Telescope – Frequently Asked Questions (FAQs)

Refraction of different colour components of white light by different amounts. This causes white light to split into a rainbow and produces false colour fringing.

Achromatic refractor telescopes that have basic quality glass and only one or two elements in the objective group can produce chromatic aberration. This means that a beam of white light coming through the lens will be split into a rainbow, which means that the different colours will focus at different distances from the lens. From the point of view of the camera sensor, a star imaged through an achromatic telescope will have a false colour fringe around it, often red or blue.

Apochromatic telescopes are made to resist chromatic aberration. They may do this by adding lenses to ensure that the different colour components of light focus at the same point, or they may do this by using extra low dispersion glass that produces less dispersion in the first place. High quality apochromatic telescopes will use both strategies.

The aperture or diameter of the primary lens or mirror of a telescope determines the light gathering capacity of the telescope.

In Newtonian reflector telescopes that have paraboloidal mirrors, off-axis light (that is, from a point near the edge of the image) will come to focus slightly behind the focal plane, and this image will also not form a point. This can be managed using a correcting lens (coma corrector) near the focuser.

In Newtonian reflector telescopes, off-axis light can form a coma (see coma). This can be corrected using a lens placed near the focuser.

The distance between the lens or mirror and the point where the focused image is formed. This determines the magnification of the telescope.

Mathematically, this is the focal length of the telescope divided by the aperture. It determines how bright the image will be. A low ratio (such as 4.0) indicates a relatively large aperture and therefore a brighter image. A high ratio (such as 14.0) indicates higher magnification and therefore a relatively dimmer image. This is sometimes known as the “speed” of the telescope.

This is the invisible surface starting at the focal point of a lens or mirror where the image (normally of an infinitely distant object) is sharp. The focal plane is perpendicular to the optical axis. In most cases the focal plane is a curve rather than a flat plain, which is why a field flattener is required for photographic work (assuming the sensor on the camera is flat).

A mechanism where a camera or eyepiece can be moved closer to or further from the objective lens or mirror. This means the camera sensor can be placed precisely on the focal plane of the objective. Some focusers can move the objective lens in a refractor or primary mirror in a Schmidt-Cassegrain or Ritchey-Chrétien to move the focal plane, having the same effect.

A lens that reduces the focal length (and hence focal ratio) of a telescope. A reduced focal length telescope is brighter and has a wider field of view than the original. This can be useful in astrophotography.

See focal ratio A focal ratio of 2-4 would be considered “fast”, or bright. A focal ratio of 10-14 would be considered “slow”, or dim.

Equatorial mounts are built to track stars as they move. At some point in the night, stars cross an imaginary north-south line called the meridian. When the star is in the eastern half of the sky, the telescope is on the western side of the tripod, the counterweights are on the eastern side, and the counterweights are rising as the mount is tracking. When the star crosses the meridian, the counterweight is level. At this point, mounts, particularly those carrying long refractors, begin to risk crashing the back end of the telescope tube into one of the tripod legs. This is A Bad Thing That You Want To Avoid.

In order to avoid this, the mount has to flip itself around so the telescope is on the eastern side of the tripod and the counterweight is on the west. To continue to track the star, the counterweight will get lower, and the back end of the telescope will start to move away from the tripod leg.

Most software packages can automate the meridian flip, although it’s a bit of a hit and miss affair when it comes to staying on the target. However, if your astrophotography rig is capable of plate solving, it becomes highly accurate.

This is a way of aligning the internal star map in your go-to mount with the actual stars. Once this is done, your mount will be able to slew to any target above the horizon. Sky alignment is a different process to polar alignment.