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.
Until April 2004, the positioning accuracy of the filter wheels has been within motor steps 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
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. Similarly, the impact of WFC elliptical haloes can be minimized by avoiding a bright star in the quadrant associated with amplifier D.
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.
The ACS/WFC detector has four amplifiers (A, B, C, D; see Figure 1.1
) 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. See Figure 4.22
and Figure 4.23
for actual examples.
The latest calacs
corrects for cross-talk in post-SM4 full frame WFC images as part of the doBlev
stage (see Section 3.4.1
). Additional information about calacs
is also available at
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.
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.
Polynomial solutions of the geometric distortion for each filter are used by the AstroDrizzle
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
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 AstroDrizzle
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.