The ACS High Resolution Channel had a user-selectable coronagraphic mode for imaging faint objects (e.g., circumstellar disks, substellar companions, quasar-host galaxies) near bright point sources (e.g., stars, quasars). The coronagraph suppressed the diffraction spikes and rings of the occulted source below the level of the scattered light, most of which was caused by surface errors in the HST
optics. The coronagraph was added after ACS construction began, at which point it was impossible to insert it into the aberration-corrected beam. Instead, the system was deployed into the aberrated beam, which was subsequently corrected by the ACS optics. Although it was not as efficient as a corrected-beam coronagraph (especially for imaging close to the occulted source) the HRC coronagraph significantly improved the high-contrast imaging capabilities of HST
A schematic layout of the ACS coronagraph is shown in Figure 6.2
The aberrated beam from the telescope first encountered one of two occulting spots. The beam continued to the M1 mirror, which forms an image of the HST
entrance pupil on the M2 mirror, which in turn corrects for the spherical aberration in the HST
primary mirror. The coronagraph's Lyot stop was placed in front of M2. A fold mirror directed the beam onto the HRC's CCD detector. The field of view was 29 x
with a mean scale of 0.026 arcseconds/pixel. Geometric distortion resulted in effectively non-square pixels. The coronagraph could be used with any filter over the entire HRC wavelength range of λ = 2000 ┼ to 10,000 ┼.
For filters F435W
, shown with logarithmic intensity scaling. The elliptical, cross-shaped patterns in the cores are due to astigmatism at the off-axis ACS aperture. The astigmatism is corrected later by the ACS optics. The sizes of the two occulting spots (D = 1.8 arc seconds and 3.0 arcseconds) are indicated by the white circles.
The occulting spots were sharp-edged (unapodized) metallic coatings deposited on a glass substrate that reduced the throughput by 4.5%. The smaller spot was located at the center of the field and had a diameter of 1.8 arcseconds. Its aperture designation was CORON-1.8. The larger spot had a diameter of 3.0 arcseconds and was located near the edge of the field (Figure 6.4
). Its aperture designation was CORON-3.0. The small spot was used for the most coronagraphic observations because it allowed imaging closer to the occulted source. The large spot was used for very deep imaging of bright targets with less saturation around the spot than would occur with the smaller spot. Its position near the edge of the field also allowed imaging of faint objects up to 20 arcseconds from the occulted source.
The Lyot stop was a thin metal mask that covered the diffracting edges in the HST
OTA (i.e., the outer aperture, secondary mirror baffle, secondary mirror support vanes, and primary mirror support pads) at the reimaged pupil. The sizes of the Lyot stop and occulting spots were chosen to reduce the diffracted light below the level of the scattered light, which was unaltered by the coronagraph. The smaller aperture and larger central obscuration of the Lyot stop reduced the throughput by 48% and broadened the field PSF. The spots and Lyot stop were located on a panel attached to the ACS calibration door mechanism and could be flipped out of the beam when not in use. The inside surface of the calibration door could be illuminated by a lamp to provide flat field calibration images for direct imaging, but not coronagraphic imaging.
In addition to the two occulting spots, an occulting finger measuring 0.8 arcseconds x
5 arcseconds (designated OCCULT-0.8) was permanently located at the window of the CCD dewar. The finger was intended to allow unsaturated imaging closer to stars than was possible with the occulting spots, but without suppression of diffracted light by the Lyot stop. It was originally aligned with the center of the large spot, but shifting of the spots during launch ultimately placed the finger near the edge of the large spot. Because the finger was located some distance from the focal plane, the field around its edges was significantly vignetted. This vignetting and the sensitivity of the occulted PSF to its position relative to the finger significantly reduced the efficacy of this observing mode.
A bright source had to 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, which is too crude to have ensured accurate positioning behind the spot. Consequently, an on-board acquisition procedure was used to provide better alignment. With the coronagraph deployed, acquisition images were taken using a 200 x
) subarray at a region of the field near the small occulting spot. The observer specified a filter and exposure time that provided an unsaturated image of the bright source. Narrowband and crossed filters (e.g., F220W+F606W, F220W+F550M, and F220W+F502N) were most often used to obtain images that contained enough signal for a good centroid measurement. However, the photometric performance of the crossed filters was not well calibrated, so estimated count rates for these configurations from synthetic photometry or the ETC were accurate only within a factor of two.
Two identical acquisition images were taken, from which the on-board computer selected the minimum value for each pixel as a crude way of rejecting cosmic rays. The result was then smoothed with a 3 x
boxcar and the maximum pixel in the subarray was identified. The centroid of the unsmoothed image was then computed within a 5 x
box centered on this pixel. Based on this position, the telescope was slewed to place the source behind the occulting spot. The combined acquisition and slew errors were approximately ▒0.25 pixels (▒6 milliarcseconds). While small, these errors required the use of subpixel registration techniques to subtract one coronagraphic PSF from another Section 6.2
. The position of the spots relative to the detector also varied over time. This movement further altered the PSF and caused subtraction residuals.
Early in Cycle 11, HRC coronagraph performance verification images were taken of the V = 0 star Arcturus (Figure 6.6
and Figure 6.7
). This star has an angular diameter of 25 milliarcseconds and was thus unresolved by the coronagraph. The images of the occulted star are atypical coronagraphic images because the occulting spots were placed in the aberrated beam. The interiors of the imaged spots are filled with a diminished and somewhat distorted image of the star that was caused by mirror M2's correction of unocculted aberrated light from the star. The image of the small spot is filled with light, while the image of the large one is relatively dark. Broad, ring-like structures surround the spots, extending their apparent radii by about 0.5 arcseconds. These rings are due to diffraction of the aberrated PSF by the occulting spots themselves. Consequently, the coronagraphic images of bright stars could saturate at the interior and edges of the spots within a short time. The brightest pixels within the small spot saturated in less than one second for a V = 0.0 star, while the pixels at edge of the large spot saturated in about 14 seconds.
The measured radial surface brightness profiles (Figure 6.8
) show that the coronagraph is well aligned and operating as expected. The light diffracted by the HST
obscurations is suppressed below the level of the scattered light—there are no prominent diffraction spikes, rings, or ghosts beyond the immediate proximity of the spots. At longer wavelengths (λ > 6000 ┼) the diffraction spikes appear about as bright as the residual scattered light because the diffraction pattern is larger and not as well suppressed by the coronagraph. The spikes are more prominent in images with the large spot than the small one because the Lyot stop is not located exactly in the pupil plane but is slightly ahead of it. Consequently, the beam can "walk" around the stop depending on the field angle of the object. Because the large spot is at the edge of the field, the beam is slightly shifted, allowing more diffracted light to pass around the edges of the stop.
The coronagraphic PSF is dominated by radial streaks that are caused primarily by scattering from zonal surface errors in the HST
mirrors. This halo increases in brightness and decreases in size towards shorter wavelengths. One unexpected feature is a diagonal streak or "bar" seen in both direct and coronagraphic images. It is about 5 times brighter than the mean azimuthal surface brightness in the coronagraphic images. This structure was not seen in the ground-test images and is likely due to scattering introduced by the HST
optics. There appears to be a corresponding feature in STIS as well.
Although the coronagraph suppressed the diffracted light from a bright star, the scattered light from the HST
mirrors could still overwhelm faint, nearby sources. It is possible to subtract most of the remaining light using an image of another occulted star. PSF subtraction has been successfully used with images taken by other HST
cameras, with and without a coronagraph. The quality of the subtraction depends critically on how well the target and reference PSFs match.
Color differences between the target and reference stars also affect the quality of the PSF subtraction. As wavelength increases, the speckles that make up the streaks in the coronagraphic PSF move away from the center while their intensity decreases (Figure 6.7
). The diffraction rings near the spot edges also expand. These effects can be seen in images through wideband filters—a red star will appear to have a slightly larger PSF than a blue one. Thus, an M-type star should be subtracted using a similarly red star—an A-type star would cause significant subtraction residuals. Even the small color difference between A0 V and B8 V stars, for example, may be enough to introduce bothersome errors (Figure 6.9
Changes in HST's
focus also altered the distribution of light in the coronagraphic PSF (Figure 6.10
). Within an orbit, the separation between HST's
primary and secondary mirrors varies on average by 3 Ám, resulting in 1/28 wave rms of defocus @ λ = 5000 ┼. This effect, known as breathing,
is caused by the occultation of the telescope's field of view by the warm Earth, which typically occurs during half of each 96-minute orbit. This heating by the warm Earth expands HST'
s interior structure. After occultation the structure gradually shrinks. Changes relative to the sun (mostly anti-sun pointings) also cause contraction of the telescope, which gradually expands to "normal" size after a few orbits.
In these cases, images of Arcturus were registered with and subtracted from similar images of the star taken a day later (Figure 6.8
.) Combined with PSF subtraction, the coronagraph reduced the median background level by factors of 250 to 2500, depending on the radius and filter. Examples of PSF-subtracted images are shown in Figure 6.11
and Figure 6.12
. The mean of the residuals is not zero; because of PSF mismatches, one image will typically be slightly brighter than the other over a portion of the field. The pixel-to-pixel residuals can be more than 10 times greater than the median level (Figure 6.13
). Note that these profiles would be worse if there were color differences between the target and reference PSFs.
A frequently used method of avoiding the color and normalization problems involved the subtraction of images of the same target taken at two different field orientations. This technique, known as roll subtraction
, was done either by requesting a roll of the telescope about the optical axis (up to 30░) between orbits, or by revisiting the target at a later date when the default orientation of the telescope was different. Roll subtraction only worked when the nearby or circumstellar object of interest was not azimuthally extended. It was the best technique for detecting point source companions or imaging strictly edge-on disks (e.g., Beta Pictoris). It could also be used to reduce the pixel-to-pixel variations in the subtraction residuals by rotating and co-adding the images taken at different orientations. (This worked for extended sources if another PSF star was used.) Ideally, the subtraction errors decreased as the square root of the number of orientations.
Residual errors from the subtraction of one image of Arcturus from another taken in a different visit (filter = F435W, D = 1.8 arcseconds spot)
Objects that were observed in the coronagraphic mode but not placed behind an occulting spot had PSFs that were defined by the Lyot stop. Because the Lyot stop effectively reduced the diameter of the telescope and introduced larger obscurations, this "off-spot" PSF was wider than normal, with more power in the wings and diffraction spikes (Figure 6.14
). Together, the Lyot stop and occulting spot substrate reduced the throughput by 52.5%. In F814W, the "off-spot" PSF had a peak pixel containing 4.3% of the total (reduced) flux and a sharpness (including CCD charge diffusion effects) of 0.010. (Compare these to 7.7% and 0.026, respectively, for the normal HRC PSF.) In F435W, the peak was 11% and the sharpness is 0.025 (compared to 17% and 0.051 for the normal F435W PSF). Users needed to take the reduced throughput and sharpness into account when they determined the detection limits of HRC coronagraphic images. Tiny Tim
may have been used to compute off-spot PSFs.