Initializing the computer

Once you've set up the mount, it's time to turn on the computer and align the encoders on one or two stars.

Make sure the computer is in equatorial ("polar") mode, of course, or great confusion will ensue.

One-star alignment

On a perfectly polar-aligned mount, one star is sufficient to locate the whole celestial sphere, because the telescope can assume that its axes correspond directly to right ascension and declination.


This is how the Meade LX200 (non-GPS) handles alignment in equatorial mode. All you really have to do is turn the telescope on, identify one star, and sync on it; the telescope does the rest. You do not have to choose "Align" on the keypad.

If you go through the alignment procedure on the Meade LX200 or Autostar keypad, what you get is a bit fancier. You start by putting the telescope in the specified home position: declination +90°, tube upside down (LX200) or right side up (Autostar). The telescope then slews to the estimated position of Polaris (slightly off the true pole) to help you adjust the mount, and then chooses an alignment star and moves to it for you to sync (see below). This is basically a one-star process because the position of Polaris is estimated, not measured.

With one-star alignment, pointing accuracy is completely at the mercy of polar axis alignment. This isn't as bad as it sounds because you can use pointing accuracy as a test of polar alignment. After initializing, go to stars in several parts of the sky and make sure they are all found successfully. If not, refine the polar axis alignment.

Two-star alignment

The Celestron NexStar takes a radically different approach. It treats an equatorial mount as a tilted altazimuth mount, and just as in altazimuth mode, it initializes on two stars (other than Polaris). This enables it to find objects accurately even if the polar axis is misaligned, but it also means you can't use pointing errors to detect polar axis error.

You can still refine the alignment by the drift method (p. 49) provided the tracking mode is set to equatorial. Otherwise the telescope will track in two dimensions, just as it does on an altazimuth mount, and the drift method won't tell you anything.

Meade Autostar telescopes, including the LX200 GPS, offer both one-star and two-star alignment. Two-star alignment gives much better pointing accuracy and I strongly recommend it.

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