[gaston (Gaston)]

Hi David,

An on axis guider (ONAG) allows to guide with the same field of view than your imager. Like an off axis guider (OAG) this solves differential flexure problems as well as the the guide scope room and extra load for the mount. However OAG are limited to a narrow donut's like shape field of view way off axis of the main ship. Also an OAG uses small prism to pick up the starlight leading to large F-numbers for the guider.
Most of the time it is challenging to find a guide star this way and most likely it will require to rotate the all camera body to reach the goal, which means you have to take new flat frames all over again each time you do so. An ONAG does not have those limitations.

Innovations Foresight (www.innovationsforesight.com) offers an ONAG, which is associated with an integrated X/Y stage, provides an easy access to your scope field of view by the guider camera. It can use on-axis as well as off axis guide stars, for a scope of 2m focal length the ONAG filed of view is in a range of 1.3 arc-degree.

The ONAG works by splitting the light between the visible range (<750nm) and the near infrared (NIR) range (>750nm). This is done with a dichroic beam splitter, known as a "cold mirror". The visible range is reflected for imaging, pretty much like a star diagonal would do, this insures perfect images, without any distortion, while the NIR light goes through toward the guider camera.
Monochrome, unfiltered CCD/CMOS camera are quite sensitive in NIR, this is why we need UV_NIR blocking filters for imaging with those.

Clear skies!

Happy new year 2012