The MultiDrizzle Handbook


6.5 STIS

6.5.1 Drizzling STIS Data

By comparison to ACS or WFPC2, planning dithered STIS observations, and drizzling STIS data is relatively simple. There is only one chip to worry about, and the differential distortion across that chip is small, so dithers are quite uniform across the entire chip. The examples provided in the ACS section will give the user a good introduction to questions concerning output pixel size and cosmic ray parameters, for drizzling as the pixel sizes and intrinsic PSFs of ACS and STIS are nearly identical.

One of the authors (Andy Fruchter) has used STIS extensively and has found for his imaging data, the default MultiDrizzle parameters work very well with a few exceptions discussed in the ACS example section: the resolution of STIS is sufficiently good such that when several dithered images are available it makes sense to use both a pixfrac and scale smaller than the native resolution. A pixfrac in the range of 0.8 and a scale of about 0.0333" work well for the final image (and the latter pixel scale also equates three pixel to 0.1"). When doing the earlier single image drizzle for cosmic ray rejection, the reduced output pixel scale should be used, but it is best to leave that pixfrac = 1.

An excellent example of dithered STIS data can be found in the HST archive under data sets o59275*. These are eight dithered STIS images of the field of the host of GRB 990123. When one has more than a few images, such as here, the user may wish to also change from the default "minmed" cosmic ray rejection (which handles small numbers of images well) to the "median" with "combine_nhigh=1" This takes the median of images after first rejecting the highest value in the various images for a given pixel and this procedure is somewhat better adapted to a large number of images than is the standard "minmed". One of the single STIS images, and the combined drizzled image using the parameters discussed above are shown in Figure 6.29 and Figure 6.30.

Figure 6.29: Single STIS Image Ready for input to MultiDrizzle

Single STIS image before MultiDrizzle has been run.
Figure 6.30: Final Drizzled STIS Image

The combined drizzled image using the parameters described above.

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