spacing question

[Steve14]

Hi all
I have a Celestron c9.25 XLT for planetary imaging but thought i'd try some DSO imaging with it as well. I have just ordered a celestron f6.3 reducer/corrector which i believe has a focal distance 105mm (back of reducer to camera chip). As i am using a Canon full frame camera with a back focus of approx 55mm, i was thinking of getting a M48x0.75 spacer made up which will sit between the reducer and the canon t mount. I figure that the effective length of the spacer should be 50mm. I think this sounds correct but before i order it, can anyone confirm that my calculation is correct?
With thanks

[Merlin66]

Spacing sounds correct, but why the M48 thread?
The rear of the reducer is the standard 2” SCT thread and the T thread adaptor for the camera is the standard 42mm T thread.

[Steve14]

Thanks for the advice. My camera is set up to use an M48 adapter instead of a standard M42

[Merlin66]

Steven,
OK.
I was misled by your wording...
""which will sit between the reducer and the canon t mount""

[Dennis]

Hi Steven

I had a Celestron Off Axis Guider lying around so I mated it with my Celestron x0.63 Reducer/Corrector and came up with the following measurements, based on the following assumptions:

Celestron designed their OAG to be used as follows:

Rear Cell of SCT -> 0.63 R/C -> OAG -> T-Ring -> DSLR.

I have a Canon DSLR so in the following example:

Canon EF Mount: distance from the mounting Flange (the metal ring on the camera and the rear of the lens) to the Film Plane = 44mm.
T-mount requires a 55mm Flange to Film Plane distance, therefore T-Ring is 11mm thick.

The 1st attached file shows the Celestron Off Axis Guider mated with my Canon 5D and the Celestron x0.63 Reducer/Corrector (RC).

When the Celestron OAG is screwed into the Celestron RC, the distance between the T-Ring flange and the interface between the OAG Flange and RC Rear threaded Face is 50mm (inside the knurled ring of the OAG).

This gives an overall measurement of 44 + 11 + 50 = 105mm.

Cheers

Dennis

[Steve14]

Thanks Dennis. Very helpful. As i am not using a OAG, the spacer should have the same distance - so 50mm it is.:)