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Author Topic: Porthole Vignette  (Read 1541 times)

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Offline Starlight

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Porthole Vignette
« on: 22:42:21, 15 January, 2011 »
I hope someone can help me.
The other night I was really pleased that guiding worked very well and I managed to get over 2 hours of 5 min shots of M31 using my QHY8, with MPCC, Astronomik CLS filter and 6”F5 Newt but when I came to process it I was really disappointed.
I used DSS to stack it, Photoshop Elements and Pixinsight to process it and tried it with Flats and without as you can see.
Although the version with Flats is I think slightly better, once I used GradientXterminator, it still shows a porthole vignette which gets much worse if I try to push it further to show the outer edges of the galaxy.
The flat has also been processed to emphasise the porthole - although I used the natural flat in DSS.
If you would like to try processing it please send me a pm and I will upload it to Dropbox or other online storeage and give you the details. The stacked files from DSS are 36Mb each !
Alternatively if you have any suggestions on processing or eliminating the porthole from the image during capturing please let me know.
Thanks in anticipation
Don
Celestron C9.25-S (XLT), HEQ5 Pro Synscan, Meade LXD75 6" F5 Newt, Meade DSI Colour, Panasonic DMC-FZ5, Starchaser 80 Guide Scope, QHY8, 28-70 Zoom Lens, 300mm Zoom Lens, 400mm Telephoto lens, DBK21 & DMK21, 8" F4 Newt, Sony DSC-HX50, Itorex semi-fish eye lens adapter, Coronado PST, ASI 120MM-S,

Offline robjn

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Re: Porthole Vignette
« Reply #1 on: 20:40:48, 17 January, 2011 »
How did you take your flats Don? Did you use darks to calibrate the image?

Rob.
Rob Johnson. 14" f5.2 Reflector, Megrez 72FD, PST. Atik383L+, Canon 350D. DMK21AU04.
Mel Bartels Drive. Guiding with the 72FD & DMK. AIP4WIN, Noel's Tools.

Offline Starlight

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Re: Porthole Vignette
« Reply #2 on: 19:11:39, 18 January, 2011 »
Thanks for your questions Rob.
I used a white sweat shirt and a lightbox for the flats.
I tried a number of slightly different exposures, position of lightbox and took 12 flats of each
I chose the ones that most closely matched the vignetting I was getting in terms of range, dustbunnies & general distribution of the light and stacked them in DSS with the lights.
I did take some darks and bias frame but they didn't make any significant difference.
Don

ps - Any more suggestions anyone or does anyone want to try processing themselves ??
Celestron C9.25-S (XLT), HEQ5 Pro Synscan, Meade LXD75 6" F5 Newt, Meade DSI Colour, Panasonic DMC-FZ5, Starchaser 80 Guide Scope, QHY8, 28-70 Zoom Lens, 300mm Zoom Lens, 400mm Telephoto lens, DBK21 & DMK21, 8" F4 Newt, Sony DSC-HX50, Itorex semi-fish eye lens adapter, Coronado PST, ASI 120MM-S,

Offline robjn

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Re: Porthole Vignette
« Reply #3 on: 22:24:51, 18 January, 2011 »
I don't think it is your processing at fault Don it is most likely something in the image train causing the flats to poorly represent the image. Try taking a look in the scope without the camera in place but with your lightbox on. Are there any reflections or unusual patterns?. It could be the MPCC - does this happen without it in place? I had the same problem when I used my camera without the filter wheel in place which gave the CCD an 'open' view of the drawtube with presumably some faint reflections, and the resulting flats just wouldn't correct the image. With the filter wheel in place  - no problem.
 
I tried a number of slightly different exposures, position of lightbox and took 12 flats of each
I chose the ones that most closely matched the vignetting
If the flats are good they shouldn't appear any different for exposure/position etc. so something else to take a look at.

Rob.
Rob Johnson. 14" f5.2 Reflector, Megrez 72FD, PST. Atik383L+, Canon 350D. DMK21AU04.
Mel Bartels Drive. Guiding with the 72FD & DMK. AIP4WIN, Noel's Tools.

Offline Starlight

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Re: Porthole Vignette
« Reply #4 on: 22:51:47, 19 January, 2011 »
Rob
Thanks for the suggestions. I've now had a look back through previous exposures with and without MPCC and CLS filter in two different scopes - Newt & C9.25 and they all seem to have vignetting to a greater or lesser extent when pushed very hard.
I think the flats looked different because I tried different exposures and different positions for the lightbox - either touching the end of the scope or at approximately 400mm from.
Am I the only person who gets this effect with a QHY8 or other large CCD ?
or are there others ............  :yes::no:
Don
Celestron C9.25-S (XLT), HEQ5 Pro Synscan, Meade LXD75 6" F5 Newt, Meade DSI Colour, Panasonic DMC-FZ5, Starchaser 80 Guide Scope, QHY8, 28-70 Zoom Lens, 300mm Zoom Lens, 400mm Telephoto lens, DBK21 & DMK21, 8" F4 Newt, Sony DSC-HX50, Itorex semi-fish eye lens adapter, Coronado PST, ASI 120MM-S,

 

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