Imaging Tips and Techniques
The Dumbbell Nebula, M27, in Vulpecula was recorded by Mike Unsold with a 14-inch Celestron Schmidt-Cassegrain telescope and a Canon EOS D60 camera set at ISO 400. Nine 3-minute exposures were stacked and processed with ImagesPlus.
Courtesy Mike Unsold.
can be a challenge, since deep-sky objects are too dim to show on a camera’s built-in LCD viewfinder. Moreover, the typical viewfinder is too small for reliable, accurate focusing. To achieve precise focus many astro imagers point the telescope to a nearby bright star and focus on it with the camera’s focus fixed at infinity and its digital zoom set at maximum. They also set the camera’s ISO rating to a high value (say, 800) to increase its sensitivity so they can focus on much dimmer stars.
Johannes Schedler took these two close-up views of the Orion Nebula and its stars using the Coolpix 995, 11-inch Celestron telescope, and 40-mm Pentax XL eyepiece. The top image is a single 8-second raw exposure; the bottom one consists of five 8-second and one 30-second exposures assembled and processed with PhotoShop. The camera was set at ISO 400, and the ambient air temperature was 32°F.
Courtesy Johannes Schedler.
Alternatively, if your camera has a video output, you can attach an external monitor or TV to increase your focusing accuracy. After focusing, the ISO and zoom settings are switched back to desired values (but watch out on some low-end cameras the zoom changes the focus slightly). Then the telescope is slewed to the target object. To do this you may need to remove the camera, center the target visually in the eyepiece without touching the focus, reattach the camera, and shoot. Some imagers take a series of test shots for centering and composing the picture.
Many of today’s telescope drives are accurate enough for you to make exposures up to a minute long without guiding. Generally, fast instruments with short focal lengths produce the best results and are more forgiving of errors in tracking and polar alignment.
The camera’s LCD viewfinder should be used only for focusing and composing; it should remain off the rest of the time to prevent heavy drain on the batteries, which adds to the heat generated within the camera.
Subtracting dark frames is very helpful for removing "hot pixels" and other artifacts. Nikon's Coolpix 995, Canon’s PowerShot G2, and others have built-in noise-reduction functions that automatically take a second exposure the dark frame with the shutter closed and use it for processing the original image. Some astro imagers use this feature, while others prefer to disable it and take separate images with the telescope covered, then manually subtract the dark frames from the originals with programs such as BlackFrame.