I have been attempting photometry of Betelgeuse using my 32cm f/6.7 telescope and CCD camera with some recent success. Of course, it and the nearby comp stars are too bright, so defocusing and stopping down are required.
I was told by a photometry expert that a single 2-inch stop is not a good idea, as it reduces the convergence of the beam to f/42, which is not what the V filter is designed for. In other words, at f/42, the V filter may be off-band. For that reason, I went with four 1.25-inch holes spaced 90 degreees apart whose edges are tangent to the edge of the primary mirror. The system still has 2 inches of efffective light-gathering aperture, but the beam converges at f/6.7, sort of like an interferometery array.
This approach is working fine, but I'm wondering if it is necessary. Does anybody know if an interference filter like my Johnson V filter would actually be working off-band if the beam came in at f/42 rather than f/6.7? Thanks.