Previously I had decided imaging in Red, Green and Blue naked - without any light pollution suppression filters - opting to instead use a light pollution suppression filter such as my Hutech IDAS LPS as Luminance, rather than a standard Luminance filter. This has two distinct advantages, though not huge ones at that:
- Less glass in the optical train.
- Light pollution suppression filters work by reflecting the rejected light back out. This can lead to a shadow on your images of the obstructions in the tube (if not imaging through a refractor). I have images showing the secondary mirror and spyder vanes on the image as a shadow when I used my Altair Astro 8" RC telescope.
The theory behind imaging in Red, Green and Blue naked is that manufacturers such as Astrodon and Baader design their colour RGB filters with a small gap between Red and Green, which therefore rejects a lot of the light pollution from Mercury and Sodium lamps.
- Red (naked): 998.49 e/minute
- Red (with Hutech IDAS LPS in front): 148.56 e/minute
- Blue (naked): 415.47 e/minute
- Blue (with Hutech IDAS LPS in front): 168.76 e/minute
There is no question however, given these tests, that if you are imaging in a light-polluted area, you must use a light pollution suppression filter in front of all four LRGB filters. Using a light pollution suppression filter as Luminance and then imaging in Red, Green and Blue naked will not do. The Hutech IDAS LPS is world-known to be fantastic in performance, including its lack of colour shift produced in images. As expected, the filter had no effect on the Hydrogen-Alpha (7nm) narrowband filter. Narrowband filters are best used naked, with the exception of Oxygen-III on Moonlit nights.