ACS Data Handbook for Cycle 20

Several circular patterns consisting of a dark ring with a bright center, are visible in the ACS flat fields, with typical diameters of ~30 pixels on HRC and ~100 pixels on WFC. These artifacts are shadows of dust on the CCD windows and are weaker on the f/25 WFC than on the f/68 HRC. The motes can be seen in WFC and HRC flats in Figure 4.16 and Figure 4.17.

Since the shapes and depths of these motes are almost independent of wavelength, their effects will be removed by the flats to << 1%, unless any of these particulate contaminants move to different positions on the CCD windows. In case of particulate migrations, the internal lamp flats have a lower f-ratio with a wider angular distribution and cannot be used to patch the flat fields because they wash out the mote shadows. To correct for new motes, patches to the pipeline flats must be made using the original laboratory flats, corrected for the low-frequency flats derived in-flight or, for short wavelengths, using observations of the bright Earth.

Larger motes are sometimes present due to dust and blemishes on several ACS filters, including F606W, POL0V, and POL60V. Because the filters are located farther from the detector windows in a converging light beam, imperfections on the filters produce an out-of-focus image at the detector, where typical mote diameters are about 350 - 400 pixels on the WFC and about 250 - 300 pixels on the HRC. One of these large motes can be seen on chip 2 of the WFC F606W flat in Figure 4.16.

Until April 2004, the positioning accuracy of the filter wheels has been within +/- one motor step of the nominal position. This delta corresponds to a distance on the detectors of ~18 HRC pixels and ~20 WFC pixels. Features with sharp transmission gradients at the filter wheels cause a corresponding flat field instability, where errors are 1% - 3% for a few pixels near the blemishes. If the filter wheel lands in a different place the dust mote will move. Blemish mis-registration is an error in the pixel-to-pixel high frequency component of the flat fields and is not related to the low-frequency L-flat correction, which has been applied for all standard and polarizing filters. For details on the L-flat correction, see Section 4.4.2.

This problem was recognized and addressed before launch by a laboratory calibration campaign to obtain flat fields at the nominal position and at plus and minus one step for the F606W + CLEAR on the WFC and for the two POLV filters in combination with the highest priority F475W, F606W, and F775W filters on both HRC and WFC. Since the resolver position uniquely determines the filter wheel step, the ACS pipeline data processing has been enhanced to automatically apply the proper flat for the wheel step position. The keyword FWOFFSET has been added to the ACS image headers to indicate the position of the filter wheel. In April 2004, an update was made to the ACS flight software and the filter wheel is now always positioned at its nominal position. For more information on flat fields for filter wheel offset positions, refer to ACS ISR 2003-11.

ACS was designed with a requirement that no single stray light feature may contain more than 0.1% of the detected energy in the object that produced it. This goal has generally been met, but during extensive ground and SMOV test programs, a few exceptions have been identified (Hartig et al. 2002, Proc. SPIE 4854) such as the WFC elliptical haloes and the F660N ghosts. While these ghosts exceed the specified intensity, their origin and characteristics are well defined and they should have minimal impact on the ACS science program. We describe below in more detail some of the optical ghosts relevant for ACS.

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WFC elliptical haloes: these ghosts are caused by reflection from the CCD surface (which lies at an ~20° angle to the chief ray) up to the detector windows and back to the CCD. They show up as pairs of elliptical annuli, aligned along the negative diagonal of the FOV (see Figure 4.19), and are observed when bright sources are placed on the lower right (D amplifier) quadrant of the WFC detector. The surface brightness of the annuli increases and size decreases with proximity to the corner. Two pairs of ghosts are seen, produced by reflection from the four window surfaces. The total energy fraction in each ghost may exceed 0.2% of the target signal.

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F660N ghosts: the F660N narrow band filter produces pairs of relatively bright circular annuli stationed near to (but radially outward from) the target image (see Figure 4.20). This is due to reflection from the two surfaces of the second “blocker” substrate back to the many-layer dielectric stack on the first substrate, which in turn reflects at high efficiency at the filter wavelength range. These haloes contain ~2% of the detected target energy and are always about 10 and 20 pixels in diameter.

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Annular ghosts: large annular ghosts 1 near their parent images are caused by reflection from the detector windows, back to the filters, then returning to the CCD (see Figure 4.19). Another type of annular ghost arises from reflections between the inner and outer window surfaces; these are much smaller in diameter, relatively low in intensity (well within the specification) and are displaced radially from the parent image by a small amount.

•	Glint: a star that falls close to the gap between the two WFC CCDs can sometimes create a reflection known as a “glint,” thin rays of light extending from the chip gap, as shown in Figure 4.21. These rays are the reflection of starlight off the residual indium solder used for chip attachment.

In general, little can be done about ghosts and blooming in the post-observation data processing phase. Instead, some judicious planning of the actual observations, particularly if bright sources are expected in, or near, the field of view, is recommended. For instance, the impact of diffraction spikes (which for ACS lie along X and Y axes) and of CCD blooming (which occurs along the Y direction) due to a bright star, can be reduced by choosing an ORIENT² which prevents the source of interest from being connected to the bright star along either of these axes. Alternatively, a suitable ORIENT3 could move the bright star(s) into the interchip gap or off the field of view altogether. Similarly, the impact of WFC elliptical haloes can be minimized by avoiding a bright star in the quadrant associated with amplifier D.

This image was obtained during ground calibration at Ball Aerospace using a HeNe laser (633 nm) through F625W (Hartig, et al.). It is a factor of ~50,000 saturated with nine highly saturated point sources in and just off of the field of view. Note the diffraction spikes along the X and Y axes of the WFC, and the severe blooming of the charge along the Y-axis. Several optical ghosts with different origins and intensities (see text below for details) are visible.

(4) “Dragon’s Breath” is a shower of scattered light from a very bright star that is just off the edge of the CCD. This rare anomaly occurs when a star falls at the edge of the mask in front of the chip; the starlight reflects off the CCD, then off the mask, and back to the detector.

This 700 second F660N exposure illustrates the F660N ghosts, namely pairs of relatively bright circular annuli stationed near to (but radially outward from) the target image. These haloes contain ~2% of the detected target energy and are always about 10 and 20 pixels in diameter.

The ACS/WFC detector has four amplifiers through which the four quadrants of the detector are read separately and simultaneously. As the quadrants are read out, electronic cross-talk between the amplifiers can be induced. As a result, an imaged source in one quadrant may appear as a faint, mirror-symmetric ghost image in the other quadrants. The ghost image is often negative; therefore, bright features on the “offending” quadrant show up as dark depressions on the “victim” quadrants.

The pre-SM4 ACS/WFC cross-talk was studied using image frames of external targets. Cross-talk ghosts from extended offending sources were observed as faint, negative images, ~2 e− per pixel relative to the background, characterized by approximately constant surface brightness. There were indications that sources in some quadrants were more effective in generating ghosts than sources located in other quadrants, and some quadrants were more susceptible to cross-talk than others.

Cross-talk turned out to be stronger for high signal offending pixels, although its most conspicuous manifestations were seen for low signal source pixels, as shown in Figure 4.22. Images taken with GAIN = 1 were more affected by cross-talk than images taken at GAIN = 2. In general, cross-talk ghosts appear to have very little impact on ACS/WFC photometry and can be ignored in most cases, especially if the observations are performed using GAIN = 2.

Cross-talk properties changed in a few respects after Servicing Mission 4 in May 2009, when the ACS/WFC electronics was replaced. Cross-talk was thoroughly characterized for GAIN = 2, which was made the default setting for post-SM4 science observations with ACS/WFC, and some limited analysis was conducted for GAIN = 1. Analysis of the GAIN = 2 case used both dark frames and external observations; for GAIN = 1, only external frames were used. In dark frames, the offending source pixels were produced by cosmic rays and hot pixels. Data from external observations also contain cross-talk generated by the observed targets.

This image (from program ID 10594, image j9ew02wlq, GAIN = 2) shows the galaxy in quadrants C and D. Its cross-talk ghosts, seen as dark oval shapes in quadrants A and B, are due to low signal offending sources of ~100 e− to 1000 e− in the area of the reddish rim of the galaxy. Also the ghosts of three largest galaxies in quadrant B are easily identifiable in quadrant A. The image in quadrants A and B is stretched within a narrow signal range centered at the sky background level, which makes the ghosts distinctly stand out against the background.

The image shows NGC6217 centered in quadrant A, obtained with GAIN = 2 (program ID 11371, frame ja7z03unq). The images in quadrants B, C, and D are stretched similarly to that in quadrants A and B in Figure 4.22. No ghosts from low signal areas in quadrant A are seen in the other three quadrants.

Unlike the distinct cross-talk ghosts seen in pre-SM4 images (Figure 4.22), no ghosts from low signal sources were seen in the post-SM4 GAIN = 2 image displayed in a similar way, in Figure 4.23. No ghosts were noticed in GAIN = 1 images either. Thus, the new ACS/WFC electronics is less susceptible to negative cross-talk at low signal levels.

This is consistent with a more quantitative analysis of the cross-talk. The pre-SM4 ghost value distribution shown in Figure 4.24 has distinct dips for source signals of 200 e− to 600 e− for the “offending” --> “victim” combinations C --> A, C --> B, D --> A, and D --> B. These ghosts are seen as dark oval outlines in the upper part of Figure 4.22. The combinations B --> A and B --> C also exhibit substantial dips in the same source signal range. These ghosts of galaxies in quadrant B are clearly seen in quadrant A. (The way quadrant C is displayed does not allow us to see the ghosts in that quadrant.) At the same time the post-SM4 ghost value distribution shown in Figure 4.25 is either positive or zero within the errors for all “offender” --> “victim” combinations.

Cross-talk from high signal source pixels was detected in post-SM4 images obtained with both GAIN = 1 and GAIN = 2. There are remarkable differences between the two gains. For GAIN = 1, the ghosts of full well pixels are ~100 e− on the same CCD and 30 e− to 40 e− on the quadrants of the other CCD. These values represent a dramatic increase from the pre-SM4 level. Below saturation, offending pixels above ~25,000 e− produce ghosts of, on average, -10 e− .

Unlike the GAIN = 1 case, no significant changes were found in the GAIN = 2 high signal cross-talk after SM4. It is observed only among quadrants on the same CCD, as noticed in pre-SM4 data, its depth of about 5 e− to 8 e− is similar to what it was before SM4, and it is about two times higher when the offender is the CCD’s left quadrant.

The red circles are from a dark frame obtained during the post-SM 4 SMOV period. The green circles are the average of 30 pre-SM4 dark frames obtained before the ACS/WFC electronics failure. The source-ghost pairing occurs only between quadrants on the same CCD. Full-well source pixels produce negative ghosts of about 5 e− to 8 e−, consistent with the ghosts from saturated stars in the image of the NGC 6217 field in Figure 4.23.

The dependence of the GAIN = 2 high signal cross-talk on the offending signal is shown in Figure 4.26 for offending pixels above 5000 e− in pre-SM4 (green) and post-SM4 (red) data. There are no significant differences between the pre- and post-SM4 behaviors. Cross-talk in both cases is seen only between the quadrants on the same CCD and becomes noticeable only for signals above 20,000 e−, with a maximum of 5 e− to 8 e− for signals near the pixel’s full-well depth. The cross-talk signal dependence is essentially linear, with post-SM4 slopes in the range of about 5E-5 to 7.5E-5.

In general, the conclusions from the ACS/WFC cross-talk characterization are as follows:

•	For the current default GAIN = 2, the post-SM4 cross-talk induced by high signal pixels remains at pre-SM4 levels (ghost-to-signal ratio ~0.01% for high signal offending pixels) and thus can be ignored in most scientific applications of ACS/WFC.

Most observers will not experience significant issues with scattered Earth light in their observations. Normally observations are scheduled only when the bright Earth limb is more than 20° from the HST pointing direction. This is sufficient to eliminate serious impacts from scattered Earth light—the most severe impact will be for observers with targets in the CVZ³ who may notice the sky background increased by a factor of 2 or 3.

It is possible to make arrangements for observations at smaller bright Earth limb angles, and these images have a potential for serious impacts from scattered light. There are two types of impact: elevated background and non-uniformity in the background. For example, at a bright Earth avoidance angle of 14°, it is possible for the sky level to be increased by a factor of 100 compared to normal pointings away from the Earth; this will of course have a serious impact on the background noise and detection of faint targets. Also, non-uniformity can arise since the scattered light is taking an increasingly non-standard path through the HST optics, and hence the flat fielding becomes corrupted. At this same angle of 14°, it is possible to have both large scale gradients across the field of view (up to ~20% amplitude in the WFC) and small scale features in the background (up to ~12% in WFC and ~30% in the HRC). See ACS ISR 2003-05 for more details.

To date, all CCDs flown in the harsh radiation environment of HST suffer degradation of their charge transfer efficiency (CTE). The effect of CTE degradation is to reduce the apparent brightness of sources, requiring the application of photometric corrections to restore measured integrated counts to their true value. Figure 4.27 shows the CTE trails in an image of 47 Tucanae observed after SM4. For a detailed discussion of CTE, please refer to Section 5.1.5.

Each ACS filter is designed to maintain confocality with both the WFC and HRC when paired with a clear aperture in the other filter wheel. To maintain this confocality when filters are paired with polarizers, the polarizers were fabricated with additional lensing power which alters the pixel scale and geometric distortion of each camera. The additional distortion is further complicated by localized optical defects (bubbles and wrinkles) in the polaroid materials (See ACS ISR 2004-09).

Polynomial solutions of the geometric distortion for each filter are used by the multidrizzle stage of the ACS calibration pipeline to produce geometrically rectified and resampled WFC and HRC images for photometric and astrometric use. These rectified images are provided as FITS images with the drz.fits extension.

Presently, distortion solutions have not been derived for HRC images obtained with the UV or visible polarizers. Such images constitute about 4% of the HRC datasets in the HST archive. Consequently, the drz.fits files produced by multidrizzle for polarized HRC images have a pixel scale that differs by ~3% from the correct pixel scale obtained for non-polarized images. A correct distortion solution will be generated before the planned creation of a static HRC image Archive. Until then, users must exercise caution when performing astrometry or surface brightness measurements with polarized HRC images.