Light Pollution Suppression Filters
Comparison between LUMICON Deep Sky and IDAS LPS-P2
We compare LUMICON Deep Sky and IDAS LPS-P2 in pollution conditions, of course:
... from my personal observatory located near the city of Toulouse (south of France). The sky is heavily polluted by light coming
from street High Pressure Sodium (HPS) lamps. The naked eye magnitude limit is approximately 3 to 3.5.
For more informations about sky pollution and observation methods in these circonstances:
http://www.astrosurf.com/buil/us/spe2/hresol4.htm (lines identification)
Filter transmission curves
Castanet-Tolosan sky spectrum
with (blue curve) and without (red curve) LUMICON Deep Sky filtering
The wide diffuse sodium line
around 589 nm is totally erased. This is the major cause of brightness of the sky from my observatory.
But the cutoff at 450 nm is a pity because the sky is moderate dark in blue (a consequence is a lower constrast in galaxies spiral arm, for example).
Castanet-Tolosan sky spectrum
with (blue curve) and without (red curve) IDAS LPS-P2 filtering
The wide and
intense Na line centered on 589 nm is only partially removed. The filter is transparent to the sodium 568
nm line, which is a shame. Note the cutoff of the infrared above 700 nm: this is not a vefry
good idea because the sky is dark in this part of the spectrum. The IDAS filter well
lines (mercury vapor), but it is of little interest to my observatory dominated by High
Pressure Na lines.
Spectrum of a A2V type star through pollution rejection filters
Aspects of the spectral profile without filter :
Aspects of the spectral profile with LUMICON filter :
Aspects of the spectral profile with IDAS filter :
Comment: The color of objects is
very modified (color balance). We can note the rejection of the blue part of the spectrum. It is a valuable astrophysical signal
lost while the light pollution is modest at this point. Hum...
this is a major shortcoming of these filters. The IDAS filter eliminates infrared, which significantly lowers detectivity of
red stars, with K
and M spectral type. The two filters are well
transparent around Halpha and Hbeba (and also
LUMICON Deep sky filter
.Comment: The attenuation
of HPS pollution is 2.82 times more efficient with LUMICON
filter then with IDAS filter. But because spectral profile aspect,
the IDAS filter is 1.61 time more transparent relative to
LUMICON filter for a set of 10 stars randomely selected in M81/M82
field (see below).
test on deep-sky object in city observation condition
Only panchromatic image is concerned here (B&W image)
Below a high contasted and negative
view reproduction of Messier 82 galaxie taken
through LUMICON and IDAS filters (March 9, 2014). Takahashi FSQ106ED refractor and QSI-583 CCD camera.
The exposure time is the same. The thresholds are adjusted for a nearly equal contrast display.
Below, a high contasted and
negative view reproduction of Messier 81 galaxie. Same setup.
The display thresholds are identical for these two reproductions.
The detectivity (the faintest stars observed) is nealy the same for the two tested filters if A, F, G stars are concerned.
The less strong attenuation of background sky with IDAS filter is compensated by a better transmission for intermediate
spectral type stars. In contrast, the LUMICON filter is superior for
emission nebulae observations
(better contrast) and for
cold stars. For a given exposure, the LUMICON
filter offers more dynamic range
of sky level
faster than the stars). In both cases, we regret the removal of blue part of the spectrum.