Vignetting Problems with Setup and Processing.

[niteman1946]

Hi Group,
I’m having continuing difficulty with my setup and/or processing, and would appreciate your insight.

http://www.astrobin.com/30572/0/?mod=none
(Click on image for larger version)

1.  Upper Left – Optical train:  12”LX200 Classic, FT Focuser, Meade f/6.3 FF/FR, Mitsuboshi OAG5, Atik EFW2 and Atik383L+mono. Approximate focal reduction is f/6.11.
2.  Upper Center – Flat Master:  Auto stretched in PI.  Vignetting evident.
3.  Upper Right – Flat Master:  Same as no.2.  Selectively stretched in PI to show sharp edge of vignette.
4.  Lower Left – Individual Sub:  Auto stretched in PI.  Note dark corners as would be expected from vignetting.
5.  Lower Center – Individual Calibrated Sub:  Auto stretched in PI.  Note that the dark corners have now turned bright.  Too bright for the balance of the image.
6.  Lower Right – Calibrated, Aligned and Integrated Image:  Auto stretched in PI.  The too bright corners from the calibration step persist into the final image.

I’ve found no way to process this corner feature out of the image.  My only solution so far is to crop the ends off, which is not what I would prefer.
Welcome any and all questions and suggestions.

Mark

[chris.bailey]

Mark

What size filters have you got in your EFW2? I found the vignetting to be pretty harsh with 25mm filters, less with 36mm and less again with 50mm.

If you look at your master flat what is the maximum ADU at the middle of the frame. I found that if I followed the normal rule of exposing for a histogram roughly in the mid range, my flats over compensated for frames from my 383 and have since dropped the exposure for a maximum reading of about 15-18,000.

Hope that helps

Chris

[pfile]

also, is your master flat calibrated? without bias or dark subtraction sometimes the flat scaling factor is computed wrong.

[niteman1946]

Mark

What size filters have you got in your EFW2? I found the vignetting to be pretty harsh with 25mm filters, less with 36mm and less again with 50mm.

If you look at your master flat what is the maximum ADU at the middle of the frame. I found that if I followed the normal rule of exposing for a histogram roughly in the mid range, my flats over compensated for frames from my 383 and have since dropped the exposure for a maximum reading of about 15-18,000.

Hope that helps

Chris

Thanks for your input Chris,

1.  I'm using 1 1/4" mounted filters (~32mm).  The supplier indicated that with the closely mounted wheel, and my shooting above f/5.0, there should be no significant vignetting.

On a side note, this "reverse" vignetting (corner brightening) was showing up prior to using the filters and wheel.

2.  Images Plus CC reports on the Flat subs: Ave - 25k, Min - 8.2k, Max - 30.7k.  Using the mouse, I get ~28k in the center and from 10k to 14k in the corners, depending on which I pick.

I'm ready to retry my flats.  Not to be too dense (me), but are you saying limit the max reported value to 18k?

Mark

[niteman1946]

also, is your master flat calibrated? without bias or dark subtraction sometimes the flat scaling factor is computed wrong.

Hi pfile, thanks for responding.

Typically I use a  modified version of Astropixel's tutorial, which I see is no longer posted.  I don't do Flats and Darks calibration until I get to the Light (image) calibration.  However, I just went back and followed Vincent Peris' tutorial to the letter (i.e. precalibrated Flats).  And got the same results.

Mark

[chris.bailey]

Mark

What size filters have you got in your EFW2? I found the vignetting to be pretty harsh with 25mm filters, less with 36mm and less again with 50mm.

If you look at your master flat what is the maximum ADU at the middle of the frame. I found that if I followed the normal rule of exposing for a histogram roughly in the mid range, my flats over compensated for frames from my 383 and have since dropped the exposure for a maximum reading of about 15-18,000.

Hope that helps

Chris

Thanks for your input Chris,

1.  I'm using 1 1/4" mounted filters (~32mm).  The supplier indicated that with the closely mounted wheel, and my shooting above f/5.0, there should be no significant vignetting.

On a side note, this "reverse" vignetting (corner brightening) was showing up prior to using the filters and wheel.

2.  Images Plus CC reports on the Flat subs: Ave - 25k, Min - 8.2k, Max - 30.7k.  Using the mouse, I get ~28k in the center and from 10k to 14k in the corners, depending on which I pick.

I'm ready to retry my flats.  Not to be too dense (me), but are you saying limit the max reported value to 18k?

Mark

Mark

That seems a huge variation in brightness in your flats. I just measured my master R Flat and it varies from around 11k in the corners to 12k in the centre (Atik 383, 36mm unmounted filters in EFW2). A max of 31k may be a little high but I think something in your imaging train is to blame.

Chris