Re: 6

Victor,

The central hole on the primary will be determined by the width of the baffling tube, which in turn is determined by the optimum location of the baffle tube front opening. You can have those parameters calculated or, you can make a scaled down drawing
and figure it out that way.

Vlad

Vic,

Why would you go with classical Gregorian, if aplantic one is not harder to make? If your secondary magnification is 6 and back focus 0.167 in units of primary's f.l., aplantic primary would have conic of -0.989 (vs. -1 for the classical), and aplantic secondary -0.54 (vs. -0.51 for the classical).

Vlad

Hello:
I put together a 12.5" cassegrain where the cental hole is 54mm in diameter but there the optimum baffle diameter is 70mm. To make my central baffle, I used soda cans stacked together with the ends cut off. The back end of the baffle is tapered a little, so that it can plug into the cental hole in the mirror. The front of the baffle is held in place with fine wires, which make a little diffraction on very bright stars but nothing too bad. So, although it is not necessary to have a central hole larger than the baffle tube, it would have been nice! In the case of this 6", it may be difficult to find a cutting tool with just the right size, so a hole a little too big or small may be necessary.

Cheers
Mike Connelley
Also interested in aplanatic gregorians...