Quote:
Originally Posted by John K
wanting to go as narrow as I can if possible to 3nm in HA.
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Omega do a very competitively priced 0.7nm filter all be it in a 1" format:
http://www.ebay.com/itm/Optical-Inte...fs99IT3LHLhxzw
Alluxa do larger (and custom) formats at a price.
The transmission curves are impressive in as much as they are mesa shaped (as distinct from a bell curve)
http://buy.alluxa.com/656-3-1-od4-ul...-bandpass.html
As with any interference filter of such narrow bandwidth, neither of these would be a sensible option at focal ratios much faster than F7. - The angle of incidence shifts the band pass too much to capture the emission line at faster focal ratios.
Also, there is inevitably some degree of variation in any given production run with respect to the wavelength at the peak of the transmission curve.
If you are given a choice, the best filter (within sensible parameters) will be the one where the the peak wavelength is approximately 1/2 the bandwidth of the filter
towards the red end of the spectrum.
For example, for a 656.3nm filter with a 1nm band pass, the best filter in the bunch will be the one with the peak transmission at 656.8nm (or there abouts)
Caveat: this advice is relevant to fast optical systems and very narrow band filters. Ignore it for 3nm and f9.