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Old 13-12-2012, 11:24 AM
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Joshua Bunn (Joshua)
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flat field frames

Hi All,

Starting to gather some of the knowledge required for deep space imaging, i came across this question; is it enough to capture a set of flat field shots with white light or are flat field exposures required for every imaging filter used to produce the final image?
I would imagine every filter would be required as each on has slightly different dust spot and light transmittance characteristics.

thanks
Josh
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Old 13-12-2012, 11:43 AM
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Octane (Humayun)
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Josh,

If you were a purist, you would flat frame for each filter.

But, I know a lot of people just take luminance 1x1 and 2x2 flat frames and use them for everything.

H
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Old 13-12-2012, 11:53 AM
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Thanks H,
So 1x1 would be for Luminance and 2x2 for the colour, if these binning settings were used?

Josh
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Old 13-12-2012, 11:56 AM
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Josh,

You may be able to get away with a single set of flats but it will depend on your optical train and how fussy you are. If you have a fast scope and the filters are far enough away from the CCD then any dust on the filters may be so far out of focus that it isn't visible. If you have a slow scope or a filter wheel that's close to the CCD plane then you may find the flats for each filter are significantly different.

You can always perform a test and see how much difference there is between your master flats. If you divide one master flat by a different master flat you will get a mostly featureless result if the flats are similar.

Cheers,
Rick.
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Old 13-12-2012, 12:34 PM
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Correct!

Also, what Rick said.

H
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Old 13-12-2012, 06:40 PM
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thanks guys,
I read somewhere that if you divide one master flat by another master flat, the result should be a perfectly flat or something like that, whats this about? Is this what you mean by a "mostly featureless result" Rick. Is it to tell if your flats are good enough?

On a different note, when subtracting the master dark and dividing dark subtracted master flats from the light frames, is it performed on each light frame or a stack of light frames (luminance stack or red filter stack for example).

thanks
Josh
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Old 13-12-2012, 06:46 PM
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Performed on each light frame.

H
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Old 13-12-2012, 06:48 PM
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cool, thanks H.
Can this be automated to some degree with ccdstack or MaximDL?

Josh
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Old 13-12-2012, 07:20 PM
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Looked through some documentation on these products and i realised it can be automated.

thanks
Josh
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Old 13-12-2012, 08:03 PM
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Quote:
Originally Posted by Joshua Bunn View Post
I read somewhere that if you divide one master flat by another master flat, the result should be a perfectly flat or something like that, whats this about? Is this what you mean by a "mostly featureless result" Rick. Is it to tell if your flats are good enough?
Josh,

To make a flat you take an image of a uniform field of light. The resulting frame measures the variations in intensity caused by the whole telescope and camera system. It captures optical vignetting, shadows caused by dust, variations in optical elements, differences in efficiency in individual camera pixels, etc.

To perform flat calibration, a light frame is divided by a master flat (each pixel value in the light frame is divided by the corresponding pixel value in the master flat). The effect of this is to "undo" the variations in illumination we measured when we recorded the flat frames.

If you perform flat correction on a flat, then you should get back the original uniform field of light that was presented to the scope. That's the mostly featureless result I mentioned. Any features or patterns you see when you divide one master flat by another are caused by inconsistencies between them (possibly caused by different patterns of dust on the filters, non-uniform transmission of light by individual filters, etc.)

If a master flat taken with a luminance filter does a good job of correcting a master flat taken with a red filter then you don't need both. If it doesn't, then you will need to take flat fields and create separate masters for each of the filters.

This is also a good way to test a "uniform" light source like a light box: take a bunch of flats, rotate your light box by 90 degrees and then take another bunch. Make two master flats and divide one by the other. If you get a nicely corrected uniform result then you have a good light box.

Hope that helps...

Cheers,
Rick.
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Old 14-12-2012, 12:03 AM
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Thanks Rick, that does help

best
Josh
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