WVAS started discussions for a new project in the fall of 1998. After months of discussions and presentations, we decided to build an 8-inch binocular telescope.
Three major issues of design came up: The first was portability. Two 8" optical tube assemblies mounted permanently together would be quite bulky. Using two separate tubes that would be separated between uses and reattached for use might make the alignment problem (second issue) more difficult. We decided to make the tubes in sections with only the rear parts permanently attached in a box frame. The upper section includes a truss tube section to increase portability and decrease the needed storage space.
The second issue was alignment: With a binocular scope there has to be a way to adjust the distance between the eyepieces. The individual scopes are Newtonian reflectors, so when the light cone comes out of the side of the tube it is bounced off a star diagonal (the "tertiary") so that the light cones and eyepieces are parallel for viewing. With this set-up, the easiest way to vary the inter-ocular distance is to use a focuser at the normal Newtonian position to vary the distance from the telescope tube to the tertiary and eyepiece. The problem with that is that it changes the distance from the primary mirror to the eyepiece, so it changes the focus. Another focuser is placed after the teriary and holds the eyepiece. We wanted to build a scope that could be used at public events, and it has been our experience that the general public is often very timid about touching the controls on a scope, so having to adjust the first two focusers and then make a significant change with the other two focusers may be too much "fussing" for the general public.
Another option is to change the inter-ocular distance by moving one or both telescope tubes. The difficulty with this is that with the magnification, the tubes need to maintain their alignment to within about 1 arcminute (at high power, and especially in the vertical direction) for the eyes and brain to combine the two images into one. This is difficult to do given the size and weight of the full tubes. Several methods to do this have been tried by other people, and it is generally regarded as one of the most difficult parts of building such a telescope. We ultimately decided to suspend the lower sections of the tubes from pivots so the tubes would swing out away from each other to the needed inter-ocular distance (much like conventional binoculars, except that each tube will have its own pivot).
The third issue involved optics: As mentioned above, an extra diagonal is needed to aim the light cones and eyepieces in a parallel direction. This means more of the light path is past the secondary, so a larger secondary is required, and that will degrade the image. A possible cure for this is to install a Barlow lens in the side of the tube where the light cone exits the tube. This streches out the remainder of the cone so that a smaller secondary can be placed farther from the primary and still achieve focus at the eyepiece. The drawback is that the scope operates at a high focal ratio, which means higher magnification, a correspondingly smaller field of view, and tighter alignment tolerance. In fact, since the light path bounces through the tertiary star diagonal before going to the eyepiece, a standard 2X Barlow will operate at about 3X. We used a "shorty" Barlow, with the Barlow tube itself acting as the extension tube between the side of the main tube and the tertiary. In our Barlow the cell holding the Barlow lens itself unscrews from the tube so it can be removed for low power viewing. There may be a bit of vignetting in this case since in choosing the secondary size, we compromised between the two options, but leaned toward the smaller size, leaving open the possibility of getting a larger second set of secondaries later for better low power viewing. The lower tube sections will have two sets of mounting holes for the primary mirror cells since the mirrors will have to be moved forward if the Barlow is not used.
It is common knowlege that using two eyes allows for easier viewing and usually provides a better view, but after reading about the extreme difficulty of the alignment problem, you may be wondering why we didn't just start with a single primary with twice the surface area and use a bino-viewer. After all, a larger primary is theoretically capable of producing higher resolution, right? There are several reasons we didn't take that route: first, the cheapest bino-viewers cost about as much as our entire budget for this project. Second, while a larger primary would achieve better resolution under perfect conditions, conditions are rarely good enough to get better resolution than an 8" mirror provides, and a larger mirror will actually be more affected by atmospheric turbulence on bad nights. Also (and not often recognized), since the two mirrors will be looking through different air masses, they will show two slightly different images, depending on how each is affected by the turbulence. When the brain gets two views of the same object, it is remarkably good at concentrating on the sharper image. Thus a true binocular scope provides good views almost twice as often as a single tube scope. Finally, another reason we made this particular type of scope (and not some other type of scope altogether) was that Jim Sattler had two 8" mirror blanks to donate to the project.
Courious about how we built this? Tour the construction...
8" Binocular Telescope