STIS Target Acquisition Simulator User Guide

Overview
Limitations of the Simulator
Step-by-Step Instructions

Overview of the Simulator

This STIS Target Acquisition Simulator (TAS) is intended to give observers a feeling for what target is likely to be selected when their observation is executed. This is particularly true for targets in crowded and/or complex fields, or spatially extended targets. The TAS implements the same algorithms as the flight software, and so should give results in good agreement what will happen on-orbit. If you want to try out the TAS without supplying an image, just use the default image "pix.fits" which is in reality the IRAF default image "dev$pix."

What the TAS will do

What you should do

Limitations of the Simulator

• Images with a spatial scale much different than that of STIS CCD images are not likely to be useful for simulating a target acquisition. Early acquisition or archival images obtained with FOC, WF/PC or WFPC-2 should be useful for the TAS, but early acquisition STIS/CCD images will obviously work best.

• Early acquisition or archival images will have been reduced differently than those obtained for an actual STIS observation. Moreover, the signal-to-noise ratio for these images is likely to be significantly higher than what can be realized during an acquisition on-orbit. Thus, for example, multiple targets of nearly equal total flux may result in a different selection on orbit than in the simulation.

• User images will almost certainly have different photometric properties than those obtained on orbit (unless the STIS CCD was used with the same filter). Thus, it is generally not be possible to simulate the S/N ratio, and therefore the rejection criteria, that will be enforced on orbit. Observers should make use of the STIS imaging exposure time calculator for this purpose. As well, if your image was not obtained with the same bandpass, the apparent target brightness may be substantially different during the actual acquisition. Thus, if your image was not obtained with the STIS CCD using the same filter, the results from the TAS may be misleading!

Step-by-Step Instructions

1. Transfer your image of the target to the STScI ftp area. The file must be in FITS format; it must not be smaller than 100 x 100 pixels nor larger than 1024 x 1024 pixels in size. Files placed in the ftp area will have a lifetime of a day or two before being deleted, so it is in general not necessary to upload your image every time you invoke the TAS. Also, fits files used by the simulator must be single image fits files, such as those outputted by "wfits", and not multi-group files.

   To transfer a file called "myimage.fit" with ftp:

   • ftp ftp.stsci.edu
   • cd outside-access/in.coming
   • binary
   • put myimage.fit
   • quit

2. Enter the name your image file in the dialog box. The file extension must be either ".fit" or ".fits."

3. Select a reference pixel within which the program will look for the target (if your image is larger than 100 x 100 pixels).

4. Indicate the spatial extent of the target. If the target is not a point-source, then you must also:
   • Select the centering algorithm, which is either geometric (brightest whole pixel), or flux-weighted centroid (fractional pixel).
   • Choose a size (in pixels) for the checkbox. The size of the (square) checkbox must be an odd number between 3 and 105 inclusive.

5. Click on the "Perform Calculation" button.

6. Repeat the steps above with different choices of target type and checkbox size to see if the result is sensitive to these parameters.

Maintained by the Spectrographs Group
Last updated: 31 Jan 1997