Filter defect(s)

[Tim Haymes]

I have a Baader Johnson/Bessel-R filter. This 1.25″ filter is a few years old and been stored in its box. It was about time i use it.

There are some interference rings within the filter construction. Would these effect the images ? The central area is clear. I have some Baader LRGB filters that dont show this effect. The R-filter is “partial dielectric design”.

I dont think any defects were visible when i bought it.

Any thoughts on this?

Cheers

[Dr Paul Leyland]

Good question, to which I do not have an answer but do have a suggestion: suck it and see.

Take images of a field rich in stars with accurately measured BVR magnitudes (a Landolt field would be ideal, otherwise find a VS at the AAVSO) and see whether your R-band measurements show a systematic dependency on the tabulated B-V and V-R values. If you have Johnson B and/or V filters, so much the better.

Regular flats should take out spatial variation.

[Robin Leadbeater]

The earlier Baader filters were conventional multilayered coloured glass filters which would explain the interference rings, with I believe a dielectric coating to block IR leaks (I have an old V filter from a different supplier which has delaminated on the edge.) The RGB and latest BVRI filters from Baader (and others) are now a single thickness of dielectric coated glass

Cheers
Robin

[Tim Haymes]

Thank you gentlemen. I have added the filter to my filter wheel and will investigate.
Tim

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