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:
- An FOS Onboard target acquisition: Used either a Binary Acquisition (ACQ/BINARY), a Peakup or Peakdown (ACQ/PEAK), an interactive acquisition (INT ACQ), an early FOS Image (ACQ) or a firmware acquisitions (ACQ/FIRMWARE).
- Blind pointing (i.e., observe at the telescope pointing immediately following the guide star acquisition without further refinement of telescope position). This was rarely used because the one sigma accuracy was on the order of 1".
- GHRS-assisted acquisition: GHRS could be used to acquire an object followed by slew and additional centering with FOS. This option was rarely used as the equivalent FOS overheads were more efficient. Conversely, FOS-assisted acquisitions for GHRS were common in Cycles 5 and 6 (see GHRS ISR 068 and FOS Instrument Handbook version 6 for details).
- WF/PC1-assisted acquisition: Similar in principle to GHRS-assisted acquisition except centering slew calculated on ground after WF/PC-1 exposure. Rarely used, this option was only available pre-COSTAR (prior to December 15, 1993).
- Reuse target offset: In order to avoid unnecessary overhead times, a new technique was developed for proposals that required more than one visit to a target within a few days (up to two months). This reuse target offset method, first employed in December 1994, allowed the instrument to use a target offset that was derived in the acquisition for an initial visit during subsequent visits so that these later visits need use only a single-stage peak-up/peak-down acquisition to reconfirm the correct centering of the target in the aperture. Limiting accuracies of 0.03" were commonly achieved.
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:
- All exposures requiring S/N>30.
- High wavelength accuracy.
- Pointing more accurate than 0.2".
- Objects too bright to acquire with the MIRROR.
- Objects too variable to acquire with ACQ/BINARY.
- Centering targets in apertures smaller than 1.0.
- Positioning bright sources behind the occulting bars.
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.
Last updated: 01/14/98 14:27:00