Design and performance of a high-throughput cryogenic detector

system

Elmer H. Sharpa,b

, Dominic J. Benforda , Dale J. Fixsen

a,d, Stephen F. Maher

a,c, Catherine T. Marx

a,

Johannes G. Staguhna,d

, Edward J. Wollacka

aNASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA;bGlobal Science and Technology, 7855 Walker Drive, Suite 200, Greenbelt, MD 20770, USA;

cSSAI, 10210 Greenbelt Rd., Lanham, MD 20706, USA;

dUniversity of Maryland, College Park, MD 20742, USA;

Abstract

Keywords: GISMO, Optical Design, High-throughput, Detector System, IRAM

The Goddard IRAM Superconducting Millimeter Observer (GISMO) is a new superconducting bolometer array camera

for the IRAM 30 Meter Telescope on Pico Veleta, Spain. GISMO uses a 3He/4He cooler mounted to a liquid He/LN2

cryostat to cool the bolometer array and SQUID electronics to an operating temperature of 260mK. The bolometer array

is based on the backshort-under-grid architecture and features 128 2mm square absorbing pixels. A 101mm diameter

anti-reflection coated silicon lens is used to define the beam. A single cold pupil stop prevents warm radiation from

reaching the array, but no other stops are used. In the beam, filters and a cold baffling and stray light suppression system

were used to define the bandpass and prevent out-of-band radiation to a very high level, including out-of-band radiation

leaking through the metal-mesh filters from extreme angles. We present a detailed description of this optical design and

its performance. A comprehensive report of the electronics and cryogenic integration are also included.

1. Introduction

With current bolometers performing at the background noise level for most ground based astronomical (sub-) millimeter

applications, it has become mandatory to increase the pixel count of the detector arrays used in bolometer cameras in

order to increase the observational efficiency and to take full advantage of the field of view provided by the telescope.

Cameras with higher mapping speeds operating at the background noise limit with pixel counts in the thousands are, or

will in the foreseeable future, go online. The most prominent of those currently being the SCUBA-2 camera at the JCMT

(Holland et al., 2006, SPIE, 6275, 45)

With this goal of having larger and larger detector arrays comes the need for systems with large windows, fast optical

systems and high optical throughput, as well as sophisticated out-of-band light rejection. A number of large metal mesh

filters are needed (thermal blocking, edge and band-pass) to reduce out of band radiation from leaking into the cryostat

and to provide the proper band-pass filter for the detectors. Lenses made from materials with a large index of refraction

can be utilized to replace cold mirrors in the optical system, which require large volumes.

In the following we describe the optical, electrical and mechanical scheme of a compact system which incorporates these

technologies to yield a very compact optical design for our two millimeter bolometer camera, GISMO, with a field of

view of 110” x 220”.

___________________________________________________Further author information: (Send correspondence to Elmer Sharp)

E-mail: Elmer.H.Sharp@nasa.gov, Telephone: +1 301 286 4229

Millimeter and Submillimeter Detectors and Instrumentation for Astronomy IVedited by William D. Duncan, Wayne S. Holland, Stafford Withington, Jonas ZmuidzinasProc. of SPIE Vol. 7020, 70202L, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.790058

Proc. of SPIE Vol. 7020 70202L-12008 SPIE Digital Library -- Subscriber Archive Copy

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2. Cryogenic Setup

The GISMO cryo-system is a 7 Liter liquid helium/ 4.7 Liter liquid nitrogen cryostat combined with a closed cycle, high

pressure, 2 stage, 4He/

3He refrigerator (from Chase Cryogenics, www.chasecryogenics.com). The final operating

temperature of the system was ~0.260K under a <5uW load. The buffered 4He stage operated at ~0.790K under <20uW

load. The maximum hold-time under load was ~20 hours with the LN2 boiling off first. Cycling the 4He/

3He cooler takes

approximately two hours and generates more than 3W of power. Initially, more power is needed to quickly raise the

charcoal sorption pumps above 30K and is later reduced in maintaining a 35K to 40K sustained temperature for ~45

minutes while driving off the 4He and

3He. 3 sheets of 38mm X 0.127mm OHFC copper are used to provide the thermal

link between the 3He reservoir and a 2 stage kinematic mounting thermal stand.

Figure 1. GISMO Thermal Stage. The bottom Y shaped plate mounts to the helium tank and the center and top stages are both

suspended with 0.356mm Kevlar thread.

The thermal support stand is a 2 level system, with one level thermally intercepting the later. It is composed of two

independent support stands nested on top of each other. Each individual support stand has three high purity oxygen-free

copper (OHFC) rings with Kevlar crosses through their centers; all three rings mounted 120 degrees apart. One stage is

flipped upside down and an intersecting plate and Stycast 2850 is used to join the parts together. The total calculated

thermal conductivity through all 12 Kevlar threads to the first stage is 2.2µW. From the first stage to the second stage,

where the detector package rests, we estimate 39nW of loading through the Kevlar. A few pull tests were done and

revealed the breaking strength of the Kevlar to be on the order of 124N, providing a very stiff support stage. We found

this design to be optimal for our setup for the following reasons: 1) An ultimately lower detector operating temperature

and 2) It removes the ringed obstructions and allows for use of larger packages.

Proc. of SPIE Vol. 7020 70202L-2

Electrical connections are made via manganin and NbTi NiCu clad wire. The thermometry/housekeeping cabling was

made with twisted pair manganin and signal cables were composed of high-speed manganin strip-lines and NbTi NiCu

clad wire. All cables connecting the 4K stage to the 0.260K stage were made of NbTi NiCu clad superconducting wire to

insure low parasitic electrical resistivity as well as low thermal conductivity. The seven, 50 ohm impedance, strip-lines

were placed inside a jig in which a Stycast 2850 mold was formed around them. The Stycast was molded in the form of a

KF-40 vacuum flange to provide electrical connections for the signal lines through the pressure jacket. The ambient

pressure side of the strip-lines is then routed into a small card cage, tower, fixed to the top of the pressure jacket and

mounted to the KF-40 flange. The strip lines terminate on the appropriate card, 3rd Stage Bias, 3rd Stage Feedback of

the series array SQUID amplifier, 2nd Stage Bias, 2nd Stage Feedback, 1st Stage Bias, 1st Stage Feedback, and Address

lines of the SQUID multiplexers. A separate card cage, the crate, is fastened to the outside working end of the cryostat

which houses a power card, 4 digital feedback cards, an addressing card, and clock card. The two card cages are inter-

connected via short SMB cables. The crate and tower are both powered by external power supplies. All connectivity to

computers is done via 10m fiber-optic cables. On the 4K, vacuum side, the strip-lines terminate on small circuit boards

with the appropriate valued resistors to bias and feedback different parts of the system. There is also a 4K address driver

circuit which allows us to digitally switch between different detectors. All of these small circuits are contained in

aluminum boxes for thermal shielding and NbTi NiCu clad wire is used to connect the terminator boards to the detector

package.

3. Optical Design and Integration

The detector package is made from a OHFC. It measures 119mm X 87mm and houses a G-10 circuit board, 4 X 32

channel NIST multiplexers and Nyquist inductors, 4 X 32 channel 2.5 mOhm shunt resistor chips, and the 128 pixel

BUG (Backshort Under Grid) array. The BUG array is glued on a copper plated alumina board to provide the optical

back-short and mounted to copper flexures which are machined into the detector package base to allow for expansion

and contraction. A 2 mm 20% Band Pass Filter is bolted to the top snout of the package and then the entire package is

wrapped in lead tape for magnetic shielding.

Figure 2. (Left) Top Lid of the GISMO Detector Package with 2mm Band Pass Filter. (Right) This photo shows the inside of the

completed GISMO Detector Package. The 8 X 16 2mm 150 GHz BUG Array can be seen in the center. The four individual SQUID

multiplexers, shunt, and Nyquist inductor chips can be seen surrounding the BUG Array.

A 101.6mm silicon convex/concave lens is used to imaging the beam onto the detector. The silicon lens was anti-

reflection coated at Princeton University on both sides with a machined Cirlex polyimide material and a thin layer of

Stycast to join the two. The lens was nested in an aluminum ring mounted above the thermal stage and detector package

by two aluminum struts. We found the mounting and heat-sinking the silicon lens to be troublesome. Our first attempt at

mounting the lens by Ti flexure failed after a few thermal cycles and chunks of silicon being removed from three sides of

the lens. Our second attempt, mounting by stainless steel springs and copper foil under pressure worked well and

brought the lens down to an operating temperature of 4.4 K.

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Above the lens are a 40% neutral density filter and a 7cm-1

edge filter, both mounted by an aluminum ring. A thin layer

of vacuum grease is applied to make good thermal contact. A bottom baffle connects the lens/filter assembly to the

detector package leaving a 1.27mm gap between the two temperature surfaces. This baffling system in nothing more

than an aluminum can with a rectangular hole cut in the bottom for the entrance of the detector package. The inside of

the baffle is coated with a steel-loaded Stycast layer. The loaded Stycast is applied to a thickness of 0.25 thick for

good absorption and is well thermally matched to aluminum to reduce stress due to thermal contraction. (Wollack et al.,

2008, International Journal of Infrared and Millimeter Waves, Vol. 29, No. 1, pp. 51-61).

Below the lens, we have a cold, 4K, entrance pupil. The pupil has an window of 91mm. The pupil and the 101.6mm lens

give us the required f/1 beam for an approximately 0.9 /D sampling, which provides a large field of view

but does not produce the customary instantaneous Nyquist-sampling of the celestial emission. This choice is intended to

optimize the efficiency of GISMO for large area blank sky surveys and has been shown (Bernstein 2002) not to

compromise the achievable point source signal-to-noise ratio, nor to sacrifice the diffraction limited angular resolution.

A series of baffles coated with the same steel-loaded Stycast is used to reduce stray light from hot sources inside the

cryostat entering the beam. We have another 10cm-1 edge filter at the entrance of the

4He shield. A series of black re-

entrant rings join the 4He shield and LN2 shield. This closes any light gaps and also acts as a housing to thermally anchor

two thermal blockers and 12cm-1

edge filter to the LN2 shield. One more set of re-entrant rings and a 300K thermal

blocking filter complete the stack to the 168mm diameter window in the pressure jacket. A 0.76mm thick sheet of

Polyethylene is used to form the vacuum window.

Figure 3. (Left) The 4 K 7cm-1 Edge Filter mounted in its aluminum housing. (Right) The 4 K Cirlex coated silicon lens. The dark

colored material is the stainless steel and Stycast mixture painted onto stepped ring surface to baffle the beam. The last ring that rests

against the silicon lens is the cold Pupil Stop in the system.

Drawing

No

Optical Element

1 Thermal Isolation Stand

2 TES 8 X 16 2mm BUG Array

3 260 mK 2 mm 20% Band Pass Filter

4 Baffle loaded with 30% Steelcast ~1 mm Thick

5 45% Quartz Neutral Density Filter

6 4.2 K Edge Filter ( 7cm-1

)

7 4.2 K Silicon Lens ( Anti-reflection coated with Cirlex )

8 Baffles loaded with 30% Steelcast ~1 mm Thick

9 4.2 K Edge Filter ( 10 cm-1

)

10 77 K Thermal Blocking Filter

11 77 K Edge Filter ( 12 cm-1

)

12 77 K Thermal Blocking Filter

13 300 K Thermal Blocking Filter

14 Polyethylene Vacuum Window ( 0.76mm Thick )

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LN2 47LITERS

4He 7LITERS

3H Cooker'

Figure 4. This is a mechanical drawing of the GISMO cryogenic and optical system. Individual elements are numbered and detailed in

the table above. For scaling, the vacuum window is 168mm and the total cryostat height is ~ 900mm.

Proc. of SPIE Vol. 7020 70202L-5

In mounting the GISMO cryostat to the 30 meter IRAM telescope, the cryostat rests on top of an aluminum I-beam

structure used to mount two aluminum mirrors. (See Fig. 4) The first mirror is an off-axis powered elliptical mirror to

match our optical beam to the telescope. The second is a folding mirror used to direct the beam back inside the cryostat.

This entire assembly, cryostat and I-beam structure, are bolted to a vibration isolation table inside the receiver cabin.

Running the system on the vibration isolated table eliminates almost all of the microphonic pickup in our system. The

remaining signals are significantly reduced in the data by our data reduction software. Alignment was done using a laser

installed by IRAM on the elevation axis of the telescope and a folding mirror. We used this to get alignment to first order

and then fine-tuned it by watching the detector in realtime as it looks at the sky. The system is aligned when the response

of the detector is maximum to celestial sources, and background level is minimized. We also made a 300K load (AN73

Eccosorb) and moved it in and out of the beam on different mirrors tracing the beam successively at different points in

the optical system. This proved useful and demonstrated that the beam was well aligned.

Figure 5. (Left) GISMO sitting on the optical stand in its imaging position at the IRAM 30 meter telescope. In the bottom right corner

of the photo, we have the aluminum elliptical mirror and directly bellow the cryostat is the fold. (Center) The IRAM 30 meter

Telescope. (Right) The fully assembled cold stage taken just before the shielding and filter installation at the telescope. The left side

displays our 4He/3He cooler, Address card box and terminator box. On the right side you can see the lens/baffle assembly mounted

above the suspension stage and detector package.

4. Summary and Conclusion

We have taken the GISMO camera to the IRAM 30 meter telescope and demonstrated its functionality. Astronomical

observations were recorded and the data are currently being analyzed at NASA/GSFC. The observed performance is

consistent with the predicted.

A second observation run is planned for the month of September 2008. We are currently working on improving our data

analysis software as well as adding a shutter mechanism and calibration source. A new detector package is being

assembled such that all silicon parts are mounted on high purity alumina boards and epoxied to copper flexures. We are

re-fabricating the lower baffle section at the entrance to the detector package such that it is heat sunk to the 260mK stage

rather than 4K.

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