How well can you do with an altazimuth mount

I hinted at the end of Chapter 4 that you can do deep-sky work successfully with an altazimuth mount. The trick is to make relatively short exposures and stack them with rotation.

The reason you must "rotate and stack" with software is that even if the individual images are sharp, there is cumulative rotation from one to the next. Over 10 minutes you get 10 minutes' worth of rotation, but if it's divided up

Sbig Efinder
Figure 9.9. The author's piggyback autoguider, consisting of an SBIG ST-V and a homemade adapter to hold a lens salvaged from a pair of binoculars, similar to the now-discontinued SBIG eFinder.
changed by enlarging the center of the picture.

into twenty 30-second snapshots, the individual images can be turned to line up with each other.

9.5.1 The rate of field rotation

How much field rotation can we tolerate? About 0.1°, in my experience, without visibly skewing the stars at the corners of the image. As long as the guide star is in the middle of the picture, this limit is unaffected by focal length or magnification (Figure 9.10).

Maximum exposure time (seconds) for 0.1° of field rotation with an altazimuth mount

(Sky seen from latitude 40° north)

NORTH

Hour angle -6hl 230 | EAST

+50o

NORTH

WEST

Declination -50o

SOUTH

Figure 9.11. Maximum exposure times in various parts of the sky if you are tracking with an altazimuth mount. Avoid the zenith; longest exposures are possible low in the east and west.

WEST

Declination -50o

SOUTH

Figure 9.11. Maximum exposure times in various parts of the sky if you are tracking with an altazimuth mount. Avoid the zenith; longest exposures are possible low in the east and west.

So how long does it take for the field to rotate 0.1°? Complex formulae for computing field rotation were presented in Appendix B of Astrophotography for the Amateur, but Bill Keicher has derived a much simpler formula that is equally accurate:

cos(Az)

Rotation rate (degrees per second) = 0.004 167° cos(Lat)

cos(Alt)

where Lat is the latitude of the observer and Alt and Az are the altitude and azimuth of the object, taking north as 0° and east as 90° azimuth respectively.1

1 Bill Keicher, "Mathematics of Field Rotation in an Altitude-over-Azimuth Mount," Astro-Photo Insight, Summer 2005, now online in http://www.skyinsight.net/wiki/. The constant in Keicher's

Figure 9.12. The nebula M27 photographed with a Meade LX90 telescope on an altazimuth mount. Stack of nine 30-second exposures with the 8-inch (20-cm) f /10 Schmidt-Cassegrain direct-coupled to a Canon Digital Rebel (300D) at ISO 400, rotated and stacked with MaxDSLR. (David Rosenthal.)

Assuming 0.1° of rotation is acceptable in a picture, then the number of seconds you can expose at any particular point in the sky is:

Rotation rate 0.004167° cos(Lat) cos(Az)

Figure 9.11 plots the exposure time limit over the whole sky as seen from latitude 40° north. (From other temperate latitudes the situation is very similar.) As you can see, most of the sky can tolerate a 30-second exposure; the areas to avoid are near the zenith, where rotation is especially fast. The rotation rate is zero at azimuth 90° and 270°, due east and west of the zenith, but only momentarily since objects do not remain at those azimuths. The "sweet spots," where you can expose for several minutes, are below altitude 40° in the east and west.

published formula was 0.004178, the earth's rotation rate in degrees per sidereal second; I have converted it to degrees per ordinary second.

Figure 9.13. This is not field rotation; it's a lens aberration. Clues: the effect is not proportional to exposure time, and bright stars are affected more than faint ones. (Film image with Olympus 40-mm f /2 lens, from Astro-photography for the Amateur.)

9.5.2 Success in altazimuth mode

Figure 9.12 shows what can be accomplished by stacking short exposures tracked on an altazimuth mount. In this picture of M27, the slight left-right elongation of the star images is probably caused by less-than-perfect tracking, since, after all, altazimuth mounts are designed for visual rather than photographic use. Each exposure was only 30 seconds long, and no field rotation is visible.

Naturally, an equatorial wedge would have made the tracking easier and smoother (because only one of the telescope's two motors would be running), but, as the picture shows, altazimuth mode is far from useless.

Presumably, you can use an autoguider in altazimuth mode, but I have yet to hear from anyone who has done so. It may not be necessary. There is no drift from polar alignment error because there is no polar alignment. There is periodic error in the gears of both motors (azimuth and altitude), but as Figure 9.12 shows, it can easily be negligible. If a few images in the series have tracking problems, they can be left out while the others are stacked.

9.5.3 What field rotation is not

Figure 9.13 shows a common lens aberration that looks like field rotation. The picture, however, was an exposure of only a few seconds on a fixed tripod; field rotation definitely did not occur.

Further evidence that this is a lens aberration comes from the fact that the brightest stars are more affected than fainter stars the same distance from the center, and the entire effect goes away if the lens is stopped down to a smaller aperture.

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Responses

  • Estella
    Can you domastroimaging with an altazimuth mount?
    2 years ago
  • bisirat
    Can you do astrophotography with an azimuth mount?
    7 months ago
  • ralf faerber
    How long of exposure can you do on a fork alt azimiuth mount?
    12 days ago

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