Off-axis guider

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OAG with astro camera (below)
Digital reflex camera and CCD camera at the OAG

An off-axis guider (about tracking outside the (main) axis , OAG for short) is an aid in observational astronomy that enables the precise tracking of stars. The aim of an OAG is to follow a guide star , with the help of which the errors in the tracking movement of the telescope are compensated. This is mainly necessary for astrophotography , as exposure is often done with times that are in the range of minutes or even hours. Errors in tracking would result in blurring of the image.

functionality

With an OAG, part of the image generated by a telescope is deflected to the side at the exit opening of the telescope and made available there for further processing. A small mirror or prism protrudes into the beam path of the telescope. A small section of the overall picture is enough; there just has to be at least one star in it. In order to increase the chances of finding a star, the prism is rotatably mounted around the longitudinal axis of the telescope and can use an angle (limited by type); approx. 100 ° are common. With some models, the prism can also be tilted from the outside using a pin. This also makes it possible to find positions that are closer to the center of the longitudinal axis, which in turn facilitates the search for a guide star. In addition, depending on the model, the distance to the image plane can also be changed in order to achieve the same focus position of the tracking and main camera.

Image processing

The image brought out laterally can now either be checked with a crosshair eyepiece and by observation, or a camera is attached to the connection instead of an eyepiece . This can be a converted webcam or a special video camera for astrophotography. She forwards the digitized image to a computer. In this a program evaluates the image and tracks the movement of the star and intervenes correctively in its tracking movement via a return line to the electrically controlled mount . In amateur astronomy, the return line is either a line directly from the computer to control the mount or a second line that starts at the tracking camera and leads from there into a special connection on the mount, which is called the autoguider port .

The rest and most of the original image is not influenced by the small deflecting mirror or prism and can be recorded by a photo camera. With large cameras (e.g. in small format ), however, the small mirror of the OAG can lead to shadowing if it is placed in the corners of the camera.

Consideration for use

advantages

  • The installation of a guide scope is avoided. An additional guide scope also means a higher load on the mount, which in turn reduces its accuracy.
  • Less material has to be purchased and, in the case of field use, also transported.
  • You look through the same optics that are used to take pictures.

disadvantage

  • The search for a suitable guide star is often difficult, since sometimes no suitable object can be found in the edge area that the deflecting mirror can sweep in regions with few stars.
  • The maximum amount of light that can be extracted is small; the star must therefore still be bright enough that it can be evaluated.
  • The object that is also being photographed cannot be tracked.
  • At the edge of the beam path, the image quality is often much worse than in the center of the image, so that actually point-like objects are distorted and are no longer suitable for tracking by an evaluation program on the computer.
  • At the edge of the beam path, the general light intensity is often much lower than in the center of the image (see vignetting ).

See also

Web links