C8 SCT - does it take 2" visual back?

[EzyStyles]

Hi Guys,

Im looking at getting a C8 and was wondering if it takes 2" visual back? From what i have heard it doesn't?

If anyone can confirm this please let me know thanks.

Eric

[h0ughy]

I can screw on my SCT 2" diagonal on it, but you are limited to the diameter hole already in it for the light to go through. I am lucky with my 12" SCT as i can unscrew this and install the Lumicon GEG which gives me "all" the light. So yes you can attach the 2" visual back but you will not gain much by doing that except standardising your accessories, like a 2" diagonal then slip in the 2" camera adapter (like what i did at duckadang - all images through the c8 were done this way) if nothing else it makes you feel better;) :whistle:

[jase]

As H0ughy states, its not that you can't connect a 2" visual back, its just that you should question the benefit in doing so. If the internal baffling tube is less than 2" in diameter, then you'd likely experience vignetting or unevenly illuminated frames (nothing a bit of image processing can't resolve). Though I'd imagine these problems would primarily be experienced on large format CCD's.:)

[[1ponders]]

If you are thinking of installing something like the Peterson eyeopener, Eric then no you can't screw the back off, I think a 10" scope is the smallest SCT that has a removable visual back

[Don Pensack]

The C8 has an opening of 38mm in the baffle tube. The maximum unvignetted field the instrument can give is 1.2 degrees visually, and 1.08 degrees photographically. The maximum size field of a 1-1/4" eyepiece yields 0.86 degrees.
So there are two ways to achieve the maximum field: a focal reducer with a 1-1/4" diagonal, or a 2" diagonal. If you use a 2" diagonal, you will have to keep the field stop of the eyepieces the size of a 35 Panoptic or smaller. The eyepieces with larger field stops (31 nagler, 40 SWA, 40 Konig, 55 Plossl) will visibly vignette (the edge of the field will appear darker).
If you choose the focal reducer, you can achieve the maximum field of view with a 32 Plossl or any 1-1/4" eyepiece with the maximum field stop for that diameter (e.g.24 Panoptic, 18mmUWA, 35 Ultima, etc.). The focal reducer has other benefits: it flattens the field as well as reducing the focal length 37%, it costs less than a 2" diagonal and eyepieces, it allows the use of a less-expensive 1-1/4" diagonal and eyepieces. The original diagonal with the scope is poor, so I'd upgrade the diagonal whichever way you go to a dielectric-coated model.
Don't think about using a focal reducer with 2" eyepieces, as the reducer limits the illuminated field to 24-27mm. This would cause vignetting with nearly all 2" eyepieces longer than about 22mm or so.
True field = 57.3 x (field stop/focal length of scope). Solving for maximum field stop says it's about 42mm for a 2" eyepiece, and 26.8mm for a 1-1/4" eyepiece used with a focal reducer. That means all 1-1/4" eyepieces should work with a reducer. That's for visual use, where sensitivity to vignetting is low.
Photographically, the field stop limits are : 2"--38mm, 1-1/4" w/FR--24mm. 35mm film would work with the 2" connection, most CCDs would work with the 1-1/4".
Hope that helps you see the big picture (pun intended).
Don

[EzyStyles]

thanks guys . would anyone know the internal visual back size for the c8?

So it can definitely screw a 2" visual back in it?

thanks

[spudrick]

Hello Eric i have a C8 Ultima and i cannot find a 2" Visual Back for it, i think the only thing is the 2" diagonal others have mentioned.
For reference i checked here and someone asked the same question as you.
http://www.telescopes.com/products/celestron-visual-back-2-inch-25845.html

[casstony]

One of the various 2" visual backs available: http://www.optcorp.com/product.aspx?pid=105-109-154-7746

[spudrick]

Ah it is best left to those who know! Thanks Tony i will order one from there.
Great price as well.

[Dennis]

Bintel sell an “Orion 2" Visual Back for SCT” for $39.00. Go here https://www.bintelshop.com.au/welcome.htm
and select “Telescope Accessories”, navigate to Page 6 then look at the first cell in the last row.

Cheers

Dennis

[spudrick]

Thanks Dennis, i go ovet to Bintel Saturday and grab one, want to try some 2" eyepieces in the SCT.
So Eric good you asked the question, so i have learned as well. :D

[Don Pensack]

Yes. Remove the 1-1/4" visual back adapter (it screws off), and what's left accepts a 2" adapter that accepts 2" accessories. The actual visual back of the scope is larger (you'll see when you remove the smaller one) than the openings in the baffle tube.

[casstony]

Terry, depending on your needs, another alternative is the dedicated 2" SCT dielectric diagonal from Bintel for $95 - good value compared to the visual back. I love helping other people spend their money :-)

[spudrick]

There you go Eric 2" fitted to the C8. Picked it up from Bintel this morning :D
Thanks for your help Tony and i decided on this over the diagonal as most of my eyepieces are 1.25" so the lower cost option was best.

[EzyStyles]

arhh thanks terry and all. great help guys.

@terry, would you mind taking a pic straight view of the visual back attached to the c8? i want to see the clearance inside.

thanks.

[spudrick]

Here you go Eric, straight shot, as the guys indicated this just allows the 2" eyepiece you can see the size of the opening is approx 38mm.