Finderscope
A finderscope (also called finderscope or finderscope ) is a small telescope with low magnification that is mounted parallel to a larger one - the main telescope. It makes it easier to find an object in the main telescope because of its weaker magnification and the resulting larger viewing angle .
application
Its optical axis is set parallel to that of the main pipe by means of adjusting screws . In the beam path of the viewfinder there is usually a crosshair that facilitates this adjustment and the centric setting of the star. Most finders are of the Kepler telescope type , so their image is upside down. For astronomical purposes, this does not interfere and avoids light loss through reversing systems . An Amici prism , which creates an upright and laterally correct image and enables a 90 ° view, facilitates orientation in the sky.
enlargement
On finderscopes (as on all binoculars or binoculars) there is information on magnification and aperture. 6 × 30 means six times the magnification and 30 mm lens aperture. Finderscopes with 5 × 25 or 6 × 30 are common, but rather faint; better results are obtained with an 8 × 50 telescope or more. At magnifications from 10 ×, however, it becomes more and more difficult to find objects, since the field of view becomes correspondingly smaller, while a larger opening has no disadvantages (apart from higher price and weight). The advantage of a larger opening is the better ability to collect light and thus easier to find faint objects. In telescopes for amateur astronomers , four to a maximum of nine times magnification is therefore common.
Attachment
Simple amateur telescopes usually only have an unstable viewfinder. A stable attachment to the telescope is a prerequisite for problem-free use of the viewfinder. To avoid transport damage, the viewfinder should be attached to an adapter rail attached to the telescope with a quick release fastener . Alternatively, another viewfinder or camera can be mounted on this adapter.
distribution
For a long time now, almost all astronomical telescopes have been equipped with setting circles or increasingly with digital control , so that the viewfinder no longer has to be used as often as it used to be. For small telescopes or those with low magnification (e.g. for satellite observation ) even a sight in the sense of a rear sight and front sight is sufficient .
There are also telescopes with which the magnification can be switched over for searching using a folding mirror. An eyepiece revolver with interchangeable eyepieces can perform a similar function .
Alternatives
There are additional setting aids especially for astronomical observations in amateur astronomy :
- The Telrad viewfinder (own page)
- The "Rigel-Quick-Finder", a device similar to the Telrad, but with only two instead of three pitch circles. In contrast to the Telrad finder, the Quick Finder is far less sensitive to dew, the pitch circles have an infinitely adjustable pulsation function. Due to its low weight, it is particularly suitable for telescopes that require a good balance (e.g. Dobsonian telescopes).
- The red dot finder is identical to the reflex sight , a device that throws a point onto a specially coated glass surface. Thanks to the coating, the red of the illuminated dot is easy to see. Due to the parallel beam path of the light source (usually an LED whose light is guided through a glass fiber ) the eye does not have to focus on the very close point, but sees the star through the "peephole" just as sharply as the luminous point. This type of setting aid is only helpful for objects that are also visible to the naked eye. The red dot finder is very precise after a (one-time) adjustment.
Special red dot finders, such as the "PDA finder", can optionally also display target circles or target crosses.
- Spirit level with goto mount : With the spirit level, the telescope is aimed at the zenith so that the synchronization of the goto mount is sufficiently accurate to hit a star safely with the main telescope or camera.