Comet Photography

Many amateur astronomers in the world waited long for the comet of the century Hale-Bopp. And it brought us a great show at the northern sky for several months. It reached perihelion in April 1997 and was a fantastic comet. Many amateurs made images to remember this great comet. Months or years after Hale-Bopp disappeared from the norther sky we can just remember the last two big comets, appearing separated by only one year. Comet Hyakutake and comet Hale-Bopp were the last really big shows. Both reached almost mag. -1 in late March 1996 and the early days of April 1997.

Since I have observed comets since 1990 I have often tried to take pictures of each comet I observed visually. So I have made some bad but also very useful experiences in taking photos from comets.
Many amateurs who have never made any pictures of the sky or only sometimes will try to image bright comets. I hope you can profit from my experiences that you take some nice images from the next bright comet.

What to know ?

If you have already taken images from deep-sky objects you should also read this because taking photos from comets needs some special requirements.
Normally you guide on a star to compensate the earth rotation. As comets move during exposing the film it is necessary to know how fast it moves and how long you can expose until you get a short streak instead of a fine and sharp image.

To know how fast the comet moves you need a program that calculates the movement from the daily positions.
When you know how fast the comet moves in one hour you can calculate how much it moves during your exposure time.

M = Movement in millimeters on the film
S = Movement of the comet in arc seconds per minute
F = Focal length of your telescope in millimeters
T = Exposure time in minutes

M = (S*F*T) / 206265

Example:

Comet Hyakutake moved on March 26th 1996 in 1 hour for about 2758 arc seconds.
Then divide 2758/60 to get the movement in one minute. You want to expose for 10 minutes using a telescope with 750 millimeters focal length.

Then calculate: M = (46 * 750 * 10) / 206265

M = 1.67 millimeter

Comet Hyakutake moves on March 26th 1996 in 10 minutes for about 1.67 millimeters on your film. You might think that it isn't much, but my experiences are that when you use TP 2415 the comet shouldn't move more than 0.1 millimeter. So you see that you must use a lens with a very short focal length or you must guide directly on the comet and compensate its movement.

Another important thing is the focal ratio of your telescope or telephoto lens to capture the tail of a comet. Since comet Hyakutake and comet Hale-Bopp were very bright you can image it with a normal 50 mm lens. But when you want to take images from fainter comets and its tails the only instrument is a Schmidt Camera, a telescope designed especially to take wide field images. The Schmidt Camera provides very sharp images without any optical failures and a very fast focal ratio from 1.5 to 3.

But don't be sad when you don't have a Schmidt Camera or something like that. It is also possible for you to take nice shots from bright comets except they are not so fast like comet Hyakutake. Because bright comets will cover the streak by its great brightness. The only thing is that you get a bit more diffuse image from the coma and the tail are not absolutely sharp.

If you want to avoid the problem with the movement then use a film that is more grainy than the TP 2415. Then you can expose some more minutes.

But as I mentioned above it is possible to compensate the comets movement during guiding. For this it is necessary to know how fast it moves and in which direction it moves. Knowing these values you can compensate the movement by making something that I call "Comet Guiding by Stars". Using this method you use a star and guides on that star. But you don't hold the star on a cross-hair. You use the star to compensate the comets movement by making calculated guiding steps. This method may be also known as "Indirect Guiding".