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Celestron SkyMaster Giant 15x70 Binoculars
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There are two models often used in astronomy, roof prisms or Porro prisms binoculars. 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 usually preferred over roof prism binoculars for astronomical viewing.
Possibly the most important consideration when choosing binoculars is their light gathering ability. Binoculars are essentially "light buckets". Tthe human eye at its widest has about a 7mm entry window. A 70mm objective lenses, as here, has over 50 times the light gathering area of the human eye. Another factor affecting the light transmitted through binoculars are the materials used in their lenses and prisms, and their lens coatings. The least expensive binoculars have uncoated lenses or single coated lenses, or may even use plastic 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 eyes' distance behind the exit pupil to see the full exit image is probably between 15mm and 20mm. These binoculars provide 18mm and additionally come with rubber eye-cups. 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 that case I leave the eyecups down. When viewing without glasses I leave the eye-cups up.
In use, I've found these binoculars' images sharp and with adequate contrast to enjoy star clusters such as the Hyades and Pleiades, along with the moon and planetary observations. Its primary negatives are its size and weight.
Owing both to their size and weight, as well as their relatively high magnification they are not comfortable to use hand-held for any but the shortest period of time. For many the best binoculars are ones that can be strung over the neck and easily hand-held. These are not definitely such a pair. 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 many the need to use a tripod runs counter to the desire to have a "portable" pair of hand-held binoculars. For these observer's a smaller 50mm pair of binoculars is probably more appropriate.
However, even recognizing 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 quite a useful adjunct even when using a telescope.
In summary, these binoculars allow for considerable additional exploration of astronomical objects compared to the naked eye. However, a tripod is required except for any extended observations. Highly recommended.