This example illustrates the orbit calculation for a WFC observation with the ACS box pattern, which implements imaging at four offset pointings. The goal of the observation is to obtain a dithered image of a field in such a way that would allow us to bridge the 50 pixel interchip gap between the WFC CCDs in the combined image. Given the WFC plate scale of 0.05 arcseconds/pixel, this requires that the offsets in the dithering pattern are larger than 2.5 arcseconds. Each offset will then take 0.5 minutes to move the spacecraft from one pointing in the pattern to another. We have determined that the exposure time necessary to reach the desired signal-to-noise ratio is 80 minutes. The visibility time at our target declination is 58 minutes. In this observation we do not want to rely on cosmic ray removal provided by the dithering data reduction package, and set CR-SPLIT=2 to be able to remove cosmic rays from the four individual images separately. As a result, the orbit calculation will involve a series of 8 exposures (two exposures at each of the four pointings in the dithering pattern) split across two orbits:
The total used time in the first orbit comes out a little bit larger than the visibility time. However, given the conservative nature of the adopted overhead times as well as a bit of flexibility in the adopted signal-to-noise ratio, the difference is not significant. It should be remembered that the purpose of the above exercises is to estimate how many orbits to request for our science program rather than to exactly design the observation.
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