filters

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 ?

"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

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

okay, thanks for the responses. The whole of the area that i live in is lit the same and the main source of light pollution is the LPS streetlights. I've seen the nonad stuff before but am not sure where to get a filter like this from? I've also been recommended a Neodymium filter, as this apparently has excellent blocking of LPS. I quite fancy the idea of darkening the background and enhancing what i'm looking at, i thought that cutting out the light from the streetlights would do this? maybe i'm completely wrong?

Originally Posted by the3rdman

okay, thanks for the responses. The whole of the area that i live in is lit the same and the main source of light pollution is the LPS streetlights. I've seen the nonad stuff before but am not sure where to get a filter like this from? I've also been recommended a Neodymium filter, as this apparently has excellent blocking of LPS. I quite fancy the idea of darkening the background and enhancing what i'm looking at, i thought that cutting out the light from the streetlights would do this? maybe i'm completely wrong?

I have a LPS streetlamp in front of my house [all my observing from rear!] and when I apply a Neodymium type filter to the eye the LPS illumination to the surrounding road and street scene vanishes! I've also found a 49mm diam Jessops 80A blue [cost=tenner] visually suppresses [but don't eliminate] LPS and likewise photographically but needs x2 exposure to compensate. Hope this helps. Nytecam www.astroman.fsnet.co.uk

Please forgive a dumb question but how does light pollution manifest
itself in the eyepiece? I have HPS to the front of the house and LPS
behind, and a *lot* of LPS to the east so I've always wondered if a
filter would improve things but every now and then I get a clear night
and dont really see a problem in the EP. Other days the view is milky
and contrast is poor.

--
Ian Newham

http://www.IanNewham.com

Ian Newham wrote:

Please forgive a dumb question but how does light pollution manifest
itself in the eyepiece? I have HPS to the front of the house and LPS
behind, and a *lot* of LPS to the east so I've always wondered if a
filter would improve things but every now and then I get a clear night
and dont really see a problem in the EP. Other days the view is milky
and contrast is poor.

It isn't a dumb question at all. It is actually rather hard to describe
accurately how light pollution appears in the eyepiece. Grey skies that
hide faint detail in objects is the short answer.

On the very clearest nights just after heavy rain from a cold front has
washed the air clean of dust and grot you will get views with sky
darkness broadly comparable to that of a filtered sky.

It is much easier to photograph it, but the camera gives an exaggerated
view of the effects compared to how the eye will see it. And on a camera
you can increase the exposure a bit to compensate for light loss.

Continuum stars, clusters and galazies are all dimmed by LPR filters,
but emission nebulae are undimmed with enhanced contrast against a
darker sky. Choose the right filter and the light pollution is dimmed
much faster than the object you want to see.

You can sometimes also choose a higher magnification to dim the sky
background and improve contrast that way.

Visually you would almost certainly benefit from Orion Optics(UK) sodium
light filter (~£60) if your environment is as you describe. I wouldn't
recommend any broadband LPR filter as a first filter for visual use.

Photographically Nonad works specifically against low pressure sodium
light, but in a mixed HPS light environment you need a more severe
filter to make an acceptable visual improvement. It gives you a rough
idea of what the effect is like but film sees things differently to the
human eye.

http://www.nezumi.demon.co.uk/nonad/nonad.htm

See also David Knisely's filter FAQ on s.a.a, but remember it has a US
bias towards eliminating light from mercury street lamps.

Regards,
Martin Brown