IV.C. Optical Performance

IRAS Explanatory Supplement
IV. In-Flight Tests
C. Optical Performance

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  1. Optical Cross Talk due to Bright Sources Crossing the Focal Plane
  2. Optical Cross Talk from Sources not directly on the Focal Plane
  3. Out-of-Field Rejection Monitoring

Diffraction and scattering of the infrared radiation from bright sources from optical surfaces or telescope structures result in potential artifacts in the IRAS data. Early in the mission an effort was made to assess the magnitude of such effects using spatially designed observations of Jupiter and Saturn. Observational procedures (discussed in Chapter III) and software (Section V.D.2.c) were used to eliminate most of the artifacts from routine observations.

C.1. Optical Cross Talk due to Bright Sources Crossing the Focal Plane

Optical cross talk is here defined to mean the detection of the flux from a source on a detector that is inconsistent with the reconstructed position of the image of that detector and the position of the source on the sky. This is distinct from electronic cross talk in that the detected signal is in fact due to infrared radiation incident on the detector.

The fact that the secondary mirror is supported by three spider arms resulted in a diffraction pattern consisting of an Airy disk and six diffraction spikes equally spaced on a circle. Two of the diffraction spikes are aligned with the scan direction. The diffraction spikes constitute a minor component of the point source diffraction pattern, but the point source detection algorithm (see Section V.C) is quite sensitive to the diffraction spikes.

C.2. Optical Cross Talk from Sources not directly on the Focal Plane

Bright sources not directly imaged onto the focal plane could produce apparent infrared signals which, if care were not taken, could be confused with real infrared sources. The attenuation of such signals as a function of angular distance from the telescope boresight is defined as the out-of-field rejection ratio, with the nominal specifications given in
Section II.C.3. A number of tests were carried out to verify the nominal out-of-field performance and to check for unexpected glints. These tests confirmed the anticipated need for operational procedures not to make routine observations closer than 1° from Jupiter, 20° from the moon, 60° from the Sun and 88° from the Earth horizon. Estimates of the in-orbit out-of-field rejection are included in the discussion in Section II.C.3.

Cross talk from Jupiter appeared to be associated with diffraction from the secondary mirror support structure. The amplitude of the spikes and associated diffuse cross talk components was generally less than that predicted from simple telescope models by about a factor of three.
Figure IV.C.1 A polar plot showing where glints from the moon were detected by the numbered detectors. The boresight of the telescope is at the center. The moon is at the cone and azimuth angle plotted with the tracks. The angle gives the angle to the sun.
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The moon tests revealed significant "glints", in addition to the anticipated amount of diffuse out-of-field radiation. Glints are well defined regions of about ½° extent, where the telescope out-of-field performance is significantly reduced. The glints are of two types, one of which propagates across the focal plane in the survey scan direction affecting all detectors along the scan and the other which affects only specific detectors around the edge of the array. Figure IV.C.1 shows a map of all presently known moon glints out to 30° from the moon. It should be noted that routine observations were carried out as close as 20° from the moon.

C.3. Out of Field Rejection Monitoring

Due to concern about possible slow degradation of the out-of-field performance during the mission, the out-of-field performance was monitored throughout the mission using two observational procedures.
Figure IV.C.2 Monitoring of the out-of-field rejection by observations of the moon 20° off axis. The observations are normalized to values of the TFPR as are the calculations in Fig. II.C.5. The dashed lines show the mean values which are consistent with the prelaunch calculations shown in Fig. II.C.5. The bracketed points are of low weight The observations in SOPs 170 and 225 required large corrections for responsivity changes in the 25 µm band.
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The first was a test using the moon as a bright source. Approximately once per month, with the moon about 90° from the Sun, the spacecraft made 60° long scans which passed within 20° of the moon. These scans were compared with reference scans over (roughly) the same strip of the sky preceding (or following) the lunar scan by one day, such that the moon was roughly 32° from the boresight at the closest approach. The difference in the flux for the same point on the sky measured in the two scans was taken to be due to out-of-field radiation from the moon. The results of this monitoring program are shown in Fig. IV.C.2. No significant changes in the out-of-field performance were observed during the course of the mission and the deduced mean value of the out-of-field radiation from the moon at 20° from the boresight is in good agreement with expected performance.

The second approach was to attempt to monitor and/or detect out-of-field radiation from the Earth, due to radiation from the Earth scattered off the inner surface of the sunshade, followed by diffraction and/or scattering through the baffle structures. The technique employed was to observe several regions of the sky, generally twice, when the orientation of the sky was such that the region could be observed while no Earth radiation was incident on the inner surface of the sunshade. The observation of the same region was then repeated typically about a month later, when the orientation of the sky was such that the inner surface of the sunshade was as nearly fully illuminated as possible without violating the Earth limit pointing constraint. The difference in observed flux, after correcting for zodiacal emission differences and changes in the baseline attributable to electronics drift is presumably due to out-of-field radiation from the Earth. The analysis is limited by the accuracy of the zodiacal emission model and baseline uncertainties. The upper limits from the tests represented a background less than 0.1 the flux from the TFPR (see Section IV.A.5), but the tests were not sensitive enough to confirm pre-launch calculations of the out-of-field rejection (see Fig. II.C.5).

The extended emission data reduction subsystem further controlled the effects of out-of-field radiation by rejecting data taken with larger avoidance cones around bright objects. Tests early in the mission indicated that the moon was the only object bright enough to warrant further avoidance and any data taken within 30° of the moon was ignored. A more detailed discussion of avoidance angles is given in Section III.C.5.

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