Five Tasks for an Astrophotographer
Used for long exposure photgraphy of the sky.
by Arvind Paranjpye (firstname.lastname@example.org)
[This article was written when the photography was still popular. Even though the digital cameras are becoming affordable and therefore popular these notes are still useful.]
The main difference between the normal photography and astronomical photography is the duration of the exposure. In the former, exposures are seldom more than half a second. . Whereas in astronomical photography, the exposures are almost always more than a few seconds.
For astronomical photography, therefore, our primary requirements are - (i) a camera with a facility to keep the shutter open as long as we want (this is indicated by mark 'B' on the shutter speed setting on all such cameras); and (ii) a sturdy tripod or a mount for the camera so that it will not shake during the entire duration of the exposure. To begin with, you may use any kind of film available in the market But, as said in the last issue, use a colour negative film, so that you can get it developed quickly. Just check the expiry date of. the film.
Task 1 : Finding the 'safe exposure' - Mount the camera on its stand and point it to some prominent group of stars, close to the celestial sphere (e.g. Orion, Gemini, Leo, etc.) and start with 30 second exposure, that is, leave the shutter open for 30 seconds. Re-centre the camera and take 1, 2, 4, 8, 16, and 32 minutes exposures. When you get the film developed you will find that up to a certain exposure, the stars on the negative will appear as dark dots. After that you will see short streaks. I call the maximum duration of exposure fot which the stars still appear as 'dots' a safe exposure. This depends on the lens of the camera.
Task 2 : Motions of the planet - Knowledge acquired from the Task 1 can now be used for recording the motions of the planets on the celestial sphere. Take a series of safe exposure photographs of a planet on different dates. When all the photographs are put together you can see the changing positions of the planet As an extension to this task you can take up a project to study the retrograde motions of planets.
Task 3 : Circumpolar and Equatorial trail - As described above, if we expose a film for longer duration, star trails will appear on the negative. Now, point the camera towards the north pole and take one and two hour long exposures and repeat it in the direction of the celestial equator. For the former set of exposures, you will see curved trails centered on the north pole and a long straight one on the later ones. Can you guess what this demonstrates?
Task 4: Meteor - On the negatives of the Task 3 you might find some streaks cutting the star trails. These could be due to a meteor, a passing aircraft or a satellite. Meteor photography, especially during a meteor shower, is a very important activity. When a comet passes by the Sun it goes through a violent activity. Its parts break loose. This debris in the form of dust and small stony chunks follow almost the same path as that of the comet, rather like a swarm behind the comet When the Earth passes close to the path of the comet some debris enters the Earth's atmosphere and burn as meteor. Since large number of meteors are seen during this period, it is called a meteor shower. The direction from which the meteor appears to originate is called the radiant of the shower; it is the direction of the orbit of the parent comet.
This picture of comet Hyaku-Take was taken by Suhas Gurjar an amateur astrophotographer from Pune. This wide angle photograph shows circumpolar stars, the star trails becoming longer as one goes away from the pole and the long tail of the comet that Gurjar could capture.
In all the above four tasks, we have not used the hand operated mount for tracking, like the Scotch Mount. You may repeat Tasks 2 and 4 with the tracking mount but giving much longer exposure than the 'safe exposure'. By giving longer exposure we can record even fainter object. For task 2, try a set of exposures (longer than your safe exposure) in the direction of planets Uranus and Neptune. Even though you cannot see them, you can record them on the celluloid.
Task 5 : Comet Photography - As said before, a comet goes through a lot of violent changes as it passes by the Sun. Sometimes a comet may also break. The shape ,of its tail changes due to its interaction with the solar wind. An astronomer would like to follow these activities as closely as possible. You might catch such an event, that others might have failed to (because of bad weather!). And a good quality photograph of yours might just be the missing link.
So, give astrophotography-a try. It is not only fun but also gives us some scientific information. And send us your photographs. We will be happy to put those on our web and would also recommend for the publication in our Khagol magazine.
Lastly, some tips: Keep the aperture of the lens one stop less than full open Avoid taking photographs during a moonlit night, it will fog your film, and if you must, then give shorter exposure, and Always take two extra exposures slightly longer and slightly shorter than what you thought you might give.