Automatic Sky Flats

If you've ever tried to take sky flats yourself, you know how difficult it is to automatically get quality sky flats with the right ADU, and still get enough flats to be useful. ACP has a service for automatically acquiring sky-flats at the proper time, and in the optimum (flattest) sky location (see Background below).

If you are a remote observer, ask the observatory operator if a standard set of sky-flats have been set up. Otherwise, before using ACP's sky flat service, you need to make sure that ACP itself is set up for the combination of imager and filters at the observatory. If you are the observatory operator, be sure to follow the directions in ACP Help, Automatic Sky Flats, to set up a standard flat plan. Advanced users, see Flat Planning.

How to Take Sky Flats

It's easy. Before saving your observing plan, enable either Take dusk flat fields or Take dawn flat fields, or both. Then save your plan as usual. There are some caveats:

• If you enable dusk flats, you must start your plan early enough to catch dusk twilight. A half hour before sunset will do it.
• If you enable dusk flats, be sure your plan starts with a wait for your first target to come into sky position. Planner does this automatically for you; just don't remove that first #wait until directive or your first images will be taken right after dusk flats, definitely the wrong time!
• If you enable dawn flats, your last target must finish up before dawn twilight. With Planner this is easy to manage by using the visual picture in the planetarium and Planner's guide display. Dawn flats start when the Sun is at -9 degrees altitude, so if you're trying to squeeze in images, check the sun altitude when your last image set finishes up.
• If you are a remote observer and you enable dawn flats, your plan will be the last one to run! You shouldn't start a plan that takes an hour's worth of images after sunset then does dawn flats! You'll hog the observatory for the rest of the night while your plan waits for dawn twilight!

If You Have a Rotator

If your imager has a rotator, and you want your flats to be taken at the various rotation angles used in your observing plan, you'll have to learn how to do Flat Planning. Planner can make a list of flats with rotation angles by scanning your plan. This list, called a flat plan, is generated and saved in a separate file along with your observing plan. Also, the generated observing plan contains links to the generated flat plan file, so you get a tailored set of flats for your rotations. Note that if the observatory has a German Equatorial Mount (GEM), rotated flats will be acquired 50% for the east-looking PA and 50% for west-facing PA. This is automatically done by ACP's flat logic.

Background

ACP acquires sky flats 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 15 degrees. This is close enough for most uses, including precision photometry. For more information and the theory behind this selection, see The Flat Sky: Calibration and Background Uniformity in Wide-Field Astronomical Images, Chromey & Hasselbacher, PASP 108: 944-949, October 1996.

At dusk, the sequence starts with the Sun 1 degree above the horizon. This is always too bright. Exposures start at the user-specified minimum duration (the shortest possible without shutter vignetting) using the first filter/binning combination in the flat plan. Minimum-duration exposures are repeatedly taken at 1 minute intervals until (at the minimum exposure duration) the mean background ADU falls within the user-desired range for flats.

At this point, flat acquisition proceeds, with the exposure duration being automatically adjusted for the decreasing light, until the prescribed number of flats for that filter/binning combination have been acquired. Before taking each flat, the telescope is moved in azimuth a random amount, up to one degree in either direction from the ideal location. This process is repeated for each filter/binning combination in the flat plan, or until it is too dark to reach the desired mean ADU range for the last binning/filter set in the flat plan. During gaps where it is too bright for the next flat set, the 1-minute wait loop is re-entered.

When it finally becomes too dark to achieve the proper flat field ADU with the most sensitive filter at the highest binning, the sky-flat acquisition process stops. At this point, the observing plan should contain a wait for the first observing target to come into the desired sky position.

At dawn the process is reversed, starting with the Sun 9 degrees below the horizon. When it becomes too light, the process ends and the observatory is left idle or shut down.

Note that if the observatory has a German Equatorial Mount (GEM), rotated flats will be acquired 50% for the east-looking PA and 50% for west-facing PA. This is automatically done by ACP's flat logic.