Prime focus DSLR on SCT

[SteveT]

I am struggling and would like some help from the experts please.

I want to connect DSLR to C9.25 SCT for prime focus imaging with F6.3 FR in place.

I also have a WO focuser connected to rear of SCT VB. I have a moonlite 2" SCT adapter (smooth bore) to attach to the FR so it can be inserted into the WO focuser which leaves me with the rear of the FR (with SCT male thread) to connect somehow to my DSLR T mount camera adapter (on T ring into camera).

The rear of the FR fully inserted into the focuser at minimum extention is 160mm from the SCT VB.

The distance from the front of the T adapter to the CCD in the camera is approx 95mm.

I am told the optimum distance for the focul point is 105mm but I dont know what this refers to, is it the FR to CCD distance, in which case i need another 10mm for some kind of joiner ?

Please help, this is getting to complicated and I now have box full of adapters already !!

Thanks.

[billhinge]

This is how mine is set up

telescope side moonlight connects to reducer, camera side reducer connects to Celestron T-adapter-SC part 93633-A (365astronomy may have them in stock)

this then connects to your normal camera T-adapter, I measured the distance from lens back to camera T-adapter flange best I could and mine is approx 85mm

then on my camera the flange to sensor distance is 20mm so 105mm in total (I know it is 20mm for a m43 camera since it is a standard) - your t-adapter and flange-sensor distance should also be magically correct as well

The moonlight then goes in the 2" focuser and keeps a constant distance between reducer and camera

I believe you can make the distance longer to get a greater focal reduction or shorter to get a smaller reduction, just that celestron claim 105mm is optimum

(I also have a variable distance adapter so in theory I could get better than f6.3 and since I'm using a smaller m43 chip I may get away without vignetting due to smaller chip size; aps-c and FF probably wouldn't or at least thats how I understand it)

[SteveT]

So the distance from the FR to Chip sensor is the 105mm critical length, OK as I thought.

The bit I need then is the Celestron T adapter SC. I assume the distance from the FR to the back cell can be anything that the focuser allows over its 30mm approx range and it keeps a good image then, obviously the shorter the better by adjusting the SCT mirror focuser to permit a short as possible focuser length reduces any flex to a minimum.

Cheers for that superb explanation.

[billhinge]

you're welcome, took me a while to work it out too

Correct, the following book is very handy, has a section on reducers and formula for working out the reduction

Digital SLR Astrophotography by M Covington

[SteveT]

So I can't get hold of one of these, out of stock everywhere and months (as usual) for expected delivery, does anyone have any alternatives which offer the same dimensions to keep the correct 105mm back focus distance?

Sent from my iPad

[billhinge]

I know, I ordered one from 365astronomy a month or so back and it came within 24hrs , I take it I got the last one?

You'll need to find something like a sct to t-thread adapter - or perhaps cobble a diy version together out of bits but I don't think you would get the exact length

[alan potts]

Try Telescope Servives in Germany, they will post and have a massive web site of bit and pieces as long as you know what you are looking for. I have had a great deal from them.

Alan.