Other
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8852g&link_type=abstract
HST Proposal ID #8852
Other
Hst Proposal Id #8852
Scientific paper
We have determined that two sets of faint ghosts are caused by a window in front of the CCD detector. One of us {RSH} has modeled the ghost position and shape for direct imagery and for spectrographic mode. We are able to predict the amplitude and shapes of the ghosts reasonably well for the G430L and G750L modes. The situation is different with M mode ghosts. These appear significantly different from the L mode ghosts in prelaunch laboratory data; however, the on-orbit data necessary to test this behavior with an external target are completely lacking to date. The calibration lamps prove not to be especially useful at this point for two reasons: their small linewidths, and the optical beam differences from the OTA. The small linewidths result in fringing by the blocking filter located on the grating structure. Light from the collimator mirror passes through this filter before and after the grating dispersion. Along the lamp emission lines as seen through a long slit, we detect fringes with amplitudes of as much as 20 percent. Although the fringes are also detectable in emission line nebulae, they are much less bothersome. For typical line widths of 20 km/s {e.g., the [O III] lines in Orion}, the modulation is no more than 3-4 percent. Other difficulties with the calibration lamps are that the lamp beam f ratio differs from that of the OTA, and that the occultation by the OTA secondary mirror is absent; therefore, details of the ghost size and shape are poorly represented. For all these reasons, a satisfactory analysis of ghosts encountered in observation requires an external source. After considerable searching, we have determined that the best demonstration of our ability to characterize these ghosts for the spectrographic mode is to observe an emission nebula, which has well-characterized velocity structure and hopefully only one fairly narrow velocity component. The easiest test is to observe the Orion Nebula at its brightest point in Balmer alpha using the G750M grating set at 6768A. We use the 0.2X0.2 aperture to define a small source in the slit plane. Several deep short-slit wavecals to be carried out in Earth occultation are also included in order to make an explicit comparison of externals and internals.
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