Space Telescope Science Institute   7.1.2 Use of Available-but-Unsupported Capabilities  7.1.4 Identify Need for Additional Exposures

7.1.3 Determine Exposure Time and Check Feasibility


Once you've selected your basic ACS configuration, the next steps are:

To determine your exposure-time requirements, consult Chapter 9 where an explanation of how to calculate a signal-to-noise ratio and a description of the sky backgrounds are provided. To assess whether you are close to the brightness, signal-to-noise, and dynamic-range limitations of the detectors, refer to Chapter 4.

If you find that the exposure time needed to meet your signal-to-noise requirements is too great, or that you are constrained by the detector's brightness or dynamic-range limitations, you will need to adjust your base ACS configuration. Table 7.2 summarizes the options available to you and steps you may wish to take as you iterate to select an ACS configuration which is both suited to your science and is technically feasible.


Table 7.2: Science feasibility guide.
Action
Outcome
Recourse
Estimate exposure time.
If too long, re-evaluate instrument configuration.
Consider use of an alternative filter.
Check full-well limit for CCD observations.
If full well exceeded and you wish to avoid saturation, reduce time per exposure.
Divide total exposure time into multiple, short exposures.1 Consider use of different Gain.
Check bright-object limits for MAMA observations.
If source is too bright, re-evaluate instrument configuration.
Consider the use of an alternative filter or change detectors and wavelength regime.
Check 65,535 counts- per pixel limit for MAMA observations.
If limit exceeded, reduce time per exposure.
Divide total exposure time into multiple, short exposures
1Splitting CCD exposures affects the exposure time needed to achieve a given signal-to-noise ratio because of the read noise.


 7.1.2 Use of Available-but-Unsupported Capabilities  7.1.4 Identify Need for Additional Exposures
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