New photometric filters - Bessell design

American Association of Variable Star Observers (AAVSO)
Mon, 03/22/2021 - 18:52

I have learned that Custom Scientific is in the process of changing the design of their photometric filters. Instead of using the classic colored glass sandwich to match the Johnson-Cousins system with Bessell prescription they plan to use a single piece of fused silica with an interference coating on one side and an anti-reflection coating on the other side. As a result the shape of the transmission bands are different compared to the old filters. I attach a pdf showing the comparison. I just wonder whether the new design is still good to do the photometry as I guess the transformation to the standard system is likely to be quite different. I had a talk with astronomer Ulisse Munari that confirms the above-metioned concern and points out additional problems when observing peculiar stars such as novae in ouburst. In those cases the Hbeta line may shift bluewards and be off the passband of the new V filter causing spurious effects. I am just thinking if Sloan filters would be a better choice as their magnitudes exist and can be retrieved by APASS.

Thank you for your comments.


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new bandpasses

I wrote a program a decade ago when I was working with Don Goldman to create the Astrodon photometric filters.  Give me until the end of the week to resurrect this and study the effect of the square bandpass filters.  Sorry this is taking so long; I am juggling a lot of projects right now.