2 Questions

1. I bought aluminum casting from Obsessiion for the side bearings of a large Dob I'm rebuilding and the bearing surface has an 8 degree draft. Do you need to remove this draft angle? If not I'd think the laminate would not go on straight.

2. Many big Dobs don't paint the top of the mirror box black but then others go crazy with flocking in their scopes. This seems contradictory. Does it really matter?

Ed Jones

Let me try at #2. You only want focused light from the mirror to reach the eyepiece, otherwise you want it pitch black for optimum contrast. So if you look through an empty focuser on an assembled truss dob, you see the other side of the cage around the secondary, and you should see a portion of mirror box around the primary mirror. These surfaces that you see are best kept as black as possible, and the flocking from Mcmaster Carr or protostar seems to be a best-bet. The stuff is so black, if you put a piece of it next to a piece of something painted ultra flat black, the painted part will look bright, light and reflective. Concerning the off axis light, that is what the shroud and upper tube extenstions are for, and flocking those parts seem to be an smaller issue, as long as thouse parts are somehow darkened.
I have an 8" tubed dob that has the entire tube and focuser lined with black velvet from McMaster carr. The contrast improvement is so good, the view matches the image brightness of a 10" scopes. Even better, when installing the velvet, no paint mess or fumes, you either glue it in, or peel and stick.