Field Flatteners

Field flatteners are used to correct for the curved focal plane that most telescopes have. This is caused by simple geometry. The distance from the lens to the focal plane is greater at the edges of a flat field, so stars actually focus in front of the sensor at the edges of the field.

Mouse-Over

Hold your mouse cursor over the image to see what the stars look like in one corner of the field with the curved focal plane of the Stellarvue SV70ED 70mm f/6 doublet refractor compared to what they look like when the scope is used with the Stellarvue 0x Field Flattener.

This can be a fairly serious problem on doublet, and even triplet, refractors, and the problem gets worse as the focal ratio gets faster. Stars at the corners of an uncorrected field will look like out-of-focus ellipses.

Some optical designs, such as the Petzval system, use four optical elements with two at the objective end, and two more in a group at the focuser end of the scope to correct for problems like field curvature. These kinds of designs do not need a field flattener and can have a fairly fast focal ratio.

Most field flatteners don't change the focal length or focal ratio of the optical system. Some telecompressors, which do change these parameters, are also designed to be field flatteners.

Barlows, Tele-Extenders and Tele-Converters

Barlows, tele-extenders and tele-converters are usually a negative optical element or group of elements that extend the focal length of the optical system and increase the magnification. They also increase the focal ratio and make the system slower photographically.

For example, a 2x barlow will make a 500mm f/5.6 optical system become a 1,000mm f/11 system. Remember, when the aperture stays the same, and the focal length increases, the focal ratio gets slower. In this case, when the focal length doubled, the focal ratio became two stops slower. It will require 4 times as much exposure at f/11 as it did at f/5.6.

Tele-converters are usually sold for camera lenses. These devices usually have electrical contacts that allow the lens to communicate with the camera body for functions such as autofocus, and aperture settings. When a lens tele-converter is used on a telescope, the camera may not fire or work correctly because it is expecting information from a camera lens, and telescopes do not have the circuitry built into them to provide this type of information. In these cases, the tele-converter can sometimes be used if you put tape over the electrical contacts.

Focal Reducers and Tele-Compressors

William Optics 0.8x Reducer (left) Astro-Tech 0x Field Flattener (right)

Focal reducers and tele-compressors are a positive optical element or group that reduces the focal length and magnification. They also decrease the focal ratio, making the system faster and field of view wider.

For example, a 0.8x tele-compressor will make a 500mm f/6 optical system into a 400mm f/4.8 system. It will require 3/4 of a stop less exposure. So if you needed a 4 minute exposure at f/6, you would only need a 3 minute exposure at f/4.8.

Because Schmidt Cassegrains are slow photographically at f/10, many astrophotographers try to use a tele-compressor to reduce the focal ratio to f/6.3, but they find that this results in vignetting. More moderate reducers, such as 0.8x models, used on refractors usually do not suffer from this problem.

You can not use a tele-compressor designed for a Schmidt Cassegrain optical system on a refractor.

Coma Correctors

Coma correctors are usually used on fast Newtonian telescopes to correct for the aberration of coma where stars take the shape of comets at the edges of the field.

Coma gets worse as the optical system gets faster. Coma is usually well controlled by the time the focal ratio is about f/8, but at faster focal ratios, such as f/4, a coma corrector is needed for astrophotography through a Newtonian telescope.

Optical Accessories - The Bottom Line There are several optical accessories that can greatly improve the optical performance of your telescope for astrophotography. A field flattener helps to improve focus in the corners of the frame because most scopes have a curved focal plane. A telecompressor can make the focal length of your scope shorter and the field wider. A coma corrector can reduce the aberration of coma in fast Newtonian reflector telescopes.