Imaging at a dark sky site and in the city

24 February 2020

Astronomy and astrophotography are always improved by going to a dark sky site.

There's a bit of a debate amongst the astrophotography community about whether you can take good shots from light polluted areas.

(There's really no argument, in truth, because Andy Campbell takes his photos from Burwood, which is, while not inner city, certainly not a dark sky site. Being a multiple international award winner, I think his shots probably do qualify as "good"!)

This goes as much for visual astronomy as it does for astrophotography - possibly even more so. But rating visual astronomy locations is so subjective - because everyone is different, you can't make a genuine objective comparison like you can with astrophotography.

Like, for example, what I've done here.

Recently I stripped, modified and rebuilt my mount. Following that, I made a bunch of adjustments to tune it up.

One of these adjustments required me to take a pile of test images. The photos were all the same exposure (5 minutes), narrowband filter (Oxygen III), camera settings (gain 70, temperature -15°) and target (a thing called SH2-308).

The first 13 exposures I got from the ASV's dark sky site on 26 January, close to the new moon.

The rest, 25 exposures, I took from by place in Kew, which is closer to the city than Andy's place. Light pollution here is pretty awful. The Moon rose at about 3am that morning, but it was after I was finished.

I've attached an example of the exposures from both locations. Frame 11 was from the dark sky site, and frame 22 was from home. The difference - only the location - is obvious. Frame 11 has a clearly higher signal-to-noise ration, with the shape of the bubble standing out far better from the background.

So having taken exactly the same photo in both locations, I think this is good evidence that if you go to a dark sky, your photos are better.

Finally, here's attached a rough "OHO" process of SH2-308, which includes both these frames, as well as 17 5-minute exposures of Hydrogen. You'll notice that the satellite tracks were both erased as part of the stacking process. The wonders of mathematics.