Shutter Map

##Input See the Observing Procedure on how to obtain the needed frames, and how to apply the necessary reduction steps.

##Assumptions

• all frames are the same shape and dimensions

ShutterCorrect will 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_shutter. The commanded exposure time is labelled as t_exp.

Assuming that the central pixels of the frame achieved 100% illumination, you can use the shutter map to determine an 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)
= (t_exp - t_shutter)/t_exp

The shutter map will be derived for each exposure time via

t_shutter = t_exp * (1  - illumination map)

In theory, the shutter map should look the same regardless of which set of exposure time twilight flats that were used to derive it. However, because the shutter time is of larger fractional importance to the shorter twilight exposures, this will have the most signal to noise. To obtain the master flat, we then average all of the shutter maps by inversely weighting by the noise in each image.

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 then be

corrected frame = (user frame) / (illumination map) 
= (user frame * t_exp) / (t_exp - t_shutter) = (user frame) / (1 - t_shutter/t_exp)

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.