The ACS High Resolution Camera has a user-selectable coronagraphic mode for imaging faint objects (circumstellar disks, substellar companions) near bright point sources (stars or luminous quasar nuclei). The coronagraph suppresses the diffraction spikes and rings of the occulted source below the level of the scattered light, most of which is 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 is deployed into the aberrated beam, which is subsequently corrected by the ACS optics. Although it is not as efficient as a corrected-beam coronagraph (especially for imaging close to the occulted source) the HRC coronagraph significantly improves the high-contrast imaging capabilities of
HST. Care must be taken, however, to design an observation plan that properly optimizes the coronagraph’s capabilities and accounts for its limitations.
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 λ = 2000 Å
to 10,000 Å using a variety of broad-to-narrowband filters.
The occulting spots are solid (unapodized) metallic coatings deposited on a glass substrate that reduces the 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 the edge of the field (
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 occulted source. The larger spot may be 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 allows imaging of material out to 20 arcseconds from the occulted 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 OTA (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 is unaltered by the coronagraph. The smaller aperture and larger central obscuration of the Lyot stop reduce the throughput by 48% and broaden the field PSF. 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 imaging. However, this configuration prevents the acquisition of internal coronagraphic flat fields.
In addition to the occulting spots and Lyot stop there is a 0.8 arcseconds x 5 arcseconds occulting finger (OCCULT-0.8) permanently located at the window of the CCD dewar. It does not suppress any diffracted light because it occurs after the Lyot stop. Its intended purpose was to allow imaging 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, which reduces its effectiveness. It was originally aligned with the center of the 3.0 arcsecond spot, but shifting of the spots during launch ultimately placed the finger near the edge of the spot. Because of vignetting and the sensitivity of the occulted PSF to its position behind the finger, unocculted saturated observations of sources will likely be more effective than those using the occulting finger.
The bright point 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, which is too crude to ensure accurate positioning behind the spot. 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. To define an acquisition image in
APT, specify
HRC-ACQ as the aperture and
ACQ as the
opmode.
Because the coronagraph is deployed during acquisition, throughput is decreased by 52.5% relative to a non-coronagraphic observation. Also, the Lyot stop broadens the PSF, 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 avoids saturating the source but provides enough flux for a good centroid measurement. A peak pixel flux of 2000 e
– to 50,000 e
– is desirable. 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
synphot1 or the
ETC may be high or low by a factor of two.
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.
The ACS data pipeline will divide images by the P-flat. P-flats specific to the coronagraph have been derived from either ground-based or on-orbit data for filters F330W, F435W, F475W, F606W, and F814W. Other filters use the normal flats, which may cause some small-scale errors around dust diffraction patterns. The pipeline then divides images by the spot flat, using a table of spot positions versus date to determine the proper shift for the spot flat. However, there is a lag in determining the spot position, so it may be a month after the observation before the pipeline knows where the spot was on that date. So, coronagraph users should note that their data may be calibrated with an incorrect spot flat if they extract their data from the archive soon after they were taken. (Spot flats for the filters listed above are available for download from the ACS reference files Web page. For other filters, the available spot flat closest in wavelength should be used. The spot flat must be shifted by an amount listed on the reference files page to account for motions of the occulting spots.)
Early in Cycle 11, coronagraphic 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 is thus unresolved by the coronagraph. The coronagraphic image of a star is quite unusual. Rather than appearing as a dark hole surrounded by residual light, as would be the case in an aberration-free coronagraph, the interior of the spot is filled with a diminished and somewhat distorted image of the star. This image is caused by the M2 mirror’s correction of aberrated light from the star that is not blocked by the spot. The small spot is filled with light, while 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 wings of the aberrated PSF by the occulting spot itself. Consequently, coronagraphic images of bright stars may saturate at the interior and edges of the spot within a short time. Within the small spot, the brightest pixels will saturate in less than one second for a V = 0.0 star, while pixels at edge of the larger spot will saturate 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 Å) 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.
As mentioned above, for any pair of target and reference PSF observations, there is likely to be a difference of 5 to 20 milliarcseconds between the positions of the stars. Because the scattered light background is largely insensitive to small errors in star-to-spot alignment (it is produced before the coronagraph), most of it can be subtracted if the two stars are precisely registered and normalized. Due to the numerous sharp, thin streaks that form the scattered light background, subtraction quality is visually sensitive to registration errors as small as 0.03 pixels (0.75 milliarcseconds). To achieve this level of accuracy, the reference PSF may be iteratively shifted and subtracted from the target until an offset is found where the residual streaks are minimized. This method relies on the judgment of the observer, as any circumstellar material could unexpectedly bias a registration optimization algorithm. A higher-order sampling method, such as cubic convolution interpolation, should be used to shift the reference PSF by subpixel amounts; simpler schemes such as bilinear interpolation degrade the fine PSF structure too much to provide good subtractions.
Normalization errors as small as 1% to 4% between the target and reference stars may also create significant subtraction residuals. However, derivation of the normalization factors from direct photometry is often not possible. Bright, unocculted stars will be saturated in medium or broadband filters at the shortest exposure time (0.1 seconds). An indirect method uses the ratio of saturated pixels in unocculted images (the accuracy improves with greater numbers of saturated pixels). A last-ditch effort would rely on the judgment of the observer to iteratively subtract the PSFs while varying the normalization factor.
Color differences between the target and reference PSFs can be controlled by choosing an appropriate reference star. 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 will expand as well. 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).
A focus change can also alter the distribution of light in the PSF. HST’s focus changes over time scales of minutes to months. Within an orbit, the separation between the primary and secondary mirrors varies on average by 3 μm, resulting in 1/28 wave rms of defocus @
λ = 5000 Å. 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 heats
HST’s interior structure, which expands. After occultation the telescope gradually shrinks. Changes relative to the sun (mostly anti-sun pointings) cause contraction of the telescope, which gradually expands to “normal” size after a few orbits. The main result of these small focus changes is the redistribution of light in the wings of the PSF (
Figure 6.10).
Plots of the azimuthal median radial profiles after PSF subtraction are shown in Figure 6.8. In these cases, images of Arcturus were subtracted from similar images of the star taken a day later. The images were registered as previously described. Combined with PSF subtraction, the coronagraph reduces the median background level by 250x
to 2500x, depending on the radius and filter. An example of a PSF subtraction is shown in
Figure 6.11. 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 (
Figure 6.12). The pixel-to-pixel residuals can be more than 10x 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.
One way to avoid both the color and normalization problems is to take images of the target at two different field orientations, and subtract one from the other. This technique, known as roll subtraction, can be 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 is different. Roll subtraction only works when the nearby object of interest is not azimuthally extended. It is the best technique for detecting point source companions or imaging strictly edge-on disks (e.g. Beta Pictoris). It can 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 works for extended sources if another PSF star is used.) Ideally, the subtraction errors will decrease as the square root of the number of orientations.
Objects that are observed in the coronagraphic mode but that are not placed behind an occulting mask have a PSF that is defined by the Lyot stop. Because the stop effectively reduces the diameter of the telescope and introduces larger obscurations, this “off-spot” PSF is wider than normal, with more power in the wings and diffraction spikes (Figure 6.14). In addition, the Lyot stop and occulting spot substrate reduce the throughput by 52.5%. In F814W, the “off-spot” PSF has 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 is 11% and the sharpness is 0.025 (compared to 17% and 0.051 for the normal F435W PSF). Observers need to take the reduced throughput and sharpness into account when determining detection limits for planned observations.
Tiny Tim can be used to compute off-spot PSFs.
