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Part II: ACS Data Handbook

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1.1 Instrument Design and Capabilities


The ACS camera is designed to provide HST with a deep, wide-field survey capability from the visible to near-IR, high resolution imaging from the near-UV to the near-IR, and solar blind far-UV imaging. The primary design goal of the ACS Wide-Field Channel is to achieve a factor of 10 improvement in discovery efficiency compared to WFPC2, where discovery efficiency is defined as the product of imaging field of view (FOV) and instrument throughput.

ACS comprises three channels, each optimized for a specific goal:

In addition to the these three prime capabilities, ACS also provides:

ACS is a versatile instrument that can be applied to a broad range of scientific programs. For example, the high sensitivity and wide field of the WFC in the visible and near-infrared will make it the instrument of choice for deep imaging programs in this wavelength region. The HRC, with its excellent spatial resolution, provides full sampling of the HST PSF at >6000 and can be used for high precision photometry in stellar population programs. The HRC coronagraph can be used for the detection of circumstellar disks and QSO host galaxies.

1.1.1 Detectors

ACS uses one or more large-format detectors in each channel:

The WFC & HRC CCDs

The ACS CCDs are thinned, backside-illuminated devices cooled by thermo-electric cooler (TEC) stacks and housed in sealed, evacuated dewars with fused silica windows. The spectral response of the WFC CCDs is optimized for imaging at visible to near-IR wavelengths. The HRC CCD covers wavelengths similar to the WFC but the spectral response has been additionally optimized for the near-UV. Both CCD cameras produce a time-integrated image in the ACCUM data-taking mode. The HRC can also be operated in target acquisition (ACQ) mode for coronagraphic observations. As with all CCD detectors, there is noise (readout noise) and time (read time) associated with reading out the detector following an exposure. The minimum exposure time is 0.1 sec for HRC, and 0.5 sec for WFC, and the minimum time between successive identical exposures is 45 sec (HRC) or ~135 sec (WFC) for full-frame and can be reduced to ~36 sec for subarray readouts. The dynamic range for a single exposure is ultimately limited by the depth of the CCD full well (~85,000 e- for the WFC and 155,000 e- for the HRC), which determines the total amount of charge that can accumulate in any one pixel during an exposure without saturation. Cosmic rays will affect all CCD exposures: CCD observations should be broken into multiple exposures whenever possible, to allow removal of cosmic rays in post-observation data processing.

The SBC MAMA

The SBC MAMA is a photon-counting detector which provides a two-dimensional ultraviolet capability, optimized for the far-UV. It can only be operated in ACCUM mode. The ACS MAMA detector is subject to both scientific and absolute brightness limits. At high local (50 counts sec-1 pixel-1) and global (>285,000 counts sec-1) illumination rates, counting becomes nonlinear in a way that is not correctable. At only slightly higher illumination rates, the MAMA detectors are subject to damage. We have therefore defined absolute local and global count-rate limits, which translate to a set of configuration-dependent bright-object screening limits. Sources which violate the absolute count rate limits in a given configuration cannot be observed in those configurations.

1.1.2 ACS Optical Design

The ACS design incorporates two main optical channels: one for the WFC and one which is shared by the HRC and SBC. These channels are illustrated in figures 3.1 and 3.2 of the ACS Instrument Handbook. Each channel has independent corrective optics to compensate for HST's spherical aberration. The WFC has three optical elements, coated with silver, to optimize instrument throughput in the visible. The silver coatings cut off at wavelengths shortward of 3700 . The WFC has two filter wheels which it shares with the HRC, offering the possibility of internal WFC/HRC parallel observing for some filter combinations. The HRC/SBC optical chain comprises three aluminized mirrors, overcoated with MgF2. The HRC or SBC channels are selected by means of a plane fold mirror. The HRC is selected by inserting the fold mirror into the optical chain so that the beam is imaged onto the HRC detector through the WFC/HRC filter wheels. The SBC channel is selected by moving the fold mirror out of the beam to yield a two mirror optical chain which images through the SBC filter wheel onto the SBC detector. The aberrated beam coronagraph is accessed by inserting a mechanism into the HRC optical chain. This mechanism positions a substrate with two occulting spots at the aberrated telescope focal plane and an apodizer at the re-imaged exit pupil.

While there is no mechanical reason why the coronagraph could not be used with the SBC, for health and safety reasons (due to brightness limits of the MAMA detector) use of the coronagraph is forbidden with the SBC.

1.1.3 ACS Geometric Distortion

The ACS detectors exhibit significantly more distortion than previous HST instruments. All ACS observations must be corrected for distortion before any photometry or astrometry is derived. For a thorough discussion of ACS Geometric Distortion, we refer the reader to Chapter 4.

The principal cause of the ACS distortion is that the optics have been designed with a minimum number of components, consistent with correcting for the spherical aberration induced by the Optical Telescope Assembly (OTA), without introducing coma. The result is a high throughput, but focal surfaces far from normal to the principal rays. The WFC detector is tilted at 22 degrees giving an elongation of 8% along the diagonal. The HRC and SBC detectors have a 25 degree tilt giving an elongation of 12%. In each case, the scale in arcseconds per pixel is smaller along the radial direction of the OTA field of view than along the tangential direction. When projected on the sky, this causes each detector to appear "rhombus-shaped" rather than square. The angle on the sky between the x and y axes is 84.9 degrees for the WFC1, 86.1 for the WFC2 and 84.2 degrees for the HRC.

The orientations of the ACS detector edges are approximately in line with the V2 and V3 coordinate axes of the telescope. Consequently, the eigenaxes of the scale transformation are along the diagonals for WFC and the apertures and pixels appear non-rectangular in the sky projection. For the HRC and SBC the situation is even more irregular because the aperture diagonals do not lie along a radius of the HST field of view. Figure 1.1 shows the ACS apertures in the telescope's V2/V3 reference frame and illustrates the "rhombus" shape of each detector. A telescope roll angle of zero degrees would correspond to an on-sky view with the V3 axis aligned with North and the V2 with East. The readout amplifiers are also indicated for each detector.

If these were the only distortions present, their impact on photometry and mosaicing/dithering could be simply computed. A more problematic effect is the variation of scale and pixel area across each detector. For the WFC this amounts to a change of ~10% in scale from corner to corner. For the HRC and SBC this variation is only about 1%, since these detectors cover much smaller fields of view. The area on the sky covered by a WFC pixel varies by ~18% from corner to corner, allowance for which must be made in photometry.

Dithering and mosaicing are complicated by the fact that an integral pixel shift near the center of the detector will translate into a non-integral displacement for pixels near the edges. This is not a fundamental limitation, but will imply some computational complexity in registering images and will depend on an accurate measurement of distortions.


 
Figure 1.1: ACS apertures compared with the V2/V3 reference frame. The readout amplifiers (A,B,C,D) are indicated on the figure. When ACS data are processed through MultiDrizzle in the HST pipeline, the resulting drizzled images are oriented with their x,y axes corresponding approximately to the x,y axes shown here. Thus, the WFC data products are oriented so that WFC1 (chip 1, which uses amps A,B) is on top (see also Figure 2.2), and the HRC data products are also oriented such that amps A,B are on top.
 

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