User:JohnOwens/Pet Projects/astrophotography

From Wikipedia, the free encyclopedia

Astrophotography is a specialised type of photography that entails taking photographs of items in the night sky such as planets, stars, and deep sky objects.

Image:MW07.jpg

Milky Way in Sagittarius. Unguided 40 sec exposure

Astrophotography is used to reveal objects which are too faint to observe with the naked eye, as both film and digital cameras can accumulate and sum photons over long periods of time.

[edit] Professional astrophotography

Astrophotography poses challenges which are distinct from normal photography in that the subject is usually extremely faint, and often small in angular size. Effective professional astrophotography requires the use of many of the following techniques:

Mounting the camera at the focal point of a large telescope
Emulsions designed for low light sensitivity
Very long exposure times (one hour or more is common)
Tracking the subject to compensate for the rotation of the Earth during the exposure
Gas hypersensitizing of emulsions
Use of filters to reduce background fogging due to light pollution of the night sky.

Since the early 1990s, most professional observatories switched to using CCD devices for astronomical imaging. They have numerous advantages, with increased sensitivity and more linear response. The principle disadvantage is a significant reduction in the field of view. CCDs require specific modifications for best effect in the low light conditions of astronomy, such as:

cryogenic cooling to reduce thermal noise
compensation for non-uniform pixel sensitivity
biasing

[edit] Amateur astrophotography

Emulsion based astrophotography remains popular with amateur astronomers, but even here CCD imaging is becoming relatively cheap and is starting to replace wet photography.

Even with simple amateur equipment, interesting photographs can be obtained. This picture of Jupiter was taken with just the built-in zoom of a Canon PowerShot A75, with no drive, but was able to capture Ganymede (the small streak coming out of Jupiter to the upper right). The only enhancement was cropping & enlarging the image 3×.

[edit] Main types of differences

altazimuth mount vs. equatorial mount w/ drive
digital camera vs. photographic film camera or photographic plate
single-lens reflex camera vs. integrated lens

[edit] Mount

[edit] Equatorial mount

Ideal for long exposures.

[edit] Altazimuth mount

Includes camera mounted directly to tripod, with no telescope/binoculars involved. Can be used for long exposures, with a GOTO and a rotating erector prism. Also useful for short exposures (typically <10s) or star trail images.