Changing backs and focussers on SCTs and SMTs

[25585]

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?

 

[Tim]

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.

[John]

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.

[Luna-tic]

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.

[Cosmic Geoff]

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".

[25585]

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 

 

[John]

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

[25585]

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!

[John]

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".

 

[Luna-tic]

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".

[25585]

Ordered the Baader!

When does FLO start giving discounts? 

[John]

20 minutes ago, 25585 said:

Ordered the Baader!

When does FLO start giving discounts? 

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

 

[25585]

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

 

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

[eekeek]

+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!

[DeepSkyMan]

On 14/10/2017 at 16:20, eekeek said:

+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!

Yeah, the dreaded SCT mirror flop...  Speaking from experience, the Crayford style focuser is a useful addition to the EdgeHDs In combination with the mirror locks.  I typically bring my scope to rough focus with the stock focuser, then apply the mirror locks, thereafter all focusing is done from the  Crayford.  I’ve also found this useful for collimation, the mirror locks allow you to collimate with more confidence against an artificial star.  Once fine collimation is achieved you set the mirror locks.' One of the draw backs with collimating classical SCTs against an artificial star is that this is typically done with the OTA parallel to the ground, of course this is not the natural orientation for the scope when observing.  When used for observing, the risk is that the primary will shift, compromising the earlier collimation.   The theory is that the Mirror locks will prevent this from happening.  Of course as stated by eekeek, the introduction of a Crayford may result in a change to the focal length of the scope, and perhaps a shift from the optimum backfocus specified by Celestron and in the case of an EdgeHD, compromising the field flattening optics in the baffle tube.  However since I’m purely a visual observer I view the convenience of the Crayford as a worthwhile addition to my setup, if you’re an imager your take may be different?

Paul 

[The Admiral]

Sorry, how can replacing the focuser, which is a non-optical device, change the focal length of the 'scope? Surely, that will be determined by the mirrors, unless you are using a reducer. Do you mean the amount of back-focus available? Sorry, I only use a refractor, perhaps I'm missing something.

Ian

[DeepSkyMan]

2 hours ago, The Admiral said:

Sorry, how can replacing the focuser, which is a non-optical device, change the focal length of the 'scope? Surely, that will be determined by the mirrors, unless you are using a reducer. Do you mean the amount of back-focus available? Sorry, I only use a refractor, perhaps I'm missing something.

Ian

Ian, hi there.  I’m no expert on optical theory, I’m just imparting information gained from chatting to more knowledgable people on SGL and CN after I replaced the stock visual back on my 8” Edge with a Baader Steeltrack focuser.  Evidently the issue relates to the optical design of SCTs which involves alteration of the distance between primary and secondary in order to reach focus.  By adding a secondary focusing component you need to alter the distance between primary and secondary, in order to reach focus.  This extends the back focus of the optical path resulting in an increase in the FL of the scope.  I’m paraphrasing here, but you can get a more informed picture from the contributions to my original SGL post:

 

 

Kind Regards

Paul.