Wierdly colored livestack after flat frame application asi294mc

[DrBobAZ]

Greetings

I have been trying in vain for the past couple weeks to generate an acceptable flat frame. I use a Vixen VMC110L at native f/9.4 with an asi294mc-pro. Vixen techs claim the image circle is 24mm. I have generated many many flats at varying ADU, exposures, gain. I am lucky to have avoided dust motes so far, so my correction is mainly vignetting. I have used both light panel and natural light at dusk. I have tried both flats with bias and flats with dark flats. I get what appears to be a reasonable monochrome flat, but Often (see results attached) when I apply it, I get a weird two region coloration: outside violet, inside region green, and the vignetting region is the same size as without a flat. I think I have attached all the files needed to see what I was doing (params and stacked lights of M104, bortle 8 sky). lights and flat converted to jpg for size. The flat at full size does not have the bullseye structure as far as I can see. That appeared when I reduced quality for posting.

Is it obvious what I am doing wrong? Any advice and critique much appreciated!

Thanks

Bob

[Menno555]

Hi Bob

I see that you save the captures as PNG. Is the same vignetting effect there when you save as FITS?
Also you could try and see what happens when you use the White Balance not on Auto and set both Blue and Red to 50.
In general it's better for deepsky not to use White Balance on Auto at all because that can cause a fluctuation during your sessions. For the 294MC it's either Red 50 and Blue 50 or Red 52 and Blue 95 (the Zwo default).

And just in case: you have some different settings with your subs and flats. Try to use for (dark)flats the exact same settings as your subs. Only change the exposure time and/or flatpanel brightness until you get around 2 or 3 seconds with around 25000 ADU. I've noticed that this worked best overall with the 294MC (which I owned in the past too).

Menno

[admin]

Hi,

some best practice for using flats (especially for deep sky, where the image will be strongly stretched)

* You must use darks with flats - just using flats will lead to problems with over or under correction
* Use bias or dark flat frames when capturing your flat - again, failure to do this will lead to problems
* Use the 'monochrome flat' option in SharpCap to avoid making a flat that will attempt to correct for colour balance - this should help eliminate any false colour rings

cheers,

Robin

[DrBobAZ]

Hi Bob

I see that you save the captures as PNG. Is the same vignetting effect there when you save as FITS?
Also you could try and see what happens when you use the White Balance not on Auto and set both Blue and Red to 50.
In general it's better for deepsky not to use White Balance on Auto at all because that can cause a fluctuation during your sessions. For the 294MC it's either Red 50 and Blue 50 or Red 52 and Blue 95 (the Zwo default).

And just in case: you have some different settings with your subs and flats. Try to use for (dark)flats the exact same settings as your subs. Only change the exposure time and/or flatpanel brightness until you get around 2 or 3 seconds with around 25000 ADU. I've noticed that this worked best overall with the 294MC (which I owned in the past too).

Menno

Hi Menno

Thanks for the reply. I usually use png for the final live stacked images. Are you suggesting I use fits for the darks and flats, and then back to png for the stacked images or fits for all of them, including the live stacks? I did temporarily try white bal on auto, but saw the same thing when I adjusted white bal.

I should mention that when I use the light panel, I needed to really turn up the blue in the white bal to get all colors with roughly the same ADUs.

I added sheets of white paper to get 25k ADU approximately with a gain of 350 and 5 second exposures for the flats, because ZWO suggested a combination like that. The weird stuff I described does occur at other gain/exp combos as well.

Thanks for the suggestions, and I'll give them all a try.

Clear skies,

Bob

[DrBobAZ]

Hi,

some best practice for using flats (especially for deep sky, where the image will be strongly stretched)

* You must use darks with flats - just using flats will lead to problems with over or under correction
* Use bias or dark flat frames when capturing your flat - again, failure to do this will lead to problems
* Use the 'monochrome flat' option in SharpCap to avoid making a flat that will attempt to correct for colour balance - this should help eliminate any false colour rings

cheers,

Robin

Hi Robin

Thanks for the reply. I did not use darks in this case, maybe that's my problem. I did use the dark flat and monochrome options for the flats. I should mention that the blue was weak with my light source so I had to increase the white bal blue to get the ADUs to the same level as the other colors. Are there ways with different materials in front of the light panel to suppress the RGL to make the blue more balanced? I'd hate to have to go through a pile of light panels to find one that is more like sunlight. (Unless you or someone else knows which inexpensive ones have the desired spectrum? Hopefully off Amazon or some such place)

Thanks for the advice, I will certainly redo everything with darks as well as flats.

Thanks also for creating such a fantastic product! I just started using the polar align tool, and it is nothing short of amazing!

Clear skies,

Bob

[Menno555]

Thanks for the reply. I usually use png for the final live stacked images. Are you suggesting I use fits for the darks and flats, and then back to png for the stacked images or fits for all of them, including the live stacks? I did temporarily try white bal on auto, but saw the same thing when I adjusted white bal.
I should mention that when I use the light panel, I needed to really turn up the blue in the white bal to get all colors with roughly the same ADUs.

Hi Bob

Yes, for the whole stacking procedure use FITS files. So subs, darks and (dark)flats in FITS. Then after the stacking save as you wish.

Adjusting the white balance for flats is not necessary. I always make my flats with Red 50 en Blue 50 and I only watch the white brightness line in the histogram to get the 25000 ADU there. This gives a green-ish flat capture but as long as it is not pure green, it doesn't matter: the master-flat is in the end used as greyscale. Best practice for me is to use the same white balance as my subs. I always capture with the R50/B50 white balance and then process it later or let a program like DeepSkyStacker do it automatically.
So if you use for your subs for example R52/B95, use that also for the darks and (dark)flats. The more things are equal, the better.

Menno

[DrBobAZ]

Hi Menno

Thanks for the rapid reply and clarification. Will try all you suggest. It's sufficiently different than what I was doing, that I'm hopeful it will solve my problems.

Appreciate you taking the time to help me.

Clear skies,

Bob

[turfpit]

Bob

Have a read of viewtopic.php?t=2552. It was written using a Altair one shot colour camera. Replace references to black level (Altair) with brightness (ZWO). The rest is the same. Take note of exposure, gain and black level settings for lights, darks, flats and bias frames. My flats when displayed in FITS Liberator have a mean ADU of around 30,000. I use FITS for capture - mixing capture types will not work. Also using FITS avoids getting hung up with colour balance at capture time.

Dave

[DrBobAZ]

Bob

Have a read of viewtopic.php?t=2552. It was written using a Altair one shot colour camera. Replace references to black level (Altair) with brightness (ZWO). The rest is the same. Take note of exposure, gain and black level settings for lights, darks, flats and bias frames. My flats when displayed in FITS Liberator have a mean ADU of around 30,000. I use FITS for capture - mixing capture types will not work. Also using FITS avoids getting hung up with colour balance at capture time.

Dave

Thanks Dave. Will do.

[ChrisR Oz]

Hi Bob,

I had similar effects in the beginning, with colour artefacts. The main cause ended up being a light leak in the 294MC pro, in through the rear past the cooling vents. This messed up the Darks, which then over subtract. I now take Darks at night, with the scope covered. I tried covering the camera. But it quickly gets to 100% Peltier power. Also, take care with Flat light source uniformity and do Dark Flats.

Now I have good Darks and Flat, and no residual colour effects in stacked images.
Cheers, Chris.