Space Telescope Science Institute   6.2 Coronagraphy  6.2.2 Acquisition Procedure and Pointing Accuracy

6.2.1 Coronagraph Design


A schematic layout of the ACS coronagraph is shown in Figure 6.2. The aberrated beam from the telescope first encounters one of two occulting spots. The beam continues to the M1 mirror, which forms an image of the HST entrance pupil on the M2 mirror, which corrects for the spherical aberration in the HST primary mirror. The coronagraph's Lyot stop is placed in front of M2. A fold mirror directs the beam onto the CCD detector. The field is 29 x 26 arcseconds with a mean scale of 0.026 arcseconds/pixel (geometric distortion results in effectively non-square pixels). The coronagraph can be used over the entire HRC wavelength range of = 2000 Ċ to 10,000 Ċ using a variety of broad-to-narrowband filters.

The occulting spots are placed in the plane of the circle of least confusion, near where the unaberrated HST focal plane would be. At this location the balance of defocus and spherical aberration provides a good compromise between maximal occulted flux and minimal spot radius. The angular extent of the PSF in this plane necessitates larger spots than would be used in an unaberrated system ( Figure 6.3).

Figure 6.3: Computed point spread functions at the plane of the occulting spots through filters F435W and F814W.

 
The elliptical, cross-shaped patterns in the cores are due to astigmatism at the off-axis location of the ACS aperture. It is corrected later by the ACS optics. The sizes of the two occulting spots (D=1.8 arcseconds and 3.0 arcseconds) are indicated. Logarithmic intensity scaled.
 

The ACS spots are solid (unapodized) metallic coatings deposited on a glass substrate (which reduces throughput by 4.5%). The smaller spot is 1.8 arcseconds in diameter and is at the center of the field. Its aperture name is CORON-1.8. The second spot, 3.0 arcseconds in diameter, is near a corner ( Figure 6.4) and is designated as aperture CORON-3.0. The smaller spot is used for the majority of the coronagraphic observations, as it allows imaging closer to the central source. The larger one may be used for very deep imaging of bright targets with less saturation around the spot edge than would occur with the smaller spot. Its position at the edge of the field also allows imaging of material out to 20 arcseconds from the central source. The Lyot stop is located just in front of the M2 aberration correction mirror, where an image of the HST primary is formed. The stop is a thin metal mask that covers all of the diffracting edges in the HST system at the reimaged pupil (outer aperture, secondary mirror baffle, secondary mirror support vanes, and primary mirror support pads). The sizes of the stop and occulting spots were chosen to reduce the diffracted light below the level of the scattered light, which is unaltered by the coronagraph. The stop reduces the throughput by 52%, and it broadens the field PSF due to the smaller aperture and larger central obscuration relative to the beam diameter. The spots and Lyot stop are located on a panel attached to the ACS calibration door mechanism, which allows them to be flipped out of the beam when not in use. The inside surface of this door can be illuminated by a lamp to provide flat field calibration images for direct-mode imaging. However, the arrangement prevents the acquisition of internal flat fields in coronagraphic mode.

In addition to the combination of the occulting spots and Lyot stop that comprise the coronagraph, there is a 0.8 arcseconds wide, 5 arcseconds long occulting finger (OCCULT-0.8) permanently located at the window of the CCD dewar. It does not provide any suppression of diffracted light because it occurs after the Lyot stop. It was to be used to image closer to stars than is possible with the occulting spots while preventing saturation of the detector. However, because the finger is located some distance from the image plane, there is significant vignetting around its edges, reducing its effectiveness. Originally aligned to cover the central portion of the 3.0 arcsecond spot, shifting of the spot relative to the beam during launch now places the finger along that spot's edge. Because of vignetting and the sensitivity of the PSF wings to the centering of the star, unocculted saturated observations of sources will likely be more effective than using the occulting finger.

Figure 6.4: Region of the Orion Nebula observed with the coronagraph in filter F606W.

 
The silhouettes of the occulters can be seen superposed against the background nebulosity. The 1.8 arcsecond spot is located at the center and the 3.0 arcsecond spot towards the top. The finger is aligned along one edge of the larger spot. This image has not been geometrically corrected, so the spots appear elliptical.
 

 6.2 Coronagraphy  6.2.2 Acquisition Procedure and Pointing Accuracy
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