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32.4 OTA Effects

32.4.1 Thermal Breathing

The change in temperature as the spacecraft crossed the terminator caused the secondary mirror support structure to expand and contract which moved the secondary mirror and changed telescope focus. This effect was termed thermal breathing and occurred on timescales equal to the orbital period of the spacecraft. The pre-COSTAR photometric error associated with thermal breathing could up to 4-7% and affected the flux in a random and uncorrectable way. The post-COSTAR effect was reduced due to the much narrower PSF and, for well-centered point source observations typically affected only the 0.3 and smaller apertures, where 2-3% variation occasionally was seen even for well-centered targets. Larger aperture point source data with misaligned Y-bases or extended source data could be affected, as well. Only in very long (> 2000 seconds) RAPID mode observations is the effect discernible (and possibly correctable).

32.4.2 Telescope Focus Changes

Systematic variations of pre-COSTAR FOS sensitivity occurred because OTA focus adjustments did not occur with sufficient frequency to keep up with the shrinkage of the graphite epoxy structure caused by outgassing (desorption) on orbit.¹ A change in focus by 15 microns led to photometric changes of up to 8% in the 4.3 aperture. The photometric variations also depended on aperture and very slightly on wavelength. The current calfos flux calibration procedure (APR_CORR, AIS_CORR, and TIM_CORR) corrects for all these variations.

Following the installation of COSTAR, OTA focus was monitored very closely and the COSTAR Deployable Optical Bench (DOB) was moved immediately to compensate for each secondary mirror move that was made. No significant focus-related FOS sensitivity variations occurred in the post-COSTAR period.

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¹ FOS ISR 102.

stevens@stsci.edu