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Version 1.5 rev 26 Sep
2016
Apogee Astronomy
Alta and Ascent Manual
7.3 Bias correction
CCD cameras typically add a bias value to each image they record. If you know that the same bias value
has been added to each pixel, it can be corrected by subtracting a constant from the sky image. However,
most CCD cameras add different bias values to their pixels therefore it becomes necessary to produce a
bias image and subtract it from the sky image, too.
Because the bias error very slowly, if ever, changes over time, it is possible to re-use the same bias image
for several months before a new one is required.
7.4 Flat field correction
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.
7.5 Further processing
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.
