Shooting with a Lens
The beautiful Eta Carinae Nebula was imaged by
John Gleason from Coonabarabran in New South Wales, Australia, with a 180-mm
Nikon lens at f/4 and an ST-10E CCD camera. This view is a composite of four 3-minute exposures taken through a hydrogen-alpha filter. Click on this image to see the entire 3.2°-wide field.
Using the camera-lens adapter and a CCD camera is easy. You simply attach a lens to the adapter; mount the setup piggyback on your telescope; center the target object in the camera's field; and focus. I check my focus by choosing a medium-bright star and adjusting the lens until the pixels that make up the star's image register the highest possible brightness value. (If you use a telephoto lens with a built-in ¼-20 tripod socket and mount it directly to the telescope, you can facilitate initial centering on the target by putting a regular 35-mm single-lens reflex camera body on the telephoto and looking through the camera’s viewfinder.)
The SBIG cameras' self-guiding feature works well with short-focus lenses. In fact, because of the large field of view, choosing a guide star is relatively easy.
I prefer lenses with fixed focal length to zoom lenses, whose optics tends to have slower f/ratios. It's also a good idea to stop down the lens at least one f/stop; this often creates sharper star images. (For example, if your lens's fastest focal ratio is f/2.8, then shoot at f/4, and so forth.) Also, you can't beat a high-quality lens for minimizing coma, chromatic aberration, and other optical flaws. And since the CCD's sensitivity extends beyond the visual range, especially at infrared (IR) wavelengths, you need to use some type of IR-blocking filter. Otherwise, a star image that appears crisp to the eye may actually end up bloated on the CCD image.
For precise focusing, the author uses a modified version of
Technical Innovations' Robo-Focus computer-controlled motorized focuser.
Steve Mandel
Precise focusing is essential since the lens's depth of field is very small at fast f/ratios. In other words, when you’re shooting at f/2.8 or f/4, focus becomes very critical — you're dealing with tolerances measured in thousandths of an inch. I soon got tired of focusing manually with my big fingers; it took me forever to find the lens's "sweet spot." To solve this problem I modified my Robo-Focus, a remotely operated focuser motor drive from Technical Innovations
(www.homedome.com), and attached it to my camera setup using a homemade bracket and a long rubber belt that ran from the gear on the motor shaft to the lens’s focusing barrel. I adjusted the bracket so the motor provides only enough pressure to focus the lens without flexing its barrel. This system allows me to focus the lens from my computer to within 1/1000 inch, using software supplied with Robo-Focus. (Details on how to modify the Robo-Focus are given on
my Web site.)
You can try the "luminance-layering technique" used by Robert Gendler to produce excellent tricolor images of galaxies and nebulae (see Sky & Telescope's July 2001 issue, page 133, or go to Gendler's Web site at http://robgendler.astrodigitals.com). Or you can shoot through a hydrogen-alpha filter to obtain black-and-white images, as Gleason and I do.
Wide-field imaging is fun and relatively easy, and it opens up — dare I say it — a wide new field for those using CCD cameras.
