Roger Hamlett wrote:

"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.

In particular for photographic or visual observing. For the former I
would tend to recommend my own Nonad filter that knocks down low
pressure sodium light without seriously affecting the rest of the
spectrum. And for the visual use I reckon Orion Optics(UK) Sodium light
filter is hard to beat.

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 are the best for maximising contrast on emission nebulae and
probably one of the better choices for a first filter provided that you
have enough aperture 6" or more to tolerate fairly brutal filtering.

Filters can only ever dim the image, but if you choose wisely they dim
the light pollution a lot faster than the nebulae vastly improving
contrast. I like Orion(US) UltraBlock.

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.

Contrast is very important. Visually none of the generic LPR broadband
filters can seriously be recommended as a first choice filter. They are
good for photography but likely to put a beginner off filters for life.

Human eye and perception is curious - the camera does not lie and the
photographic improvement may look huge but our eyes cannot see it in
realtime. Photographically you just add a half a stop to the exposure.

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.

Lumicon filters are nice pieces of glass, but the mechanical mounting is
horribly loose fitting to the point of falling out of some eyepieces.
Even the cheapest Chinese filters are threaded to the correct pitch and
diameter.

It is worth pointing out that most of the famous brand US filters are
optimised for predominantly mercury street lighting. Filtering these
wavelengths out does you no good at all in an LPS and HPS environment.

My personal recommendation for a first visual observing filter in a
mixed sodium light environment would be Orion Optics UK Sodium filter.
ISTR it is cheaper than the more obvious US brands too. YMMV

A few sample filters with spectra in front of street lamps at:
http://www.nezumi.demon.co.uk/nonad/spectra.htm

Higher resolution spectra on Maurice Gavins page:
http://home.freeuk.com/m.gavin/grism2.htm

You can see the spectrum of your local streetlamps by using a shovelware
CD as a reflection grating:

http://home.freeuk.com/m.gavin/solaspec.htm

Regards,
Martin Brown