Help needed with vignetting/gradient problem

[Glasspusher]

As you can see from the attached iamge I am having a problem with vignetting/gradient. Any help greatly appreciated. Details are: C11 with F6.3 focal reducer, self modded Canon 1000d camera, Astronomic cls clip filter. Image processed with deep sky stacker using darks and flats. The flats were made using the 'twilight' method.

Thanks,

John

[roundycat]

There is something horrible going on there. The small vignette is visible at the corners but what caused the blackish right hand edge? I don't see a gradient but there is a distinct circular pattern that could be down to poor baffling in the scope. Judging by that and the large dust shadow bottom left the flats did not work. All those effects should be taken out by a good master flat.

Overall the picture is a bit light, darkening it a bit covers a multitude of sins!

Dennis

[Glasspusher]

Thanks, I thought the problem was with the flats so I recreated them and processed again....same result!!!!

John

[carastro]

Have you examined the master flat by stretching it. See if that shows the vignetting and dust etc.

If it doesn't it could be your method of doing flats is wrong.

I speak from experience

Carole

[Merlin66]

Which version of the C11? HD?

What's the spacing between the reducer and the CCD chip? 110mm?

The edge vignetting is to be expected - your large chip will never be fully covered, but the artifact around the centre shouldn't obviously be there...

[Glasspusher]

Carole, thanks for that, I checked the master flat and it shows the vignetting and dust as expected so it looks like the flat is OK.

Merlin, not sure what the spacing is between the chip and the focal reducer, I would guess it is less than 110mm. What is the significance of this number?

Thanks

John

[Merlin66]

The optics of the reducer are optimised for a distance of 110mm.

Just think what could cause the circular artifact....

[Glasspusher]

Merlin, if that is the case then I agree it could result in the circular artifact. Oh, by the way the C11 is not HD. I will try some spacers to get the distance required and see what happens. Many thanks for that, it is clear here now but I am working tonight so will have to wait for the next opportunity!

John