Recently I expanded my ultra-small filter collection (so far only the CLS-CCD filter) with the Astronomik UV-IR block
L-2 and L-3 filter.
The CLS-CCD (City Light Suppression) filter blocks light pollution wavelengths and consequently also colors in that range.
The CLS-CCD filter blocks up to 450 nm, between 520 and 620 nm and from 690 nm upwards.
The L-2 filter blocks up to 390 nm and from 700 nm upwards.
The L-3 filter blocks up to 420 nm and from 690 nm upwards.
(click on the links above to see the graphs on astronomik.com)
Regarding the graphs the L-2 filter has the widest wavelength range. The L-3 range is very similar to the CLS-CCD without blocking the mids.
CLS-CCD vs. UV-IR L-3
The color difference is clearly visible on the following 2 images:
The first image of NGC7000 is taken 2017 with the CLS-CCD filter, the second image is taken a few weeks ago with the L-3 UV-IR blocker. Both images are taken from my home (Bortle 5). Same lens, same DSLR. The only difference is the filter and the single exposure time.
Only 40 minutes difference in exposure time, but obviously it passes more light and color through the L-3 filter. Both images are just color calibrated, histogram stretched and contrast enhanced the same way.
Since the L-3 filter doesn’t block the typical light pollution wavelengths, I reduced the single exposure time from 120 seconds to 90 seconds.
In the end I’m surprised about the result, since it was taken in a light polluted Bortle class 5 area.
I like the colors that show up with the L-3 filter 🙂
Time to test the L-2 filter… for comparison reasons hopefully on the same target. But I don’t think it will be much different to the L-3…