Taking flat images for color cameras

[Astrofriend]

Just a few days before Christmas, today I felt it was a good day to do something about my flat images. It has always been a problem. With color cameras there is a need to have the three color channels at the same signal level as the camera sensor see it. If not it can be that one color channel oversaturate and another one get too low and get noisy.

I have used a monitor where I can adjust the color and other stuff, no one was  easy to use. Now I building one with a RGB LED stripe with some extra control of the colors.

Interesting ? You have photos and text about my DIY here:

http://www.astrofriend.eu/astronomy/tutorials/tutorial-flatcalibration/07-tutorial-flatcalibration.html

This is the one that worked best yet. But I have to develope it more now when I know I can use it. The 3D-printer will come in use again.

Merry Chrstmas to all of you !

/Lars

[Stuart1971]

10 hours ago, Astrofriend said:

Just a few days before Christmas, today I felt it was a good day to do something about my flat images. It has always been a problem. With color cameras there is a need to have the three color channels at the same signal level as the camera sensor see it. If not it can be that one color channel oversaturate and another one get too low and get noisy.

I have used a monitor where I can adjust the color and other stuff, no one was  easy to use. Now I building one with a RGB LED stripe with some extra control of the colors.

Interesting ? You have photos and text about my DIY here:

http://www.astrofriend.eu/astronomy/tutorials/tutorial-flatcalibration/07-tutorial-flatcalibration.html

This is the one that worked best yet. But I have to develope it more now when I know I can use it. The 3D-printer will come in use again.

Merry Chrstmas to all of you !

/Lars

I use a OSC camera and take flat frames, but never come across an issue with colour channels…interesting…🤔👍🏼

[michael8554]

Hi Lars

You're saying that when pointed at a "white" display, one of the colours will saturate while the others are low ?

What are the values for each colour ?

Which camera ?

I've had a look at my Modded 6D Flats shot in AV Mode, which takes about 1 second exposures, displayed "As Shot", rather than "Auto" or "Custom White Balance".

My Flats are very cyan in colour (opposite colour to red), but I'd not noticed before how low the Red value is.

Which makes me feel the camera in AV Mode is overcompensating for the extra Red that the Mod gives.

I will have to investigate that.

I don't believe the Flats have to be perfectly monochrome, so the simplest solution could be to add the appropriate colour gel filter to your display.

Michael

 

 

[Stuart1971]

1 minute ago, michael8554 said:

Hi Lars

You're saying that when pointed at a "white" display, one of the colours will saturate while the others are low ?

What are the values for each colour ?

Which camera ?

I've had a look at my Modded 6D Flats shot in AV Mode, which takes about 1 second exposures, displayed "As Shot", rather than "Auto" or "Custom White Balance".

My Flats are very cyan in colour (opposite colour to red), but I'd not noticed before how low the Red value is.

Which makes me feel the camera in AV Mode is overcompensating for the extra Red that the Mod gives.

I will have to investigate that.

I don't believe the Flats have to be perfectly monochrome, so the simplest solution could be to add the appropriate colour gel filter to your display.

Michael

 

 

I did not think colour flats were debayered so the colour issue is mute, and the calibration is done before the debayering process with RAW images..🤔🤔 but I may be totally wrong…

[Astrofriend]

Hi,

I have tried to use filter to filter a light source that didn't have the correct color balance, it works but the light source was weak and I got long exposure time. Now when I can adjust the light source color temp direct I have much better control over the process and the light LED strip is powerful too.

In my test I have relative low vignetting optics because I use a medium format lens together with my full frame Canond 6D. The curves on the histogram is narrow, with high vignetting optics they are much wider. Then it's even morre important to have each color in the middle of the histogram. Many cameras compress the signal at higher level then the signal must be even lower. I have no experience of modified cameras, but the problem is the same, flat image get better with all three colors in balance (same level). I normally take 60 images and stack together and subtract the dark flat, the dark flat I do simple, use a bias because of the short exposure when taking flats. Sometimes I replace it with a constant, 2048 in the case of Canon 6D.

The examples in the graphs are raw CFA levels and linear. Of my tools the Fitswork is best to do this analyze.

First calibrate, then debyer, but to get the histogram with each color separate I have debayered it (because its difficult to explain otherwise). Can do direct on the CFA but then the levels are little bit difficukt to analyse, but that is what I normally do because the debyer proces can mess with the levels. Remember, there twice as much green pixels compare to the red and blue.

 

/Lars

Edited December 25, 2021 by Astrofriend

[michael8554]

12 hours ago, Astrofriend said:

I have tried to use filter to filter a light source that didn't have the correct color balance, it works but the light source was weak and I got long exposure time.

Interesting you found your panel too dim. 

I had to add many layers of ND Filter Gel to get my Flat Panel exposure to around 1 second.

I tried a PWM dimmer, but that flickered at the low setting that was required.

I've gone through my archive, and about a year ago my were almost monochrome.

The next session they were cyan.

I haven't found out why yet but I'll keep trying

Michael

[iantaylor2uk]

Why do you need exposures of 1 sec? When I use a light panel on my ZWO 071 Pro camera and set auto for flats on my ASI Air Pro, it typically takes flats at 40 or 50 milliseconds with a UV/IR filter, and they look ok to me.

[michael8554]

For cameras with shutters, such as DSLRs, too fast an exposure can show shutter artefacts.

Michael

[Astrofriend]

I remote control the shutter through bulb mode, the most sofware don't accept shorter then 1 second. And the time precission isn't very great with too short exposure in bulb mode.

 

Shorter exposures can also make interference with the power control of the light source.

 

/Lars

[Astrofriend]

I had some problem with my 3D-printer and this DIY Flat frame box fell asleep. Now I started it again and have got some very promising result. The key to it is the LED stripe which can be fine adjusted in its color.

Note:
This is only interesting for color cameras. like my DSLR camera.

Here are the details of the project with text and photos:
http://www.astrofriend.eu/astronomy/projects/project-flat-calibrator/01-project-flat-calibrator.html

It's working now but I 3D-print some new details that correct the small details.

/Lars