The Astronomik EOS CLS filter

This is a light pollution filter which filters out between 540nm (green) and 640 (red) so most spectral lines of mercury lamps and sodium vapor lamps are filtered out which is most of the light pollution. Metal halide and LED lamps however have a more broadband spectrum so this is not easy to filter out. But LED lamps have have lower light output in the frequencies which are allowed by this filter, so still they are more attenuated by this filter than starlight. In general, the contrast of astrophotos is better, particularly of nebulae as the wavelengths emitted by nebulae O III and H Alpha are also allowed by this filter and appears relatively brighter.

I just purchased this filter from Astronomik for my EOS 7 camera. I did some tests with high pressure sodium (the most ubiquitous light pollution source) and metal halide lamps and took some street scenes. It turned out that in broad spectrum light (daylight, starlight) the loss is about 1 stop, in metal halide and LED light slightly more 1-2 stops, but in high pressure sodium at least four stops. I used a Canon EF 50mm f/2.5 lens as I cannot use the EF-S 15-85mm f/3.5-5.6 standard zoom lens due to limitations mentioned below. This looks promising for astrophotography. Now waiting for the first moonless clear night for sky testing.

The filter can very easily be inserted into the camera body: just remove the lens and put it (without touching the filter surface!) with your fingers with the Astronomik logo facing you and to the top of the camera, into the camera just in front of the mirror.
Removing: put your fingernail carefully on the top (or bottom) and pull gently the filter out of the camera.
There are limitations:

  • Only available for Canon APS-C SLR cameras, so Canon EOS 1 and 5 Fullframe series, Nikon, Pentax, Sony owners, you are unfortunate.
  • EF-S (Canon or most third party) lenses cannot be used as the rear of the lens protrudes too deep into the mirror box so there is no room left for the only 1mm thick filter.
    Update: Some 3rd party EF-S lenses might work. On my camera the Sigma 10-20mm wideangle zoom for crop bodies does fit so I can use this for wide angle sky shots in light polluted environments.
  • You may adjust the white balance as even AWB gives a slight blue tinge to the photos which may also affect astrophotos. You can also do this afterwards after stacking and other processing of the photos.
A made an adapter for visual use of the EOS clip filter which can be used in a 2" eyepiece entry in a telescope and with 1 1/4" eyepieces only. The filter is too small to fit 2" eyepieces. I used a 50 => 32mm PVC plumbing tube adapters in two steps: 50-40mm and 40-32mm for just $2 in the local hardware shop. The PVC tubes fit loosely into the 2" telescope barrel as it is 50.8mm, so the 0.8mm can be filled by pasting felt or duct tape over the outer PVC adapter. The inner PVC adapter also has to be ducttaped as it is 32.0mm and an eyepiece is 31.7mm otherwise the eyepiece is only loosely fitted.

To be tried on real sky yet. This visual use can only be useful in heavy light polluted skies as otherwise the limiting magnitude will be lowered by 0.5-0.7 magnitude. Test have to be done yet.

The filter in its package
thumb./astronomik-cls/-tn-in eos-7.jpg
Filter mounted in the Canon EOS 7
Transmission characteristics of the filter
The homemade 2" => 1 1/4" adapter with the filter in it and a 1 1/4" eyepiece in it
Telescope side view of the adapter
The eyepiece adapter with filter replaced the original 2" => 1 1/4" adapter

Artificial lighting samples

See below the results left with filter, right without. All rows are taken at the same exposure value, so the images with filter are heavily underexposed. Except the last row is a picture of my yellow lathe which is corrected just to show the weird colors resulting from a yellow object. The forst two rows are taken outdoors. Note the very dim low pressure sodium lamps in the filter. For the MH lamp spectra picture I used a 600 W Venture metal halide lamp and for the high pressure sodium lamp spectrum and the last two rows I used a Philips Master SON 600 W lamp.
Metal halide lamp spectrum
Metal halide lamp spectrum filtered
High pressure sodium lamp spectrum
High pressure sodium lamp spectrum filtered
Railway station lighted by high pressure sodium and fluorescent lamps
Same with filter and same exposure value
street scene near same railway station with Low pressure sodium lamps in the background
Same scene same exposure value. Note the very dim red low pressure sodium lamps
Window shade lit by High Pressure sodium
Same with filter and same exposure value
Picture of my lathe lighted by a High Pressure sodium lamp
Same but now with use of the filter (EV is corrected to show weird colors)
LED lamp unfiltered
Same with filter, same EV
Halogen (tungsten incandescent) unfiltered
Same with filter, same EV
Compact Fluorescent lamp unfiltered
Same with filter, same EV
All picture pairs (except the lathe) above are taken with the same exposure value and ISO setting to show the difference.
As seen here, sodium lamps are dimmed the most, followed by warm white compact fluorescent lamp, LED lamps. Incandescent lamps are the least affected by the filter. Fortunately, they are used the least for outdoor lighting at night.

Sky samples

On 2012 March 31 I tried a few test pictures with a Canon 50mm f/2.5 macro but there was a waxing gibbous moon (56%) so this is not a representative test.
Orion 10 sec f/4 3200 ASA Canon EF 50mm f/2.5 macro (sky lit by 56% gibbous moon !) without filter
Orion same exposure (and moonlight) with filter
Bull head 10 sec f/4 3200 ASA Canon EF 50mm f/2.5 macro (sky lit by 56% gibbous moon !) without filter
Bull head same exposure (and moonlight) with filter