Taking a Picture
Ken Tapping, December 11th 2013
Just imagine you are sitting in an unheated observatory dome, looking through the eyepiece at the Moon, a planet or some other astronomical object. You sit, still, patiently, until the atmosphere steadies for a moment, and the image becomes sharp. You add a line or some shading to the drawing you are making, and then, as the atmosphere reverts to its usual turbulent state, and the image starts to shimmer and dance again, you once again patiently wait for that next special moment. Making one of those drawings may take hours or even multiple observing sessions. This is how astronomical imaging used to be done. Even today there are amateur aficionados who maintain that is the best way to capture the subtleties in astronomical images. However they are a vanishing minority. The required combination of huge patience and artistic abilities is getting rare.
It is easy to see that as soon as photography was invented, cameras would be attached to telescopes. However, those early days of astrophotography, as it was called, produced patchy results. Firstly the photographic emulsions - the sensitive chemical mixtures used to record the images - were not very sensitive, so a long time was required to collect enough light. Since the atmosphere rarely remained stable for that long, the resulting photographs were usually blurred. In addition those emulsions were not uniformly sensitive to all colours of light. They were much more sensitive to blue light than to red. The result was that in astronomical images, red stars or other red objects did not show up at all, although much fainter blue stars were prominent. One of the most striking images illustrating this problem is not astronomical. It was taken in the cab of a steam locomotive during the 19th Century. It shows the engineer and fireman standing proudly in charge of the machine. The firebox is open, and all looks dark within. It looks as though there is no fire at all. However the boiler pressure gauge shows plenty of steam is available, which proves there must be a fire. This inadequacy in photographic emulsions was that pronounced. As time passed, new emulsions were developed that were more sensitive generally, and also more sensitive to red light. Even when everybody else doing photography went on to use photographic film, where the emulsion was coated onto plastic strips, astronomers continued to use glass plates. This was not out of fuddyduddyism. It was because glass plates did not bend or stretch, so precise position measurements could be made using them. However, sensitivities were still low enough to require exposures that might be hours long. So even then the astronomer could not sneak off to spend the night in the warm, because he had to sit at the telescope to make sure it was tracking properly for the entire exposure, to avoid having the stars appear as smears.
Digital photography and computers changed everything. One big difference is you cannot easily add multiple images together if they are on film or plates. However, it is easy with digital images. So if a two-hour exposure is required, we make lots of short exposures of seconds or less in duration over a few hours. Since each one takes only a very short time to record, you don’t have to be freezing in the telescope dome; the automatic tracking is good enough. Then, at the end of the observations you go through all those images to pick out the ones that are sharp and of good quality, and then combine those to build up the required exposure. The results are the wonderful pictures we see today in astronomy journals and magazines. Amateurs are doing this too. Professional astronomers of not that many years ago would be amazed at what backyard astronomers are achieving today. What will astronomical imaging be like in another five years?
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