Version 1.4 rev 23 Oct 2017
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This procedure corrects for the variation in pixel sensitivity across the chip. As an added bonus, this procedure can also
remove the shadow of debris that may have fallen into the camera’s light path. Therefore, in addition to raw images of
celestial objects, dark images, and bias images, the astrophotographer should also collect one or more flat field images
while at the telescope.
A flat field image can be produced by pointing the telescope to a location where the sky appears uniform, such as the
zenith, during twilight after sunset or before dawn. Another effective method is by pointing the telescope at a uniformly
illuminated screen. The exposure time is typically quite short. The purpose of the flat field image is to record the pixel-to-
pixel variation in the sensitivity of the imaging system.
Once the raw image of the sky is corrected for dark current (and bias), a flat field correction can be done. The flat field
image may also need a dark correction if it required a long exposure but, usually, flat field images do not need this
adjustment.
Flat field images should be produced before or after each imaging session. It is also recommended that the flat field
image should match the same side of the sky meridian where the corresponding sky image was produced. Further, the
most effective flat field images are taken with the camera in the same rotational orientation as the sky image.
Correcting the sky image for dark current, bias and pixel sensitivity errors can be performed with commercially available
image processing software such as Maxim DL. These types of applications can automatically perform the dark current
and bias subtraction then divide the sky image by the flat field exposure.
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Many additional enhancements can be made to an astronomical image once it has been corrected. For example, it
is possible to improve the quality of the image by increasing the picture’s signal-to-noise ratio. A high signal-to-noise
image has very little “snow” (randomly varied noise from one pixel to the next) compared to the actual brightness levels
of the picture’s subject (a galaxy would be a good example).
The signal-to-noise of deep space astronomical images generally increases as the total length of exposure increases.
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One way to increase signal-to-noise is to digitally combine a set of images featuring the same subject.
To combine a set of similar images, each picture must be aligned with each other so the subject appears the same size
and in an identical rotational orientation.
Image processing software, such as Maxim DL, can automatically align and combine multiple images. This can result
in a single picture having the characteristics of an exposure with same length as the cumulative exposure time for all
pictures in the set. For example, if there are ten images, each with five minute exposures, combining them can result in
an image with similar signal-to-noise as an individual 50 minute exposure.
This is how long exposures are produced- not with a single picture but by digitally combining the photons captured by
several, images of the same subject.