Effect of Focal Ratio on Interference Filters

American Association of Variable Star Observers (AAVSO)
Tue, 01/21/2020 - 20:30

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.






Variable Stars South (VSS)
Effect of focal ratio on interference filters

My thought is that you could determine transformation coefficients with your system at various focal ratios.