Adding Crayford to SCT - Effects on light path?

[GingerNut]

Hi All,

A nice meaty question for the techies amongst you -

When a crayford focuser is added to a SCT, what effect (if any) does it have on -

1. Magnification?

2. Focal Ratio?

3. Anything else relevant?

And then if you add a 3.3 focal reducer between the SCT and the crayford what happens then?

Yours Roy

[moondog]

I tried my C8 SCT with a crayford focusser, but didn't keep it.

As far as I could tell it didn't affect the Magnification or Focal Ratio

On an Alt-Az mount a crayford plus camera can affect imaging at the zenith. It can jam on the base - I've done it! Usually not a problem on a wedge, or obviously if it's an OTA on an EQ mount. The weight of a crayford does affect the balance of the scope quite a bit as well, you may need a weights system adding to compensate.

I've just been testing the Meade F3.3 FR on my C8 (but not with the crayford). With certain set ups - filter wheels, different CCD cameras etc., I struggle with inward focusser travel.

Because the crayford is quite long, you may also encounter problems focussing due to lack of inward focusser travel.

MD

[GingerNut]

I'm already using a crayford with my SCT and I wouldn't be without it, despite having to watch out for it hitting the mount.

But my question is of a purely technical nature of the actual answers to the above questions.

I was focusing my DSI with no problem stuck staight in the crayford, but I found that when I put the 3.3 reducer in, I had to add the diagonal to the crayford to get focus............hence the questions.

Yours Roy