Everything about The Gregorian Telescope totally explained
The
Gregorian telescope is a type of
reflecting telescope designed by
Scottish mathematician and
astronomer,
James Gregory in the
17th century.
He described the design in his
1663 publication
Optica Promota (The Advance of Optics). Early attempts to build a Gregorian telescope failed, and it wasn't until ten years later, aided by the interest of experimental scientist
Robert Hooke, that a working instrument was actually constructed. Gregory's design pre-dates the familiar form of reflector which Sir
Isaac Newton first designed and made around
1670.
The Gregorian telescope consists of two concave
mirrors; the primary mirror (a concave
paraboloid) collects the light and brings it to a
focus before the secondary mirror (a concave
ellipsoid) where it's reflected back through a hole in the centre of the primary, and thence out the bottom end of the instrument where it can be viewed with the aid of the
eyepiece. This design of telescope renders an upright image, making it useful for terrestrial observations.
The Gregorian design has the advantage over
Newton's design, which brings the light to a focus
after the secondary mirror, in that it requires a shorter tube for the same
focal length. The design was largely superseded by the
Cassegrain telescope. It is still used for some
spotting scopes because this design creates an erect image without the need for prisms.
In the Gregorian design, the primary mirror creates an actual image before the secondary mirror. This allows for a field stop to be placed at this location, so that the light from outside the field of view doesn't reach the secondary mirror. This is a major advantage for solar telescopes, where a field stop (Gregorian stop) can reduce the amount of heat reaching the secondary mirror and subsequent optical components. The Solar Optical Telescope on the
Hinode satellite is one example of this design.
For
amateur telescope makers the Gregorian can be less difficult to fabricate than a Cassegrain because the concave secondary is
Foucault testable like the primary, which isn't the case with the Cassegrain's convex secondary.
Further Information
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