Space Telescope Science Institute
ACS Data Handbook V7.1
help@stsci.edu
Table of Contents Previous Next Index Print


ACS Data Handbook > Chapter 1: ACS Overview > 1.1 Instrument Design and Capabilities

1.1
ACS was designed to provide a deep wide field survey capability from the visible to near-IR using the Wide Field Camera (WFC), high resolution imaging from the near-UV to near-IR with the now-defunct High Resolution Camera (HRC), and solar blind far-UV imaging using the Solar Blind Camera (SBC). The discovery efficiency of ACS’s Wide Field Channel (i.e., the product of WFC’s field of view and throughput) is 10 times greater than that of WFPC2.
The failure of ACS’s CCD electronics in January 2007 temporary halted CCD imaging until Servicing Mission 4 in May 2009, during which WFC functionality was restored. Unfortunately, the high resolution optical imaging capability of HRC was not recovered.
ACS comprises three channels, each optimized for a specific goal:
-
-
-
-
-
-
-
-
-
In addition to these three prime capabilities, ACS also offers the following:
-
-
-
-
-
ACS is a versatile instrument that can be applied to a broad range of scientific programs. For example, WFC’s high sensitivity and large field of view at red and near-infrared wavelengths make it the preferred camera for deep imaging programs in this wavelength region.
Before January 2007, HRC provided full sampling of the HST PSF at λ > 6000 and could be used for high precision photometry in stellar population studies. The HRC coronagraph could be used for detection of circumstellar disks and QSO host galaxies.
1.1.1 Detectors
ACS uses one or more large-format detectors in each channel:
WFC & HRC CCDs
ACS CCDs are thinned, backside-illuminated devices cooled by thermoelectric coolers (TECs), and housed in sealed, evacuated dewars with fused silica windows. The spectral response of WFC CCDs is optimized for imaging at visible to near-IR wavelengths. HRC’s CCD covered wavelengths similar to WFC, but its spectral response was optimized for the near-UV. Both CCD cameras produce a time-integrated image in the “ACCUM” data-taking mode. HRC also operated in target acquisition (ACQ) mode for coronagraphic observations.
As with all CCD detectors, there is read noise and overhead associated with reading out the detector following an exposure. The minimum exposure time was 0.1 seconds for HRC, and is 0.5 seconds for WFC. The minimum time between successive identical full frame exposures was 45 seconds for HRC, and is ~135 seconds for WFC. These times can be reduced to ~36 seconds for WFC sub-array readouts. The dynamic range for a single exposure is ultimately limited by the depth of the pixel well (~85,000 e for WFC and ~155,000 e for HRC), which determines the saturation limit of any one pixel. Hot pixels and cosmic rays affect all CCD exposures. CCD observations should be broken into multiple dithered exposures to allow removal of hot pixels and cosmic rays in post-observation data processing.
SBC MAMA
The SBC MAMA is a photon-counting detector which provides two-dimensional imaging optimized for far-UV wavelengths. The MAMA can only be operated in “ACCUM” mode. SBC observations are 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 slightly higher illumination rates, MAMA detectors can be permanently damaged. Lower absolute local and global count rate limits have been imposed that define bright object screening limits for each SBC configuration. Targets that violate these screening limits cannot be observed in the proposed configuration.
1.1.2 ACS Optical Design
ACS has two main optical channels, one dedicated to WFC and one shared by HRC and SBC. These channels are shown in Figures 3.2 and 3.3 of the ACS Instrument Handbook. Each channel has independent corrective optics to compensate for HST’s spherical aberration. WFC has three optical elements coated with silver to optimize visible light throughput. The silver coatings cut off at wavelengths shortward of 3700 . WFC shared two filter wheels with the HRC, which enabled internal WFC/HRC parallel observing for some filter combinations.
The HRC/SBC optical chain comprises three aluminized mirrors overcoated with MgF2. The HRC was selected by inserting a plane fold mirror into the optical path so that the beam was imaged on the HRC’s detector through the WFC/HRC filter wheels. The SBC is selected by moving the fold mirror out of the beam and allowing light to pass through the SBC filter wheel onto the SBC detector. The aberrated beam coronagraph was deployed with a mechanism that inserted a window with two occulting spots at the aberrated telescope focal plane and an apodizer at the re-imaged exit pupil. For detector health and safety reasons, use of the coronagraph with SBC is forbidden.
1.1.3 ACS Geometric Distortion
ACS’s focal planes exhibit significantly more geometric distortion than those of previous HST instruments. All ACS images must be corrected for this distortion before any photometry or astrometry is derived. For detailed information about ACS geometric distortion, please refer to the DrizzlePac Handbook.
ACS’s geometric distortion is principally caused by its optical design, which features a minimum number of components for correcting the spherical aberration induced by the Optical Telescope Assembly (OTA) without introducing coma. The optics allow high throughput, but their focal surfaces are far from normal to the principal rays. The WFC detector is tilted by 22, so its projected diagonals differ by 8%. The HRC and SBC detectors are tilted by 25, so their projected diagonals differ by 12%. Consequently, the projected footprints of the detectors on the sky appear rhomboidal rather than square, and their pixel scales are smaller along the radial direction of the OTA field of view than along the tangential direction. The angles between the projected x- and y-axis of the detectors are 84.9 for WFC1, 86.1 for WFC2, and 84.2 for HRC.
Figure 1.1 shows the locations of the WFC and HRC apertures in the telescope’s V2/V3 reference frame, the rhomboidal projections of each detector, and the locations of their readout amplifiers (A, B, C, and D). A telescope roll angle of zero degrees corresponds to an on-sky view with the V3 axis aligned with North and the V2 with East. The orientations of the physical edges of the detectors are approximately parallel with the V2 and V3 coordinate axes of the telescope, but the eigenaxes of the pixel scale transformation of the WFC are along the projected diagonals of the detectors. The situation is even more irregular for the HRC and SBC because the aperture diagonals do not lie along a radius of the OTA field of view. Moreover, the scale and area of WFC pixels vary by ~10% and ~18%, respectively, from corner to corner. For HRC and SBC, the pixels scale by only ~1% from corner to corner because these detectors have smaller fields of view. Corrections for these scale and area differences must also be made before photometric measurements can be obtained.
Figure 1.1: WFC and HRC Apertures Compared with the V2/V3 Reference Frame
The readout amplifiers (A,B,C,D) are indicated on the figure. When ACS images are processed through AstroDrizzle in the OPUS1 data pipeline, the resulting drizzled images are oriented with their x,y axes corresponding approximately to the x,y axes shown in this diagram. Thus, the WFC data products are oriented so that WFC1 (which uses amplifiers A and B) is on top in the positive y-direction (also see Figure 2.3), and the HRC images are oriented such that amplifiers A and B are at the top in this diagram.
1.1.4 ACS Performance after Servicing Mission 4
ACS suffered component failures in its Side 1 and Side 2 electronics in June 2006 and January 2007, respectively. The latter event prevented operations of WFC and HRC cameras. SBC was unaffected by these failures and remained available for scientific use throughout this problematic period.
WFC was restored to operation after the successful installation of a replacement CCD electronics box (CEB-R) and power supply during Servicing Mission 4 (SM4) in May 2009. Unfortunately, additional damage to the HRC power harness during the January 2007 failure prevented recovery of the HRC during SM4, leaving it unavailable for scientific use.
Tests conducted shortly after SM4 showed that:
Read noise, linearity, pixel full well depth, and amplifier cross-talk of the restored WFC are as good or better than the pre-failure levels in January 2007 (Table 1.1).
WFC’s dark current, hot pixel fraction, and charge transfer efficiency (CTE) have degraded to the levels expected after extended exposure to HST’s trapped radiation environment.
All WFC images exhibit faint horizontal stripes that run across the quadrant boundaries. The stripes, constant along each row but not stable from frame to frame, are caused by low frequency (1 mHz to 1 Hz) 1/f noise on the bias reference voltage generated by the CEB-R. Global read noise statistics due to the stripes is small (standard deviation is 0.9 e), but the correlated nature of the noise may affect photometric precision for very faint sources. An algorithm for removing these stripes using an analysis of the pre-scan regions of the four WFC quadrants has been implemented in the bias correction stage of calacs in the OPUS data pipeline. Stripe removal is automatically performed only on full frame (4096 x 4096 pixel) post-SM4 WFC images; it is not performed on sub-array images. STScI provides a standalone routine (acs_destripe, in the acstools package in stsci_python2) for mitigating bias stripes in sub-array images of sparse fields. For most observing programs, however, the bias stripes will have a negligible effect on science results and their removal will provide only cosmetic benefits.
Bias frames obtained under default CEB-R operation show a 5 - 10 DN gradient spanning the rows and columns of each image quadrant. This bias gradient is stable over the time between consecutive calibration reference files, so it is precisely removed during normal image reduction and processing.
The default CEB-R mode induces a signal-dependent bias shift, an effect related to the bias gradient. The DC level of the dual-slope integrator is sensitive to changes in the CCD output voltage in such a way that the pixel bias level is shifted positively by 0.02%  0.30% (depending on the amplifier) of the signal from the previously integrated pixel. This phenomenon is well understood for full frame WFC images, and an automatic correction for it has been implemented in the bias correction stage in calacs. This correction is not applied to WFC sub-array images.
Table 1.1: Comparison of WFC Performance Before the Side 2 Failure and After SM4
Read Noise (e ; gain = 2)
Dark Current (e /pix/hr)
Cross-talk (50Ke source)
1
The Operations Pipeline Unified System (OPUS) is the name of the pipeline software that controls the processing and archiving of data at STScI, converting telemetry into FITS data products, populating the Archive catalog, and performing housekeeping on the pipelines. See Chapter 3.1 for more details.
2
stsci_python is a library of Python routines and C extensions that is being developed to provide a general astronomical data analysis infrastructure.


ACS Data Handbook > Chapter 1: ACS Overview > 1.1 Instrument Design and Capabilities

Table of Contents Previous Next Index Print