ACS coronagraphic flat fields are different from the standard flats due to the presence of the occulting spots and alteration of the field vignetting caused by the Lyot stop. The large angular extent of the aberrated PSF results in vignetting beyond one arcsecond of the spot edge ( Figure 6.4), which can be corrected by dividing the image by the spot pattern ( Figure 6.5). To facilitate this correction, separate flat fields have been derived that contain just the spot patterns (spot flats) and the remaining features (P-flats). For a full discussion see ACS ISR 2004-16
.
The ACS pipeline will divide images by the P-flat. P-flats specific to the coronagraph have been derived from either ground-based or on-orbit data for filters F330W, F435W, F475W, F606W, and F814W. Other filters use the normal flats, which may cause some small-scale errors around dust diffraction patterns. The pipeline then divides images by the spot flat, using a table of spot positions versus date to determine the proper shift for the spot flat. However, there is a lag in determining the spot position, so it may be a month after the observation before the archive knows where the spot was on that date. So, coronagraph users should note that their data may be calibrated with an incorrect spot flat if they extract their data from the archive soon after they were taken. (Spot flats for the filters listed above are available for download from the ACS reference files web page
. For other filters, the available spot flat closest in wavelength should be used. The spot flat must be shifted by an amount listed on the reference files page to account for motions of the occulting spots.)
Because coronagraphic P-flats and spot flats exist only for the few filters listed above, observers are encouraged to use those filters. It is unlikely that flat fields for other filters will be available in the future.
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