"the3rdman" wrote in message
...

My observing point (back garden) has a street light which shines down
(and up!) into one corner. Its a low pressure sodium light. I'm
thinking that a filter would help. So.......

broadband ? narrowband ? nebula filter ? sodium filter ? any ideas ?
It depends what you want to do.
Several of the terms tend to overlap in parts.
'Narrowband' filters, only let through a 'narrow band' of the spectrum.
These will block the streetlight (unless the band happens to contain this
part of the spectrum), but will also only let through this one colour of
light. Hence they are great for specific objects that produce a lot of
light in a particular part of the spectrum, but useless for anything else.
They will give much more 'generic' streetlight blocking than any of the
other types, gting rid not only of the light from your nearby light, but
also from other light designs further away.
Nebula filters, are usually filters designed to enhance the contrast of
nebula type objects. Some manufacturers, use this term as another name for
the narrowband filters, while others use it for broadband filters!. So you
have to look carefully at what specific types actually say they do. You
will find both narrowband, and broadband types, with different target
uses, under this term. Unfortunately, these will generally reduce what can
be seen of other types of object, so are great if this is what you want to
look at, but not so good otherwise...
Sodium filters, are the 'other way round'. Instead of having a narrow pass
band, they have a narrow blocking band, selected to cut just sodium light.
If your local light is a sodium light, this type will block this light,
while letting most other light through. It'll therefore have less effect
on the more general skyglow from other streetlights, but will leave things
much more visible.
Some of the 'nebula' filters, may also be called 'LPR' filters. These are
the broadband types, designed to help boost object contrast, without
cutting too much light. Generally these will have two or three passbands,
and blocking bands, with the blocking designed to get parts at least of
the spectra from high, and low pressure sodium, and mercury vapour lamps.
If you are working visually, you have to be aware of just how much less
light any of the narrowband filters passes. In general, these are much
more aimed at imaging, than visual use.
If the light pollution from other sources is not too bad, look at a sodium
filter. If it is significant, consider a LPR/broadband filter. The Lumicon
'Deep-sky' filter, is quite effective on many sites.

Best Wishes