This happens because of the natural movement of the stars around the celestial poles (+/- 90° declination). Because the stars rotate around the poles, your scope must be perfectly aligned with the poles to prevent such a rotation in your field of view. The farther away from true polar alignment you are, the more of a problem it becomes. When polar aligned properly, you may still get some rotation, but not enough to affect your exposures. If your exposures show a circular movement of stars around the center of your object, then you know that your polar aligning skills need improvement.
Quite often the beginning astrophotographer wonders why the experts (of which I'm NOT one) always say that his or her Dobsonian mounted reflector or altaz mounted refractor/SCT can't take long exposures without showing field rotation. Well, altaz alignment is about as far away from true polar alignment that you can get! How severe a problem is it? Well, judge for yourself:
All photos taken on Kodak Royal Gold 400 film
Looking at the first set of photos, both of the North America Nebula, field rotation is obvious in the first photo. Star trails become worse away from the center of the object. This will be true of any altaz mounted scope. Of course the second picture solves the problem by polar aligning on a wedge. Longer exposures can be made without field rotation.
In the second set of photos, of the Andromeda Galaxy, it's easy to see how field rotation increases in direct proportion to the length of the exposure. The first photo shows a little bit of field rotation at the edges of the field; an exceptable amount if you crop the photo. But then again, this object obviously needs more exposure time (the galaxy is actually 50% wider than the nebula, but you don't see it here). The second photo shows an unacceptable amount of field rotation at 7 minutes.
Question for you! Since the first photo of each object is the same exposure length (3 minutes), each photo uses a 200mm camera lens, and each object rests at approximately the same declination (~+41° for M31 and ~+44° for NGC 7000), why does the galaxy photo show far less field rotation than the nebula? That's tough to answer.
Well, it just goes to show that the amount of field rotation is hard to predict! The main reason for this is that field rotation is dependent upon the direction your scope is pointing in azimuth and altitude (given an altaz aligned scope). In azimuth, field rotation is greater when pointing north/south than it is east/west. In altitude, field rotation is greater when pointing up than down. And the effects of altitude on an object is much greater than the effects of azimuth.
Thus, an object with a declination that matches your latitude will show ZERO field rotation. In other words, objects that pass exactly overhead (at the zenith) will NEVER show field rotation regardless of where you shoot it. If another object is at 80° in altitude (not quite at the zenith), it will show a tremendous rate of field rotation if it is also too close to the meridian (where the azimuth becomes directly north/south instead of east/west). In other words, field rotation of this object can be reduced slightly by shooting it farther away from the meridian, which has the effect of both lowering the altitude of the scope and directing the scope farther to the east and west in azimuth.
So any object close to, but not directly over the zenith will always show the greatest amount of field rotation since altitude tends to have a more severe effect on field rotation than azimuth. Said differently, these objects that pass at a naturally higher altitude need a greater change in azimuth (to the east or west) to compensate. For this reason, it also demonstrates another fault with the field derotator: most objects directly overhead (within 20° or so) tend to spin with too much rotation for this device to be effective.
Thus, if an object passes far away from the zenith at your particular location, you can always reduce the amount of rotation by shooting the object closer to the horizon (lowering the scope in altitude), as opposed to the meridian. Likewise, this has a greater, proportional effect on objects that already have naturally low altitudes since it doesn't suffer the penalty of being too directly overhead.
So why again are those two photos so different? The Andromeda Galaxy was closer to the horizon than was the North American Nebula when I shot it. Had I shot the galaxy closer to the meridian (increasing the altitude of the scope), the amount of field rotation would have been the same as the nebula since I originally shot it at the meridian. The fact that both objects have similar declinations, producing similar scope altitudes while aligned in altaz mode, means that the amount of field rotation between these two objects should be approximate if shot in the same part of the sky.
But remember, even though you can reduce the amount of field rotation by shooting closer to the horizon, it's not all that practical. Light pollution and atmospheric turbulence become objectionable at the horizon, generally speaking.
But thankfully, some aspects of altaz photography are predictable. The longer the focal length, the more field rotation you'll get. So shooting prime focus through a 2000mm focal length scope will show field rotation MUCH sooner than shooting through a 50mm lens riding piggyback. Thus, it's good to know that wide fields photos of constellations or asterisms can easily be done without needing a polar mount or wedge. And as a rule, you can increase the length of your altaz exposures by shooting objects that will pass at the zenith (for zero rotation) or shooting all other objects while closer to the east/west horizon, if practical (for a reduction in rotation).
So how long can you shoot with an altaz scope pefore you get field rotation? Depends on the object, where you decide to shoot it, and how long your focal length is! So who really knows!
Of course, the true solution to all these problems is to either put your Newtonian reflector/refractor in an Equatorial mount or purchase a wedge/field derotator for your SCT (for which one to get, try the article What's better: wedge or field derotator?.
much do the stars really
rotate in altaz alignment? Try this
article on Doc Greiner's site for specific charts
on figuring field rotation and why he says that a
field derotator should be a last resort
Copyright(c) 2003 - 2004 ALLABOUTASTRO.com. All rights reserved.