CCD distance for Skywatcher 100ED .85 reducer

[Astro-Geek]

I have a Skywatcher 100ED .85 focal reducer and I'd like to make sure I get the camera at the right distance behind it.

I'm fitting it to my Celestron C100ED refractor, which I believe is identical optically to the Skywatcher 100ED.

The reducer has the 2 inch nosepiece adapter, so it fits nicely to the scope itself.

I'm confused about the camera distance though.  The camera back connection is M48 male, so 2 inch m48 spacers will fit ok.

I can't find any details regarding the optimum CCD distance though.  The manufacturer's instruction sheet is very limited, it just shows a series of photos showing it screwed straight on the the back of the scope's focuser via the unusual 56mm thread, (which is no problem, with the 2 inch tube adapter, which just drops into a standard 2 inch focuser eyepiece holder.

On the camera side, it simply says:

"please use SYNTA tailor-made camera adapter rings to ensure the largest aperture and the optimum imaging focus. Connect your single lens reflex camera to the "SYNTA" tailor-made adapter ring"

There's no indication of the required optimum distance, and also I would like to use it with CMOS ZWO cameras as well as EOS SLRs.

I'm thinking of buying a set of m48 extension rings and an M48 female to m42 T mount male adapter.

I found an old thread on here that mentions the First Light Optics - Skywatcher DSLR-M48 Ring Adapter   but still no mention of distance.

Edited February 27, 2020 by Astro-Geek

[wxsatuser]

SW reducers should have 55mm back focus.
Subtract the camera back focus from 55mm and that's your spacing.

For EOS 11mm t-ring should do it other cameras you will have to check.

[Astro-Geek]

Thanks Mike.

I'd found out that the back focus of the EOS body is 44mm, so I can see your logic in the 11mm t-ring making it 55mm.

I just hadn't found any specific reference to the skywatcher 100ed reducer's back focus distance.

Are all/most eos to m48 T adapters 11mm thick ?

When I use my ASI294 (non-pro uncooled) camera, that has a back focus of 6.5mm, so I assume I then need 48.5mm of additional spacing.

It has a male m42 T mount thread on its face, so I'll have to put an "up" converter to m48 T on first, then make up the rest with a 24-35mm variable m48 T extension and then an appropriate short extension piece from an M48 T four piece extension set.  (I believe) ?

 

[wxsatuser]

When I got mine some years a go there were two types of t-ring.
Normal thin one and a slightly thicker one to suit the SW reducer, if I remember right.
If you look on FLO they should have one that goes with the reducer, there are ones for EOS and Nikon.
The 55mm is measured from the flat face of the reducer, camera side.

For CCD or CMOS your correct on your calculation but if there are filters involved normally add about 0.5 to 1mm extra to the spacing.
This extra depends on the filter thickness and particular make.

[Astro-Geek]

I checked FLO and you're quite right, they list an EOS to M48 adapter specifically for skywatcher reducers.

I've just ordered one and also an M48 to M42 adapter to enable me to use my ASI294 as well.  I already have a set of m42 T extensions tubes, so I'll see how close I can get to the 55mm spacing.

Thanks for your help. 👍