The Opticstar OS-PVR is a F 0.5 focal reducer that will fit at the end of standard 1.25" eyepieces/nose pieces. The focal reducer, reduces the focal length of the main optics to half thus doubling the field of view. This is useful in visual observation and photography.
The focal reducer can also be used for imaging to reduce the focal length of the telescope to F0.5 or F0.33 and in conjunction with an extension, i.e. camera nosepiece. When used photographically the focal reduction will vary depending on the distance from focal reducer optics to the camera sensor.
Nosepiece extensions can only be used with nosepieces of very short lengths, these will further reduce a telescope's focal length but focusing may not be possible. This depends on the focal ratio of the telescope, focuser travel and length of the extension.
The Opticstar focal reducer consists of a two lens cemented system for better correction, optics are fully multi-coated. The two lens system sits is a metal case that is threaded for filters. This is an improved version with green coatings.
When attached to a telescope the f0.5 focal reducer will typically reduce exposure times to near a quarter when imaging extended objects like galaxies, nebulae and the Moon. Telescopes with low focal ratios require shorter exposures when imaging extended objects (for example an f/8 system will require four times the exposure of an f/4 system). This does not apply to non extended objects (point sources) like stars where aperture dictates what can be seen/captured.