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Quite a bit of info at the Digital yahoo group files area eg;
Quote........
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included and the contents is not changed.
This is not a complete guide but rather a checklist. For deeper
understanding of astronomical image calibration, I recommend reading a
good book, e.g. "Handbook of Astronomical Image Processing" which is a
very thorough and complete treatment of the subject.
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Here is a short reference guide into different kinds of frames:
Dark
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What
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- reflects the camera-induced noise,
- temperature, exposure time and ISO setting -sensitive,
- camera-specific,
- aperture, telescope etc. have no effect so they are not needed.
How
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- no light coming in (use lens or body cap) and block the viewfinder as well,
- exposure time and ISO setting the same as for light frames,
- near the same temperature as light frames,
- take as many as you can and average-combine them.
Use
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- whenever the temperature, ISO and exposure time is the same,
- for the camera they were taken with (with whatever scope, filters, etc),
- ImagesPlus "Auto" dark calibration matches across exposure times and
also somewhat across ISO and temperature,
- if you have perfect tracking (=autoguiding) you may need upto 5 times
as many darks as light frames,
- with nonperfect tracking (some movement between each light frame),
or dithering between each light frame even 10 darks are quite enough,
- I have been trying different approaches but have not yet settled on
one (a huge number of darks collected over longer time vs 8-20 darks
collected after each imaging session), but in both cases the "Auto"
feature is great.
Flats
====
What
--------
- records the uneven illumination of the image,
- dependent on the camera (sensor size and type) and the optical system
(scope, including barlow lenses, extension tubes etc),
- even filters can change it (usually not their color but their size
and position),
- dust in the sensor is also recorded.
How
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- use an evenly illuminated source of light (evening sky opposite the
setting sun, double T-shirt in front of the lens, a lightbox etc),
- preferably light should be bright enough to keep the exposure time
under one second (otherwise, stack more or blur the noise out),
- check that the scope/lens is focused to infinity (no need to be perfect
but should be very near),
- use the lowest sensitivity (usually ISO 100),
- use camera autoexposure (Av mode in Canon),
- take a few (4-10) and average-combine them,
- for lenses, one set for each aperture setting,
- for scopes, one set for each combination of filters/extension tubes/barlows
etc, in some cases even for different camera orientations,
- I prefer to keep the camera sensor clean - especially when taking
the flat frames, to get the dust out of the equation,
- the ImagesPlus from version 2.75 does not need changing flats into
grayscale any more, but other programs usually need that.
Use
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- whenever the optical chain is the same (scope, filters etc),
- some telescope/camera combinations "sag", so you may need to make sure
the orientation is the same,
- for lenses use the one with the same aperture,
- keep the sensor clean also when imaging,
- I have taken four sky flats, rotated 90 degrees, for each lens and
aperture (upto 1 stop closed down for 10D, full frame cameras require
more) and averaged each set together.
Bias
===
What
--------
- reflects the camera "zero" level,
- different for each pixel, usually has a pattern over the image area,
- camera-dependent,
- ISO sensitive,
- not dependent on optics.
How
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- cap the lens (or the camera without a lens),
- use the same ISO setting as for light frames,
- pick the shortest possible exposure,
- take a few (1-10) and average-combine them.
Use
------
- usually not required, because unscaled dark-calibration subtracts
the bias as well,
- if you have a remaining weak fixed pattern all over the image, bias
frames may help,
- you can always reuse them when the ISO setting is the same (for the
same camera),
- I have taken one set (10 images for each ISO, average-combined) which
I use repeatedly.
Dark and bias frames change slowly over the time, so you probably should
not use anything more than one year old.
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