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AUSTRALIAN
NATIONAL UNIVERSITY System Design Note 4.10 Created: 8 June 2000 Last modified: 17 July 2000 |
NIFS CRYOSTAT THERMAL EMISSION
Peter J. McGregor
Research School of Astronomy
and Astrophysics
Institute of Advanced
Studies
Australian National
University
Revision History
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Revision No. |
Author & Date |
Approval & Date |
Description |
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Revision 1 |
Peter J. McGregor 08 June 2000 |
John Hart 08 June 2000 |
Original document. |
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Revision 2 |
Peter J. McGregor 17 July 2000 |
John Hart 17 July 2000 |
Considered effect of lower temperature of silica transmission profile. |
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Contents
6 Implementation
of a 5.5 mm
Cutoff Detector
1 Purpose
The document quantifies the cryostat thermal emission of the Gemini Near-infrared Integral Field Spectrograph (NIFS) for both a 2.5 mm cutoff HAWAII-2 PACE detector and a 5 mm cutoff HAWAII-2 CdZnTe detector.
2 Applicable Documents
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Document
ID |
Source |
Title |
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RSAA |
NIFS Performance Model |
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RSAA |
NIFS Science Detector Trade Offs |
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3 Introduction
The Gemini Near-infrared Integral Field Spectrograph (NIFS) should have low intrinsic background in order to maximize sensitivity. Thermal emission from the cryostat enclosure must be reduced to below that of the detector dark current. This is achieved by cooling the spectrograph body and detector. The baseline detector for NIFS is a 2.5 mm cutoff HAWAII-2 PACE device (NIFS Science Detector Trade Offs, SDN0008.00). However, it is possible that a 5 mm cutoff HAWAII-2 CdZnTe device may also be available. In that case, thermal emission from the cryostat enclosure must be considered more closely.
4 2.5 mm Cutoff Detector
The 2.5 mm cutoff HAWAII-2 PACE detector has a quantum efficiency of ~ 0.6 from ~ 1 mm to its wavelength cutoff, and a dark current > 0.01 e s-1 pix-1. The NIFS performance calculator (NIFS Performance Model, SDN0004.01) has been used to determine the cryostat thermal emission that this device will detect in a 3600 s integration for different cryostat enclosure temperatures (Table 1, columns 1&2). The detected cryostat signal is below the dark current signal for all cryostat temperatures below 140 K.
Table 1: NIFS Cryostat Thermal Emission in 3600 s
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2.6 mm
Cutoff, 60% QE |
5.5 mm
Cutoff, 90% QE |
4.0 mm
Cutoff, 90% QE |
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Tcryo |
icryo |
Tcryo |
icryo |
Tcryo |
icryo |
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125 |
0 |
55 |
0 |
80 |
0 |
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130 |
1 |
60 |
0 |
85 |
0 |
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135 |
4 |
65 |
1 |
90 |
4 |
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140 |
17 |
70 |
22 |
95 |
35 |
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145 |
68 |
75 |
281 |
100 |
246 |
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150 |
249 |
80 |
2642 |
105 |
1430 |
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Dark Current |
36 |
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36 |
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36 |
5 5.5 mm Cutoff Detector
The cutoff wavelength of the potential HAWAII-2 CdZnTe detector is not known, but is expected to be ~ 5.5 mm. The quantum efficiency is expected to be ~ 0.90, and the dark current is likely to be ~ 0.01 e s-1 pix-1. The longer cutoff wavelength of this device results in higher detected cryostat thermal signals which rival the detector dark current at cryostat temperatures above ~ 70 K (Table 1, columns 3&4). Clearly, the detector enclosure must be cooled below ~ 65 K and long wavelength light must be blocked in the spectrograph camera.
The final camera element is a silica field flattener. Silica transmits near-infrared light to ~ 4.0 mm. The thermal emission from the cryostat enclosure seen through this field flattener can be approximated by considering the signal detected by a 4.0 mm cutoff detector (Table 1, columns 5&6). The detected cryostat thermal signal then rivals the detector dark current at cryostat temperatures above ~ 90 K.
6 Implementation of a 5.5 mm Cutoff Detector
Implementation of a 5.5 mm cutoff detector in NIFS can be accomplished by cooling the cryostat enclosure to below 90 K and cooling the final element of the spectrograph camera (i.e., the silica field flattener) and the detector enclosure to below 65 K.
7 Temperature Dependence
It has been suggested that the transmission cut-off of silica moves to longer wavelengths at lower temperatures. This would increase the cryostat thermal emission detected by a 5.5 mm cut-off device. The infrared transmission of crystal quartz has been measured by Stierwalt[1] (Figure 1). At a temperature of 77 K, the transmission of silica (thickness possibly 0.105 cm) falls to below 2.5 ´ 10-3 longward of ~ 5.0 mm. If we approximate this with a step function, the detector cryostat thermal emission is as shown in Table 2. Cryostat thermal emission will be below detector dark current in this arrangement as long as the camera body is cooled to below ~ 75 K. This is already being achieved in the NIRI cryostat. Consequently, the shift of the transmission profile of silica to longer wavelengths at lower temperatures is unlikely to affect the use of a silica field flattener as a blocking filter when used with a 5.5 mm cut-off detector.
Figure 1: Low temperature transmission of silica from Stierwalt.
Table 2: Cryostat Thermal Emission in 3600 s at 77 K
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5.5 mm
Cutoff, 90% QE |
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Tcryo |
icryo |
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65 |
0 |
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70 |
1 |
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75 |
11 |
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80 |
129 |
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85 |
1127 |
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Appendix A: List of Figures
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Figure 1 |
silica_trans.tiff |
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