Exposures Considering Light Pollution

Gibraltar has become increasingly light-polluted in the last few decades, with its rapidly-expanding city and nearby cities in Spain also expanding (La Linea and Algeciras). This can at times make it difficult to practice astrophotography locally. It comes down to either putting up with it or packing it all up in the car boot and driving about an hour North into Spain and getting to a darker site. There are some amazing places in Spain accessible from Gibraltar in terms of dark site but nevertheless, it involves more work than just going locally to Europa Point or up the Rock (with a Government permit for night-time access). 

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:

1. Less glass in the optical train. 
2. 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. 

Point 1 above is not particularly game-changing because it is after all just one more filter. Point 2 above can be easily remedied if you capture your flats straight after imaging right there and then. The flats then reproduce the shadow and remove it from the images. Of course, for refractors, there is no such shadow as refractors have no front obstructions. 

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. ​

Fortunately mathematics do not lie. The other night I captured 5 minute test exposures in Luminance (Hutech IDAS LPS filter was used for this), Red, Green, Blue and Hydrogen-Alpha (7nm bandwidth) on three sample points in the night sky around Europa Point in Gibraltar - South (pretty dark), East (nice and dark) and North-West (pretty light-polluted). I averaged the results of Median measurements on areas of the images deemed purely background and came up with results for electrons/minute related to the light pollution noise. Let us look at Red, the worst-affected (as is expected):

• Red (naked): 998.49 e/minute
• Red (with Hutech IDAS LPS in front): 148.56 e/minute

That is a massive ~6.72x reduction in light pollution noise by placing a Hutech IDAS LPS filter in front of the optical train, before the regular Baader Red filter. Red is the naturally the worst-affected, but Blue was also affected a fair amount:

• Blue (naked): 415.47 e/minute
• Blue (with Hutech IDAS LPS in front): 168.76 e/minute

Again, a decent reduction in light pollution noise, this time by a factor of ~2.46x. Results do of course vary not only by particular section of the night sky being imaged, but by the quality of the night sky at that particular instant you are imaging. The other night had average to good overall night sky quality and so presents a good measure of usual imaging conditions. 

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.