Space Telescope Science Institute   3.3.1 Detectors  3.4.1 Target Acquisitions

3.3.2 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. Each channel has independent corrective optics to compensate for spherical aberration in the HST primary mirror. The WFC has three silver-coated optics to optimize instrument throughput in the visible and near-IR. The silver coatings cut off at wavelengths shortward of 3700 Ć. 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 ( Section 7.9). The optical design of the WFC is shown schematically in Figure 3.2. The HRC/SBC optical chain comprises three aluminized mirrors overcoated with MgF2, shown schematically in Figure 3.3. The HRC or SBC channels are selected by means of a plane fold mirror (M3 in Figure 3.3). 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 that focuses light 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, use of the coronagraph is forbidden with the SBC.

Figure 3.2: ACS optical design: wide field channel.

 
Figure 3.3: ACS optical design: high resolution/solar blind channels

 

Filter Wheels

ACS has three filter wheels: two shared by the WFC and HRC, and a separate wheel dedicated to the SBC. The WFC/HRC filter wheels contain the major filter sets. Each wheel also contains one clear WFC aperture and one clear HRC aperture (see Chapter 5 for more on filters). Parallel WFC and HRC observations are possible for some filter combinations and these are automatically added by the Astronomer's Proposal Tool (APT) (http://apt.stsci.edu/) in Phase II, unless the user disables this option via the PAREXP optional parameter, or if adding the parallel observations cannot be done due to timing considerations. Because the filter wheels are shared, it is not possible to independently select the filter for WFC and HRC parallel observations.

Calibration-Lamp Systems

ACS has a calibration subsystem consisting of tungsten lamps and a deuterium lamp for internally flat fielding each of the optical chains. The calibration subsystem illuminates a diffuser on the rear surface of the ACS aperture door, which must be closed for calibration exposures. Under normal circumstances, users are not allowed to use the internal calibration lamps.

In addition, a post-flash capability was added to the instrument to provide the means of mitigating the effects of Charge Transfer Efficiency (CTE) degradation. We do not expect to use this facility at present (except for calibration and characterization) but in later years, as radiation damage of the CCDs causes the CTE to degrade, it is possible that some users will want to avail themselves of this facility. Since we are not yet at a stage where use of post-flash is expected to be useful for any science observations, we do not provide an extensive discussion related to it.


 3.3.1 Detectors  3.4.1 Target Acquisitions
Space Telescope Science Institute
http://www.stsci.edu
Voice: (410) 338-1082
help@stsci.edu