Space Telescope Science Institute   6.2.1 Coronagraph Design  6.2.3 Vignetting and Flat Fields

6.2.2 Acquisition Procedure and Pointing Accuracy


The central source must be placed precisely behind the occulting spot to ensure the proper suppression of the diffracted light. The absolute pointing accuracy of HST is about 1 arcsecond, too crude to ensure accurate positioning. An on-board acquisition procedure is used to provide better alignment. The observer must request an acquisition image immediately before using the coronagraph, and must specify a combination of filter and exposure time that provides an unsaturated image of the source. An acquisition image is taken by specifying HRC-ACQ as the aperture and ACQ as the opmode in APT.

Acquisition images are taken with the coronagraphic masks inserted. The star is imaged within a predefined 200 x 200 pixel (5 x 5 arcseconds) subarray near the small occulting spot. Two identical exposures are taken, each of the length specified by the observer (rather than each being half the length specified, as they would be for a conventional CR-SPLIT). From these two images, the on-board computer selects the minimum value for each pixel as a crude way of rejecting cosmic rays. The result is then smoothed with a 3 x 3 pixel box and the maximum pixel in the subarray is identified. The center-of-mass centroid is then computed for the unsmoothed image within a 5 x 5 pixel box centered on this pixel. Based on this position, the telescope is then slewed to place the star behind the occulting spot.

Because the coronagraphic masks are in place during acquisition, throughput is decreased by 52% relative to a non-coronagraphic observation. Also, the PSF is broader than in the normal imaging mode due to the larger obscurations in the Lyot stop, resulting in a lower relative peak pixel value (see Section 6.2.6). Care must be taken to select a combination of exposure time and filter that will prevent saturation of the star while providing enough flux to provide a good centroid measurement. A peak pixel flux of 2000 e- should be considered the minimum while 50,000 e- is a safe maximum (the HRC saturation limit is ~140,000 e-). Narrowband filters can be used, but for the brightest targets crossed filters are required. Allowable filter combinations for acquisitions are F220W+F606W, F220W+F550M, and F220W+F502N, in order of decreasing throughput. Be warned that the calibration of these filter combinations is poor, so estimated count rates from synphot or the APT ETC should be considered to be a factor of two off (either high or low).

Initial results from multiple on-orbit observations indicate that the combined acquisition and slew errors are on the order of ±0.25 pixels (±6 milliarcseconds). While small, these shifts necessitate the use of subpixel registration techniques to subtract one coronagraphic PSF from another ( Section 6.2.5). The position of the spots relative to the detector also varies over time. This further alters the PSF, resulting in subtraction residuals.


 6.2.1 Coronagraph Design  6.2.3 Vignetting and Flat Fields
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