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Focal Reducer for a Schmidt-Newtonian?

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#1 shark-bait

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Posted 29 May 2020 - 04:36 PM

I know this is an ignorant question, but is it possible to utilize a focal reducer to achieve a desired sampling size with a given sensor?  I am currently imaging at 0.65" arcseconds per pixel with my Canon 90D on my SN10 and am unable to image large objects such as M31.

 

Thank you.



#2 jgraham

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Posted 29 May 2020 - 05:20 PM

I also image with an SN10 and there are just a few targets that are just too large for its 1000mm focal length. M31 is particularly challenging. I have few scopes that offer a field wide enough to squeeze it in. It barely fits the the field of my 6" f/3.6 Meade Comet Tracker. I have experimented a bit using 0.5x focal reducers on my SN10, but it only performed well with a very small sensor (my Revolution Imager 2).
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#3 rutherfordt

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Posted 30 May 2020 - 08:25 AM

Many years ago, I used one of the screw-in 0.5 focal reducers with my SN-6.  It did not do a very good job, as I recall.  The center was OK, but definite issues toward the edges-- probably the same issues that John just mentioned.



#4 Peterson Engineering

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Posted 30 May 2020 - 10:38 AM

A focal reducer is actually an image size reducer.  The FR simply squeezes the image down onto the center part of the imaging chip.  While you'll certainly increase your sampling size to over 1"/pixel you'll suffer enormous vignetting with a DSLR.  If it were an astro camera you could use binning but DSLR options are quite limited



#5 shark-bait

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Posted 02 June 2020 - 03:16 PM

Thank you for all of these responses.  This is great information and tells me that it is probably time to invest in another imaging scope.  Maybe I can get a few bucks out of my SN10 to subsidize the cost a new 11" Edge HD or a RASA. 



#6 jgraham

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Posted 02 June 2020 - 04:30 PM

If you want to go wide sd something like an 80ED would be worth considering. I used a C80ED for wide field imaging and found that it not only worked very well, the images were so sharp that they scale well.

#7 photoracer18

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Posted 02 June 2020 - 05:52 PM

Since you are using a DSLR the best optic for something like M31 is a camera lens, a prime lens not a zoom (maybe a really good expensive one). From memory framing M31 is 400-450mm for FF and 300mm for APS-C sensors.




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