Telrad viewfinder
The Telrad viewfinder is a reflex sight that is used in astronomy - alone or as a supplement to the optical viewfinder - to align a telescope .
In contrast to the finderscope, or the telescope itself, the Telrad finder does not show an enlarged image. This has the disadvantage that only the same objects are visible as with the naked eye, but has the advantage that the image in the viewfinder fits into the overall view of the sky instead of just showing a section of the sky view. This makes a rough alignment of the telescope with the Telrad viewfinder very intuitive.
In order to find the exact position of an observation object, sky maps can also be used, on which Telrad circles are imprinted in addition to interesting objects (galaxies, star clusters, nebulae) .
Search example
For example, the Orion Nebula in the constellation Orion is to be found. To do this, the telescope, including the viewfinder, is first set to a well-known, well-visible bright star near this sought-after object. For example, the telescope is swiveled so that the star Rigel is in the center of the glowing red circular image that appears to be projected onto the sky. Now you can read from the appropriate map where the Orion Nebula is in the printed Telrad circle image around Rigel. Finally, the telescope is swiveled so that the new center point of the viewfinder image points to this position in the previous viewfinder image. The telescope then points exactly at the object it is looking for, provided the viewfinder was correctly adjusted to the telescope on which it is mounted.
construction
The circular image described consists of three concentric target circles which are projected from below onto a glass pane aligned at a 45 ° angle using LEDs and optics. If you look through the inclined pane from behind, the luminous image appears to be reflected in the sky at infinity; refocusing of the eye is therefore not necessary. The circles appear, regardless of the distance of the observer, with a diameter of 0.5 °, 2 ° and 4 ° in the sky, which is very precise for finding an object.
However, the Telrad finder is prone to dew . A dew cap, which also includes a mirror for a 90 ° view of objects close to the zenith , is optionally available.
In the elongated box below in front of the glass plate there is a battery holder for two AA batteries (Mignon), the LED including optics and a mirror.
On the picture that shows the narrow side of the Telrad viewfinder, you can see the mirrored target circles above, below it a lens and on the front of the housing three manually operated adjustment screws, which are used for the precise adjustment of the viewfinder to the telescope. On the right side you can see the on-switch with which you can also regulate the brightness of the target circles.
The dimensions are: 21.5 cm (length), 12 cm (height without mounting plate) and 7 cm (width including switch)
The device was developed by Steve Kufeld in the USA in the 1970s. A target device for aircraft from World War II served as the basis.