I should have mentioned that there is also a need for an energy-rejection pre-filter, ahead of the etalon in the optical train. It's purpose is to reduce the intensity and forbid IR and UV transmission (and not melt your H-a etalon or blind the observer). Interference filters are by nature very fussy about the angle of incidence of the light passing through them, due to the small spacing between coating layers. That's how they work- by reflecting certain wavelengths internally while passing others. That's why they tend to work better at longer f-ratios, where the light cone is converging at a much smaller angle than a faster system. The angle at the center of the field is much closer to the angle near the edge. Long f-ratio refractors work well not only because of the shallow convergence angle of the light cone, but also because achromatism isn't required for H-a observation (or any narrowband observation). On Mon, Apr 26, 2010 at 1:05 PM, Chuck Hards <chuck.hards@gmail.com> wrote:
On Mon, Apr 26, 2010 at 12:26 PM, Canopus56 <canopus56@yahoo.com> wrote:
So, what's an ethalon and how does the Lunt incorporate into a refractor design?
Essentially it's just narrow-bandwidth filter, working on the same principal as most of the "interference" filters in the amateur astronomer's arsenal (LPR, H-a, H-b, O-III, etc). A specialized type of Fabry-Perot interferometer. Thes spacing between the two reflective surfaces determines the wavelength bandpass.