3.4 Basic Instrument Operations

3.4.1  Target Acquisitions

For the majority of ACS observations, target acquisition is simply a matter of defining the appropriate aperture for the observation. Once the telescope acquires its guide stars, the target will be positioned within 0.5 arcseconds of the specified aperture. For observations with the ramp filters, the desired central wavelength for the observation must be specified. 

3.4.2  Typical ACS Observing Sequence

ACS is used primarily for deep, wide-field survey imaging. Important issues for observers to consider are the "packaging" of their observations, how sub-stepping or "dithering" of images for removal of hot pixels and cosmic rays is implemented, and how, if necessary, a mosaic pattern is constructed to map the target. It is also very important to consider the effects of charge transfer efficiency losses. See ACS ISR 2019-07 for more discussion of these issues in planning ACS observations.

Narrowband observations with the WFC are more likely to be readnoise-limited, requiring consideration of optimum number of readouts. Observations with the MAMA detector is not affected by cosmic rays or readnoise, but long integration times will often be needed to obtain sufficient signal-to-noise.

A typical ACS observing sequence consists of a series of 10 to 20 minute dithered exposures for each desired filter. Observers will generally not take their own calibration exposures. See Chapter 7 for more details about observing strategies.

3.4.3  Data Storage and Transfer

At the conclusion of each exposure, the science data are read out from the detector and placed in ACS's internal buffer memory, where they are stored until they can be transferred to the HST solid state data recorder (and thereafter to the ground). The internal buffer memory is large enough to hold one full-frame WFC image, or sixteen SBC images, and so the buffer typically must be dumped before or during the following WFC exposure. If the following exposure is longer than ~337 seconds, then the buffer dump from the preceding exposure will be performed during integration (see Section 8.2 for a more complete discussion).

ACS's internal buffer stores the data in a 16 bit-per-pixel format. This structure imposes a maximum of 65,535 counts per pixel. For the MAMA detectors, this maximum is equivalent to a limit on the total number of detected photons per pixel which can be accumulated in a single exposure. For the WFC, the 16 bit buffer format is not an issue when the default GAIN = 2.0 is used. The full well will be reached before the buffer is overflowed. See Chapter 4 and Chapter 7 for a detailed description of ACS instrument operations.