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Old 28-04-2013, 10:35 PM
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Nico13 (Ken)
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Preliminary Report. (Vignetting SCT & SLR)

Some time back there was a discussion re vignetting with SCT's and SLR's in particular with the focal reducer flattener in the image train.
For some time I've mulled over ways around this and recently while reading up on telescope design and considerations I saw it mentioned that the reducer correctors should be as close to the image sensor as possible.

Thinking about that and the Orion flattener I use on the ED80 is mounted directly to the camera and that works fine, it even helps on the SCT (Meade 8") So looking at my SCT setup with the reducer fitted directly to the back of the scope and then focuser before camera adapter I wondered if by any chance the camera adapter threads would be the same as the flattener and bingo they are.

So next step, move the flattener further back in the image train to just in front of the camera.
The aim is to reduce the amount of vignetting but there was a surprising additional effect as well.
See my next post shortly with some test flats ect for the initial results prior to field testing.

Image one and two Original setup
Image three and four fitting flattener to camera adapter
Image five new location of flattener
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Old 28-04-2013, 10:54 PM
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Nico13 (Ken)
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Initial test flats show a definite improvement on the vignetting but while having the good wife (read assistant) hold a white board up at about 12 metres distance for some flats I used her fingers as a focus point and a very obvious magnification was evident with the flattener in the new location.

As this is a reducer flattener this was a surprise and so it will be interesting to get some real shots in to see just how much this has effected the actual field size.

Image 1 & 2 before and after the change
Image 3 & 4 before and after the change
Image 5 checking for field flatness

Unfortunately wont be able to try it out until next week as I'm working late shift this week.
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Old 29-04-2013, 07:09 AM
Garbz (Chris)
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The magnification is evident as this only works as an f/6.3 reducer when setup with the correct focal length of the reducer itself. I think this was something like 89mm. I.e. it's an f/6.3 reducer if the distance between the sensor and the reducer element is 89mm, otherwise the number changes.

As the focal length in your setup changes you change the magnification, and naturally as a result the vignetting too. Only problem is that coma correction is only deigned to work at one focal length.

From what I've been told with the focal reducer you should be targeting minimum coma and not minimum vignetting. The vignetting will sort itself out with flats providing you're not actually obscuring the edge of the shot with the sensor (like many full frame cameras do on an SCT). This is one of the reason I always shot my Nikon D800 in cropped mode as any advantage of a 35mm sensor was lost due to the added vignetting and poor edge sharpness.
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Old 29-04-2013, 08:02 PM
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Nico13 (Ken)
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Quote:
Originally Posted by Garbz View Post
The magnification is evident as this only works as an f/6.3 reducer when setup with the correct focal length of the reducer itself. I think this was something like 89mm. I.e. it's an f/6.3 reducer if the distance between the sensor and the reducer element is 89mm, otherwise the number changes.

As the focal length in your setup changes you change the magnification, and naturally as a result the vignetting too. Only problem is that coma correction is only deigned to work at one focal length.

From what I've been told with the focal reducer you should be targeting minimum coma and not minimum vignetting. The vignetting will sort itself out with flats providing you're not actually obscuring the edge of the shot with the sensor (like many full frame cameras do on an SCT). This is one of the reason I always shot my Nikon D800 in cropped mode as any advantage of a 35mm sensor was lost due to the added vignetting and poor edge sharpness.
Thanks very much for that Chris,
That bit of info re the distance between reducer and camera sensor is something I obviously missed in the last discussion on this subject so that will save me a lot of messing around only to arrive at the same conclusion.
That's the problem I guess when purchasing a lot of second hand gear the relative info for setting it up doesn't always get passed along with it.
It's also another great thing about IIS, the info is out there with someone.
Thanks again.
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