Unguided Exposures
From TzecMaun
Huh? Unguided? What's Guiding?
When you point a telescope at the sky and take a picture, a whole lot of things need to go right in order for the result to be a good one. Tracking the apparent movement of the sky overhead is a difficult and challenging task.
The level of accuracy required is extreme. Very, very few things in most people's everyday experience comes anywhere near the level of precision required for imaging astronomical objects. In fact, the terminology involved is probably not familiar to many students. To make an astronomical image, the mount that holds the telescope has to move with arcsecond accuracy.
An arcsecond is very, very small - it's 1/1,296,000th of a circle. There are 360 degrees in a cricle, 60 arcminutes in each degree, and 60 arcseconds in each arcminute. Thus there are 60* 60 * 360 arcseconds in a circle. Thus, roughly speaking, we are talking about less than one millionth of a circle. That's very precise.
(Actually, some of our telescopes don't require quite that much precision; they only need a precision of about one quarter of a millionth of a circle.)
There are two ways to achieve this level of precision in moving the telescope mount:
- Take pictures of a star and adjust the mount's speed to keep that star steady (guiding)
- Move the mount with arcsecond accuracy (unguided)
Why is this precision important? Because astronomical objects are extremely dim, long exposures record more details than short exposures do. The ability to take long exposures is critical to success for most types of astronomical imaging. It is not uncommon to need hours and hours of exposure time in order to get high levels of detail.
Note: It's not practical to take hour-long exposures for a variety of reasons. It's more typical to take a series of shorter exposures, and then combine them. For a discussion on how to determine the optimal individual exposure duration, please see the wiki page on Exposure Duration.
Guiding in a Nutshell
Most of our telescopes use Snap-Shot Astronomy software to take pictures of the sky. This is simple point-and-shoot imaging of the night sky: slew (point) the telescope at a given object in the sky, and then take (shoot) a picture. As explained below, as long as the exposure time is reasonably short, this works. But things like wind, telescope flexure, atmospheric refraction, gear eccentricities, etc. can interfere with accurate tracking.
Guiding takes care of this problem. While the main camera is busy accumulating photons for the image you want, a second camera, called logically enough the guide camera, is also taking images. If things have been set up properly, there is an adquately bright star image on the guide camera's CCD sensor. (Adequately bright means bright enough to get a good image in 3-15 seconds of exposure time.) The guide camera takes an image, figures out if the star is centered, and if not, directs the mount to make a minute adjustment to re-center the star. (The exposure time must be short enough so that the adjustment is very small; otherwise, the adjustments will be large enough to smear the image being taken by the main camera!)
This process keeps the telescope pointed at the same place in the sky for the duration of the main camera's exposure.
Unguided Imaging Explained
All of our telescopes are mounted on high-precision mounts, so they track very well. We use Paramount ME mounts from Software Bisque. These are top of the line mounts that can, in fact, track at these very precise rates. Unfortunately, they can only stay that accurate for a relatively short period of time. The longer the focal length of the telescope (and therefore the greater the apparent magnification of the image), the shorter that time is.
And of course it follows that the shorter the focal length of the telescope, the longer you can expose without guiding.
The Foundation telescopes with the shortest focal length are the Epsilon 180s (one in Australia, one in New Mexico at the time this is being written). You can take exposures of up to 15 minutes unguided with these telescopes under ideal conditions. "Ideal" in this context means there isn't much wind, there's no bird parked on the telescope for an overnight, etc. At the other extreme, the "Big Mak" telescope in New Mexico can barely do one-minute unguided exposures, and thus is almost always use with guiding active. (The Big Mak doesn't have an enormously long focal length, but it has tube flexure and a lot of wind resistance, as well as a mirror that takes time to settle into a new position - all of this reduces the limit for unguided exposures.)
So you need everything working in your favor to take those desirable long exposures on any given telescope. We do one more thing to help you get the longest possible exposures: we use special software to adjust the tracking of the telescope to make it even more accurate than the factory specification.
This software is called TPoint. We spend an entire night sampling the pointing error of the telescope all across the sky. We use more than 300, sometimes more than 500, positions to determine how and why the telescope's pointing varies. These variations are saved, and then used later to correct telescope pointing and tracking. This is a big part of the reason why you can take those 15-minute unguided exposures with the E180s.
We use TPoint on all of the telescopes to maximize their pointing and tracking accuracy. TPoint works with the Paramount ME to correct both slewing errors and tracking errors. However, even with TPoint, there is still going to be a limit to how long you can image unguided. At some point, even very, very small errors add up, and the exposure has to end.
On any given night, on any given telescope, and for any given area of the sky, you will have to experiment to determine how long you can image unguided. Wind, telescope flexure, gear eccentricity, elevation in the sky, distance from the celestial equator and other factors all come into play. As you gain experience with the Foundation's telescopes, you'll get a feel for the kind of conditions that are good for really long unguided exposures, and which are not.
Other than the Big Mak, most of our telescopes can handle a five-minute exposure time without the stars becoming elongated. This assume little or no wind, of course. The AP 180 can typically handle 10-minute exposures. The AP206 can typically handle 10-minute exposures in normal mode, and may handle 10-minute exposures in ZOOM mode if conditions are excellent.
These are minimum times; it's possible you can get better results on any given night.
Ron Wodaski Observatory Director
