Use ACS ETC to investigate any concerns about scattered light (zodiacal light, scattered light from Earth’s limb at the beginning or end of orbits, etc.) in some or all filters which may cause a higher sky background than required to successfully achieve the scientific goals

By entering the target’s coordinates and other vital information about the observations, one can estimate the maximum sky background levels due to such things as zodiacal light to see whether it is advisable to request a LOW SKY or SHADOW requirement.

It should also be remembered that the effects of CTE (Charge Transfer Efficiency; Riess et al., WFPC2 ISR 1999-04; Riess et al., ACS ISR 2002-06) in ACS are greater at low sky background levels, so there is a potential trade-off to be considered here.

Generally, any new exposure with a sky background level of less than roughly 30 electrons (e^-) is at risk of suffering from increased CTE effects.

This is most common in narrow- and/or medium-band filters, and very short exposures of less than about 300 seconds in any broad-band filter. However, although the ACS/WFC postflash can be used to increase sky background levels to mitigate CTE issues in such cases, unlike the WFC3/UVIS postflash, the ACS/WFC postflash is highly non-uniform, varying by a factor of about 2 across the ACS/WFC field of view (Ogaz et al. 2014, ACS ISR 2014-01; Miles et al. 2018, ACS ISR 2018-02.) The improvement is greatest in the central area of the detector.

At present (December 2019), the ACS/WFC postflash is only recommended for use in very specific situations. Typically it is only currently recommended where mere detection is the goal, rather than in situations where uniform and accurate photometry is desired across the whole field of view.

Use of it is typically discouraged when making a comparison to any other photometry which is more uniform across the field.

LOW SKY and SHADOW are limited resources in terms of scheduling, and they will also cut down on available exposure time per orbit.

However, if the low sky background is truly needed, this may help avoid using more exposure time and yet still having a sky background that is too high for the desired scientific purposes.

Again, LOW SKY and SHADOW requirements must be proposed for and specified in Phase I proposals in order to use them in Phase II.