The majority of refracting telescopes have straight through optics and the physical length of the telescope is dependent on the focal length. This helps when the objective lens is acromatic rather than apochromatic. With an acromatic objective, the red and blue light are brought to the ‘same’ focus whereas with an apochromatic objective all red blue and green light are brought to the ‘same’ focus, reducing or even eliminating chromatic aberration. With a long focal length, an acromat brings the different wavelengths of light to a closer focus than with a short focal length.
Other types of telescopes such as Schmidt Cassegrains, Maksutov Cassegrains and a number of other types of telescopes use folded optics to shorten the physical length of a scope of long focal length. There are some folded optics design telescopes that are not intended to have an eyepiece, but which are designed to have a camera at the position normally occupied by the secondary mirror in say, a Schmidt Cassegrain. Such a scope is the recent Sky-Watcher HAC125 DX Minigraph which is very fast at f/2 and F=250mm and is very suitable for Electronically Assisted Astronomy, EAA.
However, we are dealing here with a class of refractors that have folded optics and we shall use 4 examples:
Example 1
The most familiar example of folded optics refractors is Porro prism binoculars.
The folded optics using Porro prisms shortens the length of the binoculars as well as placing the objectives further apart, enhancing the 3D viewing experience.
Although historically there have been a number of folded optics telescope made by scope manufacturers such as Unitron and amateur astronomers such as Dave Trott, we shall only look at a selected few:
Example 2
The Zerochromat Refracting Telescope
Peter Wise, from Pensarn, Conwy, Wales, UK, designed and made the 10" Zerochromat Refracting Telescope for the Custer Institute and Observatory in Long Island, New York State. It is Located in the main observatory dome and is the largest of its kind in the United States. Designed by award-winning optician Peter Wise and manufactured in Pensarn. Its dialyte lenses make it apochromatic. A dialyte lens is a compound optical lens where the individual lens elements are separated by a significant air space, allowing for the correction of optical aberrations like chromatic aberration. This air-spaced design offers more "refractive surfaces" for correcting distortions.
Peter Wise demonstrating his 8" Zerochromat folded optics refractor to the Swansea Astronomical Society meeting in Swansea University, UK on March 14, 2014.
Some of Peter Wise's publications:
The Dialyte refractor revisited. Journal of the British Astronomical Association. vol.127, 6, p.350-353, 2017
The retrofocally corrected apochromatic dialyte refracting telescope Wall, J. & Wise, P. Journal of the British Astronomical Association, vol.117, 1, p.29-34, 2007
Example 3
This example is a beautiful folded optics refractor made by Berger Astrogeräte.
The mechanical part was made and designed by Andreas Berger. The optical path was designed by Dr. Georg Dittié. Georg Dittié is the programmer of the program "Giotto".
The photographs of the telescope were provided by Andreas Berger.
This telescope is of a very compact design and demonstrates how it is possible to build a refractor into a small space by the use of folded optics.
Example 4
The Seestar S50 smart telescope by ZWO
The telescope is controlled by the Seestar app on an Android tablet/phone; or an iPad/iPhone.
It has a 50mm objective and a focal length of 250mm. The camera sensor is a Sony IMX462. This produces an image with a field of view of 0.73° x 1.29°.
The Seestar S50 in AZ mode
The Seestar S50 in EQ mode
Zwo have not published schematics for the Seestar but the closest they come is a promotional image representing the interior with an exploded view of the apochromatic triplet objective. This picture is derived from that image:
The folded optics are clearly visible showing the light rays converging and being reflected onto the camera sensor. The folded optics are the reason why Seestars are such compact telescopes. The design, whilst not being directly related, is very reminiscent of Andreas Berger's telescope.
It would be very interesting to see astronomical images produced by folded optics refractors other than the smart telescopes.
