removing the reducer

[Demonperformer]

When using my ccd camera (SXV-H9) with my sct (200mm f/10), if I use the focal reducer (0.5x), I need to include a 10mm tube to achieve focus. The total distance from the front of the ccd to the back of the sct [reducer+tube] is 57mm.

The instructions for using the ccd state that, if I use it with a camera lens, I need to use a 25mm tube to bring that lens to focus.

So, if I use the camera without the reducer, on the sct, is achieving focus as simple as using a 25mm tube (treating the scope like a camera lens)? Or is some other spacing required? [i'm sure there must be a formula out there somewhere to work it out myself, but I have been unable to find it.]

I realise this is not an ideal set up for a lot of objects, but, after last night's preliminary attempt to image the Uranian moons (about 10^o directly above my streetlight - hence "preliminary"!), my thinking is as follows. [if I'm talking rubbish in this bit, please let me know.]

(1) Uranus is a multi-pixel object, so by increasing the focal length the light will be spread over a greater number of pixels, making the planet less bright (per pixel) - just as if it was a DSO - and so reducing "glare".

(2) The distance between the planet and the moons will be double the number of pixels, so it will be easier to separate them [Miranda never seems to get more than about 9.4" from the planet, which is quite close with the focal reducer in place].

(3) Because the moons have angular diameters that are much smaller than the angle/pixel in either setup, the principle applied to the planet in (1) will not apply, as the light will still all fall on the same pixel and so the brightness will not be affected. [i realise this is a gross over-simplification, and any/all imperfections in my optical train will disperse the light, and the moon's position may mean the light would not fall on a single pixel anyway, but I think the theory holds true in a perfect system, and is therefore likely to improve my results.]

Thanks.