The Afocal Method
Basic, consumer-level digital cameras don’t have removable lenses; you have to go to a more expensive digital SLR camera for that. So the only way to shoot through a telescope with them is by the afocal method, whereby the camera’s lens is aimed directly into the telescope’s eyepiece. You can hold the camera by hand, mount it on a separate tripod, or make or buy a bracket to attach the camera directly to the telescope. (See our
sidebar on the afocal method to learn more about its history.)
For this view of
Jupiter, Io (the dim dot in the lower-left corner), and Europa (upper-right corner), New Jersey amateur Lester Dobos used his
Olympus C-2020 camera and 16-inch Dobsonian with a 9-mm eyepiece. Note the Great Red Spot and Europa’s shadow in transit.
Like the old eyepiece-projection technique, the afocal method dramatically increases the image size at the expense of a much slower effective f/ratio (or photographic speed). You may also get some vignetting and image distortion. Vignetting is the darkening of an image's edges that happens when light is cut off by mechanical limitations. It typically happens when the camera is held too far from the eyepiece or when the apparent angular field of the camera's view exceeds that of the eyepiece. This, however, is usually not a problem when you are photographing planets or other small objects that can be kept centered. To reduce vignetting, set the camera as close to and centered on the eyepiece as possible. Also, choose an eyepiece with ample eye relief – some short-focus eyepieces have such limited eye relief that a camera cannot get close enough to image well through them. Find the best camera position by experimentation.
Zooming in narrows the camera’s angular field and thus can minimize or eliminate vignetting (but avoid using "digital zoom," which sacrifices ’s resolution). Zooming also increases magnification, which often makes focusing easier. On the other hand, it makes finding your target tougher.
Images from digital cameras can be improved with popular image-editing software such as
Adobe Photoshop. This portrait of
Saturn was obtained by Hong Kong amateur Canon Lau with a
Nikon Coolpix 950 camera. He used a 14-inch
Celestron Schmidt-Cassegrain telescope and a 6.7-mm eyepiece. Colleague Eric Ng digitally stacked nine separate exposures and applied
Photoshop's unsharp mask and Gaussian blur procedures to enhance the final image.
Image distortion can occur due to various effects in the eyepiece and camera optics. The center of the image may be in focus while the outer areas are not, severely restricting the usable field. Be sure you keep the camera’s image plane centered on and perpendicular to the telescope’s optical axis. Also, keep the eyepiece and camera lenses free from smudges and dust, which degrade contrast.
Mounting the camera on a separate tripod can prevent vibration while avoiding the need to rebalance the telescope for the camera's weight. With such an arrangement, it helps to orient the eyepiece so that it moves directly toward or away from the camera as the telescope follows its subject across the sky. Use a black cloth or cardboard mask to shield the eyepiece and camera lens from stray light.
Although not essential, a telescope motor drive is convenient since it keeps the subject centered in the camera frame as you focus and compose the shot. For scopes without a drive, you can still take good pictures if you keep exposures short. Position your subject at the edge of your frame opposite the direction of drift, then wait for the object to glide near the frame’s center before triggering the shutter.
