Telescopes are normally designed to focus an image in a curved plane. This is because you normally use them with a small eyepiece. A camera lens is designed to focus an image on a much larger flat plane. This is because the film or sensor is a flat rectangular or square shape. If you were to attach a camera to a telescope and take a photograph, the resulting image would only be in focus in the centre, the edges would be blurred.
In order to prevent this, the image from the telescope must be changed so it is in focus across all of the film or sensor. To do this you add a correction lens. These come in three different types as follows;
Field Flattener. This is the most common. This does not alter the image magnification, angle of view or focal ratio of the lens (F number). It just flattens the image plane to alow use with a camera.
Reducer. These are a special type of corrector. In addition to providing a flat image plane they have the effect of reducing the focal length of the lens and increasing the angle of view. The by product of doing this is that they also reduce the focal ratio. Typically a reducer will change the focal length from 0.63 to 0.75x. Therefore a 1000mm / F10 telescope with a 0.63 reducer would become a 630mm / F6.3 telescope. Because of the critical relationship with the main optics, reducers are made for specific telescopes. Please ask before ordering.
Barlow. This is the opposite of a reducer. In the photographic world it would be called a teleconverter. It magnifies the image in addition to providing a flat field. Note that Barlows are available for eyepieces which do not have flat fields. As with a Reducer, a Barlow will have an effect on the focal length and F number. A 1000mm / F10 telescope with a 2x Barlow will become a 2000mm / F20. Because of the huge reduction in light gathering due to the small F number, Barlows are only really useful when used to image bright objects like the Sun, Moon and Planets. They also require very accurate guiding and optical quality because of the magnification effect. Because of this the only Barlows we offer are the world's best. Unlike Reducers, a Barlow can be used with most telescope tyoes.
Tele-Centric. This is a magnifying lens like a barlow. However a correctly designed tele-centric outputs a parallel beam rather than a diverging one. This property makes tele-centrics deal for use with solar filters which need parallel light to pass through them.
Tele-Compressor. This is a special type of reducer for use with tele-centrics. A normal reducer is expecting to see a converging light cone so when used with a tele-centric distortion occurs. Tele-Compressors avoid this problem.
Dispersion Corrector. Atmospheric dispersion changes the size, shape and location of objects according to their wavelength. Blue wavelengths being much more affected than red. A dispersion corrector compensates for this.