The Borg 100ED is a dual focal length astrophotography instrument. With its huge helical focuser offering 95mm of clear aperture, I use it for medium format work, but with additional adapters it can mount 35mm cameras as well. At its native speed of f/6.4, it offers an FOV similar to a 320mm lens in 35mm format, but with the advantage of producing huge 6x7 slides and negatives. In its f/4 configuration, the scope gives a view similar to a 200mm lens in 35mm. I chose the 100ED model from Borg's product line because it offered focal lengths comparable to my 300mm and 180mm lenses in 35mm format, and because the light falloff across a 6x7 frame was less than in their 125ED and 150ED scopes.
The switch between f/6.4 and f/4 is made by unscrewing the modular components of the scope - objective, dew shield, finder base ring, and focuser - from whichever tube length is being used and installing them on a second tube. At f/6.4, the scope uses a field flattener that is mounted to the Pentax 67 camera mount. This flattener is removed when switching to f/4, and a four-element "super reducer" is installed inside the focuser in it's place. It's pretty simple to do, but because I work mostly in primitive locations, I got worried about dust and other issues associated with making this change in the field. To make the switch less of a job, I purchased a second finder base ring, focuser, and Pentax 67 mount - now all I have to do is move the objective and dew shield from one tube to the other to change focal lengths.
The tube diameter is 115mm, close enough to use the excellent Takahashi tube holder from their FS102 scope. I had a spare doveplate left over from my original Epsilon mounting setup, so I had this plate drilled and tapped to mount the Tak tube holder. At first, I worried about flexure in the longer f/6.4 configuration and had the plate drilled and tapped to take two Tak holders in tandem, but in practice I haven't seen any flexure and continue to use a single tube holder without trouble. At f/4, there's just room for one holder, and the short scope is very happy mounted this way. At f/6.4, the fully assembled scope with a Pentax rear lens cap covering the end of the focuser just fits into a Pelican 1650 case, with room for the f/4 setup next to it, so the scope can be stored in either configuration in one well-padded, watertight, dustproof case. All in all it's a great setup for my style of astrophotography.
In the photo below, the Borg is in its f/6.4 configuration, and another Pentax 67 camera is mounted under the front of the doveplate for simultaneous wide-field imaging with the excellent Pentax 200mm 67 lens. This plate is mounted next to the trusty Epsilon, which is also carrying the Tak FC60 guidescope and STV. Even with all of this gear, the Losmandy G-11 mount is still well within its capacity and tracks well - balancing all of this stuff takes some practice, however. The photo was taken at Chiricahua National Monument in SE Arizona in April of 2001.
Borg 100ED Specifications:
Aperture: 100mm (about 4")
Focal Ratio: f/6.4 or f/4 with super reducer lens group
Focal Length: 640mm or 400mm
Field of View: Approximately 5.0 x 5.9 degrees or 8.0 x 9.4 degrees on 120 film