Just got the Antlia 3.5nm 1.25" filter and tried it out on the bright nebulas in Sagittarius (and Serpens Cauda) around midnight.
I am up against the San Bernardino mountains, so my southern exposure looks out across the urbanized valley with my worst light pollution.
I was using my camera lens line up with focal lengths from 25mm to 500mm. Of the collection of lenses I have acquired, these are the ones I use for observing:
Tamron 2/3 25mm f/1.4
Canon Nifty Fifty 50mm f/1.8
Computar V5013 CCTV 50mm f/1.3 1" format
Pentax Super Takumar Thorium lens 50mm f/1.4
Canon EF 75-300mm f/4-5.6
Nikon Nikkor 180mm f/2.8 ED
Sigma APO 400mm f/5.6
Kalimar 500mm f/8 mirror lens
I ended up with a super-abundance of fast 50mm lenses but they are all good, and light, so I keep them all in use although there is not much difference in their views (minor advantage - I can put different filters on each, and swapping lenses is nicer than fiddling with filters in the dark). Only the Canon and the Pentax Thorium can use 1.25" filters.
The most versatile is the Canon 75-300mm which is f/4 at 75mm and f/5.6 at 300mm, excellent mechanically and quite light for a telephoto. It must have plastic aspheric lenses in it to be so light. In this application the weight tradeoff against optical perfection is absolutely worth it. The others provide either a brighter/wider view, or more magnification for close ups. But definitely I started with sweeping and zooming in and out with the Canon telephoto.
The nebula pairs M17 and M16 (Swan and Eagle nebulas) and M8 and M20 (Lagoon and Trifid nebulas) showed up very well in all of the lenses with the Antlia filter. Small but detectable in the 25mm. They made a nice quartet at 75mm f/4, and nice bright pairs in the Nikon 180mm. The best close ups were with the Sigma 400mm, which balanced scale and brightness.
I tried for NGC 6357, the Lobster Nebula, in Scorpius and it might have been barely detectable, but I am not sure, it was lost in the brightness and haze nearer the horizon.
I'll try the same viewing with some of my other filters tonight (I have 5nm, 7nm, 12nm and 35nm also).
I'll also try my luck with Sh2-27 using a 50mm lens. For a large dim object like that you need to have surrounding framing to perceive it (I have learned from viewing the Triangulum galaxy). So nothing longer than about 75mm I think.
Edited by careysub, 21 June 2020 - 04:06 PM.