Equatorial mounts get rid of the field rotation illustrated in Figure 4.2. Celestial objects tilt as they rise, travel across the sky, and set. An equatorially mounted telescope tilts with them, so that everything remains stationary in the field of view, but altazimuth-mounted telescopes suffer field rotation. With an altazimuth mount, the object that you're tracking remains centered, but everything else rotates around it.
Can you make do with an altazimuth mount? Maybe. Field rotation does not affect photographic exposures shorter than about 15 seconds, so it is no obstacle to solar, lunar, or planetary imaging; only deep-sky photography is affected, and even then, altazimuth-mode exposures of several minutes are possible when imaging an object low in the eastern or western sky. Field de-rotators are available for some of the larger altazimuth-mounted telescopes; they rotate
Using equatorial mounts and wedges
Table 4.1 Altazimuth versus equatorial mounts
Simple and lightweight (no wedge)
Stable (centered load, low center of gravity)
Easy to set up (point the axis roughly straight up, then sight on two stars) Tracks using two motors Can see entire sky
Hard to maneuver when pointing near zenith (some models cannot aim straight up with a camera attached) Easy to maneuver when pointing near Polaris Unsuitable for long-exposure photography (Figure 4.2); OK for photographing Sun, Moon, and planets
Heavier and more complex
Must be aligned on Earth's axis
(by sighting Polaris and/or checking for drift) Smoother tracking (just one motor running) May not be able to see far southern sky (Figure 4.14) Easy to maneuver when pointing near zenith (also easier to reach Meade ETX focus knob, which is not hidden between fork arms) Hard to maneuver when pointing near Polaris (some models will not aim near Polaris with a camera attached) Easy to slew directly north, south, east, or west for direct comparison to star map Required for long-exposure photography the camera to match the stars but are usable only when photographing through the telescope, not when using telephoto lenses or other instruments mounted "piggyback" on it.
Even equatorial mounts suffer from field rotation if they are several degrees away from correct polar alignment. This is the subject of many misconceptions. For detailed analysis, see Astrophotography for the Amateur (Cambridge, 1999), Appendix A, and the software at http://www.covingtoninnovations.com/astro. Here are some basic rules of thumb:
(1) The field always rotates around the star that the telescope is tracking (the guide star, Figure 4.3). If the camera is aimed in a different direction than the telescope, field
4.2 Must field rotation be eliminated?
rotation will turn stars into arcs of circles centered on the guide star, which lies outside the picture.
(2) Many lenses have an aberration that looks just like field rotation but is always centered on the center of the field and is the same regardless of exposure time. Do not be misled by this. Such an aberration usually disappears when the lens is stopped down.
(3) A picture will normally look sharp if there is no more than 0.1° of field rotation, provided the guide star is near the center of the picture. Curiously, this is true regardless of the focal length or magnification, because telescopes don't magnify rotation. (A rotating wheel, viewed through a telescope, still rotates at the same speed.) Lunar and planetary work can tolerate somewhat more field rotation because the highly magnified images are not as sharp.
(4) With a telescope in altazimuth mode at latitude 40°, the rate of field rotation varies from 0.5°/minute along much of the meridian to just 0.07°/minute low in the east
TRACKING WITH ALTAZIMUTH MOUNT Image rotates; long-exposure photographs are not possible
Figure 4.3. Effect of field rotation on a photograph. The rotation is always centered on the star that the telescope is tracking. (From Astrophotography for the Amateur, Cambridge, 1999.)
or west. Thus, exposures of one or two minutes are practical in some parts of the sky.
(5) A perfectly polar-aligned telescope would have no field rotation at all. For a maximum of 0.1° rotation in a 1-hour exposure of objects high in the sky, the telescope must be polar-aligned to within 0.4°. That level of accuracy is easy to achieve with one or two iterations of the drift method (p. 49). Much of the time, simply sighting on Polaris is good enough.
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