Check out the Sky & Telescope article on the Borg astrographs
One of the fundamental cornerstones of Borg's scope designs has been excellence in performance as astrographs - high-performance objectives combined with uniquely high-performance flattener/reducer elements, specially designed for imaging.
This astrograph utilizes the new Borg 4-element ED super reducer, which is optimized for digital imagers, yet covers a full 35mm-sized frame. The astrograph also includes built-in filter holders as well as a camera rotator for flexible image composition. Borg helical focuser is standard, but motorizable Feather Touch focusers are an option.
Focal reducers are optimized for specific distances between the optical components and the image plane. For the F4 focal reducer, these are accomplished through the use of calibrated spacers for specific focal lengths. Two 63mm diameter spacers are provided for the front (between focuser and front reducer lens assembly), and two 58mm diameter spacers for the rear (between the rear reducer lens assembly and the camera adapter).
Provision for the attachment of filters is via a a 55mm diameter filter thread on the front reducer lens assembly (for semi-permanent filter installation) and via an easily removed filter holders which accommodate 52mm or 48mm filters. A camera shutter cable can be attached to the focal reducer assembly to make removal of the filter holder a simple operation which does not disturb the optical train. Additional filter holders are available for quick changes between filters.
The ED77II Package includes the following components: 77EDII Objective Assembly (2178), Tube (135mm long) and Drawtube Set for Series 80 OTA (7135), Drawtube Holder (7753), Helical Focuser M – M68.8 (7835), and F4 ED Super Reducer Set (7704).