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How large an image circle on a 10" Meade SCT?

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#1 BillD17

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Posted 08 March 2018 - 09:10 PM

I'm considering upgrading my camera and am curious how large an image circle I can get on a 10" SCT.  Mine is a circa 1990 Meade 2120, but I suspect it has an image circle comparable to a modern Meade 10" SCT.

 

What type of field flattener would be needed to keep this field flat enough to image?

 

I probably just need to take a full frame DSLR and run some tests.



#2 J A VOLK

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Posted 08 March 2018 - 10:10 PM

I don't believe anyone makes a corrector for the native f/10. Years ago I shot 35mm film through a C11 - vignetting was very noticeable at the corners. One of the Starizona products would give you good results, but they are all reducers. The large SCT Corrector for C-11/C-14 (f/7.2 - f/7.5) looks really good for full frame, possible adaptable to M2120 since the rear cells are nearly identical. You could contact them to see.

#3 555aaa

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Posted 09 March 2018 - 09:48 AM

On my Meade 10" f/10 ACF there is some curvature of field and if I recall there is about 20mm dia of very good area. Out past 30mm things go bad and then there is vignetting because the standard 2" accessories are too small. I have a post with measurements on my Meade 16" ACF which has a substantially flatter field. This link has the results for the larger scope:

 

https://www.cloudyni...de-16-f10-data/

 

I think that the radius of the field curvature scales roughly with the focal length for this data for for the 10" f/10 that would be smaller by (100/160) = 0.625, so just scale the X axis by 0.625.

 

The vignetting is limited by the adapter and in this case I was using the Orion thin off axis guider and a Canon 6D DSLR.


Edited by 555aaa, 09 March 2018 - 10:02 AM.


#4 George N

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Posted 09 March 2018 - 12:27 PM

What about the Explore Scientific 3-inch .7x flattener/reducer? Has anyone tried one on a 'classic' Meade SCT?

 

I'm rather curious because I know of a Meade 12 SCT tube in very good condition available for a good price.



#5 BillD17

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Posted 12 March 2018 - 05:01 PM

So it sounds like without a reducer my image circle won't be nearly the 43mm I need for a full-frame sensor.  What about with the 0.63 corrector/flattener?  Will that give a full-frame image circle?



#6 555aaa

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Posted 12 March 2018 - 11:39 PM

The image here is close to full frame with a 24mm x 36mm DSLR, shot with the 10" Meade ACF.

 

https://www.cloudyni...m8-lo-res-copy/

 

I think the reducer is going to give you a smaller image circle, not a larger one. You are ultimately limited by the physical aperture in the SCT baffle and the camera adapter - if you have a 50mm OD tube then the ID going thru that is more like 45mm and you are trying to fill a 43mm sensor with it and that's not going to happen. If you just look up inside the SCT from the eyepiece end you'll see what I mean - the diameter of that metal tube is what ultimately limits vignetting. But still I think it is pretty good. There are lots of other problems introduced with a reducer so I would leave it off for the time being.

 

The vignetting on this M27 image is exactly the same as I would get on the 10" ACF using the standard 2" adapter and Orion Thin Off Axis Guider.

https://www.astrobin.com/302165/



#7 BillD17

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Posted 13 March 2018 - 12:48 AM

Those images from the modern ACF Meade look great.  Unfortunately my 30-year old pre-ACF Meade is not going to have that flat a field.  Without a flattener I'm going to get weird stars in the corners.




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