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Regarding filter placement. Narrow passband or notch filters are somewhat sensitive to angle of incidence. Behind the eyepiece, for a 60 degree apparent field light can transit the filter at up to 30 degrees off axis. This will 'de-tune' the filter sufficiently so that a considerable portion of the outer field will not be properly filtered.

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Ed had previously alluded to this, thanks to both of your for explaining this. As they say, one learns something new everyday. I guess I can relax and take a nap, I learned my one things...

Do these filters work on the same principle as an Etalon?

Jon

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Regarding filter placement. Narrow passband or notch filters are somewhat sensitive to angle of incidence. Behind the eyepiece, for a 60 degree apparent field light can transit the filter at up to 30 degrees off axis. This will 'de-tune' the filter sufficiently so that a considerable portion of the outer field will not be properly filtered.

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Ed had previously alluded to this, thanks to both of your for explaining this. As they say, one learns something new everyday. I guess I can relax and take a nap, I learned my one things...

Do these filters work on the same principle as an Etalon?

Jon

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An Etalon would be an example of filter which is extremely sensitive to AOI change. For the most part this is due to the "spacer" layer. With most common interference filters the spacer is a relatively thin optical layer deposited between sets of layers, both before and after the spacer, which are referred to as the reflectors. Generally speaking the thickness of the spacer determines bandwidth of the resulting filter. With an Etalon the optical spacer is replaced with a very thin physical spacer placed between the reflectors. And, although it`s still quite thin, it is much thicker than a traditionally constructed interference filter spacer, but even more important, it is also much more consistent in terms of refractive index than an optically deposited spacer, which would require many more 1/4 waves of optical deposition than is practical, to equal the same thickness, and thereby produce the desired narrow bandwidth.

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Such filters as high transmission notch filters are certainly viable as a 'permanent' option for city-bound stargazers, if changing out eyepiece sets is considered annoying.

Regarding filter placement. Narrow passband or notch filters are somewhat sensitive to angle of incidence. Behind the eyepiece, for a 60 degree apparent field light can transit the filter at up to 30 degrees off axis. This will 'de-tune' the filter sufficiently so that a considerable portion of the outer field will not be properly filtered.

You can appreciate this simply by peering through the filter while tilting it. For a notch filter, look at a sodium street light and note the tilt which results in an observable brightening of the light. It will be rather less than 30 degrees.

The only type of filter which might deliver full-field performance when placed behind an eyepiece is a broad-band type, such as, e.g., the Lumicon Deep Sky.

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Here`s images of my cheapo Tasco 10x50s with the internal raw filter setup (GCEs). Despite the appearance in the first image, internal reflections are pretty much nil with this setup.

Dan McShane

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...Almost makes me want to buy a pair of Tasco's Almost

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LOL!!! the real treasure in the pics is my circa 1967 JHS woodshop chessboard .... I have a long time friend and fellow chess player who tries to buy it from me every time we meet ... he`s been doing it since we first met in 1974 ... the answer has of course been; no ...

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