Vignetting issue

[jamenx]

Hey

Beginner here. I managed to try out my new rig for the first time last night: Redcat 51, AM5, ASI2600MC Pro and optolong L-extreme. Spent most the evening just trying to get everything working, but I did eventually get some images of M42. I had already taken my darks (30) and biases (100) which I think are ok, but at the end of the session I took my flats using a white laptop screen held up to scope - probably not ideal.

I used the Asiair plus auto setting to take 30 flats. They came out about 10 seconds long, then I tried again at a higher brightness which came out at 2.4 seconds long. When I stack the images I get strong red and green vignetting (see below) whichever flats I use.

When I just stack the lights I don't get this issue so I think it must be my calibration images. I have been able to remove this processing the images, but I'd like to try and sort it out before that. Any ideas would be much appreciated!

Thanks

 

[Fegato]

I think with a narrowband filter and OSC something like this is almost to be expected. I do have vignetting to deal with, but also tend to get a red to green gradient across the frame when using a dual narrowband filter which has to be removed using gradient removal techniques (DBE in Pixinsight for me). Maybe you have some vignetting here as part of that too ( I do too).

It is worth trying to get good flats certainly...  never tried the laptop method. I did have some success with tracing panels, but when I got a RASA 11, I invested in an Aurora Panel which works well for me. 

Can you post a sub and a calibrated sub and your master flat? Might help to see if any issues there...

[jamenx]

Hey thanks for your reply! I've attached the files 

Also just looking at Aurora Panels - is there any particular one you would recommend?

Light_M42_300.0s_Bin1_20240126-223226_0006.fit MasterFlat_Gain100.tif sub_calibrated.TIF

[pipnina]

I think a typical laptop screen won't produce good flats because the light intensity and colour of an LCD varies by the angle you view it from, and this becomes much more drastic the closer you get.

iPads and Phones etc use AMOLED screens which do not have this problem and may produce better flats.

[Fegato]

10 hours ago, jamenx said:

Hey thanks for your reply! I've attached the files 

Also just looking at Aurora Panels - is there any particular one you would recommend?

 

Well, your flat looks OK, but whether the colour side of it is right I don't know. As pipnina says, laptop screens may not work that well. For a Redcat 51, I assume the smallest Aurora panel would be fine, which is not too costly. You will probably need to buy one or two of his "filters" too, as the panels are not dimmable, and this is the only way of dimming the light a bit (more of an issue with broadband flats, although at F/2.2 I use one of these for my dual narrowband flats too). As I said - I did have some success with a tracing panel previously, so you could pick up an A4 one of these a bit cheaper and try that.

I can't see any obvious signs of vignetting, and I guess you shouldn't have any visible with a 44mm image circle. So I think this is just about the OSC bayer filter and an effect that does happen with narrowband filters. I'm not an expert - you can probably find articles and posts about this online. Perhaps the signal is very slight weaker around the edges, and the red signal has less intensity anyway, and this is the end result (albeit your integration should be taking account of the different channel intensities - certainly Pixinsight does this). But as above, I do get this sort of red / green issue too in narrowband. I suspect you'll still see it even with a better flat panel, and you just have to use gradient removal techniques to get rid of it.

[Fegato]

sorry - should have added, if you weren't aware (not sure), Aurora panels are a particular make of flat panel by Gerd Neumann - see https://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/aurora-flatfield-panels/uebersicht-aurora-flatfield-panels-overview.html

[jamenx]

8 hours ago, Fegato said:

sorry - should have added, if you weren't aware (not sure), Aurora panels are a particular make of flat panel by Gerd Neumann - see https://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/aurora-flatfield-panels/uebersicht-aurora-flatfield-panels-overview.html

Ah ha that makes more sense. I was looking at what appear to be light ceiling panels! Thanks for your help, I have just ordered one so will see if it helps.

[TerryD]

I have always found that the auto exposure for flats with an ASIAIR are far too bright.  I manually adjust exposure to get an average brightness reading of between 26000 and 28000.  If the flats are good they will remove color gradients caused by filters.  I also agree with others that a laptop screen isn't a good choice for a light source.  I've never seen one that has an even illumination.  I use an inexpensive light panel meant for making tracings and it works great.  Following is a link to the one I use.

https://www.amazon.com/16-6×12-6inchs-Stepless-Dimming-Tracing-Painting/dp/B08MVSR7H5/ref=sxin_14_pa_sp_search_thematic?content-id=amzn1.sym.cd4900ae-382e-4968-8cca-26c773a2a0f3%3Aamzn1.sym.cd4900ae-382e-4968-8cca-26c773a2a0f3&crid=XLL159YC8J2V&cv_ct_cx=led%2Btracing%2Bpanel&dib=eyJ2IjoiMSJ9.4Zyoocl0s_Q6gg3Wi0rci6qZ14XaXkLCXwOyAhDmqEm5fo0Ply4I-z8vXvypnh02VcgAhJZO6Kj53v3ldtXvHg.H2RprCZ9c1vBT93AZSivWq8yxJRv_8xhmmYATo3Nvzg&dib_tag=se&keywords=led%2Btracing%2Bpanel&pd_rd_i=B08MVSR7H5&pd_rd_r=83c2fe93-2c01-41dd-a655-4f884d5ac9e0&pd_rd_w=3XHkl&pd_rd_wg=hYPj3&pf_rd_p=cd4900ae-382e-4968-8cca-26c773a2a0f3&pf_rd_r=QZ5PV8P7KN0ADX7VWBGH&qid=1709051598&sbo=RZvfv%2F%2FHxDF%2BO5021pAnSA%3D%3D&sprefix=tracing%2Bpan%2Caps%2C117&sr=1-2-3f7eb011-4703-48ad-8058-43dbff1e6b87-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9zZWFyY2hfdGhlbWF0aWM&th=1

Edited February 27 by TerryD

[mackiedlm]

This is a normal result with the L-extreme and is easily removed with any gradient removal tool.

This is a comparison of before (left) and after (Right) application of graXpert. Boosted stretch to show the difference better.The remaining red in the background is the dust starting to show through.

 

I dont beleive there is anything wrong with your flats or calibration.

Edited February 29 by mackiedlm