In an unbaffled telescope.
>
> The view of the lower section of tube through the secondary is *back* along
> the incident direction not in the reflected direction which was Chucks
> concern.
>
> This view of the tube walls thru the diagonal occupies a larger angular
> area than stuff on the tube end(like dust on the mirror or mirror cell)
This depends somewhat on how big your diagonal is and the diameter of your tube vs the diameter of the primary mirror. Planetary telescope (most sensitive to contrast reductions) would have bigger tubes and smaller diagonals.
> which might scatter the light from the glossy reflection towards the image.
>
> Until you can make your roughened walls as 'dark' as the dust on your
> mirror you may as well leave the lower part of the tube glossy.
>
> Hmm, I'd better check how dusty my mirror really is (:-)>
>
>
> JohnH
>
>
Very interesting.
In an ordinary baffled designs for Newtonians that I have seen, the goals are that stray light entering the telescope (which does not directly hit the primary mirror) must reflect at least twice at near normal incidence before reaching the primary mirror (near normal incidence reflections being much less reflective, about 1%, two such reflections give 0.01%). No light can enter the telescope, strike the tube wall, and then strike the primary mirror (the baffles are in the way). Also stray light which does strike the primary mirror (which does not directly enter the eyepiece) must reflect at least twice at near normal incidence before reaching the eyepiece. Light can not travel from the primary mirror, via the tube wall, to the eyepiece (via the diagonal) without being blocked by a baffle.
This kind of double bounce design works very well for refractors. It works pretty well for reflectors too, but for reflectors there is the additional light path of stray light entering the telescope, striking a baffle near the rear of the tube (at near normal incidence), and going straight into the eyepiece (via the diagonal).
So, if I read John's suggestion correctly now, the idea is to not baffle the rear of the tube and make it gloss black so the light path from stray light, side of tube, to eyepiece is low reflection since most of the light is reflected in the specular direction or absorbed. This makes me wonder about other light paths. What about stray light, side of tube (specular), back of tube (diffuse?), eyepiece? What about stray light, primary mirror (near perfect specular), side of tube (imperfect specular), eyepiece? I suspect that these other light paths would negate the advantage of reduced reflectivity directly from the side of the tube into the eyepiece. You also have to worry about dust on the side of your tube. Where did this glossy tube idea originate? I have not seen it published, has it been? Is it used by any manufacturers?
Now Sam also says "some baffle surfaces should be left glossy" which to me implies that he means some baffles other than (or in addition to) the side of the tube. This is a very interesting idea which I have not seen published. Do you have any references for this or is it new? Do any manufacturers do this? It does not seem immediately obvious what type of geometry would be best for these directed reflection baffles.
Chuck