They are similar however they don't do exactly the same thing. The O-III is a narrower (allows less types or bands of light through) filter that works better on some object than others for example the Veil Nebula looks better through an O-III than a UHC. However many other object look better in a UHC such as the Orion Nebula although both are useful here. If you stay in this hobby long enough you will probably end up with both and use both. If I could only pick one, I'd pick the UHC, although others will disagree. It will come down to what the molecular makeup of the object you are viewing is. Although they seem to do the same thing, they do not, their affect is similar but not exactly the same. Try the two filters on the same Nebula and you will see a difference. Bob -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Troy Davidson Sent: Thursday, April 28, 2011 2:14 PM To: Utah Astronomy Subject: [Utah-astronomy] Filter Help I have two filters that, as far as I can tell, do the same thing. One is an O-III filter, and the other is an UHC filter. Both of these seem like they do the same thing. Can some one help me out in understanding what these are good for, and if both do the same thing? Thanks. ++++++++++++++++++ Troy Davidson, KF7MTE www.istroyworking.com