INSTRUMENT SCIENCE REPORT - WF/PC-1 91-02:
Spacecraft Jitter: It's Effect on the HST PSF (and on the "Breathing"?)
Roberto Gilmozzi
March 04, 1991
Excerpts from the abstract:
The effect of the spacecraft jitter on the HST Point Spread Function is analyzed, with special reference to the terminator oscillations. It is found that the jitter may substantially affect the shape of the PSF, seriously impairing the chances of accurate photometry. However, the true PSF can be modelled by convolving the uperturbed PSF with the jitter PSF obtained from the FGS pointing information, provided the observations were conducted in fine lock. The best results are obtained for high telemetry rates (better sampling of the jitter). The true PSF so obtained can then be used to deconvolve WF/PC-1 images.
This leads to the following recomendations regarding WF/PC-1 observations needing careful deconvolution and/or photometry:
1. Use of Fine Lock as tracking mode (at least in PC),
since Coarse Track makes it very difficult to
transfer the jitter information from the FGSs
to the WFPC.
2. Use of high telemetry rate for proper jitter
information - necessary only for "short"
(<1000s) exposures.
3. Addition of jitter information to the PODPS
products (as in the proposed OMS outputs).
If these recommendations are accepted, it will be necessary to advise the GOs very promptly (Coarse Track has been suggested as fefault tracking mode for WF/PC-1). Work is under way to improve Coarse Track but so far the results are not yet below the 25 mas (FWHM) threshold (equivalent to a 5% effect on the PSF).
With respect to the so called "breathing" phenomenon, it is found that it appears to correlate with the jitter, although an experiment aimed at reproducing it by convolving the unperturbed images with the jitter PSF is not completely successful. This may be due to the undersampling of the jitter information used (4 Kb rate), to a possible uncertainty about the actual start time of the various exposures, or to the phenomenon really being due to a secondary mirror movement. Arguments are presented against the proposed interpretation of the phenonmenon in terms of a thermal gradient in the secondary mirror supports. Rather, the "Occam's razor" argument is invoked to attribute the breathing to the jitter, either as a direct cause or because of its effect on the position of the secondary mirror (still very tilted at the epoch of the observations analyzed). However, a dependence of both jitter and breathing on a third variable, although of lesser likelihood, cannot be ruled out without further observations.
For a paper copy of this report, please contact help@stsci.edu.