Aberrations are defects in images produced by a telescope (or optical system). They are caused by limitations in the design and manufacture of the optics. Designers and optical factories strive to produce affordable telescopes that have as few aberrations as possible.

Two major aberrations seen in telescopes are spherical aberration and coma.

Spherical aberration is caused by rays of light passing at different distances from the center of a lens or mirror not coming to the same focus. Edge rays will typically come to a focus closer to the lens or mirror than central rays. Corrector plates in Schmidt-Cassegrain telescopes are designed to fix this aberration. Coma is a related effect: it is spherical aberration from rays that come in off-axis. It shows up as little off-axis comet-shaped blobs that point inwards towards the center of the field and that get bigger as you look towards the edge of the field of view.

Aplanatic optics are designed to eliminate both spherical aberration and coma. They are superior to SCT optics that rely only on the corrector plate, since the corrector only gets rid of the aberrations in a small area around the center of the field of view.  This is especially important for astrophotography, where coma towards the edges of an image can be very noticeable.

The end result is sharper images across a much wider field of view than in other designs. EdgeHD aplanatic optics maintain diffraction-limited images across the entire field of view of many of the most popular astrophotography cameras.

As the earth rotates around its axis, the stars appear to move across the sky. If you are observing them using an altitude-azimuth (alt-az) mount, they will quickly drift out of view. Readjustments to get them back in view are awkward and frequent or require computerized tracking.

In order to avoid these problems for either visual astronomy or astrophotography, you need a different type of mount that’s oriented or aligned to make following the apparent motions of the stars much easier than with an alt-az mount.

A telescope on an equatorial mount can be aimed at a celestial object and easily track the daily motion, keeping it in your eyepiece. It works by first polar aligning or inclining it at an angle equal to your latitude and pointing one axis (called either the polar axis or right ascension (RA) axis) in the same direction as the earth’s rotational axis (towards the celestial pole). Once the polar axis is parallel to the earth’s axis and turned at the same rate of speed as the earth, but in the opposite direction, objects will appear to stand still when viewed through your scope. There is no rotation of the field of view and tracking can be extremely accurate, making the equatorial mount perfect for astrophotography.  It has two motions: in RA (east-west) and in declination (dec, north-south). With the use of setting circles, a polar-aligned equatorial mount can quickly find celestial objects.

The north celestial pole (NCP) is the point in the sky
around which all the stars appear to rotate.
The star Polaris lies less than a degree from
the NCP and it can be used to roughly polar align
a telescope. However, for accurate polar alignment,
the polar axis of the telescope’s mount needs
to be aligned to the true NCP.

Aligning the telescope to the earth’s rotational axis can be a simple or rather involved procedure depending on the level of precision needed for what you want to do. For casual observing, only a rough polar alignment is needed. Better alignment is needed for tracking objects across the sky (either manually or with a motor drive) at high magnifications. Still greater precision is needed in order to use setting circles to locate those hard-to-find objects. Finally, astrophotography will require the most accurate polar alignment of all.