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## Re: RGB and flats

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• This discussion has indeed been interesting and raised a question in my mind. I have been routinely taking flats in each color (LRGB) and decided to analyze a
Message 1 of 14 , Oct 1, 2001
This discussion has indeed been interesting and raised a question in
my mind. I have been routinely taking flats in each color (LRGB) and
decided to analyze a recent set. The flats were taken with a light
box that I had built. All pixel math was done in Maxim. I first
normalized each flat to an average of 15,000 ADU. I decided to use L
as the reference and computed L-c+2200 where c= R,G,B. The offset of
2200 was an attempt to keep the difference positive. If flats were
truly consistent over color, then the result of the subtraction would
be an average value of 2200 with minimum and maximum very close to
2200. Here are the statistics as reported by Maxim:

Color Min Max. Averag SD
L-R+2200 733 2598 2199 154
L-G+2200 76 3016 2199 410
L-B+2200 683 2590 2199 108

Based on this data taken with my f/5 scope, I don't believe I can get
away with a single set of L flats for all colors. It would be
interesting to look at similar data for different scopes and optical
trains. I also need to take either some twilight flats or overhead
flats to certify my light box radial uniformity but I don't think
this changes my conclusion.

Feel free to tell me what I'm missing in this analysis. Thanks.

John

--- In ccd-newastro@y..., "Kevin Dixon" <ksbtk@h...> wrote:
> Good evening folks:
>
> This has been a most interesting thread. I see that several people
have
> stated that flats for each color channel are not necessary.
>
> When I examine the dark-subtracted flats that I produce with my
light box,
> they each have a substantially different appearance from one
another. Based
> on this, I would conclude that indeed, flats should be created for
each
> channel. However, that is based strictly on the appearance of the
flat.
>
> Does the physics behind flat fielding render the different
appearance of
> each channel's flat, a mute point? I look forward to the opinions
of others
> in the group.
>
> Clear skies,
> Kevin Dixon
> ksbtk@h...
• It would be useful to compare the subtracted frames visually as well. Ron Wodaski The New CCD Astronomy http://www.newastro.com ... From: John Smith
Message 2 of 14 , Oct 1, 2001
It would be useful to compare the subtracted frames visually as well.

The New CCD Astronomy
http://www.newastro.com

-----Original Message-----
From: John Smith [mailto:johnzonie@...]
Sent: Monday, October 01, 2001 7:31 AM
To: ccd-newastro@yahoogroups.com
Subject: [ccd-newastro] Re: RGB and flats

This discussion has indeed been interesting and raised a question in
my mind. I have been routinely taking flats in each color (LRGB) and
decided to analyze a recent set. The flats were taken with a light
box that I had built. All pixel math was done in Maxim. I first
normalized each flat to an average of 15,000 ADU. I decided to use L
as the reference and computed L-c+2200 where c= R,G,B. The offset of
2200 was an attempt to keep the difference positive. If flats were
truly consistent over color, then the result of the subtraction would
be an average value of 2200 with minimum and maximum very close to
2200. Here are the statistics as reported by Maxim:

Color Min Max. Averag SD
L-R+2200 733 2598 2199 154
L-G+2200 76 3016 2199 410
L-B+2200 683 2590 2199 108

Based on this data taken with my f/5 scope, I don't believe I can get
away with a single set of L flats for all colors. It would be
interesting to look at similar data for different scopes and optical
trains. I also need to take either some twilight flats or overhead
flats to certify my light box radial uniformity but I don't think
this changes my conclusion.

Feel free to tell me what I'm missing in this analysis. Thanks.

John

--- In ccd-newastro@y..., "Kevin Dixon" <ksbtk@h...> wrote:
> Good evening folks:
>
> This has been a most interesting thread. I see that several people
have
> stated that flats for each color channel are not necessary.
>
> When I examine the dark-subtracted flats that I produce with my
light box,
> they each have a substantially different appearance from one
another. Based
> on this, I would conclude that indeed, flats should be created for
each
> channel. However, that is based strictly on the appearance of the
flat.
>
> Does the physics behind flat fielding render the different
appearance of
> each channel's flat, a mute point? I look forward to the opinions
of others
> in the group.
>
> Clear skies,
> Kevin Dixon
> ksbtk@h...

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