After completing my first TCT (NEWTRYAD) I decided to design and build a binocular version. The final result is a 4 1/4 inch unit operating at f/10. As a bonus this unit can be mounted on the same equatorial drive that carries NEWTRYAD.
One of the design features of interest is the method used to accommodate the variable inter-ocular distance between users. This was accomplished by way of swivel blocks (figure 2) that are centered over the exit pupil of each half of the telescope. Composed of two inclined mirrors, the image is relayed from the telescope to an eyepiece inserted into the block. Each block is swiveled to match the distance between eyes. The blocks are mounted on a common pivot arm that provides for focusing.
The collimation of a unit of this sort is not a trivial task. In fact, this became a source of immense frustration due to a problem that was ultimately tied to the swivel blocks. Initially, each half was independently collimated and produced very fine identical images. The challenge became to blend these into overlapping images to produce the binocular effect.
And here the frustration began; in spite of the identical images all attempts to bring the images together met with dismal failure. I decided the design was flawed and that what was needed was to split the common box holding the two optic sets so that each half could be brought together as a final assembly. Frustrated, I set the unit aside and went on to a new project involving a compact TCT design operating at f/8.5 with no focal plane tilt and no anamorphism. Splitting the box was now at the bottom of a long to do list.
As fate would have it, circumstances (I was asked by an S&T; reader about the status of the unit) caused me to revisit the bino unit and conclude that splitting the box was unnecessary. I managed to pop a glued connection as I was re-examining the focusing assembly. During the re-assembly I toyed around with the swivel blocks.
Sitting in my favorite chair I viewed the clock on the wall through one of the blocks in a periscope manner. With both eyes open, the two images slowly snapped together as a binocular image. The reason for the slow response was that the image in the block was not perfectly matched with the naked eye view. In spite of the slight mismatch the eyes where able to accommodate and make the adjustment. After a few alignment tweaks (each mirror in the block can rotate and translate) the snap effect was improved. I apparently overlooked this minor adjustment in the first place, most likely because the view is perfect when using only one eye.
I grabbed the second unit and was both disappointed and pleased. Pleased because I had found the source of a significant problem, disappointed in having missed it in the first place. The image was displaced and slightly rotated with respect to the naked eye. No wonder I had such a struggle with collimation! More tweaking and the bino effect kicked in.
At last the scope performed as expected. I still think that splitting the scope would be an overall improvement because collimation is still a real bear! Considerable effort was required to overlap one image over the other while maintaining individual collimation. Hindsight is always 20/20. Was the overall effort worth it? Yes, but I plan on using a bino viewer on the next project to compare the results.
J. Francis, "New Twist on Tilted-Mirror Telescopes"
Sky & Telescope, July 1999, 128-133
Curator: Hartmut Frommert
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