“The adventure is not to see new things, but to see things with new eyes.” — Marcel Proust

For a ground-based telescope, the mount moves the telescope to point to a particular object and then track the object throughout a observation.

Transit or ‘drift-scan’ telescopes do not track but observe a particular region toward the zenith all the time. The spin of the Earth brings different parts of the sky within the FoV of such a telescope.

Ground-based telescopes have two main types of mounts: altazimuth and equatorial.

Altazimuth: rotate the the vertical axis to different azimuths, and horizontal axis to different altitudes.

Equatorial: rotate the polar axis (pointed toward the north celestial pole) to different hour angles, and the declination axis to different declinations. The hour angle is the sidereal time minus the right ascension (RA). So the mount directly tracks the RA and DEC.

Equatorial mount is simpler for pointing and tracking, but more expensive and clumsy in terms of construction. All ground-based telescopes above a diameter of 5 m use altazimuth mount.

Advantage of altazimuth mount is that the gravitational load does not vary with pointing direction in this case. But the main disadvantage is the complex tracking. Both the axes have to moved simultaneously with varying speed as a function of time.

Almost all modern telescopes are reflecting telescopes.

In prime focus telescopes, the astronomer or her remote-controlled detectors or instruments are located at the focal plane of the paraboloid primary mirror. As the detectors block parts of the light going to the primary mirror, this configuration works well only if the size of the detectors is small compared to the primary mirror. For mirrors larger than around 3.5 meters, the instruments are located in a prime focus cage.

For focused images, the central obstruction has minimal effect, but out-of-focus images have a ‘doughnut’ shape. The support structure of the prime focus cage, that connects it to the side of the optical tube, also creates ‘diffraction spikes’ on very bright stars.

The length of the coma effect for these telescopes, $L_c = \theta/16\mathcal{R}^2$ and for astigmatism $L_a=\theta^2/2\mathcal{R}$.