-.. 
- 30 -

Even the worst alliular rate quoted for the Strap 5 ACS (10" jitter peak to peak 0 

0.2 Hz == 4" s·l) is within this tolerance. 

7. TECHNICAL H1GHUGHTS 

In the followini sections, we cover a number of key technical issues which contributed to interim successes and failures within the lMAPS program. These topics 

are presented here for two reasons: (a) They had a major impact on the program 

schedule and cost and also the realization of certain technical and scientific goals. 

and (b) they cover important findings which refiect on the overall capabilities of 

the IMAPS instrument and associated subsystems. 

7.1. Problems with CCD Performance 

In preparation for its first fiight, in February 1983 the IMAPS payload 

underwent the standard integration and evaluations at GSFC (e,g" tracker alignment, electrical functional tests, determination and adjustment of the center of 

mass, bend tests, spin balancing vibration tests and post-neE integration). In 

addition, we evaluated signal transmission quality of our ICCD output throughD\lt t~ 

telemetry system. Overall, the outcome of the GSFC integration tests were 

uneventful, and the payload was successfully integrated with the rest of the rocket 

systems. 

After the integration run at GSFC, we transported the payload to the Naval 

Research Laboratory (NRL) , where G. Carruthers had a test facility which permitted us to evaluate the performance of the complete payload assembly for imagery 

of spectra. The intent of this test was to determine how well the optical system 

formed a spectral image. Major items for the test were a) whether or not the collimator had objectionable diffraction spikp.s, b) How efficient were the optica.l components, c) how much scattered light carne off the gratings, d) a double check of 

-.. 

__________ .,.#1 ...... __________ .------~-.- • .a,. .. _".".:,_ ,.:.. •• ., __ __ .. ~;~ 

~
I 

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- 31 -

the focus, and e) a full-up test of the sianal processina chain. 

After about ftve days of intensive testinj and problem solvina, we ascertained 

that the payload had an enormous loss of sensitivity. We could detect a sianal only 

when the LesL apparaLus was emiLtina a nux far above what we could ever expect 

from a celestial source. At the time, we were unable to determine whether the 

problem was relatively trivial, such as from a simple mechanical interference in 

Lhe beam or a aratina beina off its blaze, or more fundamental, such as an insensitive detector or nonfunctionina optical component. Since it was clear that we 

could not fty the lllstrument under the circumstances, we postponed the tHaht and 

shipped the payload back to BASO for trouble shootina. 

Our initial suspicion was that there was no fault with the CCO, because we 

thouaht we were able to register the presence of individual photoelectrons and our 

problem was simply that there were few in number (that conclusion, as later investiaations revealed, was definitely false!). Thus, in the early phases of trouble 

shooting, we concentrated on possible sensitivity losses in the optical system 

and/or photocathode. 

After its arrival at SASO, the payload was inspected and fOWld to be baSically 

sound. Next, the alianment of Lhe optical system was verified, and a special test 

was performed to check that the grating blaze anales were indeed where we 

expected them to be. With those evaluations showing essentially no problems, we 

embarked on the more difficult task of checking the efficiency of the optical components. To accomplish this. we fabricated a special fixture for directing a bea~ 

of light from a uv source first directly onto the detector, followed by a new setup 

with the assembly illuminating the detector through the complete optical chain. 

As is usually the case with measurements of optical efficiencies. fairly elaborate 

precautions were needed to insure that systematic errors of various sorts did not 

give us invalid readings. Various practical problems, such as electrical 

t m .. __ E ... _::_' _:~-:_= ___ ... .-.."_;j_J _=_7 ._. _: ,-,-=_~._ ~: __ .. -.. 

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~ 32-

interference from a noisy power supply for the uv lamp, prevented us from obtainiIli very definitive quantitative data on the optical efficiency. However, from rouih 

comparisons of the results from the two illumination schemes, we could satisfy 

ow-selves that the optical system in the IMAPS payload could not alone be the 

explanation for our overall efficiency problem (Le., several orders of maglutude 

below a calculated efficiency for the whole system). 

After the test.s of the optical syst.em, our att.ention turned t.o locating possible 

deficiencies of the detector. From previous tests of elect.ron yields from single 

phot.oevents, measurement.s of the CCD on-chip amplifier gains, and our experience with the random noise at the output, we would have predicted that. single 

photoevents would have had a healthy 20; 1 signal-to-noise ratio. On the other 

hand, examination of the recorded data showed that the photoevents were not too 

easily recognizable, and in addition, they seemed to show a large dispersion in 

amplit.udes. In one diagnost.ic session with the camera electronics, we discovered 

an open circuit in the CCD amplifier reset line, caused by a wire which had come 

loose Crom a connector. After correct.ing this problem we found an improve;-.ient 

in the recognizability of events, but. t.he overall sensitivity of the detector had not 

improved. G. Carruthers remeasured the quantum efficiency of the photocathode 

at NRL. While some loss of efficiency had occurred, it was only by a factor of three 

at the very most -- far short of the amount needed to explain our sensitivity problem. 

The complete ICCD detector and associated electronics were shipped to 

Princeton for further study. In support of another research program at Princeton 

(evaluation of ICCD detective quantum efficiencies), we had on hand a uv source 

whose absolute flux had been calibrated. It consisted of a radioactive source which 

produced Cerenkov light in a MgF 2 crystal at the focus of a collimating mirror. The 

yield from the source should have produced 400 events per second over the 
- 33· 

(omplete ICeO frame area, yet we were unable to see any events whatsoever, when 

we viewed the video output on a monitor screen. L'ltimately, this test convinced us 

that our fundamental problem rested with the apparent inability of the eco to 

show us single photoevents. In the past, we thought we could see such events 

when the detector was exposed to briaht uv 50urces. However, these sources were 

so bri&ht that what we were indeed seeing (and interpreting as sillile events) were 

the chance pileups of multiple electron hits on top of a crowded sea of photoevents. 

This conclusion was surprising at the time, since, at least superficially, 

straiahtforward calculations of the overall sensitivity seemed to indicate that we 

would have had no difficulty in registering sillile events. Specifically, we knew that 

from previous experiments with the eco operated in slow-scan mode (lon~ int.egration times, ceo cooled to -130°C, 10 second readout time for a whole frame) that 

the charge yield for 17 keY electrons was about 4000 electrolls per event. The 

charge sensitivity for the on-chip amplifier at the high bandwidths needed to 

record at the lMAPS fast scan rates was 0.25~V Ie-when it was connected to a 3KO. 

load of the IMAPS video amplifier. While this gain was lower than the nominal value 

of about 0.5 to 1.5~V Ie - found for this and similar RCA CCOs, by itself this reduction could not explain why we were unable to see events. An additional effect, however, conspired to reduce our signal margins farther. From experiments at slowscan rates with several di1ferent CCOs viewing an optical test pattern, we knew 

that pulses only one pixel wide experienced a loss of signal amplitude by about a 

factor of two, compared to illuminations which covered a large number of contiguous pixels. Even so, we calculated that 17 keY charge packets passing thro~gh the 

lMAPS signal chain with its gain of 560 would produce pulsps with an amplitude of 

260 mY, if all of the charge were concentrated in a single pixel. Compared with 

measured rms noise levels ranging from 24 to 36 mV, we expected a signal-to-noise 

-.. 

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- 34-

ratio somewhere between 8 and 12. 10 While this was ostensibly a reasonable margin 

for detection, we nevertheless harbored an awareness that, at the higher clockina 

rates needed for lMAPS, any further reductions in the CCO's charge transfer 

efficiency (CTE) could compound the attenuation of point-llke signals and jeopardize our ability to perceive lhe evenls. 

Following lhese initial conclusions, w~ embarked on an intensive evalualion of 

many different CCOs. Our goal was first to obt.ain a better perspect.ive on lhe limitations of then currently available devices and lhen explore what corrective 

actions we could take to reslore the IMAPS ICCD to a condition where we could 

unquestionably record individual events. As a first step, we tested two CCOs purchased for another NASA grant program at Princeton (for an optical observing 

camera for ground-based telescopes). These CCDs, virtually identical to the RCA 

type 501 scientific chips selected for the lMAPS detector, were assemblies 

mounted on glass (hence they were not electron bombardable). 

While both deVices showed good responsiveness in their on-chip, dual stage 

output amplifiers, 1.5 JJ. V / &- for one device and 1.4 JJ. V / Q - for the other (which 

was a major improvement over the earlier 501 chip desiina), they still had rather 

poor parallel CTE, even after we had experimented with changes in the rise and fall 

times of the vertical (parallel) clockina waveforms. For both chips the CTE was 

bad even at high signal levels or during operation at -80 DC (often, CCDs show poor 

CTE only at low signal levels and low operating temperatures; the problem usually 

clears up for strong signals). Specifically, the poor CTE degraded a spot 

lOThil criterion for liinaHo-noile ratio il a Itandard that we have adopted, even tho\.lih the actual inatantaneoUi siiMllevel above the noile may be lell when the charie is Ipread over two or 

more adjacent pixell. Thil measure hal been Ul6Cl as a iauce for the et!ectivenell of event 

detection Itrateiiel and alleSlments of the minimum performance we needed from the [CCO 

and the sublequent liinal procellq and ampli1ication chain. [n the absence of intanslble 

dU!iculties, we calculated that we could pick up events with a SNR of about 7 without any appreciable overlookinK of valid electron hits, while at the same time we wouldn't reilster an appreciable number of SpuriOWl events from the inevitable larie noise excursions. 

... 4/i -----------........... - _____ ..olIt-.-. . .-...:..."-. _ .... ~ .-. 
- 35-

illumination of a sillile pixel with a level of 12,000 ,-/pixel to a point that it 

couldn't be seen above the noise on a repetitive oscilloscope tracilli of a singJe 

line's readout. This pixel was located near the center of the field. Another pixel 

near the top of the A rea1ster which was illuminated with the same slnale-pixel spot 

couldn't be seen until the sianal went above 21.000 ,-/pixel! The appearance of a 

bar-chart test pattern indicated that the serial CTE for these devices was alriaht 

however. We also noted that the readout noise levels were a bit on the hiah side: 

one device showed a nOlse of 200 i - rms in the active pixels and 160 i - rms in the 

overscan area, while the other showed respective values of 150 and 130 e - rm~. 

These CCOs were eventually returned to RCA because we felt that they were unsatisfactory. 

After the above mentioned tests were completed, it became clear that major 

improvements in CCO performance were needed to achieve our objective of realsteriIlj individual electron events at the IMAPS frame rates. In the swnmer of 1983, 

we were informed by RCA that a new desian of CCD would be produced whlch, they 

claimed, would have somewhat better performance. We responded by placina an 

order for this new device, with a specific request that it have the cover alass chemically etched away so that the CeD would be electron bombardable. At the time, 

we were informed that the new CCD would be shipped several weeks aftar we 

placed the order. As thinas turned out, however, it didn't arrive in Princeton until 

early December 01 that year. This delay in the procurement created a significant 

slip in the lMAPS proaram schedule. 

As with the two CCOs used for around-based observing, the amplifier exhibited 

aood aain (1.3~ V /" -) but with this device the serial CTE was not especially good. 

In contrast to the earlier two chips, however, we found that the CTE was alright 

with moderately large signal pulses, or with small signals on t.op of an elevated 

zero-level background (around 25 mY). In fact, with a signal bias from the high 

~.~ . -~ ---~':-ar '1' 

[:.;;._ . ___ ' __ .") _____ ,;;;. .. ;.;..41-1-----...01, .......... ......:. .. -.... -- . 
- 38-

dark current that we could anticipate for normal fast scan operation at room lemperature for the IMAPS camera, we ascertained the CTE would be acceptable, that 

is, individual photon events would not become unrecognizable, The n'Hse level in 

slow scan operation was equivalent to 110 ,,- rrna, which for IMAPS operatlons was 

a good ftgure. One disturbina feature of this particular CCD was that lts quantum 

efficiency showed strona variations over the fteld. A nat fteld exposure showed global nuctuations of about 50" in amplitude. While this probably would not present a 

serious operational problem for [MAPS, the lack of uniformity indicated that there 

rniaht have been some problems with quality control in the manufacturing process. 

Shortly after the evaluations of the ftrst new IMAPS CCD had begWl, we had the 

opportunity to check out a much newer CCD purchased by the imaae sensor 

development group at Princeton's Plasm'! Physics Laboratory (PPL). This CCD was 

from a brand-new batch produced at RCA. We learned from RCA that this newest 

batch had some processing improvements incorporated into their manufacturing 

procedures. It was immediately apparent to us that the PPL chip was significanlly 

better than any chip we had seen before. The CTE for this chip was excellent all 

the way down to signals of 1000 ,,-/pixel (single pixels could be seen at that level). 

A surprisinaly high gain of 2. 8 ~ VI" - was found at the output of the amplifier, and 

the noise was only 80 .- rma. The response to a ftat field illumination was quite 

Wliform, and there were no blemishes, hot pixels or bad columns. Another very 

extraordinary feature of this chip was that the dark current was quite low. In the 

slow scan test set, which normally clocks out the CCDs at a rate which gives an 

effective integration time of 10 seconds per frame, the dark current at room temperature gave a charge equal to only 3/4 of the full well capacity of the pixels! 

On the basis of the very favorable results we obtained for the PPL ceo, we 

decided to place immediately a new CCD order for the IMAPS program. We were 

especially anxious to obtain a device from the same production run as that which 

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- 37-

lenerated the device which belonled to PPL. Allo, we felt that it wal important to 

have a backup chip to support our tUlht operations. 

The second IMAPS chip arrived in Princeton in mid January of 1984. Much to 

our deliaht, we found that the Ilow-Ican performance of thil chip leemed to be 

about al 100d as the PPL one. The noile wal only about 90 • - rms, and once alain 

the thermal dark current seemed extraordinarily low at room temperaturas. 80th 

the lerial and parallel CTE atllow Ican Idemed to be excellent. We had no trouble 

seeina a linale pixel Ulwninated at a level of 2000 • - Ipixel, which il a weaker Ipot 

than we'd expect from linale 15 to 20 keY eleelrons in the IMAPS ICCO. The sensitivity of the output amplifter wal aaain al 100d al we had leen in recent CCDs from 

RCA: 1.4 JJ. V /. -. Also. a very clean format with a uniform nat field response was 

oblerved. 

When we inllalled the new eco in the IMAPS breadboard fast scan camera systern. we were plealed to see lhat the charae sensitivity of lhe output stage 

was. as clole al we could determine, as aood as the slow-scan performallce. This 

was an important improvement over the CeDs which we had purchased much earUer for the lMAPS prolram However, the fast-scan testtna also lave us a preview to 

some unanticipated technical hurdles. It was immediately apparent that the very 

nne CTE we clJwd obtain at slow Ican rales was no lonaer in evidence in lhe fast 

scan camera: in essence, thinas had really lone to pot! (We never experienced a 

lOIS of CTE 10 ina from slow scan to fast scan in our evaluations or ecos in past 

years.) 

After some inve," \tion. we determined that the source of this new eTE problem was indirecU~ _.fult of lhe new construction of the CCDs. To overcome the 

more fundamental lack of load CTE in their devices. RCA modified the construction of the lates from an open strip confiluration to a more hiahly overlappina 

sandwich-type layout. This type of construction was intended to reduce the losses 

.it, + i ;" III 
·38· 

relultina from tleld frinaina at the aate boundarles when the phas~s were 

sWltched, and the excellent CTE we were tlndina at slow scan rates contlrmed that 

the new desian strateaY wal work ina well. However, a byproduct of the new construction was that the interelectrode capacitances were sianitlcantly ruaher than 

in the old desian. AI a conlequence, with our current electronics we were unable 

to impose properly shaped waveforms on the input pins of the C~OCkl~ aates. 

Based on advice we received from RCA Laboratories, we concluded that the hiah 

capacitance rClulted in a bad impedance mismatch throuah the 3 or 4 inch leads 

between the clockina output drivers and the CCO. Thcy advised us that l~ads 

lonaer than an inch or so were bound to aive us di1ficulties in obtainina the neces· 

sal'ily sharp waveforms to do really fast transfers. Also, the output driver staaes lr1 

our camera didn't seem to have enouah currll!nt capacity to do the job. We ex¥eriml'!nted with shorter lead lenaths and extra emitter follower staaes to help us 

obtain better ~aveforms at the C;:O and hence improve the eTE, but only a mild 

improvement was seen. 

While we could have aone to more extreme measures to improve the clockina 

drives and physically relocate the circuits, such chanaes would have forced us to 

do a major rebuildina of the tliaht camera. The modU\cations would have entailed 

our puttina the output driver Itaaes very near the ima~e section .- an undesirable 

option in view of the probable oulaassina from the circuit elements which are dissipatina a larat! amount of heat. Even then, we judaed that there miaht still be 

some risk thbt we would be opera tina the chip clockina under marainal circumstances. In the liaht of this, and aaain followina the advice from RCA, we 

decided that the best c\)urse of action was to relax on the whole timina scale and 

reduce the overall frame rate of the IMAPS camera by a factor of two. Thus, we 

would operate the camera at 15 frames per second instead of 30. Indeed, when we 

experimentally reduced the master ciock frequency by this factor of two in the 

-.' • , 'H 
- 39-

breadboard camera, we were able to clock the CCO much more comfortably, and 

the good CTE returned. 

Reducing the frame rate had both some advantages and disadvantages. The 

primary disadvant.age of having a lonaer integration time per frame was that the 

rnaximwn permissible count rate was reduced by a factor of t.W(), if we wished to 

identify individ ual events. At high phot.on flux levels we would still be able to properly sense the sianals in an analog fashion, but event confusion would preclude us 

from aoing beyond our nominal wavelenath resolution (~/ t '. = 2x 106) by means of 

centroidina (i.e., by obtaining sub-pixel resolution). 

Anot.her disadvantage of t.he slower frame rate was t.hat the image smear due 

to angular motions of the rocket in the ACS limit cycle would be doublC!d. This 

didn't appear to be a serious problem, since at 30 frames per second we had a generous margin of safety, even if the ACS motions were as high as 15 arc sec per 

second of time. Even though the safety margin was to be reduced by a factor of 

two, it. st.ill seemed acceptable. 

An important advantage of going to a slower frame rate was that the dwell 

time per pixel during readout was increased, which meant that the bandwidth 

requirement on the telemetry system and our recording apparatus could be 

relaxed. In other words, we could experience a reduction by a factor of V2 in the 

nob,e per pixel attributable to losses in the signal link bet.ween our amplified video 

output on the payload and the eventual post-fiight digitization for the data reducaM. 

lion computer on the ground. We felt that this was"important con."ideration. 

Finally, a minor advantaae of going to the slower frame rate was that we could 

expect to have half as many image frames to process on the ground. At the 

present moment (when this report is being written), this factor is most prominent 

in our minds, inasmuch as we are arduously making our way through the some 

5,000 frames of data from the most recent flight of IMAPS! 

• 10 

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In summary, we felt that our position vis-a.·-vi.t CCD performance had improved 

markedly over our situation of approximately a year earlier, when we dlscovered 

that the !MAPS instrument sensitivity was faUina far short of our expectations. 

The new RCA chip desian had a number of important improvements which 

retlected very favorably on our ability to detect siIlile hi&h-eneriY photoelectrons 

in the IMAPS imaae section: 

[1] The sinale level late electrodes were replaced by multilayer, overlapping gate 

structures to improve the CTE. 

[2] The siIlile video output transistor on the chip has been replaced by two 

ampll1\ers workina in cascade. This resulted in a more predictable output 

response (from one device to the next) and seeed to prevent the signal loss 

which occurs when the sample times are shortened (at fast scan) or when the 

output impedance is lowered (the input impedance of the IMAPS camera circuils was only 3KCl). At fast frame rates the improvement in charge sensitivity seemed to be about a factor of 3 over the earlier devices. Also, the 

readout noise, expressed in terms of equi'·!ilent electrons per pixel, was ,")out 

a factor of two lower in fast scan applications. 

(3] 

[4] 

Better manufacturina procedures or higher quality silicon in the new CCDs evidently gave us devices with very much lower thermal dark currents and formals which hll\'e very few cosmeti~ problems. The flat-field response also 

seemed to be excellent. While these considerations were not too vital for 

IMAPS applications (aside frem making the data reduction a bit easier), they 

indicated that the overall quality of the product had improved, which probably 

also reflected favorat.." 'm its survivability. 

Some of the signal lines leading to and from the on-chip amplifier had better 

shielding from the clocking leads. As a consequence, the im:tantaneous levels 

detected by the IMAPS sample-and-hold modules were less subject to 

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, 

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- 41 -

interference from the tails of the clockina pulses. We lllter realized that this 

chanae gave us a vastly lower response to int.erference from noise pulses from 

the voltage reiulator5 in the electronics power supply. Evidently, the carry· 

over of the pulses into the video slanal was caused by part of the video samplina ridina up on the steep exponential tails of the. previously enormous clock 

pulses in the CCD's raw output. 

[5] The new CCDs had a much better pin design. We could expect to see il 

siinittcant improvement in the reliability of the electrical connections to the 

device, especially under the severe environmental conditions of a rocket 

tUaht. 

7.2. Flrst. Fliahl of IIIAPS OD a SoundiDa Rockel (27.071UG) 

The ftrst launch of the IMAPS instrument on a Black Brant rocket occurred on 

October 5, 1984 at the White Sands Missile Range. The target star chosen for this 

flight was £ Perseii. This star is ~specially important to observe in the ultraviolet 

with an instrument having good spectral resolution, since earlier Copenl.icus data 

indicated that there aI'e about 5 secarate components in the interstellar medium 

along this particular line of sight. With spectra taken at the velocity resolution of 

Cope rn.icus , these components are badly blended, and their existence was known 

only by careful modehng of the smeared protlles. 11 Far more definitive results 

would be forthcoming if these components could be untangled. Interstellar results 

for this star are also enigmatic: Vidal-Madjar it al. 12 found that the apparent 

ratio of atomic deuterium to hydrogen varied with the time of observation (typical 

time spacing of about a few hours).They explained this in terms of effects from 

time-variable winds from the star. Since Copem.icus scanned different lines at 

lIMartln. E. R. and York. O. G. 1982. Arwoplt.f/SU:al JOt/lrMl. 257, 135. 

lltyjdal-MadJar, A., Ferlet. R.. Laurent. C., and York. O. G. 1982. A8tr0'phy~a.l Joumal, 280, 128. 

~~------------------~"~---'------====.::~--__ ~I_-_.-_--~-·------ ---
- 42-

different times, they found it very difficult to unravel lhe lrue differential 

equivalent widlhs for lines of D and Po from lhe perlurbations caused by the steUar 

wind on the blue-shifled component. Observations by IMAPS could unravel this 

mystery, since all wavelengths would be observed Simultaneously. 

Except for some difficulties with payload testma at NRL prior to our going to 

the Missile Ranae for the launch, preparations for the tliaht were uneventful. At 

NRL, when we attached the IMAPS instrument to Carruthers'S vacuum tank and 

collimated ultraviolet source, we once qain found the payload had a drastically 

reduced sensitivity (initially, it seemed as if we were repeating our unpleasant 

experience the last time we lested IMAPS at NRL! -- see §7.1). After several days of 

troubleshooting, we discovered that the echelle grating was not set at the correct 

angle. This improper settina meant that we were observin~ light. far from the grating blaze angle. During optical alianme.!t at BASD, there had not been a realization of the importance of having this angle set exactly at the right value (the width 

of the bp.am is only about 112 of a degree because the grooves are very wide). 

We corrected the grating angle by installing a shim on the attachment to the 

arm which rotated the echelle. Much of nur ai.fficulties in it:ientifying this problem 

was attributable to our being unable to mt:!asure the alignment of the IMAPS optical axis relative to the collimator for the ultraviolet source. As a result vf this 

experience, we took note of the importance of providing fixtures for measw'ing 

this alignment in future testing at NRL. 

During the flight of lMAPS, there was a catastrophic degradation of the 

vacuum inside the instrument. As a consequence, the detectol"~ high voltage 

underwent a continuous discharge during most of the observing time. The loss of 

vacuum and its effect on the instrwnent were indicaled by three independent 

telemetry signals: 1) The readings from an ion gauge installed in the payload, 2) 

The initial lack of any high voltage level after lhe HV power supply was aclivat~d, 

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- 43-

followed by a slow, raaaed (from arcina) recovery, 2) Excessive current draw on 

the HV power supply, and 3) the CCO television sianal which was solid white. Thus 

there was Pale question about the nature of the failure. The only mystery at the 

time was what was the cause of the inrush of air. 

We now recount some of the sianificant events which happened during the 

tUaht. The prime anomaly, of course, was the breach of vacuum which must have 

occurred alter T = 0 seconds. After the vacuum seal plua was inserted at T = -10 

minutes, we monitored the vacuum inside the p"vload with our ion aauae. The leak 

rate was quite small, so we were satisfied that we had no problem whatsoever prior 

to liftoff. As a safety precaution, we turned off power to our payload at about T = 

-1 minute so that the vibration during the ride wouldn't accidently trip a relay or 

momentarily shurt something in our experiment (a timer in the instrumentation 

section restores power after burnout). Since the ion gauge ran on our experiment 

power, we had no record of when the leak occurred. After the power was restored 

at T = +62 seconds, the ion gauge indicated that the vacuwn was horrible -- see 

Figure 6 (on page 49). Alter the aperture was opened, there was a nominal drop in 

pressure, but the pumping rate seemed to be very slow. 13 At T = 103 seconds, the 

ion gauge was prc;~ra.mmed to shut down so that the detector could function 

without an ion background from the gauge . 

Alter a while, the quality of the vacuum within the instrument chamber 

improved because of the pumping action through the entrance aperture. The high 

voltage eventually stabilized at the proper value, and the CCO went from solid 

white to a condition where one could see an avalanche of ions hitting the photocathode. After about T = +420 seconds, the ion rate subsided to the point where 

genuine photoevents (Le., a real signal) could be seen. Over the interval from +420 

ISThe small uptick in pressure on top of the dElcay occurred at precisely the time the ACS valves 

tired for the main maneuver to tarlet. 

_.--_ .. ..-,. "- 1 .~ 

-. . 1" __________ ........ ..... _________ #! ... --:'- ,;. " •• 

-

- --
- 44-

to +472 seconds. the detector was resisterina a complicated mixture of resid ual 

ions (which produce very stroIli sisnals) and true photoelectrons. The altitude of 

the rocket at that time was still ruSh enouah to allow IMAPS to view an ultraviolet 

spectrum from t Per. 

It is conceivable that there is useful data buried amonsst the clutter of ions. 

Careful extraction technique! with a computer miaht recover some scientific information. However in view of the much greater quantity and quality of data obt.ained 

from the subsequent ftisht of IMAPS (described in §7.3). the importance of committin& time and resources to such an ambitious data reduction task has been diminished. 

At the time. we had no indication at all where the failure of a vacuum seal 

occurred. The break seemed to hRve been only momentary. since t.here was a reasonably good vacuum present in the payload aft.er it was recovered some 8 to 10 

hours after t.he fiight. The vacuum we found afterwards wa& consistent with atmospheric pressure at an altitude of about 30 km. which agreed with the altitude 

where the aperture door closed according to the radar tracking data (at T = +491 

sec). 

Our initial conjecture was that stresses from the powered fiight must have 

jarred loose one of our a-ring seals somewhere. Of all the vacuum seals on the 

payload. we presumed that the most vulnerable one is that associated with the 

aJierture door. However. this door has about 200 pounds of spring pressure to hold 

it closed. in addition to the force from the atmosphere which is about 900 pounds 

at sea-level pressure (this force is slightly lower at the elevation of WSMR). The 

force holding the door closed seemed to be pretty generous compared with the 

expected g-loads; the weight of the door itself is only about 1 pound. 

There were two other significant anomalies which occurred sometime during 

the flight. One was a break which occurred in a weld where the outer skin (not the 
- 45-

vacuum vessel, but the load-bearing cylinder which surrounds it) joins one of the 

bulkheads. The break was complete along about 2/3 of the circumlerence. Fortunately, the break didn't go all the way around the skin, otherwise we would have 

had a real disaster on our hands (i.e., the payload miaht have been completel} destroyed or at least seriously damaged). 

The outer skin was provided to the project by the soundina rocket division at 

GSFC. Since much of the responsibility for sounding rocket mission operations 

was beina transferred to WFC shortly after our tUaht, we returned the skin 

(inasmuch as it was GFE) to WFC for evaluation and repair. The weld was inspected 

at WFC and it was found to be defective (the bead had insWficient penetration). 

The other major mechanical problem was that the detector assembly, weighing 31 pounds, had torn loose from its mount. Fortunately, clearances in the vicinity of the detector were so tight that it did not go crashing around inside the payload, inflicting damage on itself and other instrument components, such as the 

gratings or mechanical collimator. A shock in excess of 100 G's would ha"e been 

necessary to shear the 2 1/4-inch dowel pins and 4 10-32 socket-head stainless 

steel cap screws which secured the assembly. A less severe stress could have broken the detector loose if an outer anchoring point failed before the others felt the 

load (Le., a zipper type effect). There is no evidence from the accelerometer 

telemetry that a large stress occurred during the flight, but it is conceivable that 

the payload felt a significant shock when the entire package fell over like a t.ree 

after the initial impact on the desert. 

Our corrective action for the next flight of IMAPS was to use stronger bolts to 

secure the detector assembly to its mount. The stainless steel cap screws were 

replaced with hardened steel screws of the same size, increasing the load capability by at least a factor of two. 

-.. 
, 

t 

~ 

- 48-

In the months which followed the tliaht, we investijated various possibilities 

for the cause of the vacuwn failure. The payload was inspected carefully for any 

evidence of physical damage that might indicate where the leak occurred. None 

was found. We tried to stress parts of the payload that could result in possible 

ftexure that would cause an O-rina seal to fail, but we were unable to create a 

failure. Finally, we hypothesized that trapped air in some void within the payload's 

vacuum vessel could have been released under the shock and vibration of the 

rocket night. We tried to simulate this failure mode by vibratina the payload to 

tliaht levels whlle it was under vacuum. No release of aas was detected while the 

vibration was in progress, beyond a barely perceptable increase in the normal leak 

and/or outaassina rate. 

In summary, many months of thinking and tests failed to produce a definitive 

explanation for the vacuum problem on the night. These tests could not definitely 

rule out some of the hypothetical causes, but we could not pinpoint a real problem 

area and make what we considered to be a ddfinitive fix. Up to about a month 

before the second launch, we devised the following corrective actions Which, we 

felt, would lessen the probability of the failure happening again: 

[1] We increased the spring pressure on the aperture door to 400 lbs. While we 

felt the previous pressure should have been adequate, the extra force would 

give an extra margin, in case some strange vibration built up during the fiight 

which caused enough warping or rattling of the door to cause the seal to fail. 

A change in the spring constant would also change its resonant frequency, 

which might also be beneficial if our previous misfortunes were the result of 

some buildup of vibration. 

[2] Relief holes were drilled in all places inside the payload that were suspected of 

being able to trap air. 

, !').,. 

... 
 

I 

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f 

- 47-

[3] We decided to vibrate the payload at White Sands a.jt.T the vacuum can had 

been pumped down for the last time before fliaht. If there were any trapped 

voids containing air still within the payload (i.e., corrective action [2: above 

didn't let them all). they would be very likely to be relieved during this vibration on the shaker, rather than on the ftilht itself. 

[4J A separate source of power was provided to the ion gauae, so that it could 

operate independently of our payload electronics whose supply was intentionally turned off during powered ftighl. If the failure reoccurred on tr~ second 

rught, we'd have a better indication from the telemetry of when and how 

rapidly the failure occurred. On the flrst fliaht. all we could tell was that the 

vacuum was very bad at a time well afler powered flight (but before the door 

had opened). 

[5J Silicone potting material was applied over ~ll joints on and near the ion gauge. 

The ion gaUie was housed in a sealed can maintained at normal air pressure tf) 

keep its high voltage leads from arcina at intermediate ambient pressures. 

Since the pump down of the payload after the apel."ture door had opened was 

not rapid, we surmised that a sl"w leak from this sealed can could have disastrous consequences. ThE. actual probability that this sort of failure really 

occurred seemed rather low, inasmuch as there was no leak near the ion 

gauge after the flight. Any breakage of the seal would have had to mend itself 

afterwards' 

[6] We lengthe:ned the time interval between the door opening and the activation 

of the hiah voltage power supply. This would give more time for the payload to 

pump down. if the pressure were too high at the beginning. (Once an arc is 

established. the pressure for it to be quenched is much lower than the 

minimum pressure to establish it in the flrst place.) The time interval on 

flight 27.071UG was 13 seconds; for 27.082UG it was lengthened to 20 seconds. 

l ....: ___ ..;...;.,;;,;,..;.;..-.....!.... ::...:.. ... .......;,;' .:::: ===.L:a =_ ,_ ... ,_-.. , 
- 48-

Also, the door openin& was pOlponed to a hiah.r altitude (later by 8 sec:')nds, 

which made the altitude 142 km instead of 135 km on 27.071UG). 

A sianU\cant breakthrouah in our investiaation of the vacuwn failure occurred 

about two to three weeks before the second launch. Durin& the testin& of the 

payload's optical performance at NRL, Don Bradshaw (of BASD) happened to notice 

that the vacuum pump on Carruthers's apparatus was able to pump the IMAPS 

instrument throuah the front aperture much more rapidly than the rate of pressure decrease experic!nced on the tirst ftiaht! In essence, a'simple diffusion pump 

on a laboratory setup performed better than an ostensibly perfect vacuum pump 

with unlimited pumpina speed (i.e. the vacuum of space at ftiaht altitudes)! Fiaure 6 illustrates the tremendous disparity between the two rates (compare the 

curve labeled "test pumpdown at NRL IMAPS ion aauie" with that labeled "IMAPS I 

27.071 UG" after the door opened). 

-:---: ::-. 
-- -- -- - --- - XNAM

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- 50-

ThlS tlndina aave us a clue that the sustained hiah pressure inside the vacuum 

can durina the observina lime wal not. cauled by simply a larae insertion of aas at 

some previous instant followed by a slow pumpina rate throuah the mechanical 

collimator. TIw tong d'W"ation oj high pr'lSsurtl wcu tlw r.Nt oj e& S'uste&in.d 

t.aJca,g. oj gC&S', .itlwr dir.cttll into tlw pal/toad. or in itS' imm.di4t •• nvironm.nt. 

With this insiaht as a strona hint, some further thouaht produced the followinj 

explanation - one which we now think is the most probable description of the 

source of our vacuum problem: There was no intentional ventina of the housinas 

for the ORSA, ACS, telemetry, or experiment electronics systems. Since the operation of these systems in tli&ht are unaffected by the pressure, that is, they could 

function perfectly wf!ll between a pure vacuum and one atmosphere pressure, no 

notice was taken as to whether they were perfec:Uy sealed or were allowed to vent. 

As has been the practice on other tliahts, the chambers between the nose cone and 

the experiment were allowed to leak (very slowly) throuah joints in the skin (see 

Fijure 7). Another way to relieve the pressure, however, was for jas to now 

thr0Uih the chamber which housed the experiment electronics, then thrcujh 

some narrow slots which provided thermal isolation on the rear support for the 

experiment vacuum can on the rocket skin, alona the cylindrical void between the 

vacuum can and the skin, and finally, into the open housina right in front oj the 

'lJiewing apertu.re oj tlul instrum.,nt! We feel that a partial retention of jases by 

this housina before they vented into space could result in an appreciable intlux of 

molecules into the payload . 

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- 52-

The principal restriction to the flow of a~r elona the entire circuit is the small 

aap between the vacuum veslel and the skin of the payload. Fiaure 8 shows a 

detailed cross section of this aap and illUitrates the route of the flow. Flaures 9 

and 10 show, respectively, where the thermal isolalion spacers were located and 

the sizes of 'he important dimensions. If one actually computes how rapidly the 9 

ft3 volume could decompress throuah this annular arranaement of slots, lhe rale 

is prelly hiah. However, one mUit remember that the parachute in lhe ORSA will 

release its trapped aues at a very slow (and not easily calculable) rate. 

Some additional support for the ventina theory jUlt prelented miahl be 

derived from the upUck in prelsure shown in Fiaure 6 at T = +81 sec (lhis was 

mentioned previously - see foolnot.e 13 on paae 43). Prior lo t.his explanation, it 

was hard for us lo 1maaine how ACS aases could ever be reaist.ered by an ion aauae 

inlide t.he payload. when lhe payload's aperture was so far removed from the 

thrust. jets. Now, one could imaaine t.hat. a small leak in t.he plwnb1na inside the 

ACS sect.ion could release some extra aas. which in turn would follow the other 

material alona the aforementioned rout.e. 

To overcome lhe problem discussed above for the second tuaht. of lMAPS, we 

implement.ed t.wo tlxes durina lhe t.wo weeks of preftiahl preparations allhe MissUe 

Ranae. Firsl, we drilled several 1-inch diameler holes in lhe ACS and TIM housinas 

and covered t.hem wit.h the standard st.reamlined shrouds. The number and diameters of these holes were judaed to be adequate, by a aenerous mara in, to insure 

lhat the chambers would pump rapidly durina the ascent of the rocket. As a 

second precaution, we packed the slots near the thermal isolation spacers with 

asbestos ftber cord, so that we could be certain that t.he amounl of air reacllil1i 

lhe front. of the payload was lower t.han before by a.t taa.st two orders of maanit.ude 

(remember that. t.he pressure on t.he first ftiaht was higher t.han the usual pressures where hiah volt.ages break down by about a fact.or of 30). 
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- 56· 

7.3. Second Fliaht. of IIIAPS 011 a SouDdiDa Rocket (27.082UG) 

In contrast to the drst tUaht, the second lMAPS mission nown on April 20, 1965 

(0050 MST) was a complete success, :'oth technically and scienti.ftcally. From the 

real-time TV display of the detector output telemetry siinal, it was immediately 

apparent that the payload was stabilized on target, thd.t the detector was functionina very well, and that a spectrum of about the right intensity was being 

registered. It was also clear that the Lyman-a airglow background was at about 

the expected level - Le., not strona enough to compromise the data quality. 

The target on this mission was '" Scorpii, a B1V+B2V type star (actually a 

binary system with one component very much fainter than the other) with a V 

magnitude of 2.88. While the interstellar spectrum toward this star is not as 

bizarre as that toward t Per (see 17.2), it is still of signif!.cant interest: it has a 

fairly appreciable abundance of H2 and there is a component at moderately high 

velocity, presumably from a shock passing through some of the gas. 

At the conclusion of the night, the payload descended into a mountainous 

region on the western edge of the missile rallie. Fortunately, it did not land on 

any of the nearby treacherously precipitous terrain, but the slope was moderately 

steep and rocky. After the initial impact on one end, the skin of t:le rocket and 

the payload's vacuum vessel were punctured by a rock as the entire assembly 

tipped over, landed nat, and rolled a short distance down hill. The damage was not 

serious, however; only minor structural members and a small part of the vessel 

wall were in need of repair. No internal components were broken during any phase 

of the flight. 

At the time this report is being written, we are actively working on data reduction. Sections of this report which follow give some performance data on some 

payload components and some of the NASA-supplied support systems (e.g., ACS, 

telemetry and vehicle). Some preliminary conclusions on the scientifl.c data, 

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- 51-

based on the limited information available at the moment. are presented in later 

sections. 

7.3.1. Sut.yst.em Perf ormance and Useful EqiDeerinl Data 

7.3.1.1. Rocket Vehicle Ptn'formance 

According to the radar data. the payload was boosted into a trajectory which 

reached a peak altitude of 263.8 km at T = +265.0 sec. This altitude was slightly 

lower than the prediction 14 of 266.5 km. but the vehicle probably performed nor- ..... s 

mally because extra weiaht"added to the payload. Weights were attached to the 

payload about a week before the /tight so that the re-entry attitude lVould be more 

fiat. which in turn Vi ould lead to less aerodynamic heating (on night 27.071 the 

front end of the ORSA package was badly burned). 

7.3.1.2. Tim~"l Proftle of t.he Payload's Skin TemperaLure 

A principal concern in flying the IMAPS payload is how much heat would be 

transferred to the skin of the vacuum vessel - a structure which defines the distance between the only focusing element in the payload. the cross-disperser. and 

the detector. This wall was made of ordinary aluminum which has an expansion 

coefficient of of 2.6xl0-~ ac-1.

15 Appendix B shows calculations of the rate of temperature rise of the payload wall as it is heated by radiation from the hot rockel 

skin. As the last line of the calculations show. a temperature rise of about 8 °C 

could result over a 400 sec. interval of time, and if the spectrograph were initially 

at a perfect focus, the blur of the image could increase to 37~ at the end of the 

14A.,.oballistic AMlys;s lor Pre-lntegra,tllJn Rwtnu by R. P. Buchanan (Code 841.1). March 19. 

1985. 

15At one time. we had corusidered buUdilli the optical meterillj structure out of graphite epoxy. 

We abandoned this idea becaWie the cost was more than the program could afford at the time. 

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- 58-

night. It should be apparent that these calculations were rather approximate, and 

thus we had some concern that an out of focus condition might be worse than 

predicted, 

Figure 11 shows the telemetered temperature data for sensors located on the 

vacuum vessel skin, aft bulkhead of the payload, and on the detector inside the 

instrument. The latter two sensors were to check that heating by the electronics 

was not appreciable. The skin sensor showed that the temperature increased from 

23.6 ae to 33.2 ae during the time the spectral data were beina recorded (+132 ( T 

( +457 sec.). The rate of temperature rise after about T = +220 sec. was 0.026 ac 

s"l, a figure remarkably close to the prediction of 0.020 ac s"l! 

In anticipation that we would be observing at elevated temperatures, the people at BASD who performed the optical alignment of the [MAPS instrument set the 

focus to be best at 27 ac. (Just before the flight, considerable car~ was made to be 

sure the ambient temperature for the payload was as low as we could make it, 

within a reasonable effort.) At the beginning of the exposure sequence, we were 3.4 

ac too cold, which would theoretically mean that the focus would be wrong by 0.16 

rrun. With the spectrograph's fila beam, the blur circle would be 16~ in diameter. 

As the payload heated, it should have gone through perfect focus and, at the end of 

the data-taking sequence, would be out of focus in the other direction by 0.29 mm, 

because it was too hot by 6.2 ae. In short, thermal changes should have produced 

blurs going from about half of a 30~ eCD pixel in the beginning to the full width of 

a pixel at the end. 

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7.3.1.3. ALULude CODtrol s,.t.em (ACS) Performance 

7.3.1.3.1. Taraet AcquiaiUon 

It seems reasonably certain that the ACS system pointed the payload to the 

correct star. ACS personnel stated that the telemetry indicated no anomalies in 

the maneuvers to target or in the target acquisition process. Also, the spectrum 

recorded by IMAPS seems, when viewed superficially, to be that which one might 

expect from the chosen target. However, no independent verification of correct 

pointing has been made by any analYdis of the aspect camera pictures (the aspect 

camera was intended to be used only in the event that there were serious doubts 

about the pointing). 

Figure 12 shows the stars in the field of view of the startracker. From the 

presence of several other stars in the general vicinity of the target which were 

moderately bright, the ACS group decided that there would be considerably less 

risk of acquiring the wrong star if an intermediate maneuver were made to the 

very bright st.ar a ScorpH, so that the positions of the gyros could be updated 

(see§7.3.1.5) . 

7.3.1.3.2. Stability 

The critical factor which infiuenced the quality of the observations was not the 

angular amplitude of the ACS limit cycle, but rather the rate of change of the 

pointing direction. Since IMAPS is an objective grating spectrograph, as long as 

the spectrum format was apprOximately centered on the dete\~tor and attenuation 

by the mechanical colUmator wc..s not significant, the absolute pointing direction 

could be somewhat off the target (excursions of 1 or 2 arc min coul\! be tolerated). 

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Small rates of chanae, on the other hand, were important since we must not have 

an appreciable smearing of the image In the 1115 sec. frame time. In settina our 

requirements for ACS .,erformance, we declared that a rate of less than 15" 5"1 

should be a specification for the ACS hold performance, whUe less than 5" 5"1 

should be a goal. Wlth a rate of 15" s"l the smearlna would be equivalent to 0.3 CCO 

pixels (30~ each) per frame. 

Fiaure 13 shows a plot of x and y positions of the broadband star image for a 

series of ICCD frames which have been processed up to now. Each CCD pixel 

corresponds to 3.45". The laraest peak to peak variations are about 15 pixels (or 

about 52"), for the data shown here. From a visual estimate comparing the 

dynamic behavior 01 the star during this segment of time with that during the rest 

of the rught, it. appears that the information in this plot is pretty representative of 

what happened duri~ the whole data-taking session. The excursions of almost l' 

may seem large by normal standards for the limit cycle for a STRAP 5 ACS system. 

but one must remember that we asked to trade small amplitudes for low rates, so 

the ACS people deliberately opened up the dead band and used low pressure bleed 

jets for stablization. 

Each dot on the curves represents a sinale 1/15 second frame. Consecutive 

dots which are spaced by more than 0.3 pixels in the vertical direction on the plot 

represent cases where the 15" s"l specification was violated. Figures 14 and 15 

show that this happened rather frequently. Figure 14 is a histogram showing how 

often the displacement vector changed by a given amoWlt from one frame to the 

next. These results, taken from the data shown in Figure 13, tend to overstate the 

excursions a bit, since some additional random walk is produced by errors in 

detennining the centroid of the star image. (ThB error of this determination will 

add to the pointing error in quadrature, if the two effects are Wlcorrelated). 

."l" 

-. . . ,. 

W

.~-.-~

~ ", ..... 
30 

• 

25 I I • FIGURE 14 

~ 

, ~ I I 

~ 

fI) 

Q) 

20 () 

c: 

~ 11111 In Q) 

s.... Total Motion (r + yZ)1/2 ~ 

:;j 

0 . 0 

!. 0 15 I, 

I c.-. 

: \iii 0 

~ 

cv I- III i., 

rO 

8 10 : 'l-l :;j 

Il" Z [II I I I I I I I I I I I J 

I 

I .\ 

5 

O' L.I I ILU I IL.lI.l 

; o 20 40 60 80 100 120 

Angu]ar rate (arc-sec/sec of t.ime) 

.. .,b...,-~ , .... , r ~ 't' ... -- _~_:--: ... ~.~ _ ,,_ _oIrT-' "'- E2&iiiii1f!~~~~ ~_ . ....,r;:..-" ,,_ ~ -""""......... -. __ 
40,1 TI-rTi-rTi-rTi-rTi-rTl-'TI-'T1rrllllllll~~~~~~ 

• 

, 30 ~ 

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Q) 

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L"~J"I FIGURE 15 I Q) I S-4 

l s... 

liB 

• i 0 !.I 0 

• 

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r, I ' ..... 2°H I X Motion Only 

i 0 f ~ : Q) 

,,0 

8 

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10 ~ I j I 

I n 

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~1 OI~~~~~~~~LL~~~Li~~~~~Jj~~tIJ:~Jt~-LJ 0 I I I I I I 

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e ~. .. -,~. ~ ~:... "" ..... _iiiil __ T7t7riiiiiiiiliii~iilililclliiii __ ;;;:. .... __ ~~ ..... - __ - ...._--_-__ 

• 

10 20 30 40 50 60 

Angular rate (cu'c-s~c/sPc of time) 

70 

~ 

uo 
• 

- 88· 

The quantity which really inftuencel the spectral resolVllli power of the 

recorded data is the smear alona t.he X direction only (i.e. parallel to the dispersion of the echelle aratina). Fiaure 15 shows that for a moderate fraction of the 

time. the rate was in excess of 25" s-t. which corresponds to 112 pixel of smear in a 

aiven frame. When the recorded data are eventually combined to produce a spectrum for scientiftc interpretation. we may elect to discard those frames where the 

motion was relatively rapid. to recover some of the lost spectral resolution but 

with some sacriftce in the total number of photoevents counted (and hence siinalto-noise ratio). 

7.3.1.4. Telemetry Slinai Quality 

A major concern early in the IMAPS proaram , jJ whether or not the final 

sianal-to-noise ratio would be adequate to reaister events reliably. While this 

requirement was absolute, we also felt that it was highly dlisira.ble to obtain l;i.,,;,od 

enouah signals so that events could be centroided to an accuracy of at least half a 

pixel in the dispersion direct.ion. 

From pretiiaht test results and also the outcome of the 27.082' 

was clear that the signal quality was fully adequate. The best s~nals were 

recorded at the Jia-67 site. as one miaht imaaine since the diameter of the receivina dish is rather larae (24 feet). Even the sianals recordeu at other receivina stations were adequate to meet our minimum requirement. Photoevenls were clearly 

visible on the tape recordinas derived from 10-foot dishes on the rallie. We were 

very fortunate to have at Jia-67 not only the NASA receivers with 10 MHz IFs. but 

we were also permitted by the military authorities to use one of their 10 MHz IF 

receivers. This second low-noise receiver allowed us to record both polarizations 

(LH and RH circular) and also have some backup capability that was as good as the 

primary one. 

,..... ,.. 41i ~ .-----u_ "!""·_--·~2JW"" •• _i&IiO"- ~ -. 

.. ---------------- ....... - -.-.......... ----- - ------

. ~ " i ;-:-
• 

, 

I ; 

p 

• 

- 61-

Noise measurements were p"!rformed on the YCR tape'! recordings of the night 

telemetry, and the followiIli results were obtained, 

[1 J J-67 RHC 10 MHz IF: max sijnal = 900 mY, 1(1 noise = 5,5 mY 

[2] same as [1J above except sijnal routed throuah IJ.-wave link to N200: max signal = 800 mY, 1(1 noise = 6,5 mY 

[3] J-67 LHC 10 MHz IF: max sianal ~ qno - 800 mY (rather ragged upper clipping 

level), 1(1 noise = 10 mY 

[4] 10-foot dish 10 MHz IF VAB/N200: Recorded on GE tape recorder (rather than 

Panal!t.Jnic ones for [1] - [3] above), Jt is di1Ticult to get the Panasonic YCR at 

P~nceton to st.abilize in the freeze-!rame mode for this tape, presumably 

because of some sneaky incompatibility :~ the ways the signals wp,'"e recorded, 

No areat effort was made to measure the signal properties for this tape since 

we l':lave plenty of good recordings otherwise, 

7.3.1.5. Event Timing 

On the two pages which follow, times and altitudes of key events during the 

flight are listed, This information is followed by pages which show actual tImes, 

measured "tf of the telemetry records, of events triggered by timers in the experiment and instrumentation sections, The exposures enumerated on the second 

page refer to the aspect camera in the experiment section (which ran off a timer 

inside the instrumentation section), 

I .,~ 

. 

)
• 

( 

rnm 

(.ec) 

-600 

-540 

-480 

-120 

-60 

o 

40 

60 

63 

66 

67 

74 

96 

98 

100 

104 

ALTIroDE 

(km) 

o 

o 

o 

o 

o 

o 

33 

73 

77 

79 

84 

86 

99 

135 

139 

142 

148 

-.r .. 

---_ .. . _-------

FUNCTION 

Kanually 1n.art vacuum pluS. 

Verify ;.hat vacuum 11 .tUl good. 

Withdraw vacuum rail .y.tem. 

Verify that the rate of pre •• ure ri.e within the 

payload vacuum enclo.ura i. acceptably low. 

In.trumentation .action removal axperiment pow.t. 

Launch 

Rocket burnout 

No.e cone eject. 

leatore 

active 

EchaUe 

launch) 

experiment powar; all expariment .ystema 

except for the detector high voltage. 

IDOV •• from po.ition 1 (high cam --Iafe for 

to pod tion 4. 

Yo-yo De.pin. 

Payload .eparation and ACS start. Start experiment 

tilD8r. 

Eract pitch and yaw gyro. to launch attitude and 

.ettle; eliminate re.ldual roll u.lng jets. 

Remote adjust begln: Roll +7.13 deg. (1.47 sec) 

Pltch -109.86 deg. (22.76 sec) 

Yaw 0.00 deg. (0.00 sec; 

(It launch occurs at 0000 MST • 13h 47m 598 LST) 

for maneuver to alpha Sco (ACS update star) 

Safe altltude to open experiment aperture door. (p 

~ 3 x 10-6, ln the absenca of scoop effects and/or 

outgaasing from other parts of the rocket and 

assoclated systems) ,-

'eX. - I~ 'Z.~:'" Begln settle and capture on a Sco 7' "":s.;--

Op t d iS /a.t!.-,'; -= - Z c.. '2 y 0 "2. en aper ute oor t· -,,, / 

Cage gyros 

.,.. , 

~ ~41) -. . 

. -----------.....;--------....... ""'-'---;.. ........ - --
• 

( 

.. 

, 

f 

~ ~ . , 

l ft ~ l52 

10'.1 156 

115 164 

118 168 

120 111 

130 184 

134 189 

168 225 

202 251 

239 266 

273 270 

307 263 

341 245 

378 213 

412 173 

446 123 

460 99 

464 92 

475 71 

480 61 

aalin _"e"vII" to ~ Sc,:, (ob.ervin_ carlet Itar): 

Pitch -4.0H d~~. (0.85 .ec) 

Yaw +5.~1 dMg. (1.13 (.ec) 

Maneuver to w SCO complate, begin capture and 

.ettle. 

ACS hold on ~,;O ·or re.inder of the fl1Sht. 

Turn off experiment i~n lause; .tart detector hllh 

volta.a lequencinl. 

Piflt hiSh voltase nep active; continue with 

lucceillvely hisher voltasel. 

Expariment detector hiSh voltale at full level. 

Chanse echelle tUt: POI 4 to pOI 3 

Chansa echelle tUt: POI 3 to POI 2 

Change echelle tUt: POI 2 to pOI 1 

Chanse ech .. lle tUt: pOI 1 to pOI 4 

Change echelle tUt: pOI 4 to POI 3 

Change cchelle tUt: pOI 3 to POI 2 

Change echelle tUt: pOI 2 to POI 1 

Change echelle tUt: pol 1 to pOI 4 

Change echelle tUt: pOI 4 to POI 3 

Change echelle tUt: pos 3 to POI 2 

Close aperture door and turn off HV. 

Change echelle tllt to pOI 1 (Iafe pOI) 

Aqblant prelsure reaches 10- 3 torr --HV discharge 

may occur ln lmage aectlon, lf the door weren't 

cloled and HV turned off. 

Turn off experiment power. 

lat Severance and drogue parachute deployment. 

---------------------------------~--------------------------------------._---- NOTES: 

Formula for computlng apPloxlmate altltudes: 

h(km) • [270 km - 0.5(9.2x10- 3)km a- 2 (267 8 - t)2) 
I • 

-7D- 

5-9-85 

DFB 

IMAPS TIMING AT WSMR, NM 

4-10-85 4-15-85 4-18-85 4-20-85 

PRE-HORIZ. HORIZ. VERT. FLIGHT 

T-O 0 0 0 07:49:59.651GMT 

EXP.PWR.ON 61.2 62 62 63.1 

SEPARATION 67 67 69 68.0 

ECHELLE 1+4 90.2 100 90 97.4 

APER.DOOR OPEN 101.5 102 104 101.8 

HI VOLT ON 119.5 120 121.4 119.0 

ECHELLE 4+3 123.5 130.4 

H I VOLT FULL UP 131.2 132 133.4 132.0 

ECHELLE 3+2 156.7 163.7 

II 2+1 190.0 197.0 II 1+4 226.5 233.1 II 4+3 259.5 266.4 II 3+2 293.5 299.7 II 2+1 326.5 333.0 II 1-+4 ~62.5 369.3 

II 4+3 396.2 402.6 

II 3+2 429.5 436.0 

HI VOLT OFF 458.5 457.8 459.0 457.0 

&A.DOOR CLOSE 

ECHELLE 2+1 4~&.!.5 469.2 

EXP.PWR.OFF 498.2 475.5 474.5 476.2 

-.. .... -.-..... --.---...,.---_-"

·lfl 
• 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22. 

23. 

24. 

25. 

2E. 

27. 

28. 

29. 

30. 

31. 

32. 

33. 

34. 

78.4 sec. 

92.4 sec. 

105.5 sec. 

118.4 sec. 

131. 4 sec. 

144.0 sec. 

157.8 sec. 

169.3 sec. 

181.8 sec. 

194.5 sec. 

207.1 sec. 

219.5 sec. 

232.1 sec. 

244.3 sec. 

256.8 sec. 

269.3 sec. 

281.7 sec 

294.0 sec. 

306.5 sec. 

318.S Sl~C. 

331. 2 slac. 

343.4 sec. 

355.S sec. 

3f,S.2 sec. 

380.6 sec. 

392.9 sec. 

405.4 sec. 

417.9 sec. 

430.3 sec. 

442.7 sec. 

455.1 sec. 

<I 467.5 sec. 

479.6 sec. 

493.0 sec. 

-71-

lMAPS II EXPOSURES 

OTIME+07:49:59.651GMT 

2.8 sec. PULSE DURATION 

1.6 sec " " 

1.6 sec. 

1.6 sec. 

1.6 sec. 

1.6 sec. 

1.6 sec. 

1.6 sec. 

1.6 sec. 

1. 6 sec. 

1.6 sec. 

1.6 sec. 

HI VOLT OFF 456.4 sec. 

LAST EXPOSURE 

5-9-85 

DFB 

lli.. ____ -.. __ ,·_,·-=_-_··-,_,· _..;,-;,;.-.,,_' ____ ........... f>, -."-_ ,-_ ••-.. _-~--. -,-.---......... 
• 

- 72-

7.3.1.S. tJerforDl&Dce oIlhe ICCD Detector 

The principal characteristics and operating modes of the ICCD detector in the 

IMAPS instrument are presented in §8.4 of this report. §7.1 discusses in some 

detail the properties of the CCD which was tlnally selected for flight. In sections 

below, we cover some basic attributes of the imaain& section's performance that 

are evident from the tiiaht data. 

7.3.1.8.1. Distribution 01 Event. Charle Amplit.ude. 

A proaram has been written which identifies dlscrete photoevents in areas of 

the image forma,t where the event rates are low enough that there is little confusion from chance overlaps. Events are recognized by scanning for pixels which 

exceed a certain thre3hold (well above the readout noise) in frames where the CCD 

fixed pattern backg,round has been removed, Once this threshold has been passed, 

the total charge c1JUected in a 3X3 box centered on the hot pixel is measured. 

Care is taken to insure that events are not counted twi(':;. when more than one 

pixel exceeds the threshold in a small area. Also, events which might be contaminated by a nearby event are rejected. 

Figure 16 shows the distribution of signal amplitudes which were extracted by 

the event detection program from a collection of frames recorded on the flight. 

The units given on the abscissa are arbitrary; their values correspond to the 

recorded voltages times a scale factor governed by a gain setting of the Colorado 

Video frame storage and digitization device connected to our VAX computer. The 

narrow bar at Relative Amplitude = 256 represents cases where Signals equal to or 

greater than 256 were detected. These probably correspond to exactly superposed 

events or ion events. The tail between amplitudes 200 and 250 also probably came 

from superposed events that were not recognized as doubles by the program. 

C Iiii __ - ___ ... _____ ......;.......... '*_1 _____ :....... ='---:-::=:-- ...... u· • e .?i_~-'-' 
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-,. -' -----.:;;;;; . ....... 

... , ---_. " ... 
• 

'p 

- 74-

If one rejects the extended tail of the distribution at the hiah end by substituting in the dropott at the low end, reftected about the distribution's axis of symmetry, one finds that the averaie pulse amplitude is llB.5 and the rms dispersion 

is 26.1. The peak is well separated from the random noise in the readout, which, 

on the scale given here, corresponds to an m'tS variation of 4.Bl. Ie 

The ratio of the distribution function's lu width to its mean value is of special 

significance. From the numbers given above, this relative dispersion is equal to 

22%. If we accumulate the recorri':!d signal by adding the pixel intensities from 

frame to frame instead of countina events explicitly, the net signal-to-noise ratio 

should be reduced below the one governed solely by the ~tatistical varidtions in 

counls by a ractor of only (1 + 0.222)~ = 1.024. 17 This means that in places where 

the spectrum intensity is so large that the events are inseparable, an analog accumulation will have virtually the same signal-lo-noise ratio as lhe one we would have 

obtained if we were actually able to identify and count lhe events. This is indeed 

fortunate, since on lhis flight the intensities recorded in the upper halves of the 

frames was great enough to require the analog coaddilion of signals. We do, of 

course, lose our ability centroid the events (see §7.3.1.6.2 below) when lhey are 

not explicitly recognized. 

IfIThe noise in the diiitiz~d versiona of the ftiiht frames is Ireater than measured variatioM in 

the anal0i outputs discus.ed in §7.3.1.4. (For inatance, dilitized SNR = 118.5/4.81 = 24.6. while 

the pulses seen on the raw VCR output - ranjline from apprOximately 200 - 250 mV - have a 

minimum SNR = 200/5.5 == 36.) The source of this extra noise is not understood at the moment. 

It may be attributable to either problems with the Colorado Video di,itizer or with the VCR when 

it operates in the freeze frame mode. 

\'7Thi.! assumes, of course, that readout noise is unimportant. A sienificant deter'ioration in 

lilMHo-noise ratio can occur if the mean event rate is low, since the silnel from each event 

must. on the averale, compete with the combined noise from a larie number of other frames 

containina no events in the same location. 
• 

, 

f • 

~} ~.~ 1:'1 

~ ~ i 

- 75-

1.3.1.8.2. J..t.eral Spread of Charle in Each Eyent 

Many events have their charae reaistered in more than one pixel. This is 

caused by a lateral dispersion of the secondary charaes as they miarate toward 

the potential wells which define the pixel boundaries. Only events which are very 

nfl,srly centered on a pixel have practically all of their charae reaistered in one 

pixel. This charae spreadiI1j, if not too larae or small, 'lermits us to achieve a spatial resolvina power over dimensions smaller than the 30~ pixels. Basically, one 

can compare the charae amplitudes in the neighboring pixels and solve for the 

exact position of the primary electron hit. The accuracy of the determination is 

aoverned by only 1) the sianal-to-noise ratio, 2) the size of the spread function and 

3) the predictability (and uniformity) of the spread function's shape. 

Figure 17 shows the distribution of charge amplitudes integrated along pixel 

rows divided by the total charges in the respective events. Since the events hit in 

random locations relative to the row boundaries, an analysis of the function can 

yield the a:1J8rage shape of each event's actual charge distribution in the CCD, 

Except for some smoothing due to noise, the functional form in Figure 17 is very 

nearly the same as one we found much earlier in laboratory experiments using 

CCDs in slow-scan readout. 1e The earlier study indicated that the charge spread 

behaved in one dimension as an exponential with a scale length of 0.135 pixels. 

This size f'!' the distribution is very nearly ideal for attaining an accurate solution 

for the even~ centroid, with a given signal-to-noise ratio. As an example, if the 

readout noise in a single pixel is 1/20 the total charge of an average event, we calculate the rms error in pOSition along one dimension ranges from 0.013 to 0,426 

times the length of a pixel, depending on whether the hit occurred near a pixel 

18When a CCO is read out slowly (for instance. one frame every 10 seconds iJl:!Itead of the 1/15 

second frame period for lMAPS). one can achieve a better signal-ta-noise ratio. However the CCO 

must be cooled. to keep the accumulation of dark current at a reasonable level over the relatively lone integration. time. 

~ "'~' ... , ___ ' tm-..o .... • ____ lliiiiiii _______ ..... +_, • __ ._ ........ _ ~-_-----__ -_-__ ~_ 

. ; ", .. 

__ 

, 
• 

-?8· 

boundary or in the center of a pixel, respectively. 

7.3.1.6.3. General Appearance of. Frame 

Fiaure 18 shows a sinale frame recorded on the ftiahl. The echelle orders are 

barely perceptible as linear aroupinas of event.s st.ret.china horizont.ally. At the 

bottom of the picture is the very briaht imaae of the star formed by one (Jf the 

mirrors described in §e.e. The motion of this spot from frame to frame was used 

to obt.ain the ACS point.ina in!ormation described in §7.3.1.3.2. As mentioned in 

the previous paragraph, the top half of t.he format is very busy with events and will 

need t.o be analyzed by anal0i coaddina of pixel amplitudes. Event. recoanition can 

be accomplished for the much weaker part of the spectrum in the lower half. The 

pict.ure is divided into 16 panels because there were interruptions which occurred 

during the standard TV broadcast line and vertical retrace intervals (see §6.5). 

Figure 19 shows a magnified portion of the panel which contains the broadband star imSie. In this presentation one can clearly see the intensities 

registered for the indiVidual pixels. It is possible to see examples of the charge 

sharing between adjacent pixels responding to a Single event, as discussed in 

§7.3.1.6.2 above. The thin, rectangular blocl< of pixels which appear to be 

anomalous in the upper right portion of the picture do not contain valid image 

information; these pixels were overwritten by the character insertion generator 

which was used to identify the precise time each frame was recorded. Blocks of 

this type can be seen in each of the panels displayed in Figure 18 (the character 

information is scrambled when the broadcast TV image's interlaced lines are rearranged for the IMAPS detector display -- portions of the unscrambled numbers are 

written on the right hand edge of the picture). 

r 

j 

I
• 

C) 

v 

) -

Q... 

l. 

IS 

:t 

... 

~ 

.. 

~ 

... 

(1 

... ..... e 

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t 

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0 

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0 

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.... 

0 

0 

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ORlGtNAL PAGE " 

'" POOR QUALm 

-..... ,.... '" . ... 
ORIGINAL PAGE 

~LACK AND WHITE PHOTOGRAPH 

I 

...... . - • re' .eM_ 
- 80-

1.3. t.e.4. Hale of loa EYeDta 

For 70 frame. which were recorded at an altitude of 270 km (very near apoaee). there appeared to be a total of 10 Ion evenll. The aclual number of evenll 

may be lower than thl. number becaUie .ome of the ,plke. Wllh larae ampHtude. 

""lay have been mulUple (lealUmate) photoelectroN. The 73 hit. m 70 frame. 

tranllalel to an evant rate of 1.9)( 10-4 hill P1X- 1 .-1 . 

1.3.1.7. SeuiU.t&.,oI thelDUrelut.nama&. 

A meuurement of the conUnuum of" Sco between 1141.00 and 1141 20 aClvc 

an averqe count rate of 0.75 count PUc'l frame' Thi. level refers to the intensity 

above a backaround composed of scaltered liaht plus the dUTuse La backaround 

(see 17.3. t.8 below). We elUmaled the true nux from" Sco within thiS wavelenath 

mterval by two methods: 1) The low-resolution (U2) Gbpern.;cw scans of this star 

reported by Snow and JenldN 1G• Ulina an averaae of the counts recorded al the 

two end-pOint wavelenaths and their reported sensitivity of the Gbpern.;cu. spectrometer at the time, and 2) lUE absolute nuxes near 1140A for the V = 6.15 ml1~­

nitude 81V star HD31726.

20. The IUE intensity of 7.5)ClO'lO era cm'2 s'l A'l (= 42.6 

phot cm'2 s'l A '1) was multiplied by a factor of 10°.

4(8. 115' 2.GO) to account for the 

differences in visual maanitudes and 10-0,4(0.03)(8. 1) to correct for the fact that ~ 

Sco has Es . v = 0.06 whUe that for HD31726 is only 0.05. (The factor 8.1 IS the 

averaae value of A11 40A ,viEs, v reported by Savaae and Mathis21 ,) The Cop.rnicus 

and IUE va,ues of 580 and 660 phot cm·2 s'l A'l, respectivEly, were averaaed to 

obtain a nux of 630 phot cm·2 s'l A'l from ff Sea. Sir.ce eaeh 30~ pixel on the CCD 

I'Snow, T. P. and Jenk1na, E. 8. 1m, ~NCIl JoW'fllll ~~, S\ 28;. 

IfJ/UE U&~leC Spectral M£u, [nternaUona! Ultraviolet Explorer (lUE) SASA Neftl.Her no. 22 

(Special Edition). p. 74. 

tlS-va,e, 8. O. and Uathll J. S. 1m. Anftual R~ 01 Artrono",~ aM ~"""",~Iic., 1'7,73. 

• ~ ", 
- 8t -

sub tends 0.00475A at 1140A, we ftnd that the effective area of lMAPS (Le., the product of the ieometrical aperture area, instrument's optical efficiency and the 

detector's quantum efficiency) is 3.76 cm . This value is 2.5 times a lower limit for 

the efficiency we measured at NRL before the ftiaht22, but only 0.61 times the 

ftaure of 6.14 cm2 we calculated for the product of measurl!d and expected 

efficiencies of the components in the instrument (249 cm2 aperture x 0.74 u.nobscured portion of the beam throuih the collimator x 0.83 ratio of the principal 

[zf!ro order] beam to the collimator's total throuahput, includina diffraction spikes 

x 0.25 et!iciency measured for the echelle aralina23 x 0.24 efficiency measured for 

the cross-disperser aralin,,23 x 0.67 quantum efficiency usually obtained at this 

wavelenath for a KBr photocathode on a. solid substrate). 

We suspect that the most probable reason for the actual efficiency being 

somewhat lower than expected was that the photocathode degraded slightly. In 

spitt! of our usini our best efforts to keep the KBr away frem any humidity riuring 

the installation of the photocathode, we th.ink some exposure to slightly humid air 

may have occurred. At one point, the payloed had to be opened to repair a suspicious connection on a plug associated with the det.ector electronics. This field 

repair was done at WSMR long after the photocathode was installed. It is also possible that the refiecli\tilies of the gratings could have deteriorated slightly. 

Finally, it seems to be generally true that astronomers discover that an assembled 

instrument. has an overall efficiency which seems to be noticeably lower than the 

product of the efficiency of the separate components, for reasons they seem never 

22when wo exposed the !MAPS payload to a collimated beam from an arion diacwle :source, we 

found the effective area to be 1.5 cm2. We knew at the time that this measurement was a lower 

limit. for the sensitivity of [MAPS at 1048 and 1061A because 1) The comparison monitor (a calibrated channeltron detector) also relistered La and various emission lines from other impurities in the source, and 2) A silnitl.cant percentllle of the hits which were evident on the [MAPS 

frame were probably double events (they showed Uj) with approximately twice the normal amplitude). 

23See Appendix C: notes recorded by D. Windt (U. Colorado) when he measured the efficiencies of 

our Iratinas. 

-.. 

.j

-•.... ~ .. ," ...... , 

! 

I .. 

- 83 -

7.3.1.8.2. Scattered Liaht from the Grat.in&1 

At lhe bottom of a slrolljly saturaled absorption fealure (N I absorplion al 

1134.415A) we measured a backaround of 0.17 counts piX-I frame-I. Afler sublraclina the contribution from lhe La glow, the scattered lighl nux is 0.15 counls 

pix-I frame-I, which is about 28~ as large as the local conlinuum on eilher side of 

the feature. 

Presently, it is not possible :0 say how much of the contribution of grating 

scatter comes from small angle denections by lhe echelle along its dispersion 

direction, or alternatively, how much of it is derived from much larger angle 

scntler from either the echelle or cross-disperser grating. One way to disentangle 

these two effects may be to learn whether the levels at the bottoms of the slrong 

absorption features are correlated with the immediately adjacent continuum or 

with more global intensities of the spectrum. 

7.3.1.9. Spectral Resolution 

Initially, we were concerned lhat the instrument was slightly r.i.lt of focus 

because the orders were relatively fat - about 2 to 3 pixels wide at any instant. 

However a sharp component in one of tl.e spectr' tl lines seen in preliminary 

slacked data presented in the next section (§7.3.2) seems to show that a. lower 

limit for the resolving power is about 2 pixels (FWHM) , wl:1ich corresponds to 2.5 

km s-l in radial velocity. Si!lce actual absorption features from gases in the interstellar medium may already have some natural broadening due to a dispersion in 

velocities, one could suppose that the instrumental resolution could be even somewhat better. On the other hand, one must. be very careful not to draw premature, 

overly optimistic conclusions by selecting the sharpest looking line from a lar~, ',' 

collection of examples, since noise fluctuations undoubtedly make senne ii. . .. ? 

appear sharper than they really are. A more definitive assessment of the spectral 

. I ,,,-.--.~
- 84-

resolution will need to be made after more frames have been stacked (to reduce 

the effects of noise) and more careful compensations for star motion have been 

implemented. 2f5 

From the fact that the spectrum lines seem to be narrower than the order 

widths, one is left to conclude that the imaae must have had some astiam.atism. 

The imaae points were p~ 'lbably elonaated vertically (perpendicular to t.he echelle 

dispersion direction) but still rather narrow in the horizontal dire<?tion which 

really mattered for hiah spectral resoivina power. The vertical elongation was 

probably a fortunate accident, inasmuch as it reduced the pileup of phot.oevents in 

individual frames. 

7.3.2. General Character of t.be ScienWic Data 

Presently, only a very small fruction of the frames recorded on ftight 27.082UG 

have been reduced. However, from a glimpse of the data on hand at the moment 

we can predict the character of the spectra which will ultimately be produced. 

The entire free spectral range of the echelle was covered by the four discrete 

echelle tilt seltings for which data were recorded sequentially (see pages 69 and 

70). Positions 1 and 2 were near the peak of the echelle blaze. Hence, exposures 

taken at these two tilt angles showest the most intense spectra. Positions 3 and 4 

show much fainter spectra because the angles of incidence and diffraction were 

somewhat removed from the optimum angle. 

215S0 far, corrections for pointina onsets use the instantaneoWi c,~mputaticns for star image centroids of the type shown in F1iure 13. Judiin, from the 6.ne-scala jitter in the plotted pOints. one 

miiht suppose that each determination has a an error of about 1/2 pixel. ThWi. some improvement in the oftset calculation miiht result by replaciJli the scraggly curves with best fitting 

straleht line segments bridgilli between the times when the ACS valves were known to have fired. 

Also. better registration of the stacked eVflnts miiht be achieved after an investiia~ion ot possibit, distortions of the image mappi!li. Some iroups of stacked data with larie excursions of the 

star position seem to she .. a small oftset. alter the norme.l corr'lction procedure which assumes 

that the spectrum Mm es in exactly the same manner as the star image. Finally. better resolution miiht be achieve·:! if we discard frames takp.n when the movement was too fast -- see 

§?3.1.3.2. 

~,' - .")",. ,,,,.r .. _________ -.._._ ........ 

... _______ ",I r.lL-.-. __ ,'_ ,.;. .... _. ___ -.".;~1'< - .. ------- ~ -. . 

. 
. 

• • 

- a5 -

Figure 20 shows an imaie taken at echelle position 2. This picture was created 

by stacking the pixel amplitudes from 390 frames (Le., 26 seconds worth of exposure tim\!) with the position compensaticms to freeze the star image at the bottom 

in one position. (The star image is black in the center due to integer overnows In 

the computer.) Echelle orders at the top of the picture are at about 11 ?OA , while 

extremely faint ones at the very bottom correspond to 958A. Most of the prominent absorption lines are from molt. :-ular hydrogen in the interstellar medium. 

The missing order about 2 inches from the boltom of the picture occur~ near 

1025A, where there is very strona absorption by the L~ line. 

Figure 21 shows the right-hand part of the spectrum extracted along order 

206, which is the 14th order from the top in Figure 20.26 Each dot in the picture 

represents the intensity recorded in a single ceo pixel, hence the separation 

between dots corresponds to a doppler shift of 1.25 km s·t. Assuming the principal 

contribution to the noise is from counting statistics for the photoevents (see the 

discussion in §7.3.1.6.1), the 1u uncertainty in each point should be 0.2 ~'tensity 

units. 

It is interesting to compare the preliminary reduction of IMAPS data taken 

over a brief time interval with scans of the very same spectrum lines recorded for 

1T' Sco by the high-resolution scanner (U 1) in the spectrometer of the Copernicus 

satellite. Figure 22 shows the Copernicus data. Here, the signal-to··noise ratio is 

vastly superior to that shown in Figure 21. ~ut the detail in the profile structure, 

particularly that of the Fe II absorption, is almost entirely lost, owing to the much 

lower resolution. 

In the echelle setting shown here (position number 2), we have approximately 

88 seconds worth of exposure time. Hence, we should ultimately see an _ ... _-

28The plot in Figure 21 was taken Crom a stack of 356 frames. rather than the 390 frames 

represented in Fiiure 20 . 

E! t;'II. 

~· .. ' _________ . ..;-~_-_F-. ..... #1 ___ -" .... a-.- .--.-..... -~~. 

. - .", . -.. 

" 
" " 

• 

ORIGINAL PAGE 

~K AND WHITE PHOTOGRAPH 

- - - - .ft . .. !! M 

" ..::14 

..... 
200 

Hz 1,0 P{2} Hz 1,0 R(3) Fe II 

A = 1096.439 A = 1096.725 A = 1096886 

1- 180 

't 

r-

lt30 ~-

! , ~ .....,> 

.~ 

Vl 

, C 

I Q) 

: I· ~ 

:'t. c 

~ 

'f 140 , ; 

I ,t 

I " 

I~ 

IY 

I 

-\ • 

120 

Figure 21 

100 

20 40 60 80 100 120 140 

( 

CCD Pixel Nunlber 

~--:> ." .,. .~ 
o ;4..-____________ _ 

ti 2 --:-~ \.-~;~U , .. \ I, L 

d { . 

8 

i 

'. --______ 'J; 

7 · ____ ---. 

/ 

" 

I 

'- -! _. , ., - .. j. , 

1-- -- . 

, 

I i i---~ 

I 

8 

• 

. - --'''-, 

-~ I . I r 

. '"" . -

__________ - """.;I ;..-.. _____ .MIo.-_____ ... _ ••• __ 0_ _ ~~~ _ ...... _____ ..-:-_ 

... !,t " 
-!sImprovement In the slgnal-to-nolse ratlo by a factor (88/24)* = 1 9 

for other 

echelle POSlllOns whICh covered the remauung quarlers of the free speclral r"Oile 

we had the followmg lIltegrallon tlmes. Poslllon 1 = 72 s, PosltlOn 3 = 98 S, and 

Posltion 4 = 67 s. 

~r;:... __________ ....:-;;;:.._-__ .... ___ ,..Io---_. ~."~ •.• __ . ___ .. 
, 

[ 

F 

, 

I 

1 

, 

i 

f 

- 90 . 

8. FINANCIAL STATUS OF TIlE PROJECT 

Total Award 

Expendltures 

Pnnceton . 

Subcontract: BASD 

Balance, 6/30/85 ... 

. $ 1.243,281 

. .... $ 211,905 

S 1.031.376· 

$ 1.243,281 

. SO 

• Thls figure represents the full arnol qt of the subcontract WIth Ball Aerospace 

Systems DiVlsion (BASD) and IS subJe<. to adjustment pending recelpl of a final 

invoice reflecting final burden rates through 1983 and final fee payment 

•

L" 

_ ... ____ .. ___ -___ ~ ___ Jl ___ t. _____ ..t._ ___ ._ ... • : ••• _ __.':;h. _ 
, 

- 91 -

9_ KEY PERSONNEL ON TIlE PROJECT 

Princeton University 

Prmclpal Invesllgator 

Engmeers 

Techmclans. 

Student. 

Adrrurllstrallve Officer ... 

Naval Resel\l'Cb Labora.tory 

Co-mvestlgator ...... . 

Ed ward B J enkms 

. John L. Lowrance (techmcal dlrector) 

Paul M. Zucchino (ICCD electrorucs desl~n) 

Donald C Long (ICCD electrorucs deSign) 

Michael Reale (lCCD electronics desl~n) 

.. John Opperman. Jr (vacuum system) 

Wililam G Harter (electrorucs) 

Enc M. Wilcots (data dlgltizmg) 

.Donald B. Hortenbach 

... George R. Carruthers 

Ball Aerospace Systems Division (BASD) 

Engmeers .. Dan W Simmons (program management) 

Donald F. Bradshaw (program mana,;;ement) 

Murk Bottema (opllcal ciesign) 

W Alan Delamere (ICCD image sectIOn desIgn) 

Richard P Wooley (ICCD magnet design) 

Roger A. Misch (Electronics design) 

Gerry A. 'Seigal (Production) 

Arthur A. Olson (optical alignment) 

Georg H Becker (optical alignment) 

Gary P i'~mmerson (optical alignment) 

Techniclans .............................. DavelJ L. Armstrong (electronics) 

Max G. Hedges (electronics) 

John J. Kersbergen (GSE & HVPS) 

David E. Warlick (mechanicaJ) 

Reginal L. Eckhard (machinist) 

• E= __ . _____ . ___ ... ______ .,. __ ~ _____ ._ _____ ..... -_. ___ -_~_~~:~_" __ ~._ 

~ 

~
• 

Selectlon of BASD for the {MAPS Subcontractlna Eff~~' 

rhe attached letter dated June 2, 1978 va •• ent to .ix fira. jud,ed at 

the tl .. to be capable of .upPQrt1nl the payload'. de.i,n, fabrication, and 

fiela operation.. We elected to en",e a co~any prior to the preparation of 

a propo.al to NASA, .0 tnat we could draw upon that co~any'. re.ource. and 

experti.e in defininl the technical a.pect. of the payload de.i,n. The conditionlot our reque.t for propo.al. are clear from the content. of the letter. 

The re.pon.e to our letter va. a. follov.: 

Ball Aaro.pace Sy.tema Divi.ion (BASD) 

Boulder, Colorado 

Perkin-Elmer Corporation 

Norwalk, Connecticut 

Amarican Science and Enaineer1na, Inc. 

Cambridae, Ma •• achu.ett. 

Itek Corporation 

Lexinlton, Ma •• achuaettl 

General F.lectric Company 

Philadelphia, Penn.ylvania 

proposals 

.ubmitted 

indicated that they vere too 

buay to .upport UI 

no re.ponse 

Weltinlhou.e Sy.tema Development Divi.lan 

Baltimore, Maryland 

The three propoaal. we received vere revi~ved by four p~ople at P~inceton, 

warkinl independently. To have a uniform .Yltem for judaina the propo.al., ve 

of Tltt U"U I 

aareed to Icore each of the 7 itema (excludina item 4) lilted on paae ~ and 

cter~ ·!e a grand total .core by .WllllLina vith the followina veiaht facton: 

~ . 

item number 

1 

2 

3 

5 

6 

7 

." ., 

• 

we1aht 

1.0 

0.7 

1.0 

1.0 

0.4 

1.0 
• 

Each ut the four 8valuationa gave the greatest grand score to BASD. in view 

of the fact that these independent decisions were unanimous, we felt there 

was no need to hold a formal conference to discuss the issues before we 

announced a final decision. 

. ". 

(

£

. ' 
~-, 

r 

~ ~ 

.. 1 

June 2. 1978 

Li.t of Companie. to whom tn. contrart letter. for (AO) were .ent. 

Mr. G. J. Chodll 

Technic.l Tower, 5th floor 

Ball Brothen 

Box 1062 

Boulder, Colorado 80306 

Mr. John Rua.o 

Plllrkin-Elmar Corporation 

50 D.nbury Road 

Norwalk, Connecticut 06856 

Mr. D.vid Ellenbogen 

Vic. Pre.ident of Marketing 

American Science and Engineering, Inc. 

955 ~ •• chu •• tt. Avenue 

Cambridge, M •••• ~hUlett. 02139 

Mr. Fred Gilligan 

Itek Corpor.tion 

10 Magui re Road 

Lexington, Malsachusetts 02173 

Mr. Tom Sherlock 

Room U-3225 

Ganeral Electric Company 

~.O. Box 8555 

I~iladelphi., P •• 19101 

Mr. Fred Schaff 

Westinghouse, Systems Deve~opment Division 

P.O. Box 746 - Mail Stop 431 

Baltimore, Maryland 21203 

S 

~ ~ *' ..:'1'l""~'''' ~ ..... •• 

--------------------**-----..... ... .----
r· 

I 

• 

DIPAITMIST O' UTIO'HYSICAL SCIINCIS 

Pt.YTOS HALL 

'1IHClTON, NIW 111llY 08540 

J \ tne 2, 1978 

In responae to a forthcoming Announcement of Opportunity (AO) for Spacelab payloads, Princeton University Observatory is planning to submit a proposal 

to fly an echelle Ipectrograph for observing high-resolution spectra ot bright 

stars in the range 900-1200 R. We anticipate that this AO will be for Space lab 

Flight No.3 and an indefinite number of subsequent fli~lts; the review and 

acceptence of proposals will be on a periodic basis. While the release date 

for the AO is uncertain, we expect that it could be issued as early as June 15, 

and we anticipate that our proposal would be due about 90 days later, if we were 

to meet the deadline for an initial selection of flight pr~grama. We expect that 

NASA ·.,ill announl':'~ its choice of winning proposals from the first review in early 

1979. 

The proposed instrument will consist of a mechanical collimator, objective 

echelle grating, cross dispersion grating, focusing mirror, and a windowless ~ 

image tube for data acquisition. To achieve a desi~ed resolut.ion of A/~A • 10' 

we plan to use an echelle with a high blaze angle -one possibility is a Bausch 

and Lomb 8" x 16" grating with 79 lines mm-l and a 630 blaze. The Jr.echanical 

collimator will exclude light from undesil'ed stars and the Lyman-a diffuse background. We currently have a mechanical c1Jllimator which we can salvage from a 

similar echelle spectrograph paylcl1d whi(',h was flown on an Aerobee rocket. 

In addition to proposing for SpaceJ.ab, Princeton is considering the possibility of flying this payload on small sounding rockets. Th..lse flights would 

occur well before ~he Spacelab mission and would yield valuable scientific data, 

as well as give us a head-start in the manufacture and space qualification of 

the hardware. For the imme~!ate future, however, our energy will primarily be 

directed toward responding to the Spacelab AD. With minor modifications, this 

same proposal could be submitted for consideration as c potential rocket instrument. 

Princeton wishes to assemble a proposal team, including an industrial firm, 

pr.!.or to or nearly coincident with the release of the AO. The selected industrial 

fil~ would assist Princeton in preparation of the proposal to NASA, supporting 

their proposal effort with their own funds. It would be Princeton's intention, 

.... 

~ ______ ~_. ___ ....;...,. _______ 4/' ____..io.&...__ . ____ ••• __ 
• 

2 

in the event of the contract award, to subcontract to the r~u~ industrial firm 

for the majo' effort involved in providing the instrument for flight. It is 

envisaged that Princeton will supply the imaging detector, since Princeton has 

already developed such devir_s for other programs. 

Since the plan outlined above excluded a competitive selection of a contractor to Princeton after award of the NASA contract, we plan to select the 

industrilll team member via proposals submitted by interested industrial firms. 

Thes~ prcposals are not expected to be a facsimile of the proposal to NASA. 

They will not address the technical details in any depth but will be relatively 

brief proposals covering the following areas: 

1. Name and experience of Program Manager and other key people, 

with their areas of technical responsib1l1 j'Y, who would be 

assigned to the effort. 

2. Proposed organization of the program and total magnitude 

of effort in man-months that the corporation is willing to 

devote to the preparation of a joint proposal. 

3. Expected continuity of personnel from the design and propcJal 

writing phase to th~ execution of the subcontract in the event 

NASA approves tile building of a flight instrument. 

4. Name, title and address of the corporate officer responsible 

for this work. 

5. Brief description of other relevant programs recently carried 

out by this division, including cost and schedule performance. 

5. Facilities available for fabrication and test of equipment. 

7. Present overhead rate, G&A, and fees for recent comparable 

programs. 

Firms which responded to Prin~eton's request in late 1976 to propose for 

supporting the Wide-Field Camera on the Space Telescope need not cover Items 5, 

6, and 7 above if they wish not to supplement or modify this information. It 

is Princeton's intention to include in the subcontract for the work a condition 

that any change in Program Manager will b.: mad#' \.lilly with Princeton's consent. 

While we are not planning to hold a formal pre-proposal briefing on the 

programmatic or technical details of the effort, "7e welcome inquiries or visi ts 

from those firms which have a serious interest in supporting our program. Any 

commitments made in your proposal will be contingent, of course, on whether 

or not NASA actually issues the AO; while such distribution is expected, it is 

not assured. Proposals must be received no later than June 20, 1978 to be 

considered for selection. 

continued 

& ____ - _0 •• __ -
• 

-, ?-

3 

Inquiries should be directed to John L. Lowrance. Telephone: (609) 452-3805 

(or 3803). 

Very truly yours, 

Edward B. Jenkins 

EBJ /.JIIl 

.-...o..IIt.... ______ ••• __ -
 

 

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