It’s been quite cloudy recently so I’ve been clutching at any opportunity to make the most of clear skies. Last Tuesday it wasn’t forecast to be very clear at my dark site in Somerset but it was clear at my suburban address in Bristol. Light pollution round here is pretty bad and the sky often looks like Fanta. I don’t have a CLS filter to block light pollution so visible-light astronomy of all but the brightest objects is out of the question. To make matters worse, the moon was almost at its fullest. I decided to try out my full-spectrum camera’s infrared capabilities for the first time, to see if it helps with urban light pollution.. I’ve used it for infrared landscape photography before, but not yet astronomy. I’d no idea how much light pollution there might be in the infrared band. I used an Astronomik 807nm IR filter, giving me sensitivity from approx 800-1100nm. That should kill off the vast majority of the light pollution (orange sodium street lights emit at 589nm), but won’t do anything to avoid the sun’s infrared radiation being reflected off the moon. Without a pole-finder I struggled to align the telescope precisely. It was near enough that the go-to mount was able to find every object I asked for, but exposures much longer than 30 seconds caused some motion blur.
First, I slewed to the Andromeda galaxy, Messier 31. I compared its appearance in the visible band with that in the infrared band, and was pleased to find that the infrared seeing was clearer. My camera is not quite as sensitive to infrared light as it is to visible light, and with my exposures limited to about 30 seconds I was forced to use my camera’s highest ISO sensitivity – 12800. The noise at ISO 12800 is really terrible and even stacking lots of frames doesn’t get rid of it. The final stack, showing the Andromeda Galaxy is composed from 42 frames and 5 dark frames. I used DeepSkyStacker.
The image of Andromeda is nowhere near as good as one I took recently, but that’s not surprising given that I’m in a light-polluted city, using infrared rather than visible, collecting less light overall, using a higher ISO, using a shorter shutter speed, and using a not-very-well aligned mount. Later I turned my attention to Messier 15 and Messier 2, which are both globular clusters. They are smaller and fainter than the Andromeda galaxy and the viewing conditions were far from ideal, but I had a go at imaging them anyway.
So in summary, using infrared imaging of deep-sky objects seems a reasonable technique for avoiding light pollution. However, I think I’d be significantly better off going to a dark site, or if that’s not possible, using a CLS filter to cut light pollution and leave the rest of the visible band intact – particularly the H-α emissions from stars.
Jonathan Gazeley 