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Subject: RE: Solar Viewing
From: Robert Preston
Reply To: mapug@shore.net
Date: Thu May 11 04:51:21 1995

Dave Palmer wrote: >I have the Lumicon H-alpha, which I got for about $700. It's strictly a
>prominence filter...you
...snip >reflection problem. When viewing through an eyepiece, you have to keep
>your eye angled just so.

So if the H-alpha wavelength is 656nm, then I presume these filters are multi-layer interference notch or bandpass filters. All interference filters have an angle-dependence for the position of the bandpass, as far as I'm aware, so that, for example, one can "blink" using oxygen nebula filters by tipping the filter back and forth between eye and eyepiece, so the faint nebula goes on and off as the position of the bandpass moves across the oxygen line.

Similarly, off-axis light that impinges on the filter at an angle would, I presume, "see" a different bandpass. To my limited knowledge, the off-axis effect is negligible for oxygen-type commercial "nebula" filters positioned on the objective side of the eyepiece, but it sounds like maybe the H-alpha filters have a much higher sensitivity to off-axis bandpass shift, possibly due to a narrower bandpass than the oxygen types? If so, then an f10 LX200 (there! it IS MAPUG-relevant! ;-) would perform better than an f6.3 LX200, since the angle of the f10 cone is narrower. Does this jibe with Dave Palmer's or anyone else's experience with the Lumicon filter or others?

Also, I wonder what a full-aperture 8" H-alpha would cost? I know that one-inch diameter interference filters from Omega optical in Brattleboro VT for microscopy cost about $150 a pop...wonder if they can make larger ones at a reasonable cost...possibly the 8 - 16 inch scopes could be stopped down to a couple inches for use with such a "full-aperture" filter configuration? Has anyone tried that method?

Robert Preston Pittsburgh, PA 79deg 54' 41" W 40deg 28' 11" N rapr@med.pitt.edu