Afocal Photography

When doing any sort of public astronomy, showing folks the beautiful sights available to a telescope, I often hear the question “Can I take a photo of that?” The person asking the question is usually holding the ubiquitous compact digital camera. They are often surprised when my answer is “Yes”. It is indeed possible to manage hand held shots of bright astronomical objects by simply holding the camera up to the eyepiece. There are a few tricks to making it work, but nothing that can not be demonstrated in a minute or two. The resulting photographs can be quite pleasing, definitely worth showing to friends and family along with the rest of the Hawai’i vacation shots.

The method of positioning a camera with a lens in front of an eyepiece is called afocal photography, or sometimes digiscoping. Afocal has been around for a while, but was not considered a practical photographic method by most. The advent of common digital cameras without removable lenses has changed this. Using the LCD display it is possible to get an easy look at the image before shooting and developing a roll of film. Getting a decent image is often a trial and error process with many wasted frames, not a problem with a digital camera where there is no cost involved in a bad photo beyond pressing the delete button.

Afocal PhotographyTaking a photograph of the Moon using afocal photography

Sometimes I have to explain the shot is not really possible, the object is just too dim to take the photograph without very long exposures and all of the technical complications such exposures entail. This is true of the many star clusters and nebula that we look at. Hand held afocal photography limits exposures to what the human hand can hold steadily enough to manage an unblurred exposure, about 1/8 second or shorter. It is only a few targets that are accessible to this technique, the Moon, Venus, Jupiter and Saturn, possibly a few bright stars. Of these the only one I find truly satisfying to photograph is the Moon. I have seen a few decent attempts at Saturn and Jupiter, but these are pressing the limits of what can be achieved without additional techniques that are beyond a simple handheld point and shoot attempt.

The idea is simple, hold the camera to the eyepiece, find the image on the LCD display and press the button. This works as the telescope eyepiece creates an image suitable for the human eye with it’s built-in lens. Thus we can substitute an electronic device, also with it’s lens, and be able to form an image. The idea is simple, there is some complication that comes when you try it.

The correct position to hold the camera is normally the biggest issue in taking afocal photographs. It takes a little experimentation to find the correct position at which to hold the camera. How far from the eyepiece? What zoom setting of the camera lens? The first issue is to find the image on the display. It helps to zoom the camera all the way out at first. This allows finding the rough camera position needed, but the image will usually be badly vignetted (dark all around the edge). Once the image is located zoom in until the image fills the display. Zoom in just enough, as higher zooms will become difficult to hold steady.

Nine Day MoonA nine day old Moon, taken with a 102mm refractor, a 25mm eyepiece and a Canon G11 camera

Many people naturally tilt the camera to try and find the image, this distorts the image. It is necessary to hold the camera square to the eyepiece and move the camera left, right, up or down to align it. It must be held steadily to keep the correct alignment and to prevent blurring from motion during the exposure. I find it often works to wrap one hand around the eyepiece forming a tube in which to brace the camera, both steadying the camera and holding it the correct distance from the eyepiece. The afocal technique is popular enough that you can buy purpose made brackets that will hold the camera above the eyepiece with an adjustable position. The technique will work without any aids when needed.

No simple answers to finding the correct position, maybe an optical designer could analyze the possibilities, but for most the only way is to experiment and see what works. Sometimes no matter how you hold or zoom the camera it just will not work. The issue here is the compatibility between the eyepiece and the camera lens. There are many camera lens designs and many possible telescope eyepieces. Wide field, high eye relief eyepieces are generally best. Not all combinations work as well as others, try another eyepiece if one just does not work. The smaller lens compact cameras are often easier to use use than larger lens DLSR’s for afocal photography.

Some have taken the idea and invented ways to improve the technique. You can buy brackets that will hold the camera in place, alleviating the problems of holding the camera by hand. While not as useful as a full astrophotography setup, it does allow photos to be taken with existing gear and a modest investment.

An example of a lunar shot is shown at the bottom of the article. The image was taken simply by holding the camera up to the eyepiece as described here in the article. While the image may look good on the LCD when taken, flaws, usually focus, may appear when examined later on a better display. It is best to take a number of images in the hope that at least one is decent when viewed later. This image is fairly good even when examined at full scale on the computer screen. A section of this image is shown at full magnification at the left showing that the image is reasonably well focused. There is a bit of chromatic aberration casing a slight blue fringe at the edge of the Moon.

When it works the technique can produce pleasing pictures. I find that demonstration the technique is a lot of fun at public star parties when the Moon is up. The method also works with microscopes and spotting scopes. While viewing lava flowing into the ocean this spring I allowed people to take photos using my spotting scope, many of the photos were quite decent. A few images to remind them of the beauty seen in the telescope.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on Mauna Kea, Hawai'i.

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