Parallelogram mounts solve many of the problems inherent in the use of photographic tripods and heads:
• They move the observer away from the tripod so that its legs do not interfere with the observer's body position, especially when observing at high elevations.
• They offer easily changeable eyepiece height over a wide range and can thus accommodate different observing positions and observers of different heights. The eyepiece height can be changed without changing the aim of the binoculars, making them ideal for communal observing.
• The mounting head can be designed so that binocular's center of mass can be aligned with the altitude fulcrum, thus eliminating problems associated with a changing turning moment when objects of different altitudes are observed.
• They are amenable to home construction by moderately competent wood or metal workers.
Their disadvantages are that they are relatively bulky, they require counterweights, and the long arms mean that vibrations take longer to damp down.
The simplest incarnations of the parallelogram mount have only an altitude adjustment in the binocular mounting head, thus requiring that the observer moves in a circle around the tripod in order to change the azimuth. As more degrees of freedom of movement are introduced in the head,so more sky is observable from a single position. The epitome of this development is the Universal Astro-nomics2 deluxe mounting head, which enables more than a quarter of the sky to be observed from a single position (Figure 6.13). A well-designed parallelogram mount, which has smooth motions and permits proper balancing of the weight of the binocular, almost confers the feeling that the binoculars are floating in the air in front of your eyes.
Figure 6.13A,B. A well-designed parallelogram mount allows more than a quarter of the sky to be observed without the observer having to move.
If you decide upon a parallelogram mount, you should give careful consideration to the length of the parallelogram arms. Longer arms enable a wider variety of observing postures, so you can change from standing, through sitting, to reclining without having to adjust the tripod height. Shorter arms have smaller damping times for vibrations, but require that the tripod height is adjusted for different observing postures.
Parallelogram mounts, particularly those designed for big and giant binoculars, expose the limitations of photographic tripods, particularly those with center posts. The usual solution is to use a surveyor tripod. These usually do not have leg braces, but have spiked feet that press into the ground. This is ideal, and provides an exceptionally stable platform, if you observe on a surface where this is possible. On the other hand, if you observe on a surface that is unsuitable for this, you must acquire either spreaders or leg braces (which can be retrofitted to the tripod) or an expensive accident resulting from a slipping leg is all but inevitable. Do not be tempted to rely on being able to tighten the leg hinges sufficiently to prevent this. Spreaders can be obtained from most suppliers of survey tripods, and leg braces are available from Universal Astronomics.
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