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Astronomical Telescope Tracking Modes

Depending on how technically advanced a telescope is, how it is mounted and what the required accuracy is, there are different ways of tracking sources on the sky.

Manual tracking

Obviously this is not a good option. It is ok for looking at objects through an ocular lens for a while, correcting for the objects motion only occasionally, but when it comes to obtaining images of the sky with instruments not even for amateur astronomers manual tracking is a serious option.

Clockwork (motor) tracking

The most direct way of tracking a celestial source with a telescope is that of an equatorial mount, with the primary axis aligned parallel with the Earth's rotation axis. In such a setup, by fixing the "height" ("declination") value of the telescope to that of the source, a motor can just counteract the Earth's rotation. This is a motion in only one axis, which is quite easy to control, as long as the polar axis is oriented precisely. Without any further supporting instruments, this is caused "blind tracking".

However, tracking with just a motor will never be really accurate because of difficulties to control motors to the required level of precision and misalignments between the telescope and Earth's rotation axis (which precesses and nutates with time). Therefore, for long continuous observations secondary corrections are required. These can be applied, in a very simple setup, by using a guiding telescope with an ocular piece mounted on the primary telescope and a handset with which the motion of both primary and guider telescope can be adjusted to always follow a star in the guider's field centre. In the simplest possible setup this can be done with a double cross on the ocular lens of the guider and a little handset to move the telescope up and down or ahead or back with respect to its current position. This is called "by-eye guiding".

Computer-guided tracking

Even under computer guidance there are different modes of tracking.

Auto-guider

Following up on the scenario above, a computer with astronomical guiding software can be used to keep the star at the centre of a guider's field-of-view. This is called an auto-guider. It makes use of a feedback loop in which offsets of the guide star from the field centre are measured and communicated back to the telescope drives, whose motion is then corrected for the offset.

Interferometric auto-guider

In some cases, when extreme guiding precision is required, a small interferometer made out of several star trackers serves as auto-guider. This kind of guiding is used, e.g., for the Hubble Space Telescope (HST).

Encoder-assisted guiding

For various reasons there are telescopes that don't have guiders, such as e.g. radio telescopes. Therefore, they do not have a sky reference that permits corrections of their tracking motion while an observation is on-going ("blind pointing"). However, all is not lost yet. One can use encoders, i.e. marks on the two axes of a telescope that are read by a camera/sensor, to determine the current coordinates in the telescope's restframe at any given time. By creating a so-called "pointing model", i.e. a database that matches encoder positions with positions of certain known sources on the sky, one can then use the encoder position readings during an astronomical observation to re-calculate where on the sky the telescope has been pointing. The precision of the tracking during an observation can be improved by cross-checking the match of encoder coordinates vs. sky coordinates at regular intervals (many times per second, typically). Basically all professional radio telescopes use this technique. It allows tracking to an accuracy of a few arcseconds, which is good enough for most radio telescopes, but would not be sufficient for most optical telescopes.

No tracking

Note that there are also telescopes that do not track sources at all, but rather "let the sky drift by". This, however, limits the number of observing modes that can be used (long exposures, e.g., would not be possible,unless one can employ image reconstruction techniques).