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10" RCT with 0.67x Reducer - severe vignetting

ian_bird

By ian_bird
in Imaging - Discussion

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ian_bird

ian_bird

Posted
Posted

Hi

I have a 10" RCT with 0.67x Reducer and I am getting what to me looks like severe vignetting.

Attached are flats with and without the reducer. Is this severe - or am I worrying too much?

post-26501-0-95258200-1370784575_thumb.j

post-26501-0-59622500-1370784613_thumb.j

Any advice greatfully received.

Cheers

Ian

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ian_bird

ian_bird

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You probably did a severe stretch? I've seen worse, just try it on a light frame.

Hi Jeffrey

I think you could be spot on. Just tried it with less stretch. Looks a lot better! Thanks!

post-26501-0-32759500-1370786100_thumb.j

Cheers!

Ian

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kirkster501

kirkster501

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I use the CCDT67 too on my RC8. I am at about 92mm distance by putting the CCDT67 straight onto this which then in turn connects to the DSLR T-ring. That gives 0.64 compression and I get vignetting too on a my 450d DSLR; here is an unprocessed light, straight from the camera:

post-16295-0-47413900-1370790523_thumb.j

The vignetting can be processed out easily enough. And of course this is on a DSLR; a CCD will not have the same chip size and as such the vignetting should be less if there at all.

The CCDT67 is essential on this scope in my opinion. In speeds it up so much!

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kirkster501

kirkster501

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On the 8" version of the GSO RC scope, there would be no need for the focuser rings (those that come with the scope) at all, in other words the focuser could screw directly onto the mirror cell. However, on the RC8 you need at least one ring in order to get the focuser-tipping plate installed. That is not an issue with the 10 inch version; the mechanics of the 10" are different in that respect.

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Jessun

Jessun

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On the 8" version of the GSO RC scope, there would be no need for the focuser rings

With the reducer, what distance do you have from the back plate of the OTA to the actual CCD? Without reducer the back focus is 230mm I think.

/Jesper

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kirkster501

kirkster501

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Not a lot...... I will measure over weekend. The CCDT67 moves the focus MUCH nearer the primary So much, infact, that I need to do some modifications to my focuser titler plate accessory. With my DSLR, I use 1 x one inch extension ring and the DSLR focus requires the focuser nearly all the way in. So really I do not need a one inch extender. But without the extender and with the tilting plate screwed directly onto the back of the scope I can't get access to the primary collimation screws since the tilter is in the way!

Indeed, using a CCD camera, you cannot attain focus at all with an extender.

Me and another guy on this forum are working a way around this issue. Will report back and add to my blog what I did.

Steve

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ollypenrice

ollypenrice

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Ian, looking at stretched flats can be very misleading. Look at the numbers - the black point and the white point on the flats. If they are within a few thousand ADU of each other then flats should calibrate out the difference. If you have little signal in the corners then they won't.

Olly

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