limited resource in terms of data volume, buffer storage, and buffer dumps

can also affect the primary observations

instrument use causes wear and tear on the equipment

coordinated parallels must be justified scientifically as representing compelling science.

There are a variety of ways of observing in parallel.

ACS/WFC and WFC3 in either the UVIS or IR channel are often used in parallel.

Both the WFC3/UVIS and WFC3/IR fields of view are smaller and on-axis in the HST focal plane

ACS/WFC’s larger footprint is off-axis in the focal plane

appreciable difference in field size between ACS/WFC (202 × 202 arcsec and plate scale = 0.050 arcsec/pixel) versus WFC3/UVIS (162 × 162 arcsec and plate scale = 0.04 arcsec/pixel) and WFC3/IR (136 × 123 arcsec and plate scale = 0.13 arcsec/pixel).

in the simplest of cases, perhaps there is only a single pointing and no particular parallel target location (and therefore no particular ORIENT requirement) is desired.

Or, complicating matters a bit more, perhaps there may be a specific parallel target in mind in which case a specific ORIENT requirement will be needed to have both the primary and parallel fields of view placed in the desired locations. Also, if trying to duplicate and repeat an earlier primary + parallel pointing, an exact ORIENT will need to be specified matching that of the earlier observation as executed, even if there was no ORIENT requirement on the original executed observation. (Remember that both coordinated parallel observations and ORIENT requirements also need to be spelled out as required in the Phase I proposal in order to use them in Phase II. Failure to have done so in Phase I if needed in Phase II will require a request to the TTRB with a very strong justification, and these requests are often not granted at that point.)

Some larger survey programs may employ mosaic tiling techniques with specific fixed ORIENT requirements on the primary instrument thus also creating a secondary tiled mosaic pattern with the parallel instrument’s field of view.

Taking that even a step farther, some programs then repeat the same observations roughly 6 months later at the opposite ORIENT angle when the resulting primary and parallel mosaics trade places, falling onto the field of view of the other instrument, albeit with a somewhat different footprint and coverage.

Especially when covering a loose association of pointings within some area or considering total area covered in some larger survey of many different disconnected single fields, or when creating tiled mosaics, this difference in size can add up to a significantly larger amount of spatial coverage in ACS/WFC as opposed to either of the WFC3 channels.

When comparing ACS/WFC to WFC3/UVIS in terms of throughput at given wavelengths, it is also important to remember that ACS/WFC is typically more red-sensitive, whereas the greatest strength of WFC3/UVIS is in the bluer wavelengths. For more on a comparison of ACS/WFC and WFC3/UVIS calibration and photometry, please see Deustua and Mack 2017 (ACS ISR 2017-10) and references therein.