Milton,
for emission line objects like M27 it is true that LPR filters give better
contrast. Of course for M27 a narrower UHC filter (e.g. the Astronomik UHC)
would give even better images. I have attached a comparison I did in 2004
showing M27 under Berlin skies unfiltered, with Astronomik CLS and with
Astronomik UHC.
For broadband objects (galaxies) I have found that my LPR filters
(Astronomik CLS, Lumicon Deep Sky, Baader Skyglow) dim the "useful" light by
a similar amount as the sky background. Maybe the IDAS ist better than the
filters I mentioned, but if both the object and the light pollution are
broadband it seems logical that a filter affects them similarly.
My conclusion (which took me several years of testing) is that LPS filters
are not really useful under broadband light pollution because they don't
really help with galaxies and while they are useful for emission objects
there are filters that are even better for this purpose.
Of course the situation changes if the light pollution is "narrow band",
e.g. if the light pollution comes from a single type of streetlights. If the
LPS filter blocks the light of these streetlights it can make a big
difference even for galaxies.
Stefan
Very nice M1 image Fabio.
I also use an IDAS LPS filter which really helps a lot in suppressing
light pollution in urban Calgary.
Here is a comparison between no filter and IDAS LPR filter taken with a
C8 from urban Calgary Alberta.
http://www.outcastsoft.com/AstroImag...PRFiltered.jpg
My skies were about Mag 3.5 at the time it was taken, which is a bit
better than average. I have a mix of mercury, sodium and incandescent
light sources, which make my skies a uniform darkish gray color.
What I found was I'm losing about 1.4 times the star light because of
the LPR Filter, however my background light pollution was cut by 2.3
times (compare backgrounds of Fig 1 versus Fig 2) with the filter.
Figures 4 and 5 show what sort of cutoff I would need to do to get the
backgrounds equal between unfiltered (34,000) and filtered (7,500)
images. However that pushes the contrast and star bloat up in the
unfiltered image dramatically.
I could expose the CCD for an extra 40% to compensate for the star
light lost, and would still have darker backgrounds due to light
pollution than without the filter. So in my skies situation the LPR
helps more than it hurts.
HTH..
Milton Aupperle
http://www.outcastsoft.com
http://www.outcastsoft.com/AstroImages/AstroIndex.html