Is This Vignetting, a Hot Spot, or Both?

[PRejto]

I have a new and unexpected problem - one that has really confused me.

I took an image of NGC1365 two nights ago. Only one image for 3 min just before the clouds rolled in with my new Trius + SXFW (1.25" filters) and a 2X Astro Physics barlow. I have used this barlow without problem with my G2-8300 camera (31mm filters built in). The single frame of NGC1365 shows rather severe vignetting, or something that looks like vignetting as you can see in the first picture. So, this morning I decided to take some flats. The flats exhibited even more of this rather extreme central brightness so I removed the filter wheel and inserted a 2" extension tube in place of the filter wheel. I then got a very reasonable looking flat (excuse the dirt!). Through a process of elimination I first removed the filter (no change) and then the entire carousel. Even with the entire carousel missing I get two extremely different flats! If I remove the barlow entirely and take a flat through the 1.25" filters I get a very normal looking flat that shows slight vignetting but no central hot spot.

Might someone be able to explain this behavior? I assume that the empty (of carousel) FW may slightly vignette the FOV compared to the 2" extension tube used to replace it. I fail to understand how there could be so much difference just because of this. Could it be some sort of internal relection? If so it would need to be in the FW. I thought it might be the threads but then thought no, because with the carousel installed I still get the bright center and the threads cannot be seen.

As for the comparison photos, both were 1 sec duration, same temperature, and same amount of stretch. I repeated this experiment at least 3X today always with the same result. How could the 3rd photo be so much brighter than the 2nd given the same exposure and same stretch?

Many thanks for any feedback!

Peter

[PRejto]

I've repeated the same experiment tonight.

Image 1 is IDAS LP filter/barlow/Trius (barlow to Trius is 80 mm, 1.11 arc-sec binning 2x2. Exposure 5 min.

Image 2 is IDAS LP filter/barlow/Empty filter wheel (no carousel) (barlow to Trius is 83 mm, 1.05 arc-sec binning 2x2. Exposure 5 minutes.

Both images stretched exactly the same in CCDStack.

[Shiraz]

any possibility that it is stray light getting in the Barlow/FW coupling or elsewhere? clutching at straws a bit though - very odd.

A solution may be to just ditch the Barlow and use 1x1 binning, if that works OK

[PRejto]

Hi Ray,

Thanks for your reply.

I don't think there is anything stray about the light. All the fittings are quite tight screw together, and the pattern is quite symmetrical.

I had a reply from Terry at SX. He suggests it "must" be the extension barrel threads (which run the entire length of tube from barlow to FW) and said to try putting black matt paper over the threads. Of course I don't have any on hand. He also suggested making a cardboard aperture stop just before the FW. I'll try the first idea first. The second seems quite fiddly.

BTW, are you using 1.25" filters in the SX wheel? Do you see vignetting? Your system is pretty fast so I wonder if that is more of an issue for you?

I seem to attract hard problems like a magnet. I think equipment hates me.

The tests last night at 2x2 were just for speed and small file sizes. I fully intend to use the full resolution of the barlow, otherwise, as you correctly point out why bother with the barlow at all!

Peter

[PRejto]

Terry was on the money. Black matt paper seems to have resolved the issue.

New flat through filter wheel (with luminance filter) looks pretty good. Just a bit of vignetting, but I expected that.

Peter

Edit: I think I know what might have caused this. The FW carousel is in glassy black. Perhaps that directed light back to the threads on the extension tubes. Even removing the carousel wouldn't fix the isue because the back wall of the FW housing is also glossy black. Makes me think that no filter wheel ought to have anything other that a matt finish!