Making An Adapter

My solution was to design a low-profile adapter for my CFW-8 that would hold a Nikon lens. I attached a bayonet mount for the lens to a modified CFW-8 front plate. This accessory — which I now manufacture for other astroimagers and sell as the Mandel Wide-Field Imaging Adapter — brought the lens close enough to the CCD chip to be able to bring stars to a focus on it.

Admittedly, this arrangement did not bring the stars into focus when the lens was set at the "infinity" mark on its barrel. But I quickly learned to ignore the lens marking and find focus as I would with a regular telescope and CCD camera.

Excited, I aimed my new wide-angle camera to the sky, and I was immediately astounded by the images it delivered, especially those obtained through the hydrogen-alpha filter. I began imaging deep-sky objects that I'd never even seen before. For the first time I captured the full extent of Barnard's Loop in Orion (S&T: May 2001, page 140) and the very faint emission nebula Sh2–264 between Betelgeuse and Bellatrix. With a normal (50-mm) lens I got a whopping 11.5°-by-17.1° field! You can see more of my images online at www.galaxyimages.com.

I gave my prototype Nikon/CFW-8 adapter to Gleason, who took it to Australia for a field test. He returned with some remarkable images (see his Web site at www.celestialimage.com). Based on the positive results we achieved, I started making commercial CFW-8 adapters exclusively for Nikon lenses (S&T: December 2001, page 36). Recently, Brady Johnson, an amateur astro imager based in Toronto, has made similar CFW-8 adapters for Pentax, Minolta, and Olympus 35-mm camera lenses. Of course, you shouldn't hesitate to contact the manufacturer of any CCD camera you may own to find out what solutions they may have devised since the time of this writing.

To calculate how much sky your camera will cover with a given lens, divide 57.3 by the lens's focal length in millimeters to get the lens's plate scale in degrees per millimeter, and then multiply the scale by the dimensions of the CCD's pixel array in millimeters. For example, a 300-mm lens will have a plate scale of approximately 57.3/300 = 0.191 degree per millimeter. When coupled to, say, an ST-7E camera (with a 4.6-by-6.9-mm pixel array), the camera will record about 0.9° by 1.3° of sky (see the table on page 2 of this article for an array of examples).