"Didymium" filter for LP?

[DaveS]

In another (Photography) forum it was suggested to me that a so-called didymium (There's not such thing) red enhancer filter could be used to cut light pollution.

Now, as most of the LP in London is from various Na lamps, would not this just make the problem worse? Ok, I know we're talking F-Block elements so the absorption lines are going to be relatively narrow, but it's too much to expect one of them to fall on the Na D doublet or the pressure widened yuk of the dreaded high pressure sodium lamps beloved of our councils.

Unfortunately Astronomik's clip-in filters won't work with my 5D, and although a Hutech modded 40D would be nice, I have other expenses to attend to, but could justify the £80 or so for a red enhancer, as I could use it for photography as well.

Are there any other end-of-lens filters available in, say, 77mm fitting for my 24-105 f/4, my 135 f/2 (With stepper ring) and a possible 300 f/4 that I might buy?

Thanks in advance (Esp if you've been able to wade through my waffle )

Cheers,

Dave

[Dave]

It's a Neodymium filter you need.

It works well for me.

Dave

[DaveS]

Well yes I know that, but.......I cannot find anywhere a 77mm Neodymium filter to go on the front of a camera lens, everthing I've seen so far are to go on the back-end of a 'scope, either in front of the camera or on an eyepiece, hence my question about so-called "didymium" filters for photography.

Dave

[Merlin66]

You can use any of the 2" ( 48mm) standard LPR filters on your camera using stepping rings.

The Baader UHC-S is a reasonable choice.

Lumicon also sell Ha DSO filters in larger sizes >75mm but these are really only of use imaging Ha nebulae.

[DaveS]

um....my 135 has a clear aperture of 68 mm and a filter size of 72 mm, the 300 I'm after is 75 mm ca and 77 mm filter, so stepping down to 48 mm will cause a rather severe vignetting! The 300 f/2.8 has 50 mm drop-in filters behind the lens, but even S/H is the wrong side of £1500!

I might investigate Lumicon filters though.

Dave

[DaveS]

Been an' gorn an' dunnit now aint I.

Orderd a B+W 491 "Redhancer" so-called didymium filter from WE. £84 for a 77mm jobby, plus some stepping rings from SRB-Griturn.

So we'll just have to see.

Dave