I have a Thousand Oaks OIII filter, but it's 1 1/4". If it's any indiciation though, I have been very pleased with it's performance. I also do recall looking at the Lagoon nebula using both my filter and yours last year with very similar results. --Paul ---------- From: Richard Tenney Sent: Thursday, July 03, 2003 8:20 AM To: Astronomy in Utah Subject: Re: [Utah-astronomy] More on O-III filters Chuck, The scope we tested this in has 100 inches of focal length at f/6.25; Jim's 10-inch I believe is f/7 or more which he also had no success with; I wouldn't call either of these scopes particularly fast. On the other hand, maybe we should try the TeleVue filter in a faster scope, say an f/4.5 and see what happens. But the bottom line was this -- the nebula was dramatically enhanced by the Lumicon filter, and only barely so (if at all) by the TeleVue, made even more obvious by my filter slide (both filters were loaded at the same time, and it was a simple matter of sliding each under the eyepiece (35mm Panoptic) without taking my eye away from the image. There was little if any difference between the TeleVue and the view sans filter. My Lumicon did come with transmission specs (in fact, I bought it from a dealer who had 3 of them sitting on the table, and was able to pick out the "best" of the 3). Jim, did your filter come with similar information? It would be interesting to repeat this test with other filters and/or filter manufacturers (anyone out there with a thousand oaks O-III in a 2-inch format?) and in different telescopes of varying focal lengths. -Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:

The coatings that make these narrow-band filters work are marvels of modern engineering, sometimes dozens of layers thick. They are interference coatings, so the thicknesses of these layers must be precisely controlled to 1/4 wavelength tolerances (sound familiar?). Lumicon states that their O-III filter has an 11nm bandpass, meaning that most of the energy passed by the filter consists of only wavelengths about 5.5nm either side of the center of the band. For O-III the lines we are interested in are at 496nm & 501nm, so 11nm encompasses both nicely if centered properly.

A finicky feature of interference filters is that they are choosy about the angle of incidence of the light passing through them. This makes sense, as the angle of incidence increases, the light must travel through a longer path in the coating material, thus interfering at a longer wavelength when rebounding.

There is a chance that Rich's filter is optimized for a fast Newtonian (at a higher angle of incidence)and Jims is happier at a longer focal ratio such as a "stock" SCT might work at (lower angle of incidence).

Food for thought.

Chuck

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