8.6 STIS Post-Observation Target Acquisition Analysis

Unlike with earlier HST spectrographs, it is now possible to determine what the instrument acquired during an observation. The methods described below can be used to make either a coarse (eyeball estimate) or a fine (exact pixel) determination.

8.6.1 Did the Acquisition Succeed?

Section 5.2 of the STIS Data Handbook describes in detail how to determine if an acquisition succeeded. We recommend the use of the tastis tool in stistools as a quick way to determine how well an acquisition worked. For ACQ exposures, this tool will report where the target was found at the beginning of the coarse and fine locate stages, where the reference aperture projected onto the detector, and will also give the individual and total slews in units of both CCD pixels and arcseconds. For ACQ/PEAK exposures, the tool will report the relative flux at each pointing of the peakup, the slew made, and the flux in the confirmation image. For both kinds of acquisition exposures the tastis task will issue a warning message if the fluxes or slews reported appear to fall outside the normal range of values expected for a successful acquisition.

Contents of ACQ and ACQ/PEAK image files

It can also be useful to directly examine the images provided in the acquisition data files. The STScI calibration pipeline will produce raw data files that contain images taken during the acquisition sequence as well as information about the measurements done at each stage of the acquisition.

For ACQ exposures, each raw file will contain three images, each in a separate fits image extension. The first science image extension shows the image taken at the beginning of the coarse locate phase and shows where the target was initially found. The second science image extension shows the image recorded after the coarse locate slew has been done. The third science extension shows where the image of the standard reference aperture, as illuminated by a tungsten lamp, projects onto the detector. The ACQ/PEAK raw fits file contains the final "confirmation" image of the target viewed through the aperture in the first image extension and the fluxes measured in the stepping pattern in the fourth extension. Further details of these files and their contents are discussed in Section 5.2 of the STIS Data Handbook.

Target Acquisition Simulator

While the tastis tool in stistools is normally adequate to determine whether or not your acquisition succeeded, sometimes, especially for complex extended objects, a more complete simulation of the acquisition sequence may be useful. Once you have your data, you can use the Target Acquisition Simulator to determine the exact position of the source that was acquired. The TAS will take the STIS image of the initial pointing and, using the same algorithm as the flight software, tell you the location of the target, in pixel coordinates.