[sculptor]

The council has thoughtfully provided a 10 gigawatt fluoro streetlight outside my observing site. Attached are the effects of various filters. I placed a Baader diffraction grating in front of a 70 mm zoom lens at F/10 on a Canon EOS 20D, and took 1/10 sec shots.

The top panel is with no filter. The zero order image is on the left as a reference, and the 1st order blazed image is on the right. You can see that the lamp doesn't helpfully produce a nice sodium D or mercury line, but has an almost continuous spectrum.

A Baader Neodymium filter obligingly removes the entirety of the yellow image, but leaves the other colours more or less unchanged. When looking at say Eta Carinae nebula the filter has no subjectively detectable effect good or bad. My guess is that if I lived under a sodium lamp, it would be of great benefit, but is of no use to me.

An Astronomik CLS filter severely attenated everything, but blocked orange, yellow, and violet completely. The blue of H-beta, the cyan of O-III, and the red of H-alpha are therefore selectively enhanced. Greens are suppressed but not eliminated. Subjectively, I find it increases the contrast of Eta Carinae by an appreciable and worthwhile but not miraculous amount. In my 16" Dob, the Homonculus nebula disappears.

Finally, an Astronomic UHC filter savagely cut everything except the colours associated with H-beta, O-III, H-alpha, and some nearby colours, especially nearby reds were allowed through. Subjectively, I find this filter magnificent for Eta Carinae and Lagoon from a light polluted site. The Homonculus is again utterly gone.

In conclusion, a UHC filter seems the best of the three in the presence of rather wide-band light pollution. Thank you Penrith Council.

Changing the topic very slightly, from a light polluted site where it is so bright you can read, I find an O-III filter to be far superior to a UHC on most planetaries (eg Blue Planetary, Dumbbell, Scutum Planetary), whereas the UHC is superior on general emission nebulae like Lagoon and Eta Carinae nebula. At a wild guess, I would say that this is because H-beta, CN, and perhaps H-alpha are all getting through the UHC and helping. The only exception is that the Helix is very slightly easier with the UHC. My wild guess is that this is because it is so faint that, unable to light adapt under a 6 gigawatt street lamp, I just run out of photons with the O-III.

Cheers,
Mike BJ