Skip to main content

Effect of Focal Ratio on Interference Filters

2 posts / 0 new
Last post
polakis's picture
Effect of Focal Ratio on Interference Filters

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.






arx's picture
Effect of focal ratio on interference filters

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


Log in to post comments
AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484