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Riccardi 0.75x Reducer Flattener for RC8

Ritchey-Chrétien Reducer Flattener Focuser
Ritchey Chretien telescopes GSO Family •6 replies•441 views
Brian Boyle avatar
Brian Boyle avatar
Hi, RC8 users 

Has any RC8 user out there has experience with using this reducer/flattener for an RC8?

I am looking for a reducer/flattener - and this one appears to be one of the few that is small enough to fit to a 2.5inch focuser [This is the largest focuser I can find with a M90 thread to connect to the back of the RC8].  

With the length of the focuser + corrector + required back focus from corrector to sensor, the minimum total distance from back of primary mirror to sensor is 231mm, close to the 254mm for f/8.  But this would now be f/6.  

Since I am using a full frame sensor, I do need a flattener and 1600mm is just too long a focal length for my seeing.  [I have tried the CCDT67 0.75x reducter with no flattening using an APS-C sensor, and found the curvature of the RC field an issue.] 

I currently use the TS-2inch flattener (no reduction) with a full frame sensor and it works well to field edges, even though there is fair bit of vignetting.  I have to bin x 2 to match the seeing, and would love to get more field at better sampling through some focal reduction.

Has anyone successfully done this for the RC8 and a full frame sensor.  

Many thanks

Brian
SemiPro avatar
SemiPro avatar
In theory it should work. I use a 0.8x flattener/reducer on mine. Just be warned, since a lot of them are meant for smaller refractors the typical 55mm back focus goes out the window and you'll have to find a way to get it closer.

https://www.ontariotelescope.com/sf08.html

That is the one I use.

https://www.astrobin.com/7q69z2/

That is how it does with an APS-C sized sensor. I actually need to find a way to get it closer to the flattener because on some nights the 50.5mm I have it at now is not enough.

You can also search Astrobin. That is what I did to see what kind of combos people had.
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Brian Boyle avatar
Brian Boyle avatar
Hi SemiPro,

Thanks for the link.  I didn't realise that this reducer/flattener existed.  And there is also a 0.6x version.  The limitation of a sub-55mm backfocus might prove a little challenging for me, distance from the front of my OAG to the sensor is 55mm, so I would have to give up my OAG.

I will also have a look around Astrobin, although I do find the search tool a little non-intuitive.

CS Brian
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SemiPro avatar
SemiPro avatar
Oh boy I didn't even realize there was a 0.6x one! I'd be curious to know how much vignetting that would create, or if you can even get a flat field with it. Getting the RC8 down to f/4.8 is almost too tempting to try…

With the .8x one I have I have it at 50.5mm, so that includes the 17.5 from the 2600MC, 16.5 for a filter holder and 16.5 for the OAG I use. Short of taking off the tilt plate that is as short as I can get mine.
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Mario Wunderlich avatar
Mario Wunderlich avatar
@Brian Boyle did you end up getting the Riccardi flattener/reducer?

I'm very interested in the Riccardi 0.75x M82-Large reducer/flattener for both my 12" and 16" GSO RC's, though I'm not sure how well it'll work with an RC since it's designed for refractors.
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Tobiasz avatar
Tobiasz avatar
Mario Wunderlich:
I'm very interested in the Riccardi 0.75x M82-Large reducer/flattener for both my 12" and 16" GSO RC's, though I'm not sure how well it'll work with an RC since it's designed for refractors.


Hi Mario,

check out the deep dive from Tommy Nawratil regarding the RC10: Link

At the bottom he tested different reducers with it and posted an single sub for each reducer so you can check out the stars like the one with the riccardi 0,75x: Link

Your RCs have bigger apertures, so the corrected field should be a bit better than the one from the RC10. Hope this helps.

Regards
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Brian Boyle avatar
Brian Boyle avatar
Mario Wunderlich:
@Brian Boyle did you end up getting the Riccardi flattener/reducer?

I'm very interested in the Riccardi 0.75x M82-Large reducer/flattener for both my 12" and 16" GSO RC's, though I'm not sure how well it'll work with an RC since it's designed for refractors.



Hi Mario

In the end, I upgraded to an RC12 and used the Williams Optics 0.8x reducer/flattener with my 6200MM.  It works well.

CS Brian