L 

SPACE INFRARED TELESCOPE FACILITY 

WIDE FIELD AND DIFFRACTION LIMITED ARRAY CAMERA 

(IRAC) 

P 

Grant NAG2-316 

Final Report 

For the period 1 September 1984 through 31 October 1987 

Principal Investigator 

Dr. Giovanni G. Fazio 

April 1988 

Prepared for 

National Aeronautics and Space Administration 

Ames Research Center 

Moffett Field, California 94035 

Smi t hsonian Institution 

Astrophysical Observatory 

Cambridge, Massachusetts 02138 

The Smithsonian Astrophysical Observatory 

is a member of the 

Harvard-Smithsonian Center for Astrophysics 

The NASA Technical Officer for this grant is Mr. Ruben Ramos, Mail Code 244-15, 

National Aeronautics and Space Administration - Ames Research Center, Moffett Field, 

California 94035 
ABSTRACT 

This is the final report under Grant NAG2-316 “A Wide-Field and Diffraction-Limited Array Camera (IRAC),” Dr. Giovanni Fazio, Principal Investigator. 

The camera is capable of two-dimensional photometry in either a wide-field (5 

arcminute field of view) or diffraction-limited (1.25 arcminute field of view) mode 

over the wavelength range from 2 to 30pm with a possible extension to 120pm. 

Under this grant the Smithsonian Astrophysical Observatory (SAO), in conjunction 

with the Ames Research Center (ARC), Goddard Space Flight Center (GSFC), the 

Uiniversity of Arizona (UA), and the University of Rochester (UR), with support 

from Santa Barbara Research Center (SBRC), developed a new “low-doped” indium 

antimonide detector for 1.8 - S.Opm, tested and optimized detectors for the entire 

l.rB-30pm range, developed and tested beamsplitters for the 1.8 -30pm range, and 

performed tradeoff studies of the camera’s optical system. 

This report presents data on the performance of InSb, Si:In, Si:Ga, and Si:Sb 

array detectors bump-bonded to a multiplexed CMOS readout chip of the sourcefolllower type (SBRC’s CRC-228) at SIRTF operating backgrounds (< 1 x lo8 

ph/cmf/sec) and temperatures (4 - 12OK). Some results at higher temperatures are 

also presented for comparison to SIRTF temperature results. 

sented on the performance of IRAC beamsplitters at room temperature at both Oo 

and 45O angle of incidence and on the performance of the all-reflecting optical 

system baselined for the camera. 

Data are also prei 
. 

a 

CONTENTS 

Page 

. 

SECTION 1.0 

2.0 

3.0 

3.1 

3.2 

3.3 

3.4 

3.5 

3.6 

4.0 

4.1 

4.2 

4.3 

5.0 

5.1 

5.2 

5.3 

5.4 

5.5 

6.0 

6.1 

6.2 

6.3 

INTRODUCTION .................. INSTRUMENT DESCRIPTION ............. BAND I ARRAY DETECTOR RESEARCH ........ Introduction ................... Summary of Test Results .............. Standard-Doped InSb Array FPA17 ......... Low-Doped InSb Array SCAOOl ........... Low-Doped InSb Array SCAOO2 ........... Low-Doped InSb Array RH2 ............ Low-Doped InSb Array RH4 ............ Hughes-Carlsbad Si:In Array ............ Low-Doped InSb Detector Capacitance Measurements 

QE Falloff Across Array and With Temperature 

.... ..... Effect of Clocking Changes on Array Performance .... Spectral Response of InSb .............. BAND I1 ARRAY DETECTOR RESEARCH ........ Overview of Test Results ............... Transient Response .......... Power Dissipation ........... BAND I11 ARRAY DETECTOR RESEARCH . . Overview of Test Results ........ Measurement of Detector Capacitance ... Linearity and Well Capacity ....... Effect of Guard Ring Potential on Uniformity 

Power Dissipation ........... BEAMSPLITTER RESEARCH ....... Introduction ............. Theoretical Performance ......... 

...... 

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...... Experimental Results ................ Beamsplitter 1 ................. 

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CONTENTS (Cont.) 

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6.4 

6.5 

7.0 

7.1 

7.2 

7.3 

7.4 

7.5 

Beamsplitter 2 ................. Observjng Efficiency with Beamsplitters ......... Conclusions ................... OPTICAL ANALYSIS .................. Introduction ................... IRAC Optical Specification Status ........... Refractive Work ................. Catadioptric Work ................. Reflective Work .................. Review of Work Done .............. Implementation of Magnification Change ....... Physical Layout Consideration ........... Analytical Results ................ 8 . 0 CONCLUSIONS ................... 8.1 Array Detector Performance ............. 8.2 Beamspli t ters ................... 8.3 Optical Design .................. 9.0 REFERENCES ................... 

APPENDIX A SBRC Low-Doped InSb Array RH2 Test Data Histograms 

APPENDIX B SBRC Low-Doped InSb Array RH4 Test Data Histograms 

APPENDIX C University of Arizona Test Facilities 

FIGURES 

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FIGURE 1-1 58 x 62 DRO Infrared Detector Array ......... 3 

1-2 Schematic Diagram of Multiplexer Unit Cell ....... 3 

2-1 Infrared Array Camera Functional Block Diagram ..... 7 

2 -2 Infrared Array Camera Optical Schematic ........ 

for FPA17 .................... 12 

3-2 FPA17 Variation of Quantum Efficiency ......... 13 

8 

3-1 Histograms of Quantum Efficiency. Dark Current and Read Noise 

iii 
CONTENTS (Cont.) . 

FIGURE 3-3 

3-4 

3-5 

3-6 

3-7 

3-8 4 

3-9 

3-10 

i 

3-11 

3-12 

3-13 

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6-2 

6-3 

Dark Current of FPA 17 .............. Noise 09 FPA 17 at 3.3pm and 51°K ......... Histograms of SCAOOl Performance at 4.7OK ....... Dark Current in SCAOOl as a Function of Integration Time . Gray Scale Representation of Quantum Efficiency of SCAOOl . Difference of Two Consecutive Readouts in SCAOOl as a Function 

of Time .................... Histograms of SCAOO2 Performance at 4.5OK ....... Gray Scale Comparison of Quantum Efficiencies of SCAOOl and 

SCAOO2 ..................... Gray Scale Representation of Dark Current/Noise Data ... Low-Doped InSb Detector RH2 Quantum Efficiency. NEP. Read 

Noise. and Dark Current as a Function of Detector Bias at lOoK 

and 15OK .................... RH2 Gray Scale Signal Response and Dark Current .... Low-Doped InSb Detector RH4 Quantum Efficiency. NEP. Read 

Noise. and Dark Current as a Function of Detector Bias at 

10°K ....................... RH4 Gray Scale Signal Response and Dark Current ..... Variation in Quantum Efficiency - Photoconductive Gain Product in 

Si:In Array at 13OK ................. Relative Spectral Response of an Anti-Reflection Coated InSb Array 

at 770K ..................... Si:Ga Responsivity. Noise Equivalent Power vs . Bias Voltage . Si:Sb and Si:Ga Dark Current vs . Temperature ....... Response of One Si:Ga Array Pixel to Step Change in Incident 

Flux ....................... Si:Sb 58 x 62 Responsivity. Noise Equivalent Power vs . Temperature ..................... Si:Sb Noise vs . Integration Time ............ Si:Sb Capacitance Determination ............ Si:Ga Measurement of Sense Node Capacitance ....... Linearity. Well Capacity of Si:Sb and Si:Ga Arrays ..... Unprocessed Array Output for Si:Sb Array (left) and Si:Ga Array 

Array Uniformity vs . VGRNG ............. Si:Sb Array Power Dissipation vs . VGGUC ........ Calculated Performance for Beamsplitter 1 ........ 

Beamsplitter 1 Calculated Performance with Thin Film Absorption 

Losses Included .................. 

(right) ..................... 

Calculated Performance for Beamsplitter 2 ........ 

Page 

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CONTENTS (Cont.) . 

Page 

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FIGURE 6-4 

6-5 

6-6 

6-7 

6-8 

6-9 

6-10 

6-11 

6- 12 

7-1 

7-2 

TABLE 3-1 

3-2 

4-1 

5-1 

6-1 

6-2 

7-1 

7-2 

7-3 

7-4 

7-5 

8-1 

8-2 

Beamsplitter 1 Room Temperature Transmission, Oo Angle of 

Incidenpe . . . . . . . . . . . . . . . . . . . . Beamsplitter 1 Room Temperature Transmission, 45O Angle of 

Incidence . . . . . . . . . . . . . , . . . . . . Beamsplitter 1 Room Temperature Transmission, Oo Angle of 

Incidence. UA and OCLI Test Data Superimposed. . . . . Beamsplitter 1 Room Temperature Transmission, Oo Angle of 

Incidence. IRAC Broadband Band I1 Filters Superimposed. . . Beamsplitter 2 Calculated Performance with Thin Film Absorption 

Losses Included . . . . . . . . . . . . . . . . . . OCLI Beamsplitter 2 Run No. 3: Oo Angle of Incidence . . . OCLI Beamsplitter 2 Run No. 1: 45O Angle of Incidence . . OCLI Beamsplitter 2 Run No. 3: Oo Angle of Incidence . . . OCLI Beamsplitter 2 Run Nos. 1 & 2: Oo Angle of Incidence 

Optical Schematic of Reflective IRAC System Design . . . . Worst Case Radial Energy Distribution Plots for Band I . . 

TABLES 

Band I Array Test Data and Design Goals . . . . . . . Relative Spectral Response of an Anti-Reflection Coated InSb Array 

at 77OK. . . . . . . . . . . . . . . . . . . . . Band I1 Array Test Data and Design Goals . . . . . . . Band I11 Array Test Data and Design Goals . . . . . . . IRAC Beamsplitter Specifications for 1.0-30.Opm . . . . . Beamsplitter Efficiency for Given Filter Band Pass . . . . . Geometric Analysis - 80% Encircled Energy Diameter (Microns) . . . . . . . . . . . . . . . . . . . . . 80% EESDs for Band I (Wide Field) . . . . . . . . . . 80% EESDs for Band I (Narrow Field) . . . . . . . . . Worst Case Diffraction 80% EESD (Band 11) . . . . . . . Worst Case Diffraction 80% EESD (Band 111) . . . . . . . IRAC Array Development Status . . . . . . . . . . . Status of Refractive and Reflective Design Studies . . . . . 

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V 
SPACE INFRARED TELESCOPE FACILITY 

WIDE FIELD AND DIFFRACTION LIMITED INFRARED ARRAY CAMERA 

(IRAC) 

Smithsonian Astrophysical Observatory 

Ames Research Center 

University of Arizona 

University of Rochester 

t 

Goddard Space Flight Center I 

FINAL REPORT 

for the period 

1 September 1984 through 31 October 1987 

Grant NAG2-316 

1 .O INTRODUCTION 

This final report is submitted in accordance with the requirements of Grant 

NAG2-316 for technology development in support of a “Wide-Field and DiffractionLimited Array Camera” for SIRTF. 

photometry in either a wide-field (5 arcminute field of view) or diffraction-limited 

(1.25 arcminute field of view) mode in three bands over the wavelength range from 

1.8 to 30pm with the possible addition of a fourth band from 60 to 120pm. 

The camera is capable of two-dimensional 

The camera will be applied to a number of fundamental scientific problems, 

m.ost of which cannot be addressed in any other way. Two of the most imkxtant are galaxy formation during the early stages of the Universe and the search 

far the “missing mass” in galactic halos. These would provide an important advance in our understanding of the formation and dynamics of galaxies and of the 

total mass density in the Universe. Other scientific goals include determining the 

nature of the energy sources in active galactic nuclei and measuring the properties 

OF clusters of galaxies, understanding star formation processes and evolution, both 

in our galaxy and nearby galaxies, observing the late stages of stellar evolution, 

determining the abundance distribution of heavy elements in our galaxy and in 

nearby galaxies, dust around stars, and studying the distribution of dust and gases 

in comets. The camera will also be applied to follow-up studies on IRAS sources 

and be used to assess the optical performance and operational characteristics of the 

SlIRTF telescope and spacecraft platform. 

* 

The goals of this research were to 1) Develop an improved InSb array 

detector for Band I (1.8-5.3pm); 2) Evaluate the performance of existing array 
Page 2 

detectors under SIRTF backgrounds (5 1 x lo8 ph/cm2/sec) and temperatures 

(4O-12OK) in all three bands; 

bands; by the adjustment of parameters such as bias points, operating temperatures 

and clocking methods; 

particularly read noise and dark current, could be met; 

the performance of infrared beamsplitters for the IRAC instrument; 

pare a preliminary optical analysis for the camera. 

3) Optimize the operation of the arrays in all three 

4) Demonstrate that SIRTF performance specifications, 

5) Develop and evaluate 

and 6) PreFor Band I, two detector materials, Si:In and InSb, were evaluated. The 

%:In detector array was a standard Hughes-Carlsbad product. 

were of both standard (1.1 x 1015/cm3) and reduced (1.7 x 1014/cm3) doping 

concentration. Standard-doped detectors were available from a standard SBRC 

product line; 

searclh program. “Low-doped” InSb was evaluated because it promised a factor of 

two reduction in detector capacitance and, hence, a factor of two improvement in 

read noise over normal InSb. While standard InSb has proven disappointing when 

operaked below 50°, the low-doped InSb program has yielded arrays with promising 

performance. At 8OK exceptionally low (aA level) dark currents were demonstrated 

and the anticipated factor of two improvement in read noise achieved. 

The InSb detectors 

the “low-doped” detectors were fabricated by SBRC under this reFor Band I1 (5.3-14pm) and Band I11 (14-30pm), Si:Ga and Si:Sb detectors 

Excellent results, results very close to and in some cases exceeding initial 

were chosen for evaluation respectively. 

gramis. 

SIRT’F/IRAC performance requirements, have been achieved with these arrays, 

although concern remains about their behavior in the radiation environment of 

space. 

These were developed under earlier proThe readout system for the detector arrays was the SBRC CRC-228 direct 

readout (DRO) n-channel multiplexer, a 58 x 62 pixel readout array with 76pm 

source-follower unit cells which allow random access to any of the 3,596 pixels in 

the array. One of these hybridized arrays is shown in Figure 1-1. 

A simplified diagram of the multiplexer circuit is shown in Figure 1-2. 

Seven row and seven column address lines plus other control signals allow random 

access to the 1,798 detector pairs (“unit cells”) within the array. 

read out in parallel as two interlaced 58 x 31 subarrays, each with its own readout source-follower circuit. Sampling can be performed with or without the application of a software-controllable reset pulse, allowing either destructive or nondestructive readout (NDRO). 

The array is 

‘ 

The Infrared Array Camera must reimage the infrared sky in three bands 

simultaneously. 

energy in the first band (1.8-5.3pm, Beamsplitter 1) then in the second band 

(5.3 -14.0pm, Beamsplitter 2) while passing energy in the third band (14-30pm). 

Both beamsplitters require wide-band multilayer coatings and presented a technological challenge in the development of the instrument. As a result of this research, 

A pair of wideband dichroic beamsplitters operating in series reflect 
. 

Figure 1-1. 58 x 62 DRO Infrared Detector Array 

VDOUC VRSTUC VDDUC 

mol - 0- .- 

0- IC 

IC 

- -DET SUB RA07- ROW ADDRESS 

c 

I I VOOUC VSSUC VSSUC VGGUC Y Ylllll 

COLUMN 

ADDRESS 

DECODER 

MRST AVRST 

*8:1- 

Figure 1-2. Schematic Diagram of Multiplexer Unit Cell 

Page 3 
Page 4 

the technology for these beamsplitters can now be said to be well in hand. 

Beamsplitter goals for the IRAC instrument were developed in conjunction 

with the Optical Coating Laboratory, Inc. (OCLI), Santa Rosa, California. OCLI 

subsequently fabricLted and tested one set of multilayer coated beamsplitters on 

cadmium telluride (CdTe) substrates which were further tested at the University of 

Arizona. 

Optical constraints imposed on the camera by IRAC scientific requirements 

include the necessity of independent wide-field (5.0 arcmin FOV) and diffractionlimited (1.25 arcmin FOV) modes and a resolution in both modes better than 

75pm, with the system operating at less than loo kelvin. Although no new technology will be required to meet the optical or thermal constraints, the camera will 

test the state of the art. Accordingly, optical analyses were undertaken as a part 

of this research to identify areas of concern and to select the optimum optical 

configuration. 

Optical design requirements and tradeoff parameters were established jointly 

by ,SAO, UA and GSFC. 

considered. 

because of the freedom from chromatic aberration it provides. 

Refractive-reflective and all-reflective optical designs were 

An all-reflective design was selected for the baseline optical system 

Development of hardware and software to test the arrays at SIRTF backgrounds and temperatures in support of this research constituted an important part 

of this effort. Although equipment details differ, the facilities at ARC, GSFC and 

UR are similar in purpose and design. 

operating at LN2 and LHe temperatures to cool the array under test and flood it 

with IR through a selection of filters. 

with the computer systems to control the array and read out data. 

implement a wide range of signal processing modes and clocking schemes, proc3e 

for NDRO capability and direct addressing of pixel pairs, provide direct control of 

the test conditions and record, process and display image data in a variety of 

formats. 

Each consists of a dewar capable of 

Read and drive circuits interface the array 

These systems 

The Smithsonian Astrophysical Observatory (SAO), with Dr. Giovanni Fazio 

as Principal Investigator, held overall responsibility for performance under the 

Grant, provided scientific direction, program management and support to SIRTF 

project activities, including the SIRTF Science Working Group (SWG). 

Pipher at the University of Rochester (UR) tested and evaluated the InSb and 

Si:In Band I arrays. Mr. Gerald Lamb (Co-I) of Goddard Space Flight Center 

(GSiFC) tested the Si:Ga Band I1 array initially and Dr. Craig McCreight (Co-I) at 

Amles Research Center (ARC) repeated and extended the tests later. Dr. McCreight 

also tested and evaluated the Si:Sb array for Band 111. 

(Cot-I) of the University of Arizona (UA) tested and evaluated the OCLI beamsplitten; and collaborated with GSFC in developing and evaluating optical designs for 

the camera. Optical design work at GSFC was carried out by Mr. Peter Maymon. 

Dr. Judith 

Dr. William Hoffmann 
Page 5 

Dr. Daniel Gezari (Co-I) was Instrument Scientist during this period. 

SBRC was the responsibility of, ,in order, Dr. B. T. Yang, Mr. Donald Campbell, 

Ms. Mary Hewitt and Mr. Robert Feitt. 

Work at 

This report i&mmarizes the results of this research. Detailed results are 

available in the references. 
Page 6 

2.0 INSTRUMENT DESCRIPTION 

The Infrared Array Camera images the sky on solid-state array detectors in 

three bands which &re nominally, Band I: 

and Band 111: 14-30pm. 

Beanisplitters provide simultaneous viewing in each of these bands. 

operates at two magnifications one of which provides diffraction-limited imaging and 

the other optimized wide-field imaging. 

the appropriate optical elements to change the plate scale at the detectors and filter 

wheels provide a selection of infrared filters in each band. 

1.8-5.3pm, Band 11: 5.3 - 14pm, 

A fourth band of 60-120pm .is under consideration. 

The camera 

A magnification change mechanism moves 

The major elements of the IRAC are the Optical/Mechanical Subsystem, the 

Thermal Subsystem, the Calibration Subsystem, the Focal Plane Array Detector 

(FPAD) Subsystem, the Instrument Control and Data Processor (ICDP) Subsystem, 

and the Power Subsystem (Figure 2-1).* 

The Optical/Mechanical Subsystem consists of all optical elements, the optical 

bench, and related mechanisms. 

found in Figure 2-2. 

bench which is bolted directly to the SIRTF Multiple Instrument Chamber (MIC) 

reference mounting interface. IRAC is assigned one 120° sector of the MIC and 

will utilize a single position on the SIRTF tertiary mirror. 

A schematic of the optical subsystem may be 

All of these elements are mounted on a common optical 

The Thermal Subsystem includes all temperature control surfaces and materials, heaters and related control electronics. 

The Calibration Subsystem contains all infrared reference sources, their power 

supplies and their control circuits. 

detelctor flat fielding and radiometric calibration. 

Internal calibrators will be provided for arrzy 

The FPAD Subsystem includes the detector arrays for each wavelength band; 

associated power supplies; 

and the cryogenic, mechanical, thermal and electrical interfaces with the instrument. 

In order to assume optimal thermal control of the detector arrays, they will be 

mounted directly to the independently-accessible 2OK and 7OK stations in the MIC. 

Independent thermal control can be applied directly to each detector assembly if 

necessary. Arrays in all three bands are baselined to use 75pm pixels. Band I 

will have 128 x 128 elements; Bands I1 and I11 64 x 64 elements. Signal buffers 

will be required to drive the signal cable between the cold array and warm electronics circuits. These buffers will be attached to the 7°K station. 

InSb detectors are baselined in Band I; 

drive electronics, read electronics, and A/D converters; 

Low-doped 

Si:Ga and Si:Sb in Bands I1 and I11 

*Fig,ures and text in this section are current to the date this report was prepared. 
Page 7 

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respectively. 

The ICDP Subsystem consists of all instrument control and mode change 

logic, command and telemetry formatting circuits and spacecraft interfaces, data 

accunmlators, data processors, housekeeping monitors, and FPAD control circuits and 

interfaces. The ICDP will provide control signals to the FPAD in the form of 

read enable and address commands, gain settings, and regulated power supply 

voltages and take data in the form of digital signal level words. 

ing, data formatting and mode control will be handled by the ICDP. 

interface is baselined between the FPAD Subsystem and the ICDP Subsystem. 

All data processA digital 

The Power Subsystem conditions spacecraft power and supplies filtered power 

to the rest of the instrument. 
Page 10 

3.0 BAND I ARRAY DETECTOR RESEARCH 

3.1 , Introduction 

I 

Santa Barbara Research Center fabricated and tested both standard-doped and 

low-doped InSb arrays and Hughes-Carlsbad provided a %:In array, all of which 

were mbsequently tested at the University of Rochester. 

SBRC and UR test results and discusses work on detector capacitance measurement, 

QE variation across the arrays and evaluation of alternate clocking schemes carried 

out at UR as a part of the test program. 

This section summarizes 

One standard-doped InSb array, two low-doped InSb arrays and one Si:In 

SBRC tested a total of four lowThese arrays have a useful wavelength 

array were tested and characterized by UR. 

doped InSb arrays. 

were procured under separate funding. 

range of 1.8-5.3pm and a peak response at 3.5pm. 

range of temperatures to evaluate the performance of the arrays parametrically in 

a search for the optimum operating conditions. Summary data from tests run at 

both SBRC and UR are presented in Table 3-1. 

The standard-doped InSb array and one low-doped InSb array 

Tests were performed over a 

3.2 Summary of Test Results 

3.2.1 Standard-Doped InSb Array FPA17 

The standard-doped InSb array (FPA17) was tested at 5loK, 47OK, 4loK, 

36OK, and 8OK to explore array performance as a function of temperature1. 

ating at 4OoK, FPA17 had a read noise of 900 e- and a dark current of 100 a!k. 

Quantum efficiency varied widely across the array from 0.4 to 33% in a manner 

suggestive of the unexplained dramatic low temperature falloff in QE seen in some 

standaxd- and low-doped InSb arrays and described by SBRC2 and others. At 8OK, 

this array showed the impressive drop in dark current expected at the lower operating temperature. The dark current was measured to be 5 aA at 8OK; read 

noise was 600 e-. Unfortunately QE also dropped dramatically to 0.2-2% across 

the array. 

OperHistograms of quantum efficiency, dark current and read noise for the device 

are shtown in Figure 3-1. 

be 1pF; 

The zero bias capacitance for the device is assumed to 

at 150mV bias, the capacitance is taken to be 0.77pF. 

Figure 3-2 shows the variation in quantum efficiency across the array at 

three temperatures, 51°K, 41°K, and 8OK, and clearly demonstrates the extreme 

falloff seen as the temperature is lowered. 
Page 11 I 

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(a) Histogram of quantum efficiency 

measurements for FPA 17. T = 51K; 

looking at a 295K blackbody, through 

a 3.3 pm filter. VBIAS = 150 mV; 

VGATE = -2.5 V. Pixels at left are 

dead. Quantum efficiency in %. 

(b) 

8K on FPA 17. 

include VGATE = -2.5 V, VBIAS = 

150 mV, 36 microsee/pixe.l pair read 

rate. Dark current in electrons/sec. 

Pixels at left are dead. 

Histogram of dark current, at 

Other parameters 

(c) Histogram of two subtracted 

CDS frames, in electrons, on FPA 17. 

= 150 mV. 

HWHM/ fi. 

T = 8K, VGATE = -2.5 V; VBIAS 

RMS Noise is given by 

ORIGINAL: PAGE 18 

OF POOR QufiIm 

12 

Figure 3-1. Histograms of Quantum Efficiency, Dark Current and Read Noise for FPAl7 
Page 13 

T)EIGNAC PAGE IS 

OF POOR QUALITY 

Figure 3 - 2. FPA17 Variation of Quantum Efficiency 
Page 14 

The dark current variation across FPA17 at 4OK is shown in Figure 3-3. 

The upper left-hand image is the result of a 240-second exposure to a cold dark 

slide (CDS) at 150mV bias. 

the array to the same dark side for one-twelfth of a second. 

yields at dark current display with instrumental effects removed (lower left). 

that the dark current is higher by a factor of three along the lower left and 

lower right edges relative to the center. 

further randomization as shown in the lower right-hand image. 

The upper right-hand image is the result of exposing 

Differencing the two 

Note 

At 50mV bias, the dark current shows 

Noise on FPA17 is shown in Figure 3-4. The left-hand figure is a typical 

frame taken at 3.3pm and 51OK. 

results in the image of the upper right indicating uniform noise properties across 

the array. 

Subtraction of two consecutive 3.3pm exposures 
Figure 3-3. Dark Current of FPA 17 

Page 15 

I 

DFJGINAL PAGE IS 

‘0s POOR QUALITY I 
Page 16 

ORIGINAC PAGE IS 

OF POOR QUALITY * 

Figure 3-4. Noise on FPA 17 at 3.3pm and 51OK 
Page 17 

3.2.2 Low-Doped InSb Array SCAOOl 

SCAOO1, the first of the two low-doped InSb arrays studied3 was operated at 

31OK with a measured read noise of 600 e-, a dark current of 30 aA, across the 

central portion of the array, and QE’s of 25-68% across the3 array. 

tograms taken in this array are shown in Figure 3-5. All values are based on a 

calculated detector capacitance of 0.63pF, the value appropriate for operation above 

freeze-out which occurs between 20°- 3OOK. 

0.55pF. (See Section 3.3, below.) At 8OK, the QE dropped to 2-40% across the 

array. 

(Figure: 3-6), a characteristic also noted by SBRC.2 

not observed by SBRC, was also seen in the upper left portion of the array. The 

hot spot in the lower right portion of the array is due to the relatively high 

power output MOSFETs located near that area. When VDD is decreased from 5V 

to 1V the resultant dark current in that corner decreases. This effect was ultimately determined to be caused by a dewar wiring error, not by a malfunctioning 

array. 

The hisBelow freeze-out, the proper value is 

The dark current was highest on the lower right portion of the array 

A secondary “hot spot,” one 

Also seen are small undulations on top of and parallel to the gross variation 

which are referred to as “tree rings.” 

carrier lifetime or mobility problem in the high-doped InSb, and the “tree rings” 

due to variations in the doping. 

The gross variations may be due to a 

Read noise distribution across SCAOOl may be seen in Figure 3-8. Note 

how readouts later in time improve, an effect which is not yet fully understood. 
Pagc 18 

ORLSINAL PAGE ps 

OF POOR QUALITY 

........ rrr... .. # 

x 

$? 

4R 

8 m. 

srg 

X,OL I 

>m *> 

co 

7. 

81 

YY 

a 

I.., .............. 

v ............... - - 

4 

4 t 

- ?: 

4 

8 
ORIGEUL PAGE rs 

OF POOR QUALITX 

Page 19 

F’igure 3-6. Dark Current in SCAOOl as a Function of Integration Time 

Figure 3-7. Gray Scale Representation of Quantum Efficiency of SCAOOl 
Page 20 

7 min/ 96 readouts later 

Figure 3-8. Difference of Two Consecutive Readouts in SCAOOl as a Function of Time 
Page 21 

3.2.3 Low-Doped InSb Array SCAOO2 

SCAOO2 has, to date, yielded better results than any of the other Band I 

arrays tested by UR? 

aA. 

assuming a detector capacitance of 0.55pF. Read noise was 200-300 e- at both 

6-7OIK' and lOoK and essentially the same from 83 ms to 500 s integration time. 

Dark current at 6OK is an impressively low 1.3 e-/s or 0.2 

The QE at lOoK is an acceptable 40% in the upper left portion of the array 

Histograms of quantum efficiency and dark current in SCAOO2 at 7-10°K 

may be found in Figure 3-9. 

of quantum efficiency of SCAOOl and SCAOO2. Note that while the tree ring 

structure is evident in both arrays, SCAOO2 does not show the extreme falloff 

across the array evidenced by SCAOO1. 

Figure 3-10 compares the gray scale representation 

The gray scale representation of dark current and noise reveals a good 

degree of uniformity (Figure 3-11) across the array with no hot spots evident. 
500 

M)o I 1 

scan Data: 7-NG-67 Plxala Conaldarad: 9500 

vgata: -i%o mv Rata: 4x9 ua 

Fllanmma: 7K2.m 

Vblaa: 2% mv 

chip tag: fanout tam: 6.0 K NorkSurf tag: 4.6 K 

Intapratlon limo: 30000/60tha aac. - 

I SCA U2 

Scan Data: 6-~ffi-87 Plxala Conaldarao 3597 

Fanout tam: 10.3 K NOrkSurf tag: 4.6 K 

Fllmnans: TST1OK.mo 

Vblas: 250 mv Vgato: -1500 mv 

ChlP tam: 11.9 K Integration Time: W60tha aac. 

Rata: 4x9 us 

(a) Quantum efficiency of SCA002 at 10K and 

3.3bm 

Nolam and Dark Currant 

260 

240 * 

200 . 

Plxala 160 * 

120 . I 0 

SCA 02 1 

20 so 2, lo 

(b) Dark charge in EA002 after 500 ~ee. The 

spread in the measurements corresponds to ap 

proximately 300 e‘ RMS noise. 

Page 22 

Figure 3-9. Histograms of SCAOO2 Performance at 4.5OK 
ORIGINAL PAGE IS 

OF POOR QUALITY 

Page 23 

Figure 3-10. Gray Scale Comparison of Quantum Efficiencies of SCAOOl and SCAoO2 I 

Figure 3 - 11. Gray Scale Representation of Dark Current/Noise Data 
Page 24 

3.2.4 Low-Doped InSb Array RH2 

RH2, also a low-doped InSb array, was tested by SBRC2 prior to delivery. 

At 10°K the device showed a dark current of 7.33 aA (burst mode) and a read 

noise !of 345 e- based on an assumed detector capacitance of 0.65pF. (SBRC and 

UR estimates of detector capacitance differ by approximately O.lpF, see paragraph 

3.3 below.) QE was 47.3% with a sigma of 25% across the array. 

and R.H4, described in the next section, are both AR-coated. Responsivity uniformity 'was 37% across the array. 

NEP was measured as 4.61 x lo-'* W and the dynamic range was 12.2 x lo5 e-. 

At 15OK, this array showed the expected significant increases in dark current (to 

44.5 a.A) and read noise (to 480 e-). 

(29.3% 1 u variation) while dynamic range and NEP were little changed from their 

lOoK values. Figure 3-12 shows the variation with detector bias of quantum efficiency, NEP, read 

noise and dark current of RH2. 

dark current are shown in Figure 3-13. 

the lower left-hand corner, two column defects of unknown origin and a diagonal 

defect due, possibly, to a crack in the InSb detector board. 

This array, 

Operability greater than 97% was achieved. 

QE also increased significantly to 77.3% 

Histograms supporting these figures are given in Appendix A. 

Gray scale photographs of signal response and 

Signal response shows a "cool" spot in 
Page 25 

- 40 

10" K 

100 I I 50 

75 - 

50 - 

25 - 

OLIO 

0.00 0.10 0.20 0.30 

DET BIAS (V) 

1 49-18 1 

2971 8 i 1; 19-18 3 

1 50-1 9 1 

Oe+O It 0.30 oe+O 

0.00 0.10 0.20 

Dm BIAS (V) 

-0.10 0.00 0.10 0.20 0.30 

1-1 

DET BIAS 

19-17 

0.00 0:lO 0.20 0.30 

DET BIAS (V) 

QCE 

+ SIGMA 

Q NEP 

+ SIGMA 

Q NOISE 

+ SIGMA 

Q IDARK 

+ SIGMA 

AMPERES 

15" K 

100 1 T 

75 - 

50 - 

25 - 

- 30 

- 20 

- 10 

010 

0.00 0.10 0.20 0.30 

DET BIAS (V) 

50-1 8 29-18 -. 

49-1 8 - *\; 19-1 8 

30-18- 

U 

- 19-18 9 

20-1 8 - 3 

- 5e-19 le-18- 

oe+o 

0.000 0.100 0.200 0.300 

DET BIAS (V) 

150 

1 00 ! 

0 

0.00 0.10 0.20 0.30 

DET BIAS (v) 

&-17 

3e-17 

29-17 

19-17 

Oe+O 

0.00 0.10 0.20 0.30 

DET BIAS (v) 

QE 

NEP 

NR 

ID 

Figure 3-12. Low-Doped InSb Detector RH2 Quantum Efficiency, NEP, Read Noise, 

and Dark Current as a Function of Detector Bias at 10" K and 15'K 
RH2 Signal Response Scale: *2a from the Mean 

RH2 Dark Current Scale: *2a from the Mean 

Figure 3-13. RH2 Gray Scale Signal Response and Dark Current 
Page 27 

3.2.5 Low-Doped InSb Array RH4 

RH4, also low-doped InSb, showed very similar performance to RH2 except 

that the QE was significantly higher in tests by SBRC2: 61.3% (12.6% 1 u variation) and the responsivity uniformity much better at 21%. Histograms for RH4 

are given in Appendix B and the data plotted as a function of detector bias at 

10K in Figure 3-14. 

read :noise is 331 e- and NEP is 3.68 x at the same point. Quantum 

efficiencies, NEP, and read noise are remarkably constant as bias is increased from 

0.04V to 0.25V. Dark current increases by about 50% over the same range. 

Dark current (burst mode) is a low 9.34 aA at 0.12V bias, 

Figure 3-15 shows gray scale plots of signal response and dark current. 

The excellent uniformity seen in the histograms in Appendix B is also clearly 

shown in the signal response plot in this figure, the only anomaly being a diagonal 

line ad low response across the face of the array which is attributed to a crack in 

the detector board. 

uniform in the same figure. 

Dark current gray scale response is seen to be similarly 

Of the low-doped InSb arrays tested under this program, RH4 initially 

appeared, by far, to be the best array. 

tests, developed a "salt-and-pepper" appearance due to the development of high dark 

current in about 500 pixels, the cause of which is unknown. RH2 shows no such 

behavior, but is far less uniform overall and has a column defect causing the loss 

of almost two entire columns. 

Unfortunately, the device, in subsequent 
~ 

Page 28 

2e-18 - 

10" K 

AMPERES 

- 2e-18 

. 

8 *E 

6 SIGMA 

DET BIAS (V) 

4e-18 1 e-1 8 0 

0 

U # + SIGMA 

5e-19 QNEP 

2e-19 

Oe+O 

0.00 0.10 0.20 0.30 

DET BIAS (V) 

1 00 - 01 

QE 

NEP 

READ 

NOISE 

Figure 3-14. Low-Doped InSb Detector Rk4 Quantum Efficiency, NEP, Read Noise, 

and Dark Current as a Function of Detector Bias at 10' K 
RH4 Dark Current Scale: *2u fran the M#n 

Figure 3-15 RH4 Gray Scale Signal Response and Dark Current 
Page 30 

3.2.16 Hughes-Carlsbad Si:In Array 

The Si:In array from Hughes-Carlsbad has been tested by UR3 over the 

range from 5OoK to 8OK and with biases from -55.9V to +4V. 

bias of +2V, measurements yielded a dark current of 15 aA, a read noise of 150 

e-, and a quantum efficiency-photoconductive gain product (QExg) of 1.06%. 

8OK and a bias of -55.9V, far closer to the proper operating bias of about lOOV, 

the measurements yielded a dark current of 0.5 aA, a read noise of 60 e-, and a 

QERg of 5.1%. While the low dark current and read noise of the Si:In array are 

very attractive, the low QE and high operating bias are not and further work 

must be done at higher biases to fully evaluate the performance of this array in 

the SIRTF application. 

At 30°K with a 

At 

The variation in the quantum efficiency -photoconductive gain product for the 

Si:In array at 13OK is shown in the gray scale representation in Figure 3-16. 
Page 31 

OR".! PAGE IS 

OF POOR QUALITY 

Figure 3 - 16;. Variation in Quantum Efficiency -Photoconductive Gain Product in %:In Array at 

13OK. 
Page 32 

3.3 Low-Doped InSb Detector Capacitance Measurements 

UR developed estimates for the various components which comprise the total 

detector capacitance in InSb:2 zero bias diode capacitance, sense node (DRO gate 

node) capacitance and gate overlap capacitance, based on a combination of calculated and measured data. The diode capacitance varies inversely with the square 

root of applied bias; the sense node capacitance and gate overlap capacitance are 

fixed capacitances. The estimates in the following table are for diodes in a CRC228 array at 8°K (Le., below freeze-out). 

University of Rochester Zero-Bias InSb Detector Capacitance Estimates 

Zero-Bias Diode Capacitance 0.42f0.03pF 

Sense Node Capacitance 0.07pF 

Gate Overlap Capacitance 0.06pF - 

TOTAL 0.55pF 

This value (0.55pF) was used in calculating the performance values presented 

by UR in Table 3-1 and compares well with the value of 0.57pF measured by 

SBRC: using the “noise-squared vs. signal” method on SCAOO2 at 8.1°K. Other 

measurements, including one direct measurement of the capacitance of 44 diodes in 

paralllel on a fanout board, yielded values in the range 0.68pF to 0.75pF. Taking 

the known variables into consideration, UR estimates the uncertainty in their calculation of total zero bias detector capacitance to be about O.lpF. 

3.4 QE Falloff Across Array and With Temperature 

All of the InSb arrays show variations in QE across the array and QE 

fallofl with decreasing temperature. 

such as the 2-40% variation seen in SCAOOl at 8°K. 

variation at 31°K (25-68%). 

the standard-doped array, 0.4-33% at 31°K and 0.25-2% at 8°K (Figure 3-2); 

least variations in SCAOO2, 28-40% at 10°K (Figure 3-10). 

showed variations in QE of 29.3% and 12.6%, respectively in measurements by 

SBRC!2. 

Sometimes, these variations are quite extreme, 

This array showed far less 

The worst variations were seen by UR in FPA17, 

the 

Arrays RH2 and RH4 

None of these variations are fully explained. SBRC2 has postulated that 
Page 33 

the variations across SCAOOl and SCAOO2 at a given temperature are due to detector thickness variations across the' array, and blackbody source misalignment. 

Consideration of these QE variations at UR have led to the conclusion that 

the measured detector thickness variations across SCAOOl can only partially accomt 

for the observed QE variations and, based on a measurement technique that precludes misalignment of the blackbody, misalignment can explain none of it. UR 

concludes that the observed QE variations must be due to contamination of the 

InSb3. Further work on this question will be done in the follow-on IRAC 

Defiriition Phase Contract. 

3.5 Effect of Clocking Changes on Array Performance 

Nine different clocking schemes were tried at UR to search for the method 

Changes 

that would yield the lowest read noise within IRAC power dissipation limits? 

of the nine were variations on the double-correlated sampling technique. 

in pulse widths and variations in the use of the enable and reset signals were 

tried; 

shorted). 

All 

all gave essentially the same read noise (250 e- operating with the detectors 

3.6 Spectral Response of InSb 

The spectral response of an AR-coated low-doped InSb array of identic?' 

design to those discussed above was tested at 77OK. 

in Table 3-2 and plotted in Figure 3-17. 

2.35,pm is that the response is down 19% at this wavelength compared to its peak 

at 3.5pm. Between 2 and 5pm, the shape of the response curve is largely determined by the AR-coating. The cutoff 

wavelength decreases slightly with temperature. 

The test data are presen,zd 

Of importance to QE measurements at 

Beyond 5pm, the detector cutoff dominates. 
Page 34 

- pm Resp E.2- Resp Clm- Resp 

2.0 0.73 3.4 1 .oo 4.8 0.88 

2.1 0.74 3.5 1 .oo 4.9 0.87 

2.2 0.76 3.6 0.98 5.0 0.87 

2.3 0.8 1 3.7 0.98 5.1 0.85 

2.4 0.86 3.8 0.96 5.2 0.85 

2.5 0.88 3.9 0.96 5.3 0.81 

2.6 0.92 4.0 0.96 5.4 0.78 

2.7 0.92 4.1 0.96 5.5 0.76 

2.8 0.95 4.2 0.95 5.6 0.43 

3.0 0.96 4.4 0.92 5.8 0.03 

2.9 0.94 4.3 0.93 5.7 0.12 

3.1 0.94 4.5 0.91 5.9 0.00 

3.2 0.96 4.6 0.90 6.0 0.00 

3.3 0.96 4.7 0.89 

Table 3-2. Relative Spectral Response of an Anti-Reflection Coated InSb Array at 77OK 

W 

cn 

5 

a 

cn 

W 

K 

W 

L 

a 

G 

W 

MICRONS 

- RELATIVE 

RESPONSE 

Figure 3-:17. Relative Spectral Response of an Anti-Reflection Coated InSb Array at 77OK. 
Page 35 

4.0 BAND I1 ARRAY DETECTOR RESEARCH 

4.1 Overview of Test Results 

Results with the Si:Ga array for Band I1 are sufficiently encouraging that, if 

necessary, an existing array could be flown and come very close to meeting present 

SIRTF performance requirements. Based on earlier GSFC tests4 and recent tests on 

the Band I11 arrays at 

performed were 43V bias and a temperature of 8OK. 

useful wavelength range of approximately 4-18pm with peak response at 15pm. 

Test (conditions have not yet been fully optimized for this array. 

the baseline conditions established for the tests 

The Si:Ga array has a 

Under baseline conditions, responsivity is approximately 5.1 A/ W and systemlimited readout noise is about 190 pV (100 e-) (Figure 4-1). Typical noise equivalent power (NEP) measurements are 2.0 x lo-'' W/root-Hz for a 0.205 s integration at the same temperature and bias. Signal-to-noise continues to improve up to 

the highest bias voltage available in the current test setup (43V). The temperature 

dependence at lower bias (1OV) has been measured at 0.2 A/W at 5OK and 0.5 

A/W at ll°K where the dark current becomes unacceptably high. Dark current 

as a function of temperature is displayed in Figure 4-2 where the slope corresponding to Sb impurity generation-recombination is included for comparison. 

summarizes these results. 

Table 4-1 

Dark current, though higher than that observed in the InSb and Si:Sb arrays, is still excellent at 30 aA at 8OK. 

8% (11 ,/mean) over a contiguous 58 x 59 region of the array. 

capacitance was measured to be 0.07pF, slightly higher than that of the Si:Sb 

arrays. 

The uniformity of this array is excellent: 

Sense node 

4.2 Transient Response 

Initial studies of the transient response of the Si:Ga array (see Figure 4-3) 

have shown some of the irregular behavior often exhibited by bulk photoconductors 

under low background conditions. 

does not settle to its final value until 20-40 s after application of the step input, 

depending upon background level. 

In response to a step input of IR, the array 

4.3 Power Dissipation 

Power dissipation of the array is less than 1 mW under the stated operating 

conditions with VGGUC at 0.6V. 
Page 36 

Array Designator: "Goddard Array" - 

Type: Si:Ga 

Tested by: Goddard Ames 

Test Temperature (OK) 5.4 

Detector Capacitance (pF) 0.1. 

Quantum Efficiency (%) --- 

Operability (%) -_- 

Res po ns iv i t y (AMI) 4.7 

Responsivity Uniformity (%) --_ 

NEP (WaWRoot-Hz) @ Tir0.2 s 

Well Size (e-) --- 

AR Coated No 

All values means unless otherwise indicated 

'Assumed 

Dark Current (aA) @ 8 K 50 

Read Noise (e-) 255 

--- 

6 - 10 

30 

100 

0.07 

>99.5 

5.1 

8 

2.OE-17 

>5.OE+05 

No 

--- 

~ ~ ~~~ 

Table 4-1. Band I1 Array Test Data and Design Goals 
Page 37 

)r 

> 

v) 

c 

0 

Q 

v) 

.- 

.- 

d 

Figure 

5 

T = 8 K, background P 2.885 ph/s 

Q Responsivity 

4 

3 

2 

1 

I I I 1 

0 10 20 30 40 

Bias (V) 

$ 

2. 

a 

w 

z 

4 - 1. Si:Ga Responsivity, Noise Equivalent Power vs. Bias Voltage 

d5 

10l6 

3 

lo 3 

rill8 

c 

c 

2 

* 

n 

-17 L 

m 

100 

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 

1lT (1/K) 

Figure 4-2. Si:Sb and Si:Ga Dark Current vs. Temperature. 
Page 38 

8’ 3 

10 

8 

- 

(II 

c 

cn 

.- 04 

2 

0 

0 

= 8.7 x 10 phis 

6 $ = 5.8 x 10 phk 

6 $ ~3.2~ 10 phis 

5 10 15 

Time (s) 

20 

Figure 4-3. Response of One SiGa Array Pixel to Step Change in Incident Flux. 
Page 39 

5.0 BAND I11 ARRAY DETECTOR RESEARCH 

5.1 Overview of Test Results 

As with the Band I1 Si:Ga array, the Si:Sb arrays evaluated for Band I11 

showed excellent performance, performance which virtually meets SIRTF requirements. 

31pm with peak response at 27pm. 

The Si:Sb arrays are useful over the wavelength range from about 15 to 

Optimum operating conditions, in terms of signal-to-noise ratio, were found 

to be at a temperature of 8OK and an applied bias of 2V5j6. 

tions, peak-wavelength responsivity is 3.8 A/W, average dark current is 2.1 aA (13 

e-/s), and the (system-limited) readout noise is about 220 pV rms (100 e-) for a 

0.205 s integration. Figure 5-1 shows responsivity and NEP as a function of temperature. Noise was studied as a function of 

integration time (Figure 5-2) and shows only a factor of 2 increase at 200 s over 

the noise at 0.2 s. 

W/root - Hz. 

Under these condiTable 5-1 summarizes these results; 

A typical NEP for this integration time is 2.7 x 1O-l' 

Dark current levels in these devices can be very small (about 0.3 aA at 

5OK), and integrations of 30 minutes and longer have been achieved with the array 

blanked off without approaching well capacity. As temperature increases above 8OK 

dark current increases rapidly, reaching 175 aA by ll°K. 

5.2 Measurement of Detector Capacitance 

Measurement of the input capacitance of the sense node was made by 

monitoring the current on the reset line (VRSTUC) as a function of signal and 

resulted in a value of 0.061pF. 

through the current vs. signal voltage characteristic in the detector substrate line 

(DETSUB) and through a noise-squared vs. signal plot were also made and reasonable agreement obtained (Figure 5-3). The resulting value is less than the O.lpF 

value predicted by Hughes, although this was based on their experience at higher 

temperatures. 

Corroborating measurement of the capacitance 

Capacitance measurements were also made on the Si:Ga array (Figure 5-4) 

whidi yielded a value of 0.069pF. 
Page 40 

Array Designator: SBRC 1 -- IRD -- 

Type: Si:Sb 

Tested by: Ames 

Test Temperature (OK) 

Read Noise (e-) 100 

Operability (%) >99.5 

Responsivity Uniformity (%) --_ 

5 - 12 

Dark Current (aA) 2.07 

Detector Capacitance (pF) 0.06 

Quantum Efficiency (%) 

Responsivity (A/W) 3.8 

NEP (Watts/Root-Hz) @ Ti= 0.2 s 2.70E-17 

Well Size (e-) 2.OE+05 

AR Coated No 

All values means unless otherwise indicated 

--- 

Table 5-1. Band I11 Array Test Data and Design Goals 
Page 41 

I 

v, 

cu 

cu 

0 

0 

7 
Page 42 

5.3 :Linearity and Well Capacity 

Measurement of the well capacity of Si:Sb and Si:Ga arrays was made by 

plotting the variation in the output signal as integration time was increased (Figure 5-5). Assuming a responsive QE of 0.25 and a photoconductive gain of 0.5, 

the first significant departure from linearity is seen at an integrated flux of about 

1.4 x lo6 photons which corresponds to a well size of 2 x lo5 e-. 

also shows the results of measurements made on the Si:Ga array which showed a 

larger well capacity of >5 x lo5 e-. 

up to twice the integrated flux before hard saturation is reached. 

Figure 5-5 

Note that the array could be operated with 

5.4 Effect of Guard Ring Potential on Uniformity 

Bright edge effects are displayed by these arrays under certain conditions. 

By setting the guard ring potential to its optimum value, 

The guard ring potential (VGRNG or VGATE) was found to be effective in controlling these effects. 

the excellent uniformity figures achieved (8% in Band I1 and 15% in Band 111) are 

realized (Figures 5-6 and 5-7). 

5.5 Power Dissipation 

Array power dissipation is primarily determined by the voltage applied to . the output FET’s (VDD) and the unit cell FET’s (VGGUC) (Figure 5-8). 

operating the arrays at low VGGUC, at 0.6V, power dissipation in both the Si:G? 

and Si:Sb arrays can be held below 1 mW without degrading performance over 

that obtained at higher values of VGGUC. 

By 
ORiGiTMAI: PAGE IS 

OF POOR QUALIT% 

Pa@ 43 

t A'. 1 .oo 

o.80 t 

4 Si:Gaarray 

0 Si:Sbanay 

0.00 o*20t 

0 2 4 6 8 

Generated Charge (1 OS electrons) 

Figure 5-5. Linearity, Well Capacity of SiSb and SiSa Arrays 

SBRC Si:Sb array SkGa array 

Figure 5-6. Unprocessed Array Output for Si:Sb Array (left) and Si:Ga Array lright) 
Page 44 

s 

E 

Y 

& 

3 

a 

0 

0: 0 0.5 1 .o 1.5 2.0 

VGRNG (V) 

Figure 5-7. Array Uniformity vs. VGRNG 

30 

20 

10 

0 

0.0 0.5 1 .o 1.5 2.0 

VGGUC (V) 

Figure 5 -8. Si:Sb Array Power Dissipation vs. VGGUC 
Page 45 

6.0 :BEAMSPLITTER RESEARCH 

6.1 ' Introduction 

The Infrared Array Camera utilizes two dichroic beamsplitters for the separation of the incident infrared energy into three wavelength ranges. 

ance of these elements is a key determinant of the overall performance of the 

camera. 

The performA pair of beamsplitters for the 28-30pm range was designed and fabricated 

by the Optical Coating Laboratory, Inc. (OCLI) to the IRAC specifications shown in 

Table 6-1. 

The purpose of this effort' was to design appropriate coatings for beamsplitters 1 and 2, followed by trial deposition of the coatings on cadmium telluride 

(CdTe) substrate material. Measurements of the transmission in the wavelength 

bands of interest would then allow a realistic performance level to be set for 

these elements. 

Beamsplitter Wavelength Range Design Goal I 

1 

2 

1 -5.Opm R 2. 90% average 

5.2 -30.Opm T 2 55% average 

5.0 - 18pm R 1 90% average 

18.5 - 30.0pm T 2 55% average ~ ~~~ 

Table 6 - 1. IRAC Beamsplitter Specifications for 1.0 - 30.Opm 

6.2 Theoretical Performance 

The calculated performance of the coating design for beamsplitters 1 and 2 

are shown in Figures 6-1 and 6-2. 

the design goal specifications; 

absorption in the thin film structure. 

perfor.mance will be lower than projected, especially in the transmission regions due 

to unmodelled interactions between the layers, random errors in the coating deposiIn both cases, the performance levels exceed 

however, the model does not include losses due to 

Past experience dictates that the realized 
45.0 

.DEG 

1.000 5.000 

WAVELENGTH 

Page 46 

Figure 6 - 1. Calculated Performance for Beamsplitter 1 
z. 

w 0 

I- o W 

-1. 

IL 

W a 

n2 

+ o? 

1 SCAN #4 

BEMSPLITTER #2 

W 0- 

sun I5 

BENSPLITTER 12 

2 

45.0 

DEG 

18.00 30.00 

WAVELENGTH 

, 18 WAVELENGTH 

Qooo 45 0 

DE G 50 

47 

L 

Figure 6 - 2. Calculated Performance for Beamsplitter 2 
Page 48 

tion, and the fact that accurate absorption coefficients for thin film materials at 

the operation temperature are not known. 

The broad wavelength ranges (from 1.Opm to 5.0pm for beamsplitter 1 and 

from 5.2pm to 18.0pm for beamsplitter 2) over which reflectance is required resiilt 

in reasonably complex designs. 

6.3 Experimental Results 

Both beamsplitter designs were calibrated in the OCLI coating chamber and 

Following the deposition an analysis was made 

Since OCLI does not 

then deposited on CdTe substrate. 

of the spectral and environmental performance that resulted. 

have the capability to measure reflectance at cold temperatures, the performance 

cold has been inferred from the ambient temperature data. Future tests at the 

University of Arizona will verify these cryogenic temperature figures. 

temperature the absorption of the thin films is reduced, and therefore the reflectance at this temperature is expected to be equal to or greater than the performance at ambient temperature. This section includes test data taken by both OCLI 

and the University of Arizona? 

Report on this work.7 

At cold 

OCLI test data plots may be found in their Final 

6.3.1 Beamsplitter 1 

The spectral characteristics of beamsplitter 1 were measured at both ambient 

temperature and 15K in accordance with the parameters listed in the table bekw 

and appended to the OCLI Final Report.' 

- Scan Range Type Angle Instrument Temperature 

1 1.0-2.5pm R 45O DK-2 Ambient 

2 2.5 - 30pm R 45O PE-983 Ambient 

3 2.5 - 30pm T 00 PE-983 Ambient 

4 2.5 -30pm T 00 PE-983 15 Kelvin 

5 2.5 - 30pm T 450 PE-983 Ambient 

From this spectral information, the actual performance of the dichroic at 45O 

is estimated by OCLI to be: 
Page 49 

- Wa,velength Range Design Goal Measured Performance 

1-5.0pm R 2 90% Average 84.3% (Ambient) 

5.2-30pm T >_ 55% Average 65.6% (Ambient) 

76.7% (Calculqted at 15K*) 

The previously identified technical concern regarding transmission from 5.2 to 

30.0prnt is alleviated by this dichroic design. 

made in the 1.0 to 5.0pm reflectance region. 

data7 indicates the loss in reflectance is due to absorption dips between the reflectance multilayer stack centers. This has been verified by measurement. 

transmission level in this region is virtually zero. Absorption coefficients for the 

thin film materials were estimated and included in the computer design. 

perforrnance is plotted in Figure 6-3 and indicates behavior similar to the actual 

performance. 

However, further effort needs to be 

Examination of the spectral test 

The 

This 

For manufacture of acceptable flight dichroics, a more complex coating 

design may be required. 

sorption dips may be minimized. 

rial will lower the in-band transmission. 

moving some of the reflectance stacks to the rear surface of the beamsplitter in 

order ,to decouple the interference effects which emphasize these dips. 

nately this would complicate the optical design of the camera. 

By adding thin film structures to the design these abHowever, the increase in coating thickness mateAn alternate approach to consider is 

UnfortuAs part of this study, the coating was also tested for environmental performance at OCLI with the following results: 

Test Result 

24-Hr. Humidity per MIL-C-675A Pass 

Slow Tape per MIL-M- 13 508B Pass 

50 Rub Cheesecloth per MIL-M-13508B Pass 

Subsequently, beamsplitter 1 was retested at the University of Arizona. 

Figures 6-4 and 6-5 give the OCLI beamsplitter 1 transmission from 5 to 

35pm at Oo and 45O incident angle respectively a@ measured by UA. Figure 6-6 

shows the Oo measurements with the OCLI test measurement data superimposed 

*Cold temperature transmission at 45O is calculated as follows: 

45O T at 155 Oo T at 15K 

45O T at Amb Oo T at Amb r 
Page 50 

0 il 

0oa/o~/86 IO. 22.45 

BEAMSPLITTER #1 

45.0 

OEG 

5.000 

WAVELENGTH 

Figure 6-3. Beamsplitter 1 Calculated Performance with Thin Film Absorption Losses 

Included 

as a dashed line. There is excellent agreement between the curves. 

Figure 6-7 shows beamsplitter 1 transmission with IRAC Band I1 broadband 

filters superimposed as hatched columns. 

percent transmission of the individual filters. 

The height of the columns represents the 
Page 51 

SIRTF-IRAC OCLI Beamsplitter No. 1 RUN No.5 

Room temperature 0 DCG. 10-21-86 

-. 

5 10 15 90 25 30 35 

Wavelength (microns) 

Figure 6-4. Beamsplitter 1 

Room Temperature Transmission, Oo Angle of 

1n.cidence 

SIRTF-IRPC OCLl Beamsplitter No. 1 RUN No.5 

Room temperature 0 DEG. 10-21-86 

"1 

5 10 15 20 25 30 is 

Wavelength (microns) 

Figure 6-6. Beamsplitter 1 

Room Temperature Transmission, Oo Angle of 

Incidence. UA (solid) and 

(ELI (dashed) Test Data 

Superimposed. 

SIRTF-IRAC OCLl Beamsplitter No.1 RUN No.6 

Room temperature 45 DEG. 10-21-86 

Irnm 

5 10 15 20 15 30 

Wavelength (mlcronr) 

Figure 6-5. Beamsplitter 1 

Room Temperature Transmission, 45O Angle of 

Incidence 

i 

SIRTF-IRAC OCLl Beamsplitter No. 1 RUN N0.5 

Room temperature 0 DEG. 10-21-86 

lrn

io 

Wavelength (microns) 

Figure 6-7. Beamsplitter 1 

Room Temperature Transmission, 0" Angle of 

Incidence. IRAC Broadband 

Band I1 Filters Superimposed. 
Page 52 

6.3.2 Beamsplitter 2 

The spectral performance of beamsplitter 2 was measured in a manner 

analogous to beamsplitter 1, and is summarized in the table below and in the 

OCLI Filial Report.' 

Scan Range - Type Angle Instrument Temperature 

1 2.5 -30.Opm R 450 PE-983 Ambient 

2 2.5 - 30.Opm T 00 PE-983 15 Kelvin 

3 2.5 - 30.0hm T 00 PE-983 Ambient 

4 2.5 - 30pm T 450 PE-983 Ambient 

From this information, the actual performance of the dichroic at 45O can be 

estimated as follows: 

Wavelength Range Design Goal Measured Performance 

5.0- 18.Opm R 2 90% Average 88.4% (Ambient) 

18.5 -30.Opm T 2 55% Average 58.8% (Ambient) 

62.7% (Calculated at 15K*) 

As in the case of beamsplitter 1, the lower than desired reflectance from 5.0 

to 18.0p:m is due to absorption losses in the thin film structure. 

been modelled in Figure 6-8. 

the desig,n to correct the observed deficiencies is riskier than for beamsplitter 1 

since the physical thickness of the layers is greater at these long wavelengths. 

Thus the! effect of such changes on transmission needs to be considered thoroughly 

and there is the additional risk that the stress buildup in this thicker coating could 

lead to adhesion failures. 

This effect has 

The effect of adding additional coating material to 

Beamsplitter 

results: 

2 was evaluated for environmental durability with the following 

*Cold te:mperature transmission at 45O is calculated as follows: 

45O T at 15K Oo T at 15K - 

45O T at Amb Oo 'T at Amb 
Page 53 

0 

0 

2 

0 

A0 0 

5.000 18.50 WAMLENOTH 

Figure 6-8. Beamsplitter 2 Calculated Performance with Thin Film Absorption Losses 

Included 
Page 54 

- Test Result 

24:-Hr. Humidity per MIL-C-675A Pass 

Slow Tape per MIL-M-13508B Pass 

50 Rub Cheesecloth per MIL-M-13508B Pass 

Corroboration of these results at the University of Arizona produced the 

data displayed in Figures 6-9, 6-10, and 6-11. 

Figures 6-9 and 6-10 give the transmission of OCLI beamsplitter 2 transmission from 10 to 35pm at 00 and 45O incident angle respectively. Figure 6-11 

shows the 00 measurements with the OCLI measurement superimposed with a 

dashed line. 

wavelength shift. 

spectrophotometer wavelength scale against a reference absorption cell. 

There is also very good agreement with the exception of a slight 

Before making additional measurements, UA will recalibrate the 

Figure 6-12 shows the product of the transmission of beamsplitters 1 and 2 

with two Band I11 broadband filters superimposed as hatched columns. It is clear 

that the transmission through both beamsplitters between 25 and 30pm is undesirably low. 
Page 55 

SIRTF-IRAC OCLl Beamsplitter No2 Run No.3 

Room temperature O'DEG. 10-20-86 

lWW 

10 15 20 25 30 35 40 

Waveltngfh (microns) 

Figure 6-9. OCLI Beamsplitter 2 Run No. 3: Oo Angle of Incidence 

SIRTF-IRAC OCLl Beamsplitter No2 Run No. 1 

Room temperature 45 DEG. 10-20-86 

80 

Wavelength (microns) 

Figure 6-10. OCLI Beamsplitter 2 Run No. 1: 45O Angle of Incidence 
Page 56 

10 15 20 s a 40 

, 

' SIRTF-IRAC OCLl Beamsplitter No2 Run N0.3 

Room temperature 0 DEG. 10-20-86 

80 

- 

i SIRTF-IRAC OCLl Beamsplitte'k No,l & 2 

- 

n 

- 

E g 40 

e 

t20 

0 

10 15 20 25 30 40 

Wavelength (microns) 

Figure 6-12. OCLI Beamsplitter 2 Run Nos. 1 &- 2; 00 Angle sf Incidence 
Page 57 

6.4 Observing Efficiency with Beamsplitters 

8 

The purpose of the beamsplitters is to permit simultaneous observation of 

1) certain positional 

the Eiame focal plane in the three IRAC bands. 

simultaneous versus sequential imaging in the three bands: 

registration in the three bands; 

reduced instrument mode changes and spacecraft maneuvering; 

cient use of observing time. 

the transmissions and reflection losses of the beamsplitter, the beamsplitter approach does indeed make more efficient use of observing time. 

There are four advantages to 

2) fewer instrument moving mechanisms; 3) 

and 4) more effiIn this section the question of whether or not, given 

Given: 

€1, f2, €3 are efficiencies of each band with the beamsplitters. 

Ti, 'T2, T3 are the required integration times with sequential (no 

beamsplitters) observations. 

71, 'r2, 73 are the required times for switching beams and 

stabilizing (no beamsplitters). 

n1, n2, n3 are 1 for detector readout br sky background noise 

2 for dark current noise 

For Ti, the integration time to reach a required signal-to-noise ratio without 

the beamsplitters, and T, the time to reach a required signal-to-noise ratio with 

the beamsplitters, we find: 

Detector readout noise limited: T = Ti/Ei 

Background noise limited: T = Tj/€i 

Dark current noise limited: T = Ti/€+ 

In general: 

Then the ratio of 

observations is: 

observing time with the beamsplitters to the time for sequential 

maximum of (3, 3, 

T(Beamsp1itters) - t1n1 e2n2 €3113 

T(Sequentia1) T, + T2 + T3 + r1 + r2 + 5 

Table 6-2 gives the beamsplitter efficiencies as measured by UA and OCLI in 

various filter bands. 
Page 58 

Band Filter -- 

1 3 

8 

11 

2 12 

15 

16 

18 

22 

3 23 

26 

- x 

1-3 

3-4 

4-5.3 

5.1-6.9 

6.5-8.8 

8.5-11.2 

10.7-14.5 

13.8-18.6 

17.6-23.8 

22.8-30 

No. 1 

.84 (R) 

.84 (R) 

.84 (R) 

.69 (T) 

.81 (T) 

.82 (T) 

.83 (T) 

.77 (T) 

.65 (T) 

.55 (T) 

No, 2 

.84 (R) 

.84 (R) 

.84 (R) 

.84 (R) 

-84 (R) 

.60 (T) 

.50 (T) 

Product 

.58 

.68 

.69 

.70 

.65 

.39 

.28 

Table 6-2. Beamsplitter Efficiency for Given Filter Band Pass 

The following are three examples comparing the beamsplitter to sequential 

times. 

be zero so the resulting ratio is an upper limit. 

less observing time when the ratio is less than 1. 

In each example, the switching and recovery times (71, 72, 73) are taken to 

The beamsplitter approach takes 

In the first example, equal sequential time in all broadband filters is required, Le., T1 = T2 = T3 = To 

Configuration 1 2 3 4 5 

Band Filters 

1 

2 

3 

3 3 8 11 11 

15 16 18 12 22 

23 23 23 26 26 

Then, 

= 7.6/10 = .76 -- "split - To (1/.68 + 1/.69 + 1/.70 + 1/.58 + 1/.65) 

Tseq. To (10) 

- 
Page 59 

. 

In the second example sequential times equal to 4 To in Band I, filter ll,* To in 

Band 11, filters 15, 16, and 18, and To in Band 111, filters 23 and 26, are used. 

Then, 

In the third example equal sequential times TI, = T2 = T3 = To are used in one 

filter and each band. 

Then, 

Band 1 

2 

3 

Filter 11 4 - 5.3p 

18 10.7 - 14.5 

26 22.8 - 30 

-- Tsplit max (T0/.84, T0/.70, TJ.28 ~ 1.19 

Tseq. 3 To + (71 + 72 + 73) 

- 

6.5 Conclusions 

Coating designs for the dichroic beamsplitters of the IRAC camera have been 

produced and experimental depositions have been performed. Test results at OCLI 

and UA indicate that the transmission requirement of beamsplitter 1 has been 

achieved, but that the reflectance at the shorter wavelengths must be improved. 

Beamsiplitter 2 demonstrated acceptable reflectance at the shorter wavelengths but 

less than desirable transmission. 

attempts could not be made to tune the coating deposition for maximum reflected 

performance. A certain amount of improvement can be expected from that effort 

when it is undertaken later in the program. 

23pm is highly desirable and short wavelength reflectivity must be improved but 

not at, the expense of long wavelength transmission. 

Within the confines of this study, extensive 

Transmission improvement beyond 

*References are to filters defined in IRAC-202, “IRAC Instrument Requirements 

Document. 
Page 60 

Further work should concentrate on refinements in the design that will 

enhance the broadband reflectance. 

designer certain critical spectral ranges can be identified and the performance in 

those areas maximized over other less critical regions. 

In addition, through dialogue with the system 

An analysis of the dichroia approach showed that it is more efficient than 

sequential measurements when data must be taken in three bands. Taking instrument settling times, ignored in this simple calculation, into consideration makes the 

advantage of the dichroic approach even greater. 
Page 61 

7.0 OPTICAL ANALYSIS 

7.1 Introduction 

. 

The GSFC Optical Design Section working in conjunction with Dr. William 

Hoffmann of the University of Arizona has performed conceptual baseline optical 

analyses for IRAC. This analysis modelled refractive, reflective, and catadioptric 

configurations and evaluated them for system performance by utilizing computeraided ray tracing techniques (ACCOS V).g 

7.2 IRAC Optical Specification Status 

The pixel sizes originally chosen for IRAC were 50pm (128 x 128) for Band 

I and 100pm (64 x 64) for Bands I1 and 111. 

an ongoing tradeoff study being performed at SBRC. 

size is 75pm for all three bands. 

Pixel size is being reconsidered in 

The current baseline pixel 

Image quality for IRAC is specified in terms of Encircled Energy (EE), 

Each specification applies over the entire wavelength band and field of view of the 

instrument. For the wide field of view, 80% of the point spread function energy 

must fall either within a single pixel or within a circle defined by the Airy Disk 

diameter (D = 2.44 X/f). For the narrow field of view, 80% of the same function must fall within a circle of diameter equal to or greater than two pixels so 

as to' avoid undersampling. It also must fall within a circle defined by the Airy 

Disk diameter. 

7.3 Refractive Work 

At the onset of the IRAC optical analysis effort for FY1986, the refractive 

cesium iodide singlet design initially proposed was evaluated. 

suffe:red from astigmatism, chromatic aberration, and curvature of field. This was 

especially true in Band I. 

widefield spot sizes for Bands I1 and I11 met the spec for a 100pm pixel size at 

300°K. 

tion. No other material was found that performed better than cesium iodide for 

all three bands. Therefore a silicon/germanium air-spaced achromatic doublet was 

designed for Band I for a 50pm pixel size at 300OK. Geometric ray tracing has 

been performed on these designs for the wide- and narrow-field cases. This was 

done by tracing 300 rays through the system for each particular field angle. 

geometric image quality for all three bands met specification for both the wideand narrow-field modes. 

The image quality 

Optimization was performed such that the geometric 

The Band I singlet could not be optimized to meet the wide-field specificaThe 
Page 62 

To determine the idealized performance of these IRAC designs, a diffraction 

image quality analysis has been performed for both the wide- and narrow-field 

modes for all three bands for this system. 

performance of the system is limited by diffraction and determines the real spot 

sizes and MTF’s of the system. 

A diffraction analysis determines if the 

An investigation into broadband antireflection coatings for the Band I achromatic air-space doublet at room temperature was also performed. A triple-layer 

AR. coating of MgF2, CeO2 and Si each was assumed on a substrate of germanium. 

maximum transmittance values of 95% at 2.35pm and 98% at 5.2pm. 

AR. coating can be produced on a substrate of silicon. 

4°K operating temperature has not been performed yet. 

The coating increased the transmittance from 46% to about 92% with 

A similar 

An investigation at the 

The 4°K operating temperature of IRAC will cause the refractive index, 

element thickness, and element spacing to change from the baseline (room temperature) values. This in turn will cause the image to go out of focus as the instrument is cooled down. The depth of focus for Bands I, I1 and I11 are respectively 

f86~m, f236pm, and fl7Opm. This was determined by defocusing the image 

plane until the worst case geometric 80% Encircled Energy Spot Diameter (EESD) 

exceeded the specification. Thermally induced focus shifts that exceed these values 

would have to be corrected by actively refocusing the lens or detector. 

cornpensation for thermal focus shifts would require the knowledge of the cryogenic 

optical properties of the lens material. 

prclperties of infrared materials at 4°K are rare. 

Passive 

Unfortunately cryogenic data of optical 

7.4 Catadioptric Work 

Catadioptric designs were considered only briefly due to the success of the 

all-reflective two aspheric mirror design concept. 

been identified. 

No viable catadioptric design had 

7.5 Reflective Work 

7.5.1 Review of Work Done 

Reflective IRAC designs were looked at initially out of concern for chromatic 

aberration and cryogenic uncertainties of refractive indices and thermal expansion 

coefficients at 4OK. 

Hoffmann’s original spherical tilted collimator. 

first order limitations. 

A number of two-mirror concepts were studied using Dr. 

All of these designs suffered from 

Most of these limitations were due to image inaccessibility 

*‘ . and/or obscurations. Tilting and decentering the mirrors to avoid these Arst order 
Page 63 

limitations resulted in poor image quality. 

emerged for the Band I 50pm pixel size configuration. 

No feasible all-reflective approach 

An unobscured two off-axis aspherical design was always within the realm 

of possibility for Bands I1 and I11 (100pm pixel), but it was once again revisited 

for use in Band I due to a change in the baseline pixel size to 75pm for all 

three bands. 

creased in focal length. 

with fold mirrors was generated for the baseline 75pm pixel case for all three 

bands. This design accommodated both wide- and narrow-field modes. 

bea.msplitters and filter wheels were also allotted. 

matic of this reflective IRAC system design. 

The collimator was allowed to be an aspheric mirror and was inA baseline unobscured two off-axis parabola system design 

Space for 

Figure 7-1 is an optical scheThe first parabola serves as a collimator for all three bands. There is a 

second parabola for each band which serves as an imager. 

pixel array was used. 

corresponding IRAC wide-field magnification of 0.339 (F/8.129) for the baseline 

pix.el size of 75pm. Figure 7-1 also shows that space has been allotted for a 

fou.rth band (60pm -, 120pm) beamsplitter between the wide-field primary and 

first wide-field fold mirror. Therefore Band IV would only operate in the widefield mode. 

75pm (therefore different size arrays) then the specific magnification required for 

each band will be accommodated by each band's secondary focal length. 

A 134 x 134* baseline 

This resulted in a 10.05mm detector array size and a 

If Bands 11, 111, and IV have larger pixel sizes than the baseline 

7.5.2 Implementation of Magnification Change 

The magnification switch from wide- to narrow-field mode or vice versa is 

accomplished by moving the following optical components via a mechanism: 

(1) Wide-Field Fold Mirror - following the wide-field primary (collimator). 

(2) Narrow-Field SIRTF Fold Mirror 

(3) Narrow-Field Fold Mirror - following the narrow field primary (collimator)' 

(4) Narrow-Field Pupil Stop' 

- 

*A.llows for six inactive pixels between four 64 x 64 arrays butted together. 

tIn the current design (January 1988) only two mirrors must be moved to effect 

the mode change.1° 
Page 64 

1 VERTEX 

IRAC -- INCOMING OPTICAL AXIS RAY 

FROM SIRTF TELESCOPE UNOBSCURED TWO OFF-AXIS PARABOLAS 

WIDE AND NARROW FIELD MODE CONFIGURATIONS 

I \ *NARROW FIELD 

! NARROW FIELD SIRTF ,#'SIRTF FOLD MIRROR 

VERTEX - 

WIDE FIELD 

SIRTF FOLD MIRROR 

I 

NARROW FIELD 

PRIMARY MIRROR .' r- - - - - - - 

I/- FY,MIRROR $1 *NARROW FIELD PUPIL 

WIDE *WIDE FIELO FIELD 

PRIMARY MIRROR FOLD MIRROR BAND 4 B.S. 

(WIDE FIELD ONLY) 

-8.5.12 

VERTEX 

WIDE FIELD PUPIL (ONE FOR EACH BAND) 

SECONMRY MIRROR FOLD MIRROR 

(COMMON TO WIDE AND 

NARROW FIELD MODES, 

ONE FOR EACH BAND) DETECTOR 

NOTE: COMPONENTS WITH "*"s WILL BE 

ATTACHED TO THE MAGNIFICATION 

CHANGE MECHANISM. 

lRRC -- 2 PRRRBOLtIEj 75MICR PIX 110 fLD 5Y5T 

50.000000 MM LENS Y-2 PROFILE 

?Tire T' 

Figure 7-1. Optical Schematic of Reflective IRAC System Design 
Page 65 

7.5.3 Physical Layout Consideration 

A conclusion can not be made on the space availability for the three beamsplitters, four filter wheels, and the detector until this design has been laid out 

within the MIC. 

The beamsplitters and filter wheels are placed in collimated light (between the 

primary mirror and secondary mirror of each band where there would be no 

wavelength spread in the passband of an interference filter for any one point on 

the focal plane). 

side of the secondary mirror. 

telecentric (approximately two degrees in the narrow field). There is one narrowfield pupil which is common to all three bands. To insert the narrow-field mode 

into the system along with the wide-field mode necessitated having a separate widefield pupil for each of the four bands (4 pupil stops). This will be a problem if 

it is necessary to place the calibration sources at the edge of the wide-field pupils. 

The alternative is to have these calibration sources placed at the center of the 

SIR’I‘F secondary (aperture stop). 

experiments. 

Space must be allotted for mounts and alignment adjustments. 

The filter wheels probably could not be located on the image 

This is due to the design ’being slightly nonThis is where they could serve all of the SIRTF 

7.5.4 Analytical Results 

The first step in determining this design’s performance was to perform a 

geometrical analysis. 

tem for each particular field angle. 

the wide- and narrow-field modes respectively. 

arra,y size of 134 x 134 pixels. 

This was done by simply tracing 300 rays through the sysTable 7-1 lists the geometric 80% EESDs for 

All tables are based on a Band I 

A diffraction image quality analysis was performed to determine the mal 

spot, sizes and MTF’s of the system. 

EESDs for the Band I wide- and narrow-field modes respectively. 

shows the worst case (corner of field) radial energy distribution plots for the Band1 

widefield case. Band I does not meet the wide-field specification for wavelengths 

between 2.76pm and 4.09pm. The Band I narrow-field specification is not met for 

wavelengths less than 2.63pm. Tables 7-4 and 7-5 list the worst case diffraction 

80%1 EESDs for Bands I1 and I11 respectively for both the wide- and narrow-field 

modes. Both Bands I1 and I11 are well within the wide- and narrow-field specifications. The geometric and diffraction analyses represent the IRAC optical performance independent of the SIRTF telescope. 

Tables 7-2 and 7-3 list the diffraction 80% 

Figure 7-2 

Further optimization of the design may achieve full Band I success. The 

Band I secondary mirror may have to be a nonparabolic asphere. 

sensitivity analysis is currently being performed as part of the optimization of this 

design. 

worst case 80% EESD exceeds the specification. 

An alignment 

This involves misaligning each optical element, one at a time, until the 
Page 66 

Wide-Field Case (FOV=300 arcsec; F#=8.13) 

X: Field Y Field Angles (arcsec) 

- An.gle (arcsec) -150 arcsec 0 arcsec 

0 arcsec 

75 arcsec 

1510 arcsec 

Narrow-Field Case (FOV=75 arcsec; F#=32.52) 

IC Field Y Field Angles (arcsec) 

- Angle (arcsec) -75 arcsec 0 arcsec 

0 arcsec 49.8~ 

75 arcsec 68.7~ 

37.5 arcsec 55.4p 

150 arcsec 

38.81.1 

40.1~ 

43.2~ 

75 arcsec 

34.7p 

35.2~ 

38.4~ 

Notes: (1) Encircled Energy Values are for a flat and titled focal surface. 

(2) Spot Sizes are symmetric about the Y-Z plane. 

(3) These Geometric Spot Sizes are measured in reference to the 

optimized geometric focal-plane position. 

Table 7 - 1. Geometric Analysis - 80% Encircled Energy Diameter (Microns) 

Infrared Array Camera (IRAC) Optical Design Evaluation 

Configuration: Two Off -Axis Unobscured Parabolas 

For all 3 Bands: (Pixel Size = 751.1) 
Page 67 

Band I 

j2F - 5/41 

X=lr/.L 

X=2!.76pm 

X=3p 

k4p 

X=4.09p 

X=!jp 

Band I: 75 Micron Pixel Optical Design Data 

Wide-Field Case (FOV = 300 arcsec) 

Diffraction Analysis 

Wide-Field 

Diff. Limit. 

39.7p 

54.81.1 

59.5p 

79.3p 

81.1~ 

99.2~ 

Specification X Field 

Requirement Angle 

[arcsec) 

<75.0~ 0 

75 

150 

<75.0p 0 

75 

150 

c75.0p 0 

75 

150 

<79.3p 0 

75 

150 

<81.1p 0 

75 

150 

<99.2p 0 

75 

150 

Y Field Angles (arcsec) 

-150 

67.2~ 

67.0~ 

67.0~ 

75.0~ 

69.4~ 

60.8~ 

75.6~ 

76.1~ 

77.4p 

77.9p 

78.4~ 

81.5~ 

81.1~ 

62.1~ 

61.5~ 

75.6~ 

75.5p 

75.7p 

0 - 

60.6~ 

60.2~ 

74.2~ 

69.4~ 

58.1~ 

71.6~ 

7 1.4~ 

71.2~ 

69.4~ 

67.3~ 

63.9;1 

65.9~ 

61.3~ 

75.0~ 

59.7p 

77.9p 

74.7p 

74.4p 

Notes: (1) Encircled energy values are for a flat and titled focal surface. 

(2) Spot sizes are symmetric about the Y-Z plane. 

(3) These diffraction spot sized are measured in reference to the 

diffraction optimized focal-plane position. 

- 150 

51.3~ 

51.8~ 

54.0~ 

74.0~ 

69.6~ 

58.4~ 

58.4~ 

61.9~ 

60.1~ 

60.0~ 

60.4~ 

68.2~ 

61.3~ 

73.8~ 

73.8~ 

74.0~ 

57.9p 

77.3j.4 

Table 7-2. 80% EESDs for Band I (Wide Field) 

Infrared Array Camera (IRAC) Optical Design Evaluation 

Configuration: Two Off -Axis Unobscured Parabolas 

Band I: (Pixel Size = 75p; Wide Field F#=8.13) 
e 

Page 68 

~ ~ 

Banti I 

[2P -- 54 

x=2,u 

X=2,63p 

x=3,u 

x=4p 

x=5p 

Notes: 

Band I: 75 Micron Pixel Optical Design Data 

Narrow-Field Case (FOV = 75 arcsec) 

Diffraction Analysis 

Narrow-Field 

Diff. Limit. 

<158.7p 

<208,7p 

<238.0p 

<317.4p 

<396.7p 

Sampling X Field 

Requirement Angle 

[arcsec] 

> 150p 0 

37.5 

75 

> 150p 0 

37.5 

75 

> 150p 0 

37.5 

75 

> 150p 0 

37.5 

75 

> 150p 0 

37.5 

75 

Y Field Angles (arcsec) 

- -75 

117.5~ 

117.8~ 

119.41.1 

153.0~ 

153.1~ 

154.1~ 

174.0~ 

174.0~ 

174.8~ 

230.8~ 

230.7~ 

23 1.2p 

288.0~ 

287.6~ 

287.8~ 

(1) Encircled energy values are for a flat and titled focal surface. 

(2) Spot sizes are symmetric about the Y-Z plane. 

(3) These diffraction spot sized are measured in reference to the 

diffraction optimized focal-plane position. 

Table 7-3. 80% EESDs for Band I (Narrow Field) 

Infrared Array Camera (IRAC) Optical Design Evaluation 

Configuration: Two Off -Axis Unobscured Parabolas 

Band I: (Pixel Size = 751.1; Wide Field F#=32.52) 
I 

w. 

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V A. 

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Page 69 

.. 

: &.-' 

BAND 1 REFLECTIVE IRAC SYSTEM WIDE FIELD DIFFRACTION RADIAL ENERGY 

DISTRIBUTION PLOTS FOR WAVELENGTHS OF 3, 3.5, AND 4 MICRONS (WORST CASES) 

o.*oeG mn 

SPOT RADIUS (INCREMENTS OF 0.04mn) 

IRAC - 2 PARABOLRS 7SflICR PIX WD fL.U SY5T DIFF 

Figure 7-2. Worst Case Radial Energy Distribution Plots for Band I 
Page 70 

Ban'd I1 Wide-Field 

/5p->18p) Diff. Limit. 

X=5p 99.2~ 

X=7p 138.9~ 

X=lOp 198.4~ 

X=l3p 257.9~ 

X=3.6p 317.4~ 

A= 3.8p 357.1~ 

Band I1 Wide-Field 

[5p->18p) Diff. Limit. 

-s>p 1- <396.7p 

x=7p <555.4p 

A= lop < 793.5p 

X=13p <1031.5p 

A= 16p <1269.6p 

X=18p <1428.3p 

Worst Case Diffraction 

Specification 

Requirement 

<99.2p 

<138.9p 

<198.4p 

<257.9p 

<3 17.4~ 

<357.1p 

Sampling 

Requirement 

> 150p 

>150p 

>150p 

> 150p 

>150p 

> 150p 

Wide-Field Case (FOV=300arcsec;F#=8.13) 

Worst Case Diffraction 80%EESD (microns) 

75.7p 

103.2~ 

145.6~ 

188.2~ 

231.1~ 

259.7~ 

Narrow-Field Case (FOV=75arcsec;F#=32.52) 

Worst Case Diffraction 8O%EESD (microns) 

288.0~ 

402.4~ 

746.3~ 

918.11.1 

103 3 .Op 

574.3p 

Notes: (1) Encircled energy values are for a flat and titled focal surface. 

(2) Spot sizes are symmetric about the Y-Z plane. 

(3) These diffraction spot sized are measured in reference to the 

diffract ion optimized focal-plane position. 

Table 7-4. Worst Case Diffraction 80% EESD (Band 11) 

Infrared Array Camera (IRAC) Optical Design Evaluation 

Configuration: Two Off -Axis Unobscured Parabolas 

Band 11: (Pixel Size = 75p) 
Page 71 

- 

Worst Case Diffraction 

Band I11 Wide-Field Specification 

[18/.~->30p) Diff. Limit. Requirement 

A=l@ 357.1~ <357.1p 

A=22p 436.4~ <436.4p 

X=24p 476.1~ <476.1p 

A=26p 515.8~ <5 15.8~ 

X=28p 555.4p < 55 5.4p 

X=30p 595.1~ e595.1p 

Band I1 Wide-Field Sampling 

(18p->30p) Diff. Limit. Requirement 

A= 18p C1428.3p > 150p 

X=22p e 1745.7~ > 150p 

X=24p e 1904.4~ > 150p 

X=26p <2063.1p > 150p 

X=2 8p <2221.8p >150p 

X=30p C2380.5~ > 150p 

Wide-Field Case (FOV=3OOarcsec;F#=8.13) 

Worst Case Diffraction 80%EESD (microns) 

259.7~ 

317.0~ 

345.7p 

374.4p 

403.2~ 

43 1.8~ 

Narrow-Field Case (FOV=75arcsec;F#=32.52) 

Worst Case Diffraction 80%EESD (microns) 

1033.0~ 

1262.3~ 

1377.1~ 

149 1.8~ 

1606.5~ 

172 1.2p 

Notes: (1) Encircled energy values are for a flat and titled focal surface. 

(2) Spot sizes are symmetric about the Y-Z plane. 

(3) These diffraction spot sized are measured in reference to the 

diffraction optimized focal-plane position. 

Table 7-5. Worst Case Diffraction 80% EESD (Band 111) 

Infrared Array Camera (IRAC) Optical Design Evaluation 

Configuration: Two Off -Axis Unobscured Parabolas 

Band 111: (Pixel Size = 75p) 
Page 72 

8.0 CONCLUSIONS 

8.1 Array Detector Performance 

. 

The goals of the IRAC project and the results of the detector research 

carried out under this Grant are shown in Table 8-1 and summarized below. This 

table shows the best performance across the array achieved in each band.* 

1. Read Noise: 

in Band I where the best read noise measured to date is 240 e- in low-doped 

InSb. 

IRAC proposed goals (<lo0 e-) have been demonstrated except 

2. Dark Current: IRAC proposed goals (100e-/s or 16 “A) have been demonstrated in all bands at 6OKelvin. Bands I and I11 have demonstrated dark 

currents <2.1 aA at 8O Kelvin. 

3. Responsivity: 

and I11 but are lower than required by a factor of 2 in Band I. 

IRAC proposed goals have been met or exceeded in Bands I1 

4. Power Dissipation: 

been demonstrated in all bands with 1 mW per array a typical value for the 

array alone. 

temperature control circuits and line drivers. 

IRAC proposed goals (<26 mW per 64 x 64 array) have 

Additional power may be required for operation of heaters, 

5. Integration Time: 

Integration times up to 500 s have been demonstrated in Band I with no 

read noise increase and up to 30 minutes in Band I11 with read noise at 

200 s only a factor of 2 higher than at integration times of 0.2 s. 

IRAC proposed goals (1000 s, all bands) are within reach. 

6. Linear Range Well Capacity: IRAC proposed goals are within reach. Linear 

range well capacities of 2 x lo5 e- have been demonstrated in all bands. 

7. Operating Temperatures: Optimum array operating temperatures have been 

determined to be in the 6O-1Oo kelvin range. 

*At the time of the SIRTF Instrument Requirements Review (December 1987). 

Subsequent work in Bands I1 and I11 have demonstrated read noise results at the 

60 e- level. 
Page 73 

IRAC Band 

Detector Material 

Detector Type 

Wavelength Range (pm) 

Format 

Pixel Size (pm) 

Multiplexer 

Capacitance/pixel (pF) 

Quantum Efficiency* (YO) 

Operability (Yo) 

Responsivity (AMI) 

Responsivity Uniformity (YO) 

NEP*'* (WatWRoot-Hz) 

Read Noise (e-) 

Dark Current (aA) 

Well !Size for Linear Range (e-) 

Manufacturer 

IRAC Contact 

Low-doped lnSb Si:Ga Si:Sb 

Photovoltaic** Photoconductor Photoconductor 

1.4 - 5.3 

58 X 62 

75 

Direct Readout 

Switched MOSFET 

0.55 

42 @3.3pm 

> 98 

1.2 @ 5pm 

38 

(CRC-228) 

1.50E-17 

240 (tYP.1 

0.2 @ 8" K 

1.5E+05 

SBRC 

J. Pipher 

W. Forrest 

University of 

Rochester 

5.3 - 14 

58 X 62 

75 

Direct Readout 

Switched MOSFET 

0.07 

30 (est.) 

> 99 

5.1 @ llpm 

8 

(CRC-228) 

2.00E-17 

too 

30 @ 8" K 

5.OE+05 

SBRC 

C. McCreight 

Ames 

Research Center 

14 - 30 

58 X 62 

75 

Direct Readout 

Switched MOSFET 

0 06 

30 (est.) 

> 99 

2 3.8 @ 25pm 

5 

2.70E-17 

100 

2.07 @ 8" K 

2.OE+05 

SBRC 

(CRC-228) 

C. McCreight 

Ames 

Research Center 

Not AR Coated. AR Coated Arrays RH2 and RH4 have shown QE values in the 50 - 77% range 

** For SCA002 

***Integration Time = 0.2 s 

Table 8-1. IRAC Array Development Status 
Page 74 

8.2 Beamsplitters 

IRAC proposed goals have been demonstrated in all bands except in the 

20 - 3Opm range where improved transmission efficiency is required. 

8.3 Optical Design 

Table 8-2 summarizes Le status of refractive and reflective design stuc es 

relative to meeting IRAC design specifications. 

ana1:yses on the refractive designs, only the Band I silicon/germanium doublet 

design was able to meet the specifications for the 50pm pixel size case. The 

unohcured two off-axis parabola 75pm pixel size design met the wide- and narrow-field specifications for Bands I1 and 111. 

tive design should also meet specification for all of Band I for the 75pm pixel 

case. 

generated. 

this time. 

75p:m is the preferred pixel size due to its cryogenic and achromatic advantages. 

If there is a Band IV (60pm + 100pm), it would be very difficult to design a 

refriactive imager due to the lack of optical materials that both transmit in Band 

IV and have low dispersion across the band. 

appears that a pixel size smaller than 75pm will be very difficult $0 accommodate 

in this reflective design.* 

After performing the diffraction 

With further optimization this reflecA 75pm pixel size refractive design for Bands I, I1 and I11 still needs to be 

Therefore a direct comparison of the two designs can not be made at 

A strong recommendation must be given to the reflective design if 

From this design study it now 

*Current (January 1988) assessments indicate that the Bands I1 and I11 performance will be worse than stated due to a reduction in the physical sizes of the 

Band I1 and Band I11 arrays from those analyzed here. Pixel sizes are being 

reconsidered as part of on-going detector design work at SBRC. 

tions of performance may be found in Reference 10. 

The latest projec- 
Page 75 

1 Field 

I 

Narrow 

Wide 

Narrow 

Wide 

Narrow 

1 Wide 

Band 

REFRACT1 

Hoffmann 

Design 

CSI 

No 

No 

Yes 

No 

Yes 

No 

#1 

Achromatic 

Design 

Si/Ge 

Yes 

Yes 

#2 

Parabolic Collimator 

and 

Parabolic Imager 

No 

No 

#3 

I Spec DL 

’ (wide field) or 

Band I 50pm ‘ Band 2 100pm 

Band 3 100pm 

pixel 

size 

DL 

or 

50pm 

Diffraction Limit 

or 

7 5pm 

75pm 

75pm 

VE DESIGNS I REFLECTIVE DESIGNS 

Yes 

Yes 

Yes 

Yes 

A Y:ES or NO addresses whether or not the specification is met. 

how much Umarginn exists for Yes answers is not quantified in the specification 

and therefore is not used as a specification requirement at this stage in the investigation. Optical margin will be examined in detail for feasible designs (Le., designs 

that meet the top level specification). 

Issues such as 

Table 8 -2. Status of Refractive and Reflective Design Studies 
Page 76 

9.0 REFERENCES 

1. ‘“Technology Development of Band I Arrays for the Infrared Array Camera,” Final 

:Report, Contract SV5-55005, University of Rochester, Rochester, NY, May 1987. 

2. ‘,‘Fabrication and Testing of InSb 58 x 62 Element Multiplexed Infrared Detector 

.Arrays and a Preliminary Investigation into Si:In Detector Arrays with the Same 

Configuration,” Final Report Contract SO7-01338 (SBRC Report 71945), Santa 

:Barbara Research Center, Santa Barbara, CA, December 1987. 

3. ‘“Band I Detector Testing Summary,” University of Rochester, Rochester, NY, 

November 1987. 

4. “IRAC Test Report Gallium Doped Silicon Band I1 Read Noise and Dark Current,” 

(2. Lamb, et al., NASA Technical Memorandum 87813 and IRAC-302, January 1987. 

5. “Characterization of Direct Readout Si:Sb and Si:Ga Infrared Detector Arrays for 

!Space Based Astronomy,” McKelvey, et al., August 1987. 

6. ‘“Summary of ARC FY87 DRO Test Results,” Ames Research Center, Moffett Field, 

CA, December 1987. 

7. ‘*SSIRTF Beamsplitter Study,” August 1, 1986, prepared by Optical Coating 

]Laboratory, Inc., under Contract S06-08418. 

8. “Pre-Definition Phase Activities for the Infrared Array Camera (IRAC),” January 

11988, Final Report, Contract SV5-55002, University of Arizona, Tucson, AZ. 

9. “FY86 Optical Design Activity by GSFC,” P. Maymon, IRAC-402 Revision A, 

February 1987. 

10. “Instrument Requirements Review Viewgraph Package,” IRAC-114, December 1987. 
APPENDIX A 

SBRC Low-Doped InSb Array RH2 Test Data Histograms 

Test Data Histograms 
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APPENDIX B 

SBRC Low-Doped InSb Array RH4 Test Data Histograms 

Test Data Histograms 
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APPENDIX C 

University of Arizona Test Facilities 
Page C-1 

II MID INFRARED SPECTROPHOTOMETER 

For the beamsplitter measurements from 2 to 30 microns we arranged for 

the use of the National Optical Astronomy Observatories (NOAO) Beckman 

Acculab 6 Dual Beam Spectrophotometer. This spectrometer is a grating 

instrument specifically designed for room temperature transmission 

measurements. By rapidly "chopping" between the beam through the sample 

and ii beam bypassing the sample and nulling the difference with a servo 

controlled aperture on the bypassed beam, the unit provides percent 

transmission measurements as a function of wavelength, relativity immune 

from drifts in source strength, detector sensitivity and amplifier gain. 

Figure 1 shows the optical diagram of the Beckman Acculab 6 

Speclirophotometer. 

the center line of the sample well. 

directing the beam outside the spectrophotometer for use as a 

monochrometer. 

spectrophotometer in the vertical aspect. In the horizontal aspect, the 

beam size at the sample and slits S1 and S2 is considerably smaller. 

diagram is unfolded in that it shows all reflecting elements as normal 

incidence transmitting elements. 

The beamsplitter sample goes in the sample beam near 

M12 is a movable mirror which permits 

Figure 2 shows an unfolded ray diagram of the 

This 

The characteristics of the Acculab 6 are given in Table 1. 
~~ ~ 

Page C-2 

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Page C-3 

7.08 9.46 10.68 12.64 

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A1 H4 

15.69 21.33 20.07 

1.49 .216 1.7 

ti9 

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54.47 56-64 

1.83 -864 

. H11 

BECKMAN ACCULAB 6 RAY DIAGRAM 

VERTICAL UNFOLDED DUAL BEAM MODE 

Figure 2. 
~~ 

TABLE 1 BECKMAN ACCULAB 6 SPECIFICATIONS Page C-4 

Spectral Range 

%T Range 

Scan Times 

AccuSet Meter 

Ordinate Expansion for 

Auxiliary Recorder 

Purge Fitting 

Weiaht 

Wavenumber 

Accuracy 

Wavenumber 

Repeatability 

Resolution 

Slit Control 

Double-Beam 

100% Line 

Formatted Chart 

Paper Presentation 

Monochromator 

Stray Light 

_- 

4000 to 250 cm-1 

0 to 100~aT on 

integral recorder. 

Any desired portion of 

the 0 to 100%T range 

with span of lO%T 

or greater may be 

selected for expansion 

to fullscale on a 

100-mv auxiliary 

recorder. 

3.2. 9.5. or 28 minutes 

(60 Hz) 

3.8. 11.4 or 34.2 

minutes (50 Hz) 

Yes 

Yes 

Yes 

75 pounds 

Better than 10 cm-1. 

4000 to 2000 cm-1 

Better than 5 cm-1. 

2000 to 250cm-1 

Better than 5 cm-1, 

Better than 3 cm-1, 

4000 to 2000 cm-1 

2000 to 250 cm-l 

Better than 5 cm-1 

at 3000 cm-1 

Better than 3 cm-l 

at 1000 cm-1 

Two Programs: routine 

md high-resolution 

k2%. 4000 to 600 

cm-1, excluding 

atmospheric 

absorption bands. 

+3%. 600 to 300 

cm-1. excluding 

atmospheric 

8bsorDtion bands. 

5" x 13%" grid 

1%" x 1l"grid 

totebook (Optional) 

I rotating wedge filter, 

LOO line/mm and 

IO line/mm gratings 

.ess than. 1%. 4000 

Less than 3%. 600 

to 650 cm-1 

to 300 cm-1 

OPERATING MODES RECORDER 

Automatic recording; double.beam optical null: linear transmittance 

versus linear wavenumber. 

Flatbed, strip-chart. Repeating format. 

Optional notebook4ze format available. 

Linear transmittance versus time. at lixed wavenumber. 

701 ACCURACY 

Better than *2%. 

YaT REPEATABILITY 

Better thnn 1%. 

SOURCE 

Nichromc wire. 

CHART PRESENTATION 

Percent transmission versus linear cm-1. 

DIMENSIONS 

26 inches wide;. 11 inches high; 24 inches deep. 

RECOMMENDED OPERATING ENVIRONMENT 

Less than 80% relative humidity; 1S'C to 40°C. 
Page C-5 

111 BEAMSPLITTER TEST CRYOSTAT 

Since the beamsplitters must be tested at liquid helium temperature, 

it was necessary to design a cryostat for holding the beamsplitter samples 

which was compatible with the spectrophotometer sample wall. 

We have therefore designed and constructed a beamsplitter test 

cryostat satisfying the following constraints. 

measluring reflection and measuring beyond 30 microns with a Fourier 

Transform Spectrometer, were added after the initiation of this contract. 

Two of the requirements, 

Sample temperature at room temperature and at N 15K 

Compatible with spectrophotometer sample we1 I 

Compatible with fourier transform spectrometer sample area 

Test sample at Oo or 45O incident angle 

Sample size to 1 1/2 inch diameter 

Measure transmission and reflection 

Switch from sample to open (transmission) or gold reference mirror 

(reflection) to determine instrument baseline 

maintain reference mirror co-planar with sample 

The cryostat was designed as a square addition to an existing Infrared 

Laboratories HR 8 vapor cooled radiation shield, liquid helium dewar. 
Page C-6 

During this contract the transmission only configuration was completed and 

succe,ssfully tested. The design changes for the reflection configuration 

were initiated. 

Figure 3 shows the test dewar, transmission configuration for 45O 

incidence angle. The geometry is arranged so that the lower part of the 

dewar sits in the spectrophotometer sample well with the windows aligned 

with the spectrophotometer sample beam and the beamsplitter sample holder 

located at the spectrophotometer beam waist. 

paper, the windows and paddle are offset so that the dewar case clears the 

nearby spectrophotometer reference beam. 

In the direction into the 

The dewar case windows are 1 inch diameter 2mm thick KRS5. The sample 

area and mechanism are enclosed in a helium temperature shield to maintain 

the IIDW temperature of the sample holder. 

shield is painted with Primer (3M Nextel Primer #911-P4 Light Grey) and 

black paint (3M ECP-2200 Solar Absorber Coating) to absorb the radiation 

entering through the dewar window. The inside of the dewar case, both 

sides of the vapor cooled shield and the outside of the helium shield are 

coated with aluminum foil. The vapor cooled shield and inner shield 

openings have baffle plates with openings approximately 1/8 inch x 1/2 inch. 

The interior surface of this 

Not shown is a mechanical feed-through mechanism which rotates the 

paddlle in and out of the spectrophotometry beam. The support post and 

paddlle have a second monitoring posltion for 0' incident angle. 

The electrical header connects to seven temperature sensors listed in 

Table 2. 
a 

I 

/TOP COVER 

Page C-7 

I68 

-3.800 

BEAM SPLITTER TEST DEWAR 

45 DEGREE TRANSMISSION CONFIGURATION 

Figure 3. 

E 

CELL 

'LE DIA) 
Page C-8 

TABLE 2 TEMPERATURE SENSORS 

Location Type 

1. Sample 

2. Sample Cell 

3. Paddle 

4. Post 

II 5. Cold Work Surface 

6. Inner Radiation Shield Diode ( 1 N4002) 

7. Outer (Vapor Cooled) Radiation Shield II 

Resistor (1K 1/8W Allen Bradley) 

I1 

It 

It 
~ ~~ 

Page C-9 

Figure 4 shows the dewar cool-down, with liquid helium after 

precooling with liquid nitrogen. The sample reaches equilibrium in 1 1/2 

hours. The radiation shield takes - four hours to stabilize. The hold 

time after the second LHe fill is eight to twelve hours depending on the 

number of manipulations of the feed through mechanism and the time under 

the thermal load of the spectrophotometer beam. 

Figure 5 shows the detailed thermal performance with a fused silica 

sample in the sample holder. 

out of room temperature radiation entering the dewar. 

to 14 degrees when exposed to the spectrophotometer beam. 

The sample cooled to 7'K whether it was in or 

Its temperature rose 
Page C-10 

IRAC-BEAMSPLITTER KWAR LlQUiD HELIUM COOLDOWN 

\ 

\ 

OUTER RADIATION 

SHIELD 

'\ 

\ 

\ 

\ 

FUSED SILICA SAHPLE 

INNER RADIATION SHIELD 

COLD WORK SURFACE AND POST 

0 2 3 4 5 

TIME (HOURS) 

Figure 4. 
Page C-11 

\ 

0 

IRAC-SIRTf DEWAR LHe TEMP. PFRFORMANCE . 

Spectrometer thermal 

source initially 

blocked, eerrple 

in beam FUSED SILICA SAHPLE 

I INNER RADIATION 

SHIELD 

COLD WORK SURFACE 

and POST I_ 

Spectrometer theraal /y ‘&le aoved 

into beea I eource unblocked 

Saaple out of bee8 

- 

I I I I 

1 2 

TIME (HOURS) 

4 5 

Figure 5.