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ASTROPHOTOGRAPHY CAMERAS, PHOTOS, FILM

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Astrophotography can be done quite simply by using a camera whose shutter can be left open, exposing the film to the light of the night sky. The longer the film is exposed to the night sky, the more light is collected. These types of cameras are called SLR (Single Lens Reflex) and a lot of 35mm cameras are this type. Really all that's needed is a box to hold the film! Just like a box camera made of cardboard which is a basic school project. The ability to expose the film to the night sky for different lengths of time means a large amount of light can be gathered producing colours or expose faint objects which cannot ordinarily be seen, even through a telescope. When the camera is used in conjunction with a telescope the

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Light Gathering Power (LPG)

is awesome.

The difficulties of astrophotography come in the tracking of the objects. The night sky is always moving because of the Earth's rotation. When the camera film is exposed to the night sky for any length of time, the resultant image shows the stars as dashes of light. The longer the film is exposed, the longer the dashes will be across the image until the stars move right out of the field of view of the camera. The more an object is made bigger

magnification,

through

the smaller the field of view becomes. Generally, if

you double the magnification of an object, the field of view is halved. This will give the the impression the object is moving twice as fast just because of our perspective. eg. a person walking in front of you will move past your field of sight very quickly but if they walked past at a distance in front of you, it will appear as they are moving very slowly past your field of sight. Tracking the object becomes harder, and requires more accuracy, the higher the magnification is, and the longer the the film is exposed.

The easiest way of tracking the night sky is by attaching a camera to the

optical tube of a telescope with a equatorial mount.

Equatorial mounts

have slow-motion controls which are knobs that turn slowly and smoothly to move the telescope in both axes. The idea is to point the camera in the same direction as the telescope and look through the telescope's eyepiece. By centering a bright star in the center of the field of view (guide star), and keeping it in the center of the field of view with the slow-motion controls, the camera's shutter can be opened and the film is exposed. It doesn't matter if the camera isn't pointed directly at the guide star otherwise the images you take will always have a bright star right in the middle of it and this may look out of place on every shot. It takes a bit of practise to guide a telescope accurately with the slow-motion controls, but as you get better, the exposures can be made longer.

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