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I think what Don said about it being impossible was a filter that produced white stars and not referring to what you see.
When Tele Vue first introduced their original Bandmate Nebustar (UHC equivalent) they made a point of showing how their filter didn’t produce red/blue fringing around stars like a competitors filter did. I have seen this in a filter and I much prefer the Tele Vue solution which gave mildly green stars of uniform colour.
I think the Astronomik filter (8+ years old) shows red/blue fringing and I’ll check next time I’m observing. (It’s either Astronomik or Lumicon but i’m fairly sure it’s the former and Tele Vue didn’t name which competitor it was). The original Tele Vue Nebustar is fairly gentle compared to the UHC from Astronomik, Lumicon, Thousand Oaks but gives much better contrast than say a Lumicon Deep Sky filter. I much prefer the original Tele Vue Bandmate O-III filter which provides a nice presentation offering good contrast and more stars albeit with a mildly green tinge. Many put down the original Tele Vue Bandmate O-III saying that the bandpass was too wide and not quite centred on the double Oxygen lines but I wonder how many of those commenting have actually used the filter and not just looked at the bandpass curves.
It is interesting that Tele Vue now paired with Astronomik to produce the Type 2 Bandmates which to my understanding have tighter and more accurate bandpass with high transmission and significant quality control. My take is that Astronomik improved their own filters and teamed with Tele Vue to offer a superior product to what either had before. I’m not sure if the red/blue fringing was addressed and when I get some spare funds I’ll invest in the newer Bandmate Type 2 Nebustar to see for myself through the eyepiece and not have to rely on interpretations of curve data.
Note contrary to what many believe UHC & line filters work best under dark skies but of course perform well under light pollution. Exit pupil too and telescope aperture determine how each filter behaves and sometimes it is only by experimentation that you find the best solution.
When Lumicon first marketed these filters in the 1980s (I can remember seeing the ads) the UHC was the recommended filter. The O-III was available to those seeking to find elusive planetary nebulae whilst the H-Beta was simply called the Horsehead nebula filter. Note the UHC includes both O-III and H-Beta but to have any chance of seeing the Horsehead you need the narrow band pass of the H-Beta filter alone. I believe that under heavy light pollution the narrower bandpass of the O-III would serve you better but the UHC would still provide a significant contrast boost.
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