Correct Vignetting

@marc68 Hi Marc. I shoot with an ASI6200MM as well and my pictures also suffer from considerable vignetting. What calibration files do you use? The one and only good way to correct for vignetting is to use good flats and yes, you will need them for all filters. When you use those, the dark corners will be lifted to the same background level as the center of the image.

Many thanks for the reply. I have prepared 25 flats for each channel. Around ADU 33000

Here one of those https://1drv.ms/u/s!AvDvDcozziy93mJc84q0cSLzCy8w?e=GNwZUF

My calibrated and integrated image (so wo vignetting correction)  shows very bright corners so i must be doing something wrong

Marc

@marc68 It can be very difficult to shoot proper flats. What light source are you using? Do you aim at 33000 ADU manually or do you use automation from some software for that? Since you have combined LRGB data, it is very hard to tell where the problem exactly lies. It requires investigating the color channels separately and investigating for which channel the flats produce a good and for which a not so good result.

I am using an Alnitak Flip-Flat Electroluminescent Flat Fielding Device and sequence generator pro. Adu is read from the image statistic tab. What would be a good result next to the achieved adu? On all r and g and b flats i see more energy on the center that then reduces when moving to the border creating some cercle that are somehow "applied" on the combined lrgb above, very visible on the top left side. I will continue experimenting around your advice to use flats for removing vignetting. 

This is one flat from the green Channel. They all look similar (L R G B flats). When i integrate all to create the combined LRGB, as just stated, i have those "cercles" visible (image above, top left) and the corners with too much light.

@marc68 Hi Marc. The flats look like what you should expect to me but it is hard to judge from a screenshot. ADU 33.000 usually is OK but depending on the caqmera you may want to aim at, say, 55.000 or so to make even better use of the full bit depth of the sensor. What version of APP are you using? What other calibration files are you using?

Hi Wouter, 

I use APP 1.083 beta 2

I have a set of 25 flats per channels, 51 bias, 31darks 

Thanks

@marc68 Hi Marc, could you please upload, say, 5 lights and flats for L, 5 darks and 5 bias to

https://upload.astropixelprocessor.com/

username and password both are: upload

Create a folder called marc68_flat_overcorrection and put them there. Then I'll have a look at your data. Thanks!

Many Thanks for your time! Files uploaded.

@marc68 Thanks a lot. I have downloaded the data and processed them. Apart from maybe the lower right corner all looks OK to me:

Perhaps you can process each color channel separately to see where the issue is? Note that once processing of the lights is done, you can load a light in the APP preview window by double clicking on a file in the files list. Then in the upper part of APP there is a drop down list that allows you to through all the steps that were performed by APP:

That way you can inspect one or more individual subs to see how they contributed to the final stack. That should help you sort out which are the bad frames.

Thanks a lot, i will process each color channel separately in order to find the issue.  

Hi Wouter

I have calibrated all lights separately. The issue comes from the RGB channels (all).

Bellow a single calibrated green. Vignetting is overcorrected and the "circles" (left side) back.

I allowed myself to load this calibrates green on the link you gave me (subfolder Greencal) with the uncalibrated fits and a couple of green flats as well as the master dark and bias. If you have time to have a look...

Marc

@marc68 I am downloading the data and will have a look.

@marc68 Can you add a few more raw green light subs please?

Done. I have only 4 raw greens in that session. Thanks for your help.

Hi Wouter, i have uploaded all my 4 green frames. Let me know your calibration result. Many Thanks for your help. Marc

@marc68 Marc, sorry for the late reply. I’ll have a look today and will let you know. 

@marc68 I can reproduce the issue but don't know exactly what's causing it. I'll ask Mabula for help. Stay tuned!

Having a look as well, I noticed a gain of 0 in the green lights is that correct? If I then look at the histogram at 30% stretch, I also seem to notice some data lacking in the background. Could it be that there are some true black pixels in those corners? If I'm not mistaken that could produce issues when trying to calibrate as there then would be no data.

Hi vincent, gain is 0 as per zwo preset in the ascom driver for this camera model. I dont think having black pixel in the corner as we can clearly see stars there on the light calibrated image as the one above from wouter. Also the issue is not only in the corners i also have the circles from the flats taken over in the cals. Those are besides the corners.

Ok, just wondering, I don't see gain 0 a lot. But things like the offset I didn't see in the header, what did you set that at?

Gain is 0 and offset 50

Ok thanks for checking, that all seems fine. I'll continue investigating as well.

So indeed, seeing the same. I think it must be lying in the way the flats are shot. Over-correction usually is caused by a difference between how the light enters the setup normally and when the flats setup is used. Like Wouter also mentions, it may also be worth it to shoot at a higher ADU.

Thanks Vincent, ok i will try higger adu.

Hi Marc @marc68,

I am investigating your data.

Regarding the Lum data that you uploaded:

After data calibration I see the same, the vignetting profile is overcorrected in the corners. It seems that the gain and sensor offset are all okay and the same for the lights and the bias, darks and flats.

So I had a good luck at the actual calibration data and I see several issues which can and probably will explain why calibration is not working optimally 😉

First have a look at the bias and darks, this is a single dark frame where I set the Black slider to zero to see if the sensor offset is set high enough :

Clearly it isn't because the histogram is clipping on the left side. The Same holds for your bias frames. So that needs to be fixed and can easily be done by using a higher sensor offset using your camera's driver.

Now if I look at your Luminance flats, I see immediate problems I think, the vignetting profile is not uniform from center to the corners, it has ripples in it and that will never be okay I would expect.

So.. probably you will need to change/improve the way in which you make your flats for this setup. But first try to fix the sensor offset and check if things have improved 😉 already.

Will check your green data as well now.

Mabula

Hi Marc @marc68,

Just had a look at the green flats. I see the same issue as with your lum flats, they show ripples in the vignetting profile and that needs to be solved/fixed. You expect a smooth illumination profile in the flats from the center to the corners without those ripples superimposed on it.

Mabula

Hi Mabula, thanks a lot for having looked into this. Got your points and will apply. I was not sure about those "circles" in my flats, i guess this is what you name ripples. I will try to find out why my alnitak flip flat device does produce this and also increase offset.

Just to confirm something, your flats were shot at 0 gain as well? I recently had a very similar issue with my ASI2600, flats shot carefully at 100 gain (Sky flats and with a panel) looked fine and the master flat looked fine but it over corrected the corners. I reshot really rough flats at 0 gain by just guessing exposure times as the sky darkened and they corrected really nicely, so I reshot proper sky flats at 0 gain. The illumination is quite flat to begin with and all I can think of is that the 5ADC/e of 100 gain (About the same as your 6200) introduced too much quantisation uncertainty for the flats to work well. 

I didn't do that until after I spent quite some time looking for light leaks of course!

Hi, yes gain 0 and offset 50 for all flats. I will try the points from mabula, what i dont yet get i where those ripples on my flats are coming from. Need to check my setup. Thanks