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Guiding Camera

The guiding camera is not to be confused with the actual imaging camera at the prime focus of

the telescope. Rather, it will be inserted where the guiding reticle eyepiece would have been

placed. The guiding camera can be any one of a variety of imaging devices. It can be a small

CCD designed specifically for autoguiding, as with the SBIG ST-4 autoguider. It could also be a

dedicated CCD camera, an Orion StarShoot, a Meade Deep Sky Imager (DSI) or Lunar Planetary

Imager (LPI), a Celestron NexImage or LPI, or just a modified webcam (Figure 4). In fact, the

Meade and Celestron cameras are basically modified webcams. The guiding camera detects the

guide star on its imaging chip and sends the image to the computer for interpretation. Generally,

webcams can detect and track guide stars as faint as magnitude 7 fairly reliably. Cooled CCD

cameras can detect and track still fainter guide stars.

Figure 4. Examples of guiding cameras. From left to right, a modified webcam, the Meade

DSI, the Orion StarShoot, and the Celestron NexImage.

Computer

Clearly, the guiding camera takes the place of the astrophotographer‟s eye. The computer takes

the place of the astrophotographer‟s brain. It is responsible for determining the position of the

center of the guide star on the guide camera‟s chip, comparing that to the intended position of the

guide star on the chip, calculating the differences, if any, and translating those into the requisite

commands to be sent to the mount to make any necessary guiding corrections. It can do a better

job of this than the human brain and can even do it with a comma shaped star in an off-axis

guider, as it can accurately determine the position of the point of maximum brightness within the

distorted star image and track that consistently. The computer is often a laptop computer, but

could also be a desktop computer. The electronics boxes that came with the ST-4 and V units

were stand alone computers that did not require the astrophotographer to provide a separate

computer. The computer does not need to be a particularly fast computer to accomplish these

tasks. Often, an older laptop can be relegated to this task after one purchases a faster new one for

image processing, for example. However, the number of other applications that the computer has

running during autoguiding should be limited as much as possible.

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Celestron Compustar Bedienerhandbuch Seite 11