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29.5 Target Acquisition

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In order for FOS to have observed a target, that target must, naturally, have been located in the instrument aperture. Several techniques for positioning the target in the aperture were available to FOS observers, including:

The overwhelming majority of FOS target acquisitions used the onboard acquisition modes ACQ/BINARY and ACQ/PEAK. A brief description of each of the FOS acquisition modes is provided below, and a more complete description of the acquisition modes, the data they produced and how to determine the target centering accuracy for a particular observation is provided in "Assessing FOS Acquisitions" on page 30-26.

Researchers will want to remember that FOS instrument performance (flatfields and sensitivity, especially) is best understood for the very center of the apertures. The accuracy inherent to the target acquisition strategy employed will therefore determine the calibration accuracy that can later be reached with a particular dataset. For each set of FOS observations of a given source, the first dataset taken was the FOS target acquisition image.1

The target acquisition employed for a given observation will determine the accuracy of the target centering in the science aperture, which in turn affects the calibration accuracy of the science data itself. Analysis of FOS observation data quality is incomplete without retrieval and assessment of the target acquisition exposures.

29.5.1 FOS Onboard Acquisitions

BINARY acquisition (ACQ/BINARY)

In a BINARY acquisition, single raster images of the target were made with the MIRROR in each of the upper, middle, and lower thirds of the 4.3 aperture. The aperture segment containing the brightest image was determined and the brightest pixel provided the x-position of the target. Next a scheme involving a series of up to eight successively smaller electronic deflections was employed to find the Y-base that placed the target on the edge of the diode array. During this procedure all image offsets were performed electronically-the target was not moved until the actual aperture centering slew was calculated by the algorithm. Due to its efficiency (but with limited centering accuracy), ACQ/BINARY was the acquisition method of choice for point sources fainter than V~15 in programs for which S/N < 30 was sufficient. The method had a restricted dynamic range of brightness and, in the pre-COSTAR period, was severely limited by the aberrated PSF. Acquisition of faint sources that required high pointing accuracy often began with ACQ/BINARY and concluded with one or more ACQ/PEAK sequences.

Peakup or Peakdown Acquisition (ACQ/PEAK)

In an ACQ/PEAK sequence, NXSTEP=1 integrations were performed at a series of defined positions on the sky with a science aperture and either MIRROR or a disperser in place. At the end of the slew sequence, the telescope was returned to the position with either the most (peakup) or fewest (peakdown) counts (return to brightest or return to faintest). Overheads for ACQ/PEAK slew patterns were large (30-45 seconds per dwell). ACQ/PEAK was generally required for the following types of observations:

Interactive Acquisition Image (INT ACQ)

In an interactive acquisition, the camera MIRROR was used to obtain an image of the 4.3 aperture field. The image was downlinked and analyzed in real time to determine the required slew to center the target. The slew was uplinked and FOS science observations proceeded, generally at the start of the next target visibility. This method was employed for a number of moving target acquisitions.

Early FOS Acquisition Image (ACQ)

When taking an early FOS acquisition image, a camera MIRROR image of the 4.3 aperture was acquired in similar fashion to that described for INT ACQ above, but, in this case, the science observations were performed at a later date. Often the early FOS image was used to determine geometry of crowded fields or precise offsets from a nearby bright target likely to be acquired by ACQ/PEAK or ACQ/BINARY.

Firmware Acquisition (ACQ/FIRMWARE)

Firmware acquisition was an engineering mode that mapped the camera MIRROR image of the 4.3 aperture in x and y with small, selectable y raster increments. The microprocessor filtered this aperture map in real time and found y-positions of the peaks by fitting triangles through the data. ACQ/FIRMWARE was less efficient than ACQ/BINARY and failed if more than one object were found. This mode was used for a small number of pre-COSTAR planetary satellite acquisitions and was not used in the post-COSTAR science program.


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1 If a blind pointing was done, there will not have been a target acquisition image, unless one was specifically requested.

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Last updated: 01/14/98 14:27:00