Over the past few decades there have been various designs of binocular mount that use a first-surface mirror arrangement to circumvent the problem of an uncomfortable observing posture. These usually need to be placed on a table or tripod and the binocular is secured to the mount. Either the binocular and mirror, or only the mirror itself, can be rotated about a horizontal axis for altitude, and the entire mount may be provided with a lazy-susan type, or other rotatable, base for azimuth adjustment. Such an arrangement is the popular Sky Window1 and is also amenable to do-it-yourself construction (Figure 6.12).
These mounts are like Marmite—nobody seems to be ambivalent about them and observers seem to divide strongly into two diametrically opposed camps: those who extol their virtues and those who loathe them. In the interests of enabling you to assess my objectivity, I have to declare myself to be a member of the latter camp, although I have only briefly used a mirror mount and have never used Sky Window.
The advantages of mirror mounts are obvious and simple: they permit observation, particularly of higher altitudes, from a normal seated position and with the head at a range of angles for which the human body seems to be naturally designed (the "microscope position"), thus eliminating neck and back strain and the resulting fatigue. The table, if one is used, also provides a rest for the elbows. There is no doubt that, from an ergonomic point of view, they are exceptionally comfortable
and are an ideal solution for those observers for whom this is a major consideration. They are also compact and relatively light, so they are relatively portable. If tables and chairs are not available at the observing location, it is no greater hardship to carry a portable/collapsible table and chair than it is to carry a tripod.
The obvious disadvantages of mirror mounts are that they provide an inverted image of the sky and that, unless they have some sort of heater, they are prone to fogging. Some observers find them difficult to aim. I have not seen any that are suitable for use with binoculars bigger than about 80-mm aperture. There are also disadvantages of using an additional optical surface. There will be some light loss, although, as long as the mirror surface is of reasonably good quality, it will not be to the extent that it will be noticed in use by most observers. The mirror will also impose a limit to the amount of magnification used, due to the difficulties and concomitant expense of making a large optically flat surface. This is not usually a problem with magnification of less than about x15. Finally, there is the problem of cleaning. It is inevitable that such a large exposed surface will accumulate dust and debris; as with all first-surface mirrors, cleaning must be undertaken with extreme care so as not to damage the surface.
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