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.
Here are some links with tests to help compare them:
http://www.astrosurf.org/buil/filters/curves.htm
http://www.astroamateur.de/filter/
http://home.freeuk.com/m.gavin/grism2.htm
http://www.sas.org.au/filters.htm
And here are some links with good information about filters and their uses:
http://pages.sbcglobal.net/raycash/filters.htm
http://www.cloudynights.com/item.php?item_id=387
http://www.cloudynights.com/item.php...d=63&pr=2x9x42
http://www.cloudynights.com/ubbthrea...t=1&PHPSESSID=
And here's a link to a buyer's guide to all nebula filters on the market (go down the page to the guide to filters):
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
Memorize all the information. There will be a test.
Seriously, I post all this so all who read it can link to the sites mentioned and learn about nebular filters, what they do, how they work, and how they are different.
Don Pensack