C11 Visual Back - please explain...

[DannyHarvey]

On the C11 there are two circular fittings on the visual back, the inner one I recognise as the SCT screw fit that is the same as my C6 - but what is the outer (larger one) for?

thanks

Danny 

[jambouk]

I wondered about this too, so will watch this thread....

James

[CodnorPaul]

I believe you can remove the 2" sct thread and fit a 3.25" - gives and even larger FOV

[Merlin66]

Interestingly it doesn't mention Celestron sct's......Hmmmm

http://www.petersonengineering.com/sky/EO.htm

found this one:

http://www.nexstarsite.com/OddsNEnds/N112InchAdapters.htm

[DannyHarvey]

This is great info thanks!

[Dunkster]

The C11 has a 3.25" SCT thread on the rear cell compared to 2" on the smaller models. The C14 and Edge HD models 9.25" and larger also have the 3.25" rear cell. I chucked the reducing ring in the bin the day my C11 was delivered...and bought the Baader Clicklock visual back. Highly recommended if you wish to approach the max FOV of your scope, because the reducing ring is constricted to 38mm.

[jambouk]

What sort of FoV can you now get?

Mine is predominantly for planetary and lunar, so i'm less worried about the FoV but would be interested to know what you can achieve.

James

[Dunkster]

It's 0.95 degrees at f/10, but the key thing is it's as fully illuminated as the design allows. I also have the reducer (0.7x as it's an Edge HD C11) and that bumps it up to 1.35 degrees, enough to swallow the Orion Nebula and it's quite a sight! Eta Carina nebula doesn't all fit in of course, but that's huge

[Tinker1947]

I'm not sure how much would be gained using the 3.25" on a C11 if the Celestron 6.3 FR is used this would need to connect to the Click Lock (extra spacing)  then have around 100mm distance from the rear of the 6.3 FR to the sensor on the Edge type OTA this is all ready built in and would work well....

[Dunkster]

The 3.25" thread is required because the baffle diameter is greater than 2". At least one manufacturer has announced (but not yet shipped afaik) a 3" eyepiece. The 0.7x reducer for Edge HD (irrelevant here) is 3.25" inch at both ends and doesn't alter backfocus.

I believe the clear aperture on the 0.63x reducer for normal SCTs has clear aperture less than that permitted by a 2" optical path also, so depending on how steep the light cone is there may be some drop off of illumination there. At f/10, either C11 is capable of up to 0.95 degrees. For planetary and lunar this is moot of course