Physics – Optics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.8051b&link_type=abstract
HST Proposal ID #8051
Physics
Optics
Hst Proposal Id #8051
Scientific paper
The Tiny Tim software has been used in the past by a significant percentage of the HST user community to plan observations, test photometric and image restoration algorithms, perform photometry and compute photometric corrections, etc. The quality of Tiny Tim PSF models {and hence the science which is derived from their application} is directly related to our knowledge of the aberrations present in the camera being modelled. Significant mismatches between observed and model PSFs result from inaccurate aberration values. The implementation of STIS PSF modelling in Tiny Tim currently presents significant difficulties, due to the number of unknowns related to aberrations. Aberration values for a camera are measured from images of stars using phase retrieval software. This software iteratively computes a monochromatic PSF model using a set of aberrations, compares it to an observed star image, and revises the aberrations. This process is repeated until the residuals are minimized. For the algorithm to work well, the stellar image being fitted should have as high a signal-to-noise ratio as possible, be well sampled, and be {nearly} monochromatic. This has typically been done in the past by taking out-of-focus star images through narrow-band filters {see Krist & Burrows, Applied Optics, 34, 4951}. The more high signal- to-noise ratio pixels which can be fit, the better. In-focus PSFs only have a few such pixels in the core, with most of the information in the wings. Overexposing the PSF to increase the wing intensity will saturate the core. Defocussed PSFs have more uniform values, meaning that more pixels can be obtained with high S/N values. To date, no significantly defocussed data has been obtained using STIS {the only HST camera in which this is the case}. Defocussed data would also reduce the effect of undersampling, which occurs in the STIS CCD mode. And since the width of the PSF core is directly proportional to the wavelength, it is important that the PSF be obtained in a narrow-band filter, otherwise the software will confuse PSF changes over a broad passband for defocus or spherical aberration. It is known that STIS has field dependent focus, as well as other aberrations which cannot be measured using the current in-focus data sets. Defocussed star images in various parts of the field of view will allow these field-dependent changes to be measured. The amount of defocus which will likely occur during the next NICMOS 3 campaign will be sufficient to allow accurate coma, astigmatism, and focus value to be measured. Also, since the direction of defocus will be known, there will not be the uncertainty in the sign of astigmatism, a problem with in-focus data. For a defocus of 20 microns, the peak pixel intensity at 350 nm goes from about 24% to 2%, and at 120 nm it goes from 8% to 2%. At this amount of defocus, the PSFs are doughnuts.
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