Cryogenics Requirements

In order to maintain simplicity, no image quality tests are performed and therefore no optics are installed on the instrument. However it is possible to put a pinhole grid mask in front of the detector mosaic to illuminate selected areas on the detector for crosstalk tests. Extremely low background radiation was the major goal of the facility and instead of a window, a cryogenic blackbody has been implemented. To determine the cryogenic requirements, the thermal flux seen by the detector from 2n steradian was calculated as a function of instrument temperature for detectors with a cutoff wavelength of 5.3 microns. To obtain a photon flux <0.1 electrons/sec/pixel assuming a pixel size of 27 micron, the warmest part of the testfacility needed to be less than 77 Kelvin. Figure 1 shows this curve for the CRIRES Aladdin mosaic [1].

Figure 1. Background flux from 2n steradian as a function of instrument temperature.

The thermal flux observed by the detector from the cryogenic blackbody can also be calculated with Planks law of photon emittance by including the geometric properties and the detector mosaic to be tested. Figure 2 shows the flux in electrons/sec/pixels in the case of the CRIRES mosaic. To calculate flux for QE measurements the filtercuves must be included.

temperahjte in |K]

Figure 2. Flux seen by the detectors as a function of blackbody temperature.

temperahjte in |K]

Figure 2. Flux seen by the detectors as a function of blackbody temperature.

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