Observing Procedure

ShutterCorrect works by using a series of images of varying exposure time to infer the motion of the shutter and correct for the difference in exposure time the moving shutter will generate across the detector of the camera.

For astronomical applications, this will require "twilight flats," which are exposures of the twilight sky after the sun has set. During this time, the sky as imaged by the camera is roughly a uniform brightness. As the night gets darker, to achieve a given exposure level will require a longer exposure time. For very long exposure times, the perturbation on exposure time induced by the moving shutter is vanishingly small. For more on the actual derivation process of the shutter correction, see [[]]

Follow these steps during the twilight period of night to obtain the necessary frames.

What are twilight frames? See Phil Massey:

Then, "reduce" these frames in a standard manner using a program such as IRAF (or this could be done with custom routines written in IDL or Python). The reduction means that each twilight frame should be corrected for bias (pixel-to-pixel variation), overscan (variation of count "pedestal"), and dark current. These should be as close to your "science frames" before actual flat fielding, since this program is actually designed to determine the illumination correction, which flat fielding is designed to remove.

Twilight flats should have a decent amount of counts (> 20% saturation level), since this determination is difficult (impossible) to do accurately with an underexposed image.

Corrections:

• Nonlinearity: good to keep twilight flats to the 10,000 or lower count level, otherwise we would possibly need to correct for non-linearity effects, which adds another step to the process.