CLS light pollution filter for making astronomical images in suburban skies:

In November 2014, I bought a CLS (city light suppression) filter from China to improve deep sky imaging of faint objects. In this page I documented some results using this light pollution filter.

I live in at the edge of a town of  67000 inhabitants in the north of the Netherlands. The light pollution is not too bad. When the sky is clear I can see the Milky Way at moonless nights. The sky in direction west to the city center is not good.

The 2" filter was bought for US$ 75,- from China. It doesn't indicated the manufacturer but similar filter with the same white band have an indication "AstroOptic CLS" or "Optolong CLS". The box indicates Q1-CLS2-01


Against the cloudy sky. For the human eye it is more blue:

  CLS against cloudy sky

Transmission curve of the Optolong CLS filter:
         Transmission curve CLS

As a overlay on the resonse curve of the QHY8 CCD camera, the rey part is blocked by the CLS filter. Note that most of the green and part of red is blocked. This  will block the orange light pollution:
CLS and QHY8

Important emissions lines:

Compact camera test on a spectrum made with compact disk (CD) and an incandescent light bulb  The violet, yellow & orange colours dissapear:
no filter  filtered

I measured the light drop of the CLS filter in front of the QHY8 facing a white wall illuminated by an incandescent light bulb and a compact fluorescent lamp

Darkening of a  incandescent light bulb by CLS filter measured with the QHY8 CCD camera Darkening of a compact fluorescent lamp by CLS filter measured with the QHY8 CCD camera
red 46% left
11% left
green 35% left 12% left
blue 48% left 16% left

The overall more then 50% drop of a wide spectrum light of the incandescent light bulb shows that this filter will increase the exposure time star objects such as globular clusters at least twice. However it will block most of the light from  fluorescent lamps and therefore increase the contrast with about a factor 3. More or less the same results can be measured in above image of the filter held against sky. The compact camera measures a drop in intensity for red of 50%, green  74% and blue to 73%. All caused by blocking a part of  the solar spectrum. The blocking will not occur for nebula transmitting in one of the emission lines.

Here some promising test results:

First an image of NGC1499 with 135 mm lens without filter 13x100 seconds exposure a later resized to 50%. The image is only flattened and not further processed. The main emission line of the nebula is H alpha, that is hydrogen emitting at the wavelength 656.3 nm, at the red end of the visible light spectrum.
NGC1499 without filter

The result with the CLS filter placed (loose) in front of the 135 mm lens. No focus adjustment and same 13x100 seconds exposure. Image later resized to 50%.
Note that the intensity of stars has reduced more then 50 % but the contrast of NGC1499 nebula (emitting at 656.3 nm)  improved a lot.  Sky light pollution changes from green brown to blue.The image is only flattened and not further processed:
NGC199 with CLS filter

Both images processed and zoomed in at NGC1499:

Without filter:
NGC1499 without filter

With CLS filter in place:
NGC1499 with CLS filter

Written in 2014-11

Note October 2015:
During an imaging session with the CLS filter, I had unsharp stars in one corner. This problem did not occur in the past.  After some tries, I solved it by unscrewing the filter a little. If I tied the filter softly, I presume it bends the filter resulting in unsharp stars in one corner. Bright stars have still a small halo but that is because it is not a high quality filter.

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email, 2015-10