Wed, 07/31/2024 - 20:43
I have been doing photometry with only a V filter. I have recently purchased a B filter and a filter wheel to use both filters when gathering data for a star. I am currently looking at the focus off-set for the B filter. How close do the FWHM of the star in the two filters need to be to be considered in focus?
I try to make them the same. I use the filter focus offsets in MaxIM DL.
Ray
The B filter often has a slightly worse fwhm, due to many factors. The seeing is usually a bit worse. The optics often don't correct quite as well. The exposures are longer, so your telescope might not track as well, or you get more wind shake. However, there are methods used by most software to get the best focus possible. One such method is the V curve, where you start out of focus, measure the fwhm, and then step through focus to the other side, measuring focus at each step along the way. When plotted, the fwhm values follow a "V", and you can easily see what position was best. That said, if you bought your B filter from the same vendor, and you are using a reflector, the filters should be parfocal and the B focus position nearly the same as the V focus.
Arne