Moondark for July: Easy "Afocal" Astrophotography
This odd-sounding word describes what is without a doubt the easiest route into astrophotography. All that's needed is a telescope and a digital camera. In visual use, your eyeball functions as the camera, and your retina as the film. In an afocal setup, you replace your eye with an actual camera. Once you’ve located and focused the target, aim the camera into the eyepiece and snap the shot. What could be simpler?

Afocal photography has been around a long time. Technically, it refers to matching the object and image distances to achieve a sharp focus. This used to require tedious calculations, complicated adapters and close tolerances—all without being able to see what you’re shooting! The LCD preview screens on today's digital cameras have brought a revolution in afocal imaging: you can see what you get, and the results can be amazing.

The camera can be hand-held very close to the eyepiece or mounted on a separate tripod, free from any direct connection to the telescope. Alternatively, you can make or buy a bracket to attach the camera—a timer shutter release will help avoid jittering the scope while capturing a shot. The camera’s viewscreen lets you be assured of the results, tweaking the focus as necessary. Keep shooting pix since some frames will be blurry, but the "keepers" will be sharp. No film is wasted, just delete unwanted photos from memory. One tip: LCD displays drastically shorten battery life. Try rechargeable, nickel-metal hydride batteries if your camera doesn’t come with its own. And keep a spare set handy.

The moon and the sun are easy marks for afocal astrophotos. For the moon, just shoot with autoexposure: as long as the moon fills most of the frame, the exposure reading should be accurate enough. Use an eyepiece with long eye-relief (that is, the full field of view is visible with eyeglasses on) and wide apparent field of view, and use the full optical zoom to fill the camera’s frame. Shoot in a high resolution format.The moon's terminator provides stark images of craters and an easy way to check the focus. For the sun, use the same safe-filtering practices you would use for visual use: an off-axis mask of Baader solar film will allow you to record sunspots and faculae on the photosphere.

My photos could use improvement in focusing and perhaps a better eyepiece, so I’ll keep experimenting. You can read more about afocal methods in Edwin Aguirre’s article in Sky and Telescope, August 2001 (p. 128 and also available online), Michael Covington’s Astrophotography for the Amateur (revised edition, pp. 18-19 and 60-61) and even Sam Brown’s All About Telescopes (pp. 58-61).

The limited experience you see here has convinced me that in this way anyone can explore innumerable regions of the moon and record how illumination angle dramatically alters the appearance of lunar features. On the sun, sunspots rotate into and out of view, growing and disappearing as they cross the solar disk. These changing views can be easily documented with the afocal technique. 

Afocal has always been the simplest route into telescopic astrophotography. Digital cameras have now made it practical and ever more affordable. Modest telescopes, small refractors (even binoculars) maximize the portability of a real astrophoto setup. Fine results can be had with modest equipment with a bit of practice. Terrestrial wildlife subjects can be photographed just as easily. Whether you are a veteran prime-focus imager or a astrophoto newbie, afocal imaging is an easy and enjoyable way to capture and share your views of the Moon and the Sun.

Plenty of afocal photos are posted on the web: try a google.com search. Moondark is written by Doug Miller, published on the web, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 23 June 2002. All text and images copyright © 2002 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.