We can do this simply by correctly polar aligning, and then just letting an equatorial mount's right ascension axis motor move the scope at the sidereal rate. The sidereal rate is usually the default tracking rate on modern motorized mounts.

The gears, motors and mechanical parts of a telescope mounting are not perfect however. Every worm and gear assembly has what is called periodic error. This is based on the period of the worm gear - how long it takes to make one revolution, usually about 4 to 12 minutes, depending on the gear. Any errors in this gear will repeat once per period. Instead of tracking perfectly, a star will drift east and west due to periodic error.

This type of error is present even in the most expensive mounts that use a worm and gear, no matter how well they are made. Of course, on more expensive mounts, this periodic error is usually less. But even on the best mounts, if the exposure and focal length get long enough, stars will trail due to periodic error.

There may also be other non-periodic errors and atmospheric effects that affect tracking.

Stars trail in the east-west direction in this image because of poor tracking due to periodic error in the mount's right ascension gear.

At some point, as exposures or the focal length get longer, we will have to decide whether to just let the mount track, or attempt to correct for these errors by guiding.

Guiding means following a star precisely by manually or automatically making corrections in right ascension and declination to produce higher tracking accuracy. This is usually done with an off-axis guider or a piggyback guidescope. We'll cover this later in section 806, Guiding and Autoguiding.

Our basic advice for astrophotographers just starting is to shoot with a short focal length telescope and just let it track unguided at the sidereal rate. Depending on the quality of your mount and polar alignment, some of the exposures will be ruined by bad tracking. We will throw those out, and use just the good ones. This is much easier for a beginner than trying to master the complexities of guiding.

We are going to start out simple and easy, shooting with short focal length camera lenses riding piggyback on top of our scope and not guiding. At these short focal lengths, tracking should not be a problem even with an inexpensive mount. Later we will move up to longer focal lengths and tracking will become more critical.

This method of shooting unguided at longer focal lengths, however, assumes that your mount can track well enough for a long enough period of time to produce a minimum exposure that is required to produce a usable image. We will discuss how to determine the correct exposure in a subsequent chapter.

If your telescope has such a slow focal ratio, or such a long focal length, that you need an exposure so long that you get trailing in every image, you will have to re-evaluate your setup. If your mount is not well made, or has a lot of periodic error, you may be throwing a lot of exposures away. This can be quite a waste of time and effort. You will need to get a faster optical system, or a shorter focal length telescope, or a better mount, which is usually the best solution.

Note - even though we are not going to guide, it is still important to be correctly polar aligned.

Tracking - The Bottom Line Tracking describes how accurately your mount will follow the stars. It is more critical with longer focal lengths. Every mount has inaccuracies in the gears that cause less-than-perfect tracking. Beginner astrophotographers should start out with shorter focal lengths where tracking is less critical. For longer focal lengths, shoot a lot of exposures and throw away the ones that are trailed due to poor tracking.