Top positive review
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Excellent binoculars which incorporate the most important features for astronomical viewing
on 9 December 2009
This posting begins with a discussion of some core information about binoculars for astronomical viewing that should help potential purchasers make a more informed purchase decision.
If you have other viewing objectives than astronomical objects, or are already familiar with binocular specifications, you may want to stop here or just read the latter part of this review before going on to other postings; otherwise, read-on.
There are two main styles of "true" binoculars. Here, binoculars that do not use prisms such as opera glasses, are not discussed further. Most binoculars use one of two type prisms, either roof prisms or Porro prisms. Roof prisms are more modern and have a straight through appearance, i.e., the binocular cylinders form straight tubes. Porro prism binoculars (named after Ignazio Porro) have a tell-tale right angle bend. These usually are manufactured with two prism on each side of the binoculars, i.e., double Porro prisms. Although considerably larger in size, because of their improved optical qualities Porro prism binoculars, such as the model reviewed here, are preferred over roof prism binoculars for astronomical viewing.
Another important aspect of binoculars is the size of their exit pupil. Younger folks have pupils that can open, dilate, to a maximum size of slightly over 7mm. However, as one gets older the size of this window into the eye reduces. Over the age of thirty most folks have a reduction in their dark adapted pupil size of approximately 1mm every 20 years. The exit pupils for a pair of binoculars should ideally approximate the entry pupil of the observer's eye. Although some studies suggest an even smaller exit pupil size, see below. The size of a binoculars' exit pupil is found by dividing aperture by magnification. For example, common 7 x 50 binoculars (7 power by 50mm) have an exit pupil of approximately 7.14mm. In practice, this exit pupil size is larger than many adult's dark-adapted pupil size, particularly when some extraneous light is also present. In most viewing environments such as in or near a city such extraneous "light pollution" is almost always present. In addition, the periphery of the eye's lens exhibits some inherent optical degradation. Thus, an exit pupil size around 5mm may be preferred, although some experimental evidence suggests an exit pupil even less than 4mm may be most appropriate. These 15 x 70mm binoculars have an exit pupil of approximately 4.7mm resulting in more of the light exiting the lenses entering the eye than might occur with e.g., 7 x 50mm binoculars.
One of the most important considerations when choosing binoculars is their light gathering ability. Binoculars are essentially "light buckets". The human eye at its widest has a 7mm plus entry window. The 70mm objective lenses here have over 50 times the light gathering area of the human eye. Another factor affecting the light transmitted by binoculars are the materials used in their lenses and lens coatings. The least expensive binoculars have uncoated lenses or single coated lenses. Multi-coated binocular lenses and BaK-4, barium crown glass prisms, as in these Celestrons, are typically more expensive but improve light transmission resulting in sharper and brighter images.
The best eye relief, i.e., the distance your eyes needs to be behind the exit pupil of a binocular to see the full exit image is probably between 15mm and 20mm. These binoculars provide 18mm and additionally come with rubber eyecups. Thus, I've been able to use these both with and without glasses. I use lightly tinted sunglasses when viewing the moon to see more detail. In this case I leave the eyecups raised. When viewing without glasses I leave the eyecups down.
In use, I've found the images sharp and with adequate contrast to enjoy star clusters, the moon and planets. This pair's primary negative is its size and weight. These binoculars are really big. Owing both to their size and weight and as well as the relatively high magnification they are not comfortable to use hand-held for any but the shortest period of time. Because of their magnification, the slightest shake moves the astronomical object out of the field of view. Fortunately, they come with a tripod adapter. However, for some the need to use a tripod may defeat the value of having a "portable" pair of hand-held binoculars. For these observer's a smaller 50mm binocular is more appropriate. A minor problem is the carrying case, mine arrive with missed stitching on about a 1" section of a vertical seam, letting light through and possibly rain. The case is also a bit tight making it more difficult to easily insert and remove the binoculars, a better degree of quality control for the case, and a slightly larger size would seem more appropriate. Simply holding the case to the light and looking inward will reveal any stitching missed.
However, even recognizing that these binoculars cannot be hand-held for any extended period, they are probably one of the best choices for astronomical observers who need relative portability compared to a probably more cumbersome and expensive telescope. Perhaps surprisingly, they are also an extremely useful adjunct for those who use telescopes.
In summary, these binoculars allow for considerable additional exploration of astronomical objects compared to the naked eye. However, a tripod is required for any extended observations. Highly recommended.