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25585

Changing backs and focussers on SCTs and SMTs

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25585    58

I read about people who have changed the back on a SCT or SMT. 

Does that mean the whole metal dish? 

For older light folders is changing back necessary to get a full FOV in long FL eye pieces without vignetting using a 2 inch diagonal?

Can a back change be done DIY or is it better to be done professionally?

Also focussers can be upgraded. If focussing is done by moving the mirror, how complicated is changing a focusser?

 

Edited by 25585
focussers

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Tim    2,536

Not sure about the backs, haven't heard of that one, but I changed the focuser on my C11 EdgeHD for a Feathertouch variety. It was as easy as unscrewing the old one, and replacing it with the new one, you don't touch the mirror or anything, it is all done from the back.

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John    17,077

The visual back can be switched simply by unscrewing the stock one and replacing it with a 2" unit. The inside diameter of the rear port and internal light baffle of these scopes is much harder to do anything about though. I've not heard of folks replacing the whole rear end section of the scope, at least not as a regular modification.

Edited by John

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Luna-tic    136
13 hours ago, 25585 said:

I read about people who have changed the back on a SCT or SMT. 

 

 

The "visual back" on a SCT or Mak is simply the small barrel that threads to the rear cell of the scope. Your diagonal, extension tubes or T mount fit to the visual back. Most SCT's, at least up to 11", so far as I know, come with a 1.25" back, which can be replaced with a 2". You would also remove the visual back to mount a reducer/flattener to the rear cell (the rear cell is the whole back end of the telescope that contains the primary mirror and focusing mechanism). The visual back would then attach to the reducer.  Not sure if a crayford-style focuser would mount directly to the cell, or to the visual back, but I imagine it would have a collar that screws to the cell, to maintain as much back focus as possible.

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Cosmic Geoff    210

A while ago there was a post, probably in the beginners' hardware help section, which detailed in a table the size of the hole through the back of various SCTs and MaKs of different apertures.  This helps one to decide whether replacing the 'visual back' for e.g. a 2" would be useful, or a waste of effort and money. Smaller instruments generally have a smaller hole. For instance the size of the hole in my Celestron C8 203mm SCT is 1.5".  You will note that this is more than 1.25" but less than 2".

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25585    58
20 minutes ago, Cosmic Geoff said:

A while ago there was a post, probably in the beginners' hardware help section, which detailed in a table the size of the hole through the back of various SCTs and MaKs of different apertures.  This helps one to decide whether replacing the 'visual back' for e.g. a 2" would be useful, or a waste of effort and money. Smaller instruments generally have a smaller hole. For instance the size of the hole in my Celestron C8 203mm SCT is 1.5".  You will note that this is more than 1.25" but less than 2".

I will measure up on my C8. Its the older orange tube model so expect the hole is less than 2 inches, but a 2 inch visual back would allow use of my 35mm Panoptic losing only 0.7mm according to 

 

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John    17,077

I found it worthwhile when I had a C8. Mine was a 1990's C8+ so reasonably old.

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25585    58
41 minutes ago, John said:

I found it worthwhile when I had a C8. Mine was a 1990's C8+ so reasonably old.

Can you recommend a 2 inch vb that will take heavy eps, good quality threads!

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John    17,077
2 hours ago, 25585 said:

Can you recommend a 2 inch vb that will take heavy eps, good quality threads!

I used the Telescope Services one. Like this:

http://www.ebay.co.uk/itm/Schmidt-Cassegrain-SCT-Adapter-to-2-Visual-Back-f-Meade-Celestron-TSSC-2-/371327305759?hash=item5674d5081f:g:shwAAOSwr81URfSp

Baader do one of their twist lock types for the SCT and these are excellent and very strong but somewhat more expensive:

https://www.firstlightoptics.com/adapters/baader-click-lock-2-for-celestron-meade-sct.html

When I had my SCT I used 40mm and 50mm eyepieces with a 2" diagonal but they were not my current "heavyweights".

 

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Luna-tic    136
On ‎10‎/‎11‎/‎2017 at 07:08, 25585 said:

Can you recommend a 2 inch vb that will take heavy eps, good quality threads!

Baader 2" Clicklock. Once you install one, you'll never want to remove it. No thumbscrews to tighten, it uses a twist collar to tighten the diagonal or EP in place, only requires 20 degree turn. It holds well enough that you could pick the OTA up by the diagonal and it will hold it in place. One model (BA-CLCS-2) will fit 2" threads on Celestron and Meade SCT's. There is a larger model for 3.25" threads, which I believe are the C11 and C14. That one may or may not fit Meade scopes, depending on their thread size. The BA-CLCS-2 is $104 USD. They are very well made and look great. Here's one on my C6, holding a 2" diagonal and DSLR (shooting the total eclipse back in August). The small pin you see isn't a thumbscrew, but something to grab to loosen or tighten the collar. While they may not improve the view on smaller SCT's, it's worth going to a 2" back if you plan on AP, they are more solid and can carry more load than a 1.25".

Celestron C6-A on GEM with Nikon at prime focus (3).JPG

Edited by Luna-tic
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John    17,077
20 minutes ago, 25585 said:

Ordered the Baader!

When does FLO start giving discounts? :hiding:

The margins are already tight on astro equipment. FLO's pricing is as competitive as I've seen in any UK dealer and their customer service is superb :icon_biggrin:

 

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25585    58
13 hours ago, John said:

The margins are already tight on astro equipment. FLO's pricing is as competitive as I've seen in any UK dealer and their customer service is superb :icon_biggrin:

 

I completely agree ☆. Surprised they don't sell Tele Vue, sure that would help 😊

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eekeek    11

+1 for the baader clicklock!

Be aware though, you are almost certainly changing the focal length of the scope which will have an effect on infocus/backfocus, camera imaging circles, bringing bino viewers to focus etc.

There are 2 styles of focuser upgrade I'm aware of. First is replacing the existing mechanism with a dual speed version. Note this is for moving the primary mirror.

The second (instead), is to add a crayford focuser at the rear cell. You would use the mirror mover for rough focussing and fine focus with the crayford. The crayford moves the eyepiece and not the mirror and supposedly avoids the dreaded mirror flop!

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