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 H
igh 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/pollution/measure.htm
http://www.astrosurf.com/buil/us/spe2/hresol4.htm (lines identification)
Filter transmission curves


Comparison of 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).
 


Comparison of 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 eliminates Hg 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 [OIII]).
 



Relative sky and stars flux attenuation

For LUMICON Deep sky filter
Measured sky background with filter / Measured sky background without filter =
0.17  (i.e. pollution attenuation factor)

For IDAS LPS-P2
Measured sky background with filter / Measured sky background without filter = 0.48  (i.e. pollution attenuation factor)
 

.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).
 


Dectectivity 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.


Messier 82 with LUMICON Deep Sky filter
FSQ106ED F/5 + QSI-583 (4 x 300 sec. exposure)


Messier 82 with IDAS LPS-P2 filter
FSQ106ED F/5 + QSI-583 (4 x 300 sec. exposure)

Below, a high contasted and negative view reproduction of Messier 81 galaxie. Same setup.
The display thresholds are identical for these two reproductions.


Messier 81 with LUMICON Deep Sky filter
FSQ106ED F/5 + QSI-583 (4 x 300 sec. exposure)
 


Messier 81 with IDAS LPS-P2 filter
FSQ106ED F/5 + QSI-583 (4 x 300 sec. exposure)
 



CONCLUSION

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 (reduction of sky level faster than the stars). In both cases, we regret the removal of blue part of the spectrum.

Clearly, IDAS LPS-P2, is more adapted to mercury light pollution and Low Pressure Sodium street lamp.
Finally, for my peculiar High Pressure Sodium sky pollution (but frequent in France - a standard for your country city), the LUMICON Deep Sky filter is preferred.
 

 

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