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Old 16-08-2015, 11:39 AM
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Atmos (Colin)
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How much vignetting is normal?

Very recent telescope purchase, haven't really had the weather to play with it outside so I thought I'd do SOME testing with flat fields.

Currently using a QHY9 (KAF8300) on a 0.8x flattener, spacing should theoretically be within 0.5mm of correct from what I have been told, seems to pass the ruler test.

Added is my flat field test run through CCD Inspector. The flattener has a corrected field of an APS sensor (36x24) so I was expecting less vignetting with a KAF8300. Is this just me expecting more than what I should or do I possibly have to go and revisit the spacing calculations?
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Old 16-08-2015, 12:14 PM
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The flattener corrected field refers to the size of the sensor that it can project corrected stars onto, not the vignetting free field. 17% vignetting is perfectly fine.
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Old 16-08-2015, 12:20 PM
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Atmos (Colin)
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Quote:
Originally Posted by AlexN View Post
The flattener corrected field refers to the size of the sensor that it can project corrected stars onto, not the vignetting free field. 17% vignetting is perfectly fine.
I was thinking more along the lines of causing issues with field curvature and vignetting (which changes with field curvature). I'd throw my D700 (full frame) on the back of it but I don't have the correct spacers for that yet.

Wasn't sure how 13-20% vignetting translated to a "normal" system.
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Old 16-08-2015, 12:56 PM
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Vignetting and curvature are largely independent of each other.. there is no real "normal" as every system is different.

I would not be concerned about it untill you point it at the sky. 17% vignetting will easily correct with flats.
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Old 16-08-2015, 01:12 PM
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that looks fine to me Colin. After flat fielding, you will have a very slight reduction in SNR in the corners (about 10%) but that will be right on the edge of visibility, so forget it.
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Old 16-08-2015, 02:33 PM
SpaceNoob (Chris)
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I see about 23% with a CDK 12.5 and 16803, completely disappears with flats.
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Old 17-08-2015, 08:05 AM
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I agree with all the posters. That's not a lot of vignetting. Vignetting varies a lot between types of scopes and models. Also with the size of the camera sensor. KAF8300 is not particularly big but big enough to see a drop off. Some optical designs like CDK trade off increased vignetting for faster F ratios. So the optical designer makes a choice sometimes between parameters about what characteristic is more desirable to the user.

Flats can correct some really severe vignetting. An FSQ 106ED with reducer and a 16803 camera is a miracle of flat fielding. Its really heavily vignetted.

Greg.
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Old 17-08-2015, 07:33 PM
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Quote:
Originally Posted by gregbradley View Post
An FSQ 106ED is a miracle of flat fielding.
There, fixed it for you Greg
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Old 17-08-2015, 07:35 PM
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Octane (Humayun)
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Agreed, re: FSQ. Unbelievable.

H
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Old 19-08-2015, 10:46 PM
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Took it out briefly for the first time tonight, only briefly due to one of the tripod legs sinking slightly making actual polar alignment night impossible! Didn't have the energy to dismantle the system, move and revelled the tripod and then setup again.
Mistake learned from, use concrete and not grass Grass position has a better view of sky, concrete has less tripod sinkage however.

Anyway, I have a strong feeling that I must have the spacing wrong with my flattener reducer, getting ~40% curvature! From the attached mosaic it is VERY evident. I guess my theoretical 0.5mm off is far more than that!

When I've got time (hopefully in a week or so) I am going to knuckle down and write a review of the scope. Preliminary playing with says it is a very solid well constructed scope Now I've just got to figure out how to get the most out of the damn thing!

I am actually quite curious as to how much of the vignetting has been caused by the heavy curvature. This is actually one of the reasons for the initial post on vignetting, it seemed higher than I was expecting and maybe this is a contributing factor.
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Old 20-08-2015, 09:17 AM
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Try using some pavers on the grass, see if that helps.

H
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Old 20-08-2015, 02:07 PM
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That's what I've done.. Distributes the weight over 3x 30x30cm pavers instead of 3x 1x1cm tripod legs. Mine hasn't sunk into the ground since.
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