The Crescent Moon with Earthshine goes past the Pleiades. Taken with a Canon 1D Mark II DSLR Camera through a 102mm aperture f/8 refractor working with a telecompressor to make the optical system have a focal length of 612mm at f/6. The exposure was a composite of a 2 second exposure for the Moon at ISO 400, and a 16 second exposure for the Pleiades both at ISO 400.

This photo was taken with Takahashi FS 102 Fluorite Doublet Apochromatic refractor generously lent by Dan McCauley, a friend. At prime focus, it normally has an aperture of 102mm at f/8, making the focal length 820mm. For this photo, a Tele Vue telecompressor made the system f/6, with 612mm of focal length.

A tele-compressor is a positive optical element or group that is inserted into the optical system, usually just before the camera, that shortens the focal length and reduces the focal ratio, making the system faster and field of view wider.

For example, a 0.75x tele-compressor will make a 1,000mm f/8 optical system f/6 with a focal length of 750mm. This increase in system speed will require exposures just 1/2 as long, can help record faint objects with a shorter exposure. The wider field of view can allow larger celestial objects to be photographed.

Globular Cluster M13 in Hercules was shot with a refractor with 130mm of aperture and a focal length of 1040mm at f/8. This image was made with a 2x Barlow, increasing the focal length to 2080mm and the f/ratio to f/16.

A tele-extender or teleconverter (or Barlow) is a negative optical element or group that is inserted into the optical system, again, usually just before the camera, that lengthens the focal length and increases the focal ratio, making the system slower with a smaller field of view.

For example, a 2x tele-extender will make a 1,000mm f/8 optical system f/16 with a focal length of 2,000mm. This system will require exposures 4 times as long and the increased magnification will increase the image scale.

A 1.4x teleconverter will make the system one stop slower and increase the focal length by a factor of 1.4x. For example, a 400mm lens f/2.8 lens will become a 560mm and f/4 with a 1.4x teleconverter and will require twice as much exposure.