MadSci Network: Astronomy

Re: How does a rotator help an alt/azmth telescope take images?

Date: Tue Jan 9 11:25:05 2001
Posted By: Erika Gibb, Grad student, Physics & Astronomy/Origins of Life, RPI
Area of science: Astronomy
ID: 978632636.As

Hello Dan!

The altazimuth mount is one in which the telescope can rotate around the vertical axis or move from horizon to zenith (directly overhead). An equatorial mount can rotate a telescope about an axis pointing to the celestial pole (right ascension) as well as moving north-south (declination).* See the images at

The altazimuth mount is simpler and cheaper but runs into problems when trying to take astrophotographs which may require lengthy exposures. When an altazimuth mount is tracking an object, it must move about both axes at the same time. Since the polar telescope has one axis in right ascension and one in declination, it only has to move one axis while tracking--right ascension. This means the equatorially mounted telescope rotates at the same rate as the objects in the field. During long exposures with an altazimuth mounted telescope, even if the telescope is perfectly pointed, the stars in the outer portion of the field of view appear to rotate around the field. This occurs even if the pointing is perfect and occurs because the telescope is not polar aligned. The end result are arcs in images, similar to those you get when pointing a camera at the pole without tracking and taking a lengthy exposure.

The rotator is a device which compensates for this apparent rotation. Hence, the rotator is needed when an altazimuth mount is used but not for an equatoral mount.

*Note: Astronomers use coordinates called right-ascension and declination, which are similar to longitude and latitude projected onto night sky. Declination is in the north-south direction and goes from -90 to +90 degrees. Right ascension is in the east-west direction and is measured in hours, minutes, and seconds with 24 hours in a full circle. This means that as earth rotates, stars move east to west across the sky (so right ascension appears to move E-W across the sky), but are at a constant declination.


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