Quote:
Originally Posted by Don Pensack
A true UHC filter will broadcast the passband containing the 486nm line of Hydrogen Beta and the two O-III lines at 496 and 501nm and not much more. Ideally, these 3 lines should be broadcast at 90% strength or higher.
The filter should broadcast those frequencies and have a steep cutoff outside of them.
The UHC filters with a somewhat too-broad passband are Celestron, Astronomik, and Baader. The UHC filters with a normal passband are Meade, Lumicon, Orion. A UHC filter with a narrower than normal passband (but still adequate) is the DGM NPB. But that's only 7 of the 20 brands of them available.
Don Pensack
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That's pretty subjective Don. It's a bit like saying the correct thickness ratio for a newtonian primary mirror should be 1/6 because thats what Isaac Newton said it should be 340 years ago when they were made from entirely different materials.
Similarly, filter technology has changed dramatically in recent years. I own the Astronomiks UHC, the Lumicon UHC and the DGM Optics NPB filters and they all do different things on different targets due to their slightly different bandpass characteristics. Over a wide variety of targets each of the three filters can outperform the other two on several different targets. To say the Astronomiks isn't a true or good UHC filter is inaccurate because it works within its design parameters (which is wider than some others, with the H-Alpha pass as well) and works well on certain targets
Cheers,
John B