Afocal What?

Unless you’re old enough to have learned astrophotography from the Sam Brown booklets (above) that Edmund Scientific sold for less than a dollar during the 1960s, chances are you haven’t had much experience with afocal photography and may not even be familiar with the term. Nevertheless, every clear night amateurs with digital cameras are rediscovering this technique, which was happily forgotten by more than a generation of astrophotographers. Now, as then, the reason for choosing the afocal method of mating a camera to a telescope is the same – some cameras have nonremovable lenses. Unlike then, however, the results amateurs are achieving now are consistently spectacular.

To the uninitiated, nothing could be more logical than afocal photography: if you look through a telescope’s eyepiece, why wouldn’t you point a camera into it? And before single-lens reflex cameras became popular in the 1960s, that’s what most of us did when we wanted pictures of the Moon and, for the very adventurous among us, planets. The problem was, we were shooting almost blindly. A host of subtle optical issues made it difficult to know how a picture was focused and framed until after the film was developed. Today these issues are all moot – you can see exactly how the image looks on a digital camera’s viewing screen before pressing the shutter button. (In computer lingo it’s called WYSIWYG – "what you see is what you get" – and it alone has lifted much of the curse from afocal photography.) And, of course, there’s instant feedback when the shutter closes. Don’t like the result? Just delete the image; there’s no wasting film (or money).

Another pleasant aspect of afocal work in the digital age is that we can ignore all the laborious measuring and calculating that made the process so intimidating in the past. Adapting a phrase from a popular advertising campaign, I simply tell people, "Just try it." I think most will be amazed by the results.