Visual Astronomy of the Deep Sky
by Roger N. Clark

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Book Reviews of Visual Astronomy of the Deep Sky

Book Review: Not for the casual reader, but very informative
Summary: 4 Stars

Roger Nelson Clark's book gives the Serious observer of the deep sky the skills necessary to milk the most information out of every last photon of light when observing visually. He gives an extremely complete recounting of studies of the human visual system, most specifically in terms of its ability to adapt to dark conditions and perceive subtle differences in contrast, and its acuity under low-light conditions.

He asserts that there is, statistically, not a very wide range of light perception ability in the human population (amounsgt those without obvious visual disease) and that techniques he outlines in the book will help any committed student of the sky see more details.

His conclusions? The bottom line is: most of us aren't using enough magnification. Many assume that the "richest field" view of a scope, providing the brightest image per unit area, will also allow the for the greatest amount of observed detail. Clark, however, shows that this is not the case. Indeed, at low light levels, the human visual system is not very acute, (unlike daytime vision) and that many of the details in our favorite "faint fuzzies" simply cannot be seen unless they are magnfied enough for their light to spread out over a larger portion of our retinae.

Also, magnifying has the added effect of dimming the background around an object as well as the object itself. The human eye is excellent, it turns out, in perceiving subtle differences in contrast, and that an overall reduction in light per unit area does not affect this as much as one would think. What's more, when reducing the overall amount of light entering the eye, dark adapation improves. Clark outlines the results of studies that show that in complete darkness, the dark adapted human eye can detect points of light equivilant to an 8.5 mag star! So, in looking trough an eyepiece, one's eye can, in fact, become more dark adapted than it would be under the ambient light of the sky.

He demonstrates how to compute the "optimum detection magnification"- not too high not too low- for a given object, given its brightness per unit area and its size.

An excellent feature of the book are Clark's drawings of a host of astronomical objects through scopes of various sizes. This gives an excellent indication of what one should be able to see under decent atmospheric conditions.

One weakness in the book, I find, is the charts of "recommended optimum detection magnifications". While the concept that such a magification exists is a good one to know, and its exposition in this book complete and useful, the execution of this chart is full of glitches. While the overall gist of the book suggests that many of us aren't "cranking it up enough", many of suggested optimum magnifications are absurdly high. Many are useful, however. You'll just have to find out which is which for yourself. I chalk this up to sloppy editing.

Don't, however, let this dissuade you from getting this book. If you fit the desciption above, then this book will change the way you use your scope.

Summary of Visual Astronomy of the Deep Sky

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