Vignetting Correction and bad pixel map

Posted by: Susan

First question:

When I use vignetting correction it works great on single raw frames but whe I want to do the same with a stacked frame the result is terrible.

Did not make any flats, darks or bias this time but do want to use the bad pixel map, which can't be used after the vc on single frames because this changes the image size as expected.

What am I doing wrong here?

 

Second question:

When I try to make a bad pixel map for an imx224 sensor APP gives me an error or it will freeze. Tried different settings with and without rejection and used two different sets of darks and flats.

Just making master files is not a problem, it will create a master flat, master bias and master dark. Even tried to make the bpm by using the master files only. It will either hang/freeze or give me an error.

Making the bpm for the dslr was no problem at all. So again what am I doing wrong here?

 

Any help will be appreciated,

Susan

Hi Susan,

Running the correct vignetting tool on a stack will only work well, if no flat calibration was done on the lights. Was this the case?

If flats were used, then modelling vignetting properly using the Kang Weiss models is not really realistic, because the flats already tried to correct it. The illumination problem left is not a real vignetting model anymore. Simplest other way to correct a stack is to use the "remove light pollution"tool. That usually works very well.

I you want to correct single frames for vignetting together with a Bad Pixel Map, then you would need to create an artificial master flat from one of your lights in the correct vignetting tool. This artificial master flat should have the same image dimensions as the original raw light frames. You can then calibrate the original light frames with the Bad Pixel Map and the artificial master flat. The creation of the artificial master flat is a select box in the "correct vignetting" tool. 

Regarding the BPM problem with the imx224 sensor, this bug is not on my radar so it could be that something's wrong here that I am not aware of. It will be most helpful if you can send these frames to me, so I can have a look and locate this problem? You can send them to mabula@astropixelprocessor.com using wetransfer or dropbox. Just a couple of frames that trigger this bug would be enough so I can replicate the problem.

Kind regards,

Mabula

Hello Mabula,

As I said I didn't use any flats, the light pollution tool did work but not to correct vignetting off course. I will try to make an artificial master flat and try that.

I've read elsewhere on the forum that it would not work if flats were used and I had some laptop problems so did not take any.

The result of the vignetting tool (on/with the stacked image)  does look the same as when you use it on an image that has been corrected with a master flat.

Will send some frames later for the bpm problem.

Could it have anything to do with the amp glow?

Thanks,

Susan

Hi Susan,

Okay, my bad, I understand. Yes, if you don't have flats at all, then the artificial master flat method is the recommended way 😉 ( It won't correct dust spots, as you probably have noticed )

Yes, if flats were applied but weren't able to correct all vignetting, then the KangWeiss modelling does not work. You would need to use the damped thin plate spline modelling of the "remove light pollution" tool because the residual correction that is needed simply can't be modelled by a Kang Weiss model. You would need a very flexible model that can bend in all sorts of arbitrary shapes, hence the thin plate modelling 😉

Thanks, that would be great, I am curious to see what I'll find with your frames. I guess amp glow could be cause for error here, if it's really strong... is it?

Thank you Susan,

Mabula

The artificial master flat did the job! By the way I did try all three models and the results where more or less the same.

Later I stacked some other frames without flats and the vignetting tool did work on that stack so it's trial and error to see when the tool works or when to use an artificial flat frame.

Most of the times I do use calibration files so it won't be a problem.

3 Flat frames and 3 dark frames were send yesterday so I hope it's enough otherwise tell me and I can send more.

There is a bit of amp glow in there don't think it's very strong but I don't have anything to compare it with.

Also the normal frames do have a strange pattern as do the flat frames, they do correct very well. Looks like a very dirty sensor but the next day it was all gone (same camera and same optics), didn't make flats then.

Maybe it's just a combination of both.

Curious what you think.

Thanks,

Susan

Hi Susan,

Great ;-), okay, the difference between the 2 elliptical Kang Weiss models is only apparent when you are using optics that consist of a lot of optical elements usually. Like a regular photography objective with 12 pieces of glass. The first model is a special model, if both vignetting and light pollution are strong, then in most cases it would give a better correction for both vignetting and light pollution.

"Most of the times I do use calibration files so it won't be a problem."

Excellent, making good calibration frames is always to be preferred I think 😉

3 Flat frames and 3 dark frames were send yesterday so I hope it's enough otherwise tell me and I can send more.

There is a bit of amp glow in there don't think it's very strong but I don't have anything to compare it with.

Also the normal frames do have a strange pattern as do the flat frames, they do correct very well. Looks like a very dirty sensor but the next day it was all gone (same camera and same optics), didn't make flats then.

Maybe it's just a combination of both.

Curious what you think.

I'll let you know as soon as I know 😉

Cheers,

Mabula

Hi Susan,

I have found the BPM bug. You are the first to use RGB images that are already debayered, so this will be fixed in the next version.

Regarding the darks, where they already bias subtracted possibly, or did you set a very high offset in the camera settings?

The darks aren't showing a complete gaussian peak in the histogram, so it could cause suboptimal calibration.

And is 8bit the only option to save your frames with this camera?

For best calibration results, I would also advise to use undebayered calibration frames.

Cheers,

Mabula

Hi Mabula,

Thank you for the information about the vignetting tool, I understand the difference now.

And also about the rgb image.

No bias was used and I didn't check the offset.

I bought this camera about a month ago for guiding and a bit of moon and planetary work but read on the internet about the dso live view with sharpcap. I set sharpcap to save all the individual frames and made some darks and flats to see the difference in the live stack and stacking afterwards. And a good way for me to try APP with a different camera.

Most of the testing with that camera I did I can not try with APP before converting the files from png to tif. But sharpcap can also save in tif and that was the stack I tried.

Yes the camera can do 12 or 8 bit undebayered but for testing RGB was easier and as far as I know it says 24 bit color (8 bit per channel) but that's not important because the images taken are not award winning anyway.

My moon mosaic and jupiter pictures were taken undebayered so when it's not for testing purposes I do use undebayered frames.

Thank you for looking at the files and all the information.

Greetings,

Susan

Hi Susan,

You're most welcome 😉 and I understand, you did help me solve a bug so thank you!

Cheers,

Mabula