DSLR cameras provide a cost-effective route into astronomical imaging, especially as the entry-level models are often just as capable as mid-level ones as far as astrophotography is concerned. However, their Achilles heel is poor sensitivity to deep reds. While not a problem when imaging galaxies and star clusters this greatly limits their ability to image emission nebula, whose light mostly comes from clouds of ionised hydrogen. The glowing hydrogen emits in a specific wavelength with a deep red colouration, and as much as 75% of this light can be blocked by the camera’s infra-red filter.
The solution is to remove or replace this filter. For the brave, there are various online guides for performing camera surgery yourself. Alternatively there are individuals who will perform this service for you. I had my Canon 1100D modded by Cheap Astrophotography and am very pleased with the result.
Here’s two images that show the difference that removal of the filter makes. The first shows Comet Jacques and the Double Cluster in Perseus, taken with the 1100D before modification, using a 135mm lens.
Comet Jacques is the green streak at the lower left, with the Double Cluster right of centre. Above and below the comet some nebulosity is faintly visible. The next image shows a very similar field of view taken using the same lens fitted to a modded camera.
Using the modded camera the Heart & Soul nebulae are clearly visible. The comparison is not an entirely fair one as the second image was taken at a site with very little light pollution, but in other experiments I've found it difficult to get much colour from an unmodded camera even from the darkest site.
While summer isn't the best time for astronomy - hindered by short nights and a lack of full astronomical darkness – there is at least one compensation. For observers at high latitudes in the northern hemisphere the season of noctilucent (night-shining) clouds is almost upon us. These tenuous ultra high-altitude clouds are only visible during deep twilight, being lit directly by the Sun while the observer stands in darkness. They are too insubstantial to be seen when standing directly underneath.
Last Summer I was lucky enough to observe and photograph this rare phenomena, only a week or two after learning of their existence. It took me some time to realise what I was seeing, at first I assumed it was a low and near cloud lit-up by light pollution, but eventually I realised it didn't appear to be moving.
This image, taken at about 1:30AM, was featured on the BBC News website. It’s an 8 second exposure taken with a Canon 1100D DSLR fitted with a 50mm lens.
I was curious as to how far away the clouds were, so I plate-solved the image to identify the visible stars and used a software planetarium to find their angles above the horizon. Noctilucent clouds form at an altitude of about 50 miles, so with a quick bit of trigonometry I was able to work out that they were several hundred miles away. From a location a few miles north of London I was looking at clouds somewhere off the west coast of Norway!
Plate-solved image shows the constellation Auriga on the horizon, with the bright star Canopus at upper right.
So if you find yourself at a dark site between June and August it’s well worth taking a moment to scan your northern horizon. A bright display like the one pictured above is clearly visible even from a moderately light-polluted site, while fainter ones may only show up on long exposure photographs.