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