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FR/FF for 8" F/3.9 Astrograph?

astrophotography equipment eyepieces reflector
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#1 Jim Waters

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Posted 18 January 2020 - 06:52 PM

Would it be possible to add a FR/FF to an astrograph like the Orion 8" f/3.9 Newt?  Something like a 0.8 or 0.75x FR/FF.  I will be doing astrophotography with an APS-C Camera.

 

https://www.telescop...60/p/101450.uts

 

 



#2 Gipht

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Posted 18 January 2020 - 08:31 PM

I have tried using my Stellarvue 0.8 FF/FR on my 10" f/4 reflector and could not reach focus.  There are FR/CC designed for newtons, but they would be more expensive then the telescope your looking at.  Here is a link to some of them, they are towards the bottom of the page: Some of them require a lot of back focus , and may not cover an APS-C sensor.

 

I think you might be better off using a larger pixel, full frame camera such as the Canon 6D,  with the Orion 8"  f/3.9.  Kind of accomplishes the same thing, wider field of view and more light gathering.

 

You probably don't have one of those, though.



#3 Eddgie

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Posted 19 January 2020 - 09:37 PM

Would it be possible to add a FR/FF to an astrograph like the Orion 8" f/3.9 Newt?  Something like a 0.8 or 0.75x FR/FF.  I will be doing astrophotography with an APS-C Camera.

 

https://www.telescop...60/p/101450.uts

Probably the best reducer for f/4 Newtonians is the ASA .73x/Coma corrector. This will produce quite excellent results, but it requires 90mmm of back focus.

The Orion 8" will almost fully illuminate an APS-C size sensor, but to reach focus with the ASA reducer, you would have to raise the mirror by about 25mm.  If the fully illuminated circle of about 25mm (Orion does not say, but since this is the size of the smaller APS-C sensors, it is probably about right) and you raised the primary mirror with extension screws, you would decrease the fully illuminated circle by (25 / 3.9 =) by about 6.4mm so you would only have about 18.6mm of fully illuminated circle.   Now if you did not mind cropping some this might work, but I can't tell you if that would be a satisfactory  outcome. 

 

To make it work though, you need 90mm of back focus (and this will be true for just about any reducer that is going to give this kind of reduction.. It is just geometry.

 

The ASA is a nice unit though, and the Orion is a nice reflector.  I have an ASA that I use in my 6" and have considered modding my 8" to be able to use it, but I only need 18" illuminated circle, so what works for me may not work for you.  



#4 Jim Waters

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Posted 19 January 2020 - 09:57 PM

Thanks Eddgie and Gipht.




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