Crosshair eyepiece

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The crosshair eyepiece is a special form of the eyepiece , as it is used, for example, in observing astronomy on telescopes . It is used to sight and hold an object in the middle. Its reticle in the form of a crosshair is characteristic .


There are crosshair eyepieces with single crosshairs and with double crosshairs or with various setting and measuring aids. Many crosshair eyepieces can also be illuminated.


Two crosshair eyepieces (illuminated on the left)
Double crosshairs, illuminated

Outwardly, a crosshair eyepiece hardly differs from an ordinary eyepiece. Its other optical values ​​such as focal length and field of view are also different like those of normal eyepieces. In addition, with the crosshair eyepiece, a crosshair made of wire is stretched in front of the field-side lens or it is scratched or etched in an additional glass surface . In order to be able to see these crosshairs clearly, the eye lens of the eyepiece can be adjusted; their distance to the rest of the eyepiece lenses can be changed by turning. However, this also changes the overall sharpness, which must then be corrected again. Since many crosshair eyepieces can be illuminated , they also have a lateral hole and a thread.


Small guide scope with illuminated crosshair eyepiece
Markings in a polar finder

In astronomy, crosshair eyepieces are used either to position the target area exactly in the center of the image with low magnification; in this case, the object does not disappear from the field of view at higher magnification. Or crosshair eyepieces are used on a second telescope, a so-called guide scope . While photographic recordings are often made on the main instrument, an attempt is made to hold a star in the center near the object of the main pipe using the guide scope. With an eyepiece with double crosshairs, there is a small, square field in the middle; Such a guiding star can be held there and its drifting is quickly noticeable, as the brightness of the star behind the wire drops sharply. Since the star in the exact center would be covered by the wire crossing with a simple crosshair, the eyepieces with double threads are more practical in astronomy. If you have to center a star on an eyepiece with a simple crosshair, you adjust the focus to a slight blurring. As a result, the star's diameter increases to a small disk and its light shines past both threads at the intersection when ideally congruent.

The eyepiece is usually rotated at its base so that one of the threads (or one of the pairs of threads) is aligned parallel to the direction of movement of the sky. If the crosshairs are aligned, small errors in the transverse axis can be easily detected and corrected.

A special form of the crosshair eyepiece can be found in the polar finder . A ring has been inserted here instead of a cross. There is a marking on the radius of this ring. The polar star is positioned in this position by setting the polar height and azimuth , and then the right ascension axis of the telescope is set up very precisely on the celestial north pole.


Lighting, individual parts

The crosshair is illuminated through a slot on the side that is positioned at the level of the wires. The lighting is usually in the form of a kind of tiny flashlight, which can be removed and screwed onto the slot. Inside this lighting part there is a red LED at the front , behind it the batteries (button cells) and at the back a switch with brightness control. The red color of the LED prevents the sensitivity of the eyes from being reduced.

However, the setting range for the brightness is often too small, with the result that the lighting is still too bright even in the weakest setting. Then changing the supply voltage down helps. For example, three button cells with a voltage of 1.5 volts each are installed in the lamp shown in the picture . However, if you only use two button cells, which are slightly thicker, the maximum voltage is reduced from 4.5 volts to 3 volts, which extends the brightness setting range downwards.

See also