Autoguiders

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What do I mean by "guide?" Either of two things. You can watch the guide star through an eyepiece with crosshairs, holding a control box and slewing the telescope at the slowest possible speed to keep the star centered on the crosshairs. (The art of doing this is discussed in Astrophotography for the Amateur.) Or you can let electronics do the work.

Traditionally, an autoguider is a small astronomical CCD camera, with built-in circuitry to produce commands to slew the telescope so that the guide star stays in a constant position on its sensor. I use an SBIG ST-V, which is one of the best

Figure 9.8. The Leo Triplets (M65, M66, NGC 3628). Stack of six 10-minute exposures at ISO 800 with Canon 20D and 10.2-cm (4-inch) f /5.9 apochromatic refractor (Takahashi FS-102 with focal reducer) on an equatorial mount, guided with an SBIG ST-402 CCD camera as an autoguider on a separate telescope piggybacked on the main one. (William J. Shaheen.)

Figure 9.8. The Leo Triplets (M65, M66, NGC 3628). Stack of six 10-minute exposures at ISO 800 with Canon 20D and 10.2-cm (4-inch) f /5.9 apochromatic refractor (Takahashi FS-102 with focal reducer) on an equatorial mount, guided with an SBIG ST-402 CCD camera as an autoguider on a separate telescope piggybacked on the main one. (William J. Shaheen.)

autoguiders of this type ever made. Its control box is slightly larger than a laptop computer and includes a video screen.

A newer and much cheaper approach is to use an astronomical video camera as an autoguider, with a laptop computer interpreting its signals. The camera can be a modified webcam (as in Appendix B), or a specifically astronomical device such as the Meade Deep Sky Imager or Celestron NexImage. The laptop computer runs GuideDog (free from www.barkosoftware.com) or a general-purpose astronomical software package that supports autoguiding, such as MaxDSLR.

There are two ways the autoguider can issue slewing commands to the telescope. Older autoguiders use relays that simulate the buttons on a traditional control box connected to a six-pin modular socket (sometimes called an SBIG-type or ST-4-type port); for a wiring diagram, see How to Use a Computerized Telescope (2002), p. 158. The newer approach, favored when the autoguiding is controlled by a laptop computer, is to issue slewing commands to the telescope through its serial port.

Autoguiders normally achieve subpixel accuracy. That is, each star image can be located with a precision that is a fraction of the pixel size. This is possible because each star image falls on more than one pixel of the sensor, and the auto-guider calculates the position of its central peak by determining the proportion of the image that reached each pixel. This means that a good autoguider can achieve accuracy limited only by atmospheric steadiness even when attached to a relatively small guidescope.

It is seldom possible to guide more precisely than 1 arc-second because of atmospheric steadiness. In fact, you probably don't want the autoguider to perform unnecessary movements with every slight shimmer of the air. You can generally adjust the aggressiveness of the autoguider to tell it whether to try to correct the whole error each time or only part of it. If the telescope seems to be slewing constantly back and forth, to either side of the ideal position, turn down the aggressiveness.

9.4.5 A piggyback autoguider

Figure 9.9 shows the lowest form of guidescope - a high-quality autoguider connected to a 30-mm-diameter binocular objective. I built this myself; it is similar to a now-discontinued SBIG product called the eFinder. The tube consists of a Meade eyepiece projection adapter (which is T-threaded at one end and is adjustable in length) and a custom lens mount made for me by Pete Albrecht (www.petealbrecht.com; he also makes other custom telescope accessories).

Using a cheap lens does not cost me any guiding accuracy because of the way the autoguider computes centroids. Further, the real advantage of using a small f/4 lens is that there are almost always stars in the field; that is, the SBIG ST-V autoguider can almost always find a guide star immediately, without requiring me to shift the guidescope in various directions looking for one.

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