Focuser
An eyepiece extractor (OAZ) is a device on an optical instrument ( telescope , microscope, etc.) that is used to generate a sharp image.
description
The focuser is attached to the instrument or is permanently installed and takes at its open end the eyepiece or a photographic or photometric device such as a (nowadays usually digital) camera , a webcam , a spectrometer or the like. One or more knurled screws on the focuser shift its position along the optical axis (the longitudinal axis of the light path) until the position of the sharp image is found. For a more precise setting, some focusers also have a reduction. Depending on the type of telescope, there are different types of focuser:
- as the main extension that covers the entire focusing path
- as an auxiliary extension to enable high-precision focusing. E.g. on a Schmidt-Cassegrain telescope in which the main focus position is achieved by moving the main mirror .
Main extensions are generally much longer (in the lower picture over 8 cm) than auxiliary extensions; these are partly only 2.5 cm long (upper picture).
species
Rack extension
Rack pull-out is an easy way to shift focus. The problem here is the play that the drive gear has with the rack. It is often counterbalanced by a layer of sticky fat that dampens the movement of the parts.
Crayford extract
The Crayford extract was developed by John Wall in Crayford in 1971 . Its drive is based on a smooth steel shaft which is pressed onto a rough metal surface on the drawer or directly onto the drawer. The small diameter of the shaft enables very precise adjustment. In addition, a second rotary control is often attached to one side , which enables a reduction to approx. 1:10 via a planetary gear and thus enables even more precise setting. Crayford pull-outs are available with 2-inch, 2.5-inch and 3-inch receptacles and corresponding reducers down to 1.25 inch eyepiece plug-in dimensions .