The NURO Autoguider
Dave Shaffer's Astronomy Site
The National Undergraduate Research Observatory (NURO) provides undergraduate students from a consortium of colleges and universities the opportunity to gain hands-on experience using a research quality telescope. NURO uses a 31" optical telescope owned by Lowell Observatory.
Despite its relatively small size, the 31" telescope, with its modern electronic detectors, can make very high sensitivity observations. One of the limits to how faint the 31" can observe is set by how long it can make a CCD photograph of the sky. Due to mechanical variations in the pointing of the telescope, the longest unaided exposures are around 4 minutes. During longer exposures, the telescope's pointing can vary enough that images are smeared out.
Most smaller (apertures less than a few meters) or older (built before 1990 or thereabouts) optical telescopes have polar mounts. In a polar mount, one of the support axes of the telescope is aligned with the Earth's polar axis). Once the telescope is pointed towards an object, it can then track that object in the sky as the Earth rotates by counter rotating the telescope around this polar axis. However, due to the size and weight of the telescope and various minor manufacturing and/or mounting errors, the telescope will generally not track exactly - it doesn't stay pointed exactly at the same point on the sky. If you try to take a long-exposure picture of an astronomical object, these pointing variations will cause the image to be smeared out.
To cure this tracking problem, the telescope can use an autoguider. The Autoguider, as its name indicates, can guide the telescope automatically during an exposure to compensate for pointing variations, thus allowing much longer exposures that will not smear out the object(s) of interest.
Basically, before an exposure, the autoguider is used to select a fairly bright pointing star and position it on the autoguider detector. If the star image begins to wander around during the exposure, the AG detects that wander, and commands the telelescope to move so that the star is returned to the reference position. In theory, at least, the telescope should then be able to make exposures as long as desired with little loss in image quality.
The picture shows the autoguider that I assembed for the 31" telescope operated by Lowell Observatory at their Anderson Meas site. A fair fraction of the observing time on this telescope is used by the NURO consortium, and it is for NURO that I assembled the autoguider.
For more information about NURO: the National Undergraduate Research Observatory, click here.