It is very difficult to get a good set of sky flats so most people resort to dome or lighted diffuser flats. Neither are as good as "good" sky flats(*).

ACP pioneered the automatic acquisition of high-quality sky-flats at dusk and/or dawn, using the minimum gradient position in the sky(**). This position is on the solar circle near the zenith, offset in the anti-solar direction by about 15 degrees. This is good enough for precision photometry. As flats are taken, the telescope position is dithered to prevent any stars that might leak through from overlaying each other on successive flat fields. Median combining will remove any stars.

Because the process is automated, the number of sky flats that can be taken is much greater than manual operation could possibly achieve. As this capability has evolved along with ACP, it has been improved so that more time is available for flat acquisition and less time is devoted to observatory overhead.

Unlike other "black box" automation tools, ACP's auto-flat system can be adjusted by advanced users to closely meet even the most demanding applications. ACP Planner has an advanced capability to generate a flat set plan from its observing plan and ACP can use this transparently to acquire just the flats needed for that run. More generally, however, a standard set of flats will be set up (particularly if multiple observers will be using ACP via its web service).

** For more information and the theory behind the selection of the sky-point, see The Flat Sky: Calibration and Background Uniformity in Wide-Field Astronomical Images, Chromey & Hasselbacher, PASP 108: 944-949, October 1996. Click on the link then scroll down to Printing Options, select Print Whole Paper, then click Send PDF. The paper will appear in your browser if you have Acrobat Reader installed. Thanks to the NASA Astrophysics Data System for making this available online.