Shutter Map

Deriving the shutter map.

Input: Give shutter correct well exposed and stacked twilight flats at different lengths of exposure. Frames should have already been subjected to standard processing, bias subtraction, overscan correction, and dark current correction. In addition, it will help the statistics if many frames are averegade togethere to reduce the noise. Assume that all frames are the same shape and dimensions.

Suggested exposure times: These may vary based upon how long your shutter travel time will take, but assuming a shutter travel time of 0.010 seconds, some exposures of 0.1, 1.0, 5.0, 10.0 and 60 seconds or longer will be helpful. Make the longest exposure as long as is practically possible, since for this frame it is assumed that the shutter correction is negligable.

ShutterCorrect will then deliver a 2D fits frame that is a map of the amount of time (in seconds) that each pixel is missing in the exposure due to the shutter travelling, called the "shutter map," or $t_{\rm shutter}$. The commanded exposure time is labelled as $t_{\rm exp}$.

Assuming that the central pixels of the frame achieved 100% illumination, you can use the shutter map to determine the illumination map, which shows that the illumination percentage that the rest of the frame achieved::

illumination map = (Amount of time exposed)/(Amount of time commanded to expose)
= $\frac{t_{\rm exp} - t_{\rm shutter}}{t_{\rm exp}}$

One can use the shutter map to correct any given frame to a uniform illumination. This is desireable for any flat field images or science images taken during an astronomical observing session, called "user frame." The corrected frame will be::

corrected frame = \frac{user frame}{illumination map} = \frac{user frame \times t_{\exp}}{t_{\rm exp} - t_{\rm shutter}}

From the shutter map it is also possible to infer the shutter travel time. Essentially, the central pixels should be approximately 0.0 seconds, while the edge pixels will read a value approximately the total amount of time it takes the shutter to open and close.