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Q: Calibration seems to be very complicated. Do
I need to do it?
A: Yes! If you do not at least take a single
dark frame you are giving up a lot of your camera's performance. If your
camera has a shutter, at the very least turn on the "Simple
Auto-Dark" feature. However, we strongly recommend using at least
some of the more advanced calibration techniques for best results.
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Q: Should I average dark frames?
A: It is not absolutely essential, but you will
get up to 41% more noise if you don't average dark frames. It's easy to
do. Just set up the camera to take a sequence of dark frames (if your
camera does not have a shutter, just cover the telescope) at the same
temperature and duration as your intended light exposure. Then open the
Set Calibrate dialog, click the Select Files... button under Dark Frame,
and choose all the frames you just took using the mouse and shift or CTRL
key. Click OK, and you're ready to calibrate.
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Q: Do I need to take bias frames?
A: That depends. If you are just doing dark
frame calibration without scaling, then you don't really need them. The
bias is included in each dark frame.
If you are taking flat field frames or using dark frame exposure
scaling, then you must use bias frames. Flat-field frames need to be bias
corrected (and dark corrected using Auto-Scale/Apply to Flats if you are
taking long flat-field exposures) otherwise they will not scale the image
properly. If you are scaling dark frames, the bias part of the dark frame
should not be scaled, and therefore must be subtracted first.
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Q: Do the light frames have to be of the same
duration as the dark frames?
A: That is the simplest and often best way to
go, but there are other ways to do it. You can use the Auto Scale or Auto
Optimize features to compensate for different exposure lengths. This works
better on some cameras than others (In particular, the hot pixels in Kodak
KAF series CCD sensors tend not to scale properly).
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Q: My camera doesn't have temperature
regulation. How can I make sure the dark frames subtract properly?
A: This can be a major problem since the
ambient temperature usually drops during the night. As the dark current
drops, the calibration frames "over-do it." The answer is to use
the Auto-Optimize feature. It automatically scales the dark frames to
minimize the RMS noise in the image.
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Q: Are flat-field frames necessary?
A: That depends. Flat-field frames are the
hardest calibration to do correctly; in some cases, you can get by without
doing them, but at some loss of performance.
If you are doing photometry, you absolutely need flat-field frames. If
you are doing "pretty pictures," then it depends on what you are
doing. If you have bright skies due to light pollution, you'll probably
want to flat-field. If you have vignetting, then you'll want to do it. If
you have dust spots, a can of compressed air is actually more effective!
If you are going for the faintest possible limiting magnitude with a
long exposure, then the limiting factor is caused by the fact that your
target is fainter than the sky background (even at a dark site). In that
case, the flat fielding error will set the ultimate sensitivity
limit.
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Q: What is the "ultimate" calibration
method?
A: Take at least 25 bias, dark, and flat-field
exposures. Use the same temperature as the light frame for all, and use
the same exposure for the dark frame as the light frames. Turn on the Auto
Scale feature, and use the "Apply to Flats" option for dark
frame scaling. Use median combine if you have any hot pixels or cosmic ray
hits. If you are doing median combine, it's a good idea to increase the
number of calibration frames somewhat.
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Q: Why would I want to do a "sky
flat"? How is it done?
A: The spectral response of the CCD camera
varies slightly from pixel to pixel, so the flat-field only works
perfectly if the spectrum of the flat-fielding light matches that of the
target. When shooting for very faint objects, the sky background
dominates, so the color of the sky background is what you need to match.
This is best done with sky flats.
The usual way to take sky flats is to turn of the clock drive and let
the stars trail. Take a LOT of exposures; 50 is a good number. Use the
median combine option, which will drop out the stars since they will not
match locations between images. The calibration command automatically
"normalizes" the flat-field frames to the same intensity, so if
there is a slight shift in light levels it won't hurt anything.
Since sky flat exposures tend to be long, be sure to turn on the Auto
Scale feature and activate Apply to Flats. This will apply both bias and
dark-frame subtraction to the flat fields.
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Q: Is it better to do all the calibration in
one step, or to do each in turn?
A: MaxIm DL is designed to do all the
calibrations in one step; there are a number of subtleties to calibration
and the software knows how to handle them, but only if it knows all the
calibration steps you are planning to do. It is strongly recommended to do
all calibrations in one step, and do NOT perform any modifications on the
raw calibration frames beforehand.
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Q: How do I do dark frame subtraction for STAR2000 cameras?
A: Take your dark frames with the STAR2000 guider running, and with the image type
set to Light, not Dark. Also, some tweaking may be necessary to get the best possible
amplifier glow suppression. To do this, open the Pixel Math command (Process menu).
Enter the light frame for Image A. Enter the dark frame for Image B. Select Subtract.
Set Add Constant to 100. Set Image A Scale Factor to 100%. Zoom out and pan
the image preview window until you can see the amp glow area. Now
adjust the Image B Scale Factor to minimize the glow. Click OK.
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