The rules have changed

The rules of the tracking and guiding game are not what they used to be. In the film era, the telescope had to track perfectly for 20 or 30 minutes at a time. Only equatorial mounts could be used because an altazimuth mount can only go a minute or two without excessive field rotation. Guiding corrections had to be made constantly, either by an autoguider or by a human being constantly watching a star and pressing buttons to keep it centered on the crosshairs. One slip and the whole exposure was ruined.

It was also important to guard against flexure and mirror shift. During a half-hour exposure, the telescope and its mount could bend appreciably. Also, notoriously, the movable mirror of a Schmidt-Cassegrain telescope would shift slightly. For both of these reasons, guiding was usually done by sampling an off-axis portion of the image through the main telescope.

Today, we commonly take 3- to 5-minute exposures and combine them digitally. That makes a big difference. Tracking errors that would be intolerable over half an hour are likely to be negligible. If there is a sudden jump, we can simply throw away one of the short exposures and combine the rest. We can even work with very short exposures on an altazimuth mount, using software to rotate as well as shift the images so that they combine properly.

9.2 The rules have changed

TRACKING WITH ALTAZIMUTH MOUNT Image rotates; long exposure photographs are not possible

TRACKING WITH EQUATORIAL MOUNT Telescope rotates with image; long exposures work as intended

Figure 9.2. Tracking the stars with an altazimuth mount causes field rotation, which can be overcome by taking very short exposures and doing a rotate-and-stack. (From How to Use a

Computerized Telescope.)

Figure 9.2. Tracking the stars with an altazimuth mount causes field rotation, which can be overcome by taking very short exposures and doing a rotate-and-stack. (From How to Use a

Computerized Telescope.)

Figure 9.3. The author's DSLR astrophotography setup. Meade LX200 telescope with piggyback autoguider (Figure 9.9), 8 x 50 finderscope, Canon 300-mm f /4 lens, and Canon XTi (400D) camera, mounted equatorially on a permanent pier in the back yard.
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