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. 

RA (right ascension) and Dec (declination) are the coordinates on the sky that correspond to longitude and latitude on the earth. RA measures east and west on the celestial sphere and is like longitude on the earth. Dec measures north and south on the celestial sphere and is like latitude on the earth.

RA is measured in hours minutes and seconds of time. The reason for this is the sky turns once a day to the west as the earth rotates to the east. The celestial sphere moves one hour of RA west per hour of time and 24 hours of RA during the course of the whole day. Since this is a 360-degree rotation, one hour of RA is equal to 15 degrees of turning (360/24 = 15). Just like lines of longitude, RA lines are also great circles converging on the north and south celestial poles.

Longitude has the Greenwich meridian as the zero line dividing east and west. On the sky, the zero meridian in RA is labeled 00h00m00s. It intersects the celestial equator at a point called the vernal equinox (where the sun crosses the celestial equator in late March of each year).

Measurements north and south on the sky are called declinations (commonly abbreviated as Dec, DEC or dec). Just like latitude, declination is measured in degrees, minutes and seconds north (positive) and south (negative), with 60 minutes in each degree and 60 seconds in each minute of declination.

The celestial equator is 0 degrees declination, and the north and south celestial poles are +90 and -90 degrees. The North Star Polaris is nearly at the north celestial pole at +89.2 degrees. The celestial equator is a full 360-degree circle splitting the celestial sphere into the northern and southern celestial hemispheres or simply the northern and southern sky. It’s the projection of our equator in space. It is directly overhead at the earth’s equator.

You can use a star’s declination to figure out how high it will get in the sky. The star Vega has a declination of +39 degrees, so it passes directly overhead at north latitude 39 degrees on the earth (approximately the latitude of Denver).  At 47 degrees north latitude (approx. Seattle or Vancouver), Vega will never reach 90 degrees altitude, but will peak out eight (47-39) degrees south of the zenith.