History

As an astrophotographer using film for many years, I loved to do wide-field imaging.  It was less demanding than high-resolution work and gave good results quickly.   I started by doing piggyback astrophotography and was very excited when I published my first image in Sky&Telescope in 1984. It was a piggyback shot using a 50mm lens, mounted on top of a C8. 

I purchased an 8” Celestron Schmidt camera and continued to pursue wide-field imaging.  The excellent resolution, fast speed (f/1.5) and wide field (4.5 x 6.5 degrees) made it a wonderful instrument to capture large (and faint) objects with short exposures.  As better films, notably Kodak Technical Pan and then Kodak Pro 400 for color work, became available, the limitation of resolution by film grain with the Schmidt disappeared, and I was now limited only by optical resolution and seeing.  I had a great time shooting with the Schmidt, culminating with Comet Hale-Bopp images--the resolution was so good that I have a 20x36 color print in my office, with no visible grain, from a 35mm negative.

I next purchased a medium format Pentax 67 body and lens for piggyback images, packed a telescope and mount, and along with my friend John Gleason, took the setup to Australia. We came back with stunning, wide-field color images of the Milky Way and Southern hemisphere objects.  I was now a certified wide-field junkie!

CCDs Arrive
About this time, CCD cameras started to gain in popularity.  I was intrigued and got an SBIG ST6 from SBIG.  As I quickly learned, the small size of the chip and the large size of the pixels didn't make for a good match for wide field imaging.   The camera worked fine with the long focal length instrument, a C-14, that I how had in my observatory.  But I had to wait for a chip with smaller pixels to match the shorter focal lengths I wanted to shoot with.

I didn’t have to wait long.  With the release of the ST7, ST8e and ST10e cameras, the size of the chips slightly increased but the size of the pixels shrank from 24 microns (ST6) to 9 microns (ST7-8), and then to 6.8 microns with the St10e, and with the "e" chips, the blue sensitivity increased substantially.    With the smaller pixels the short focal lengths work fine, even though it doesn’t meet the Nyquist criteria.  To paraphrase, the proof is in the final image, and those images were excellent as you can see.

If you want to produce a color image with a CCD camera, you must shoot images with red, green and blue filters and combine them digitally. No more “one shot” jobs as with film. The filters must be between the camera lens and the chip so as not to alter the light path as it comes through the lens elements.  I experimented with a hydrogen-alpha filter made by Custom Scientific because of its ability to greatly resolve faint detail in hydrogen clouds. I quickly found that I was seeing even more detail than I did with the very fast Schmidt camera and hypered Tech Pan film. However, there was one problem introducing a filter holder,  the CFW8, supplied by SBIG, would put the chip out of the focus range of a 35mm camera lens.  Kludging together the hydrogen-alpha filter, a camera lens and the ST10e combination worked for early testing.  But it was not stable and I could not use the RGB filters in the filter wheel.

The Adapter
To solve this problem I designed a low profile adapter for the SBIG CFW8 that would hold a Nikon lens, The Mandel Wide Field Imaging Adapter. I machined the lens mount and I then mounted it to a modified CFW8 front plate.  My adapter brought the lens close enough to focus on the chip. Although I could not get it to focus at the infinity point on the lens, I learned to ignore the lens markings and find focus as I would with a telescope and CCD camera.
 

Shooting with a Lens
Using a camera adapter is easy. You simply attach the lens to the adapter, center the object (you can attach a 35mm camera body to the lens first for centering purposes) and then focus.

Fixed focal length lenses, as opposed to zoom lenses whose optics tend to be more subject to distortion, work best.  It’s also a very good idea to “stop down” one stop at least, this results in sharper stars, i.e., if your fastest lens speed is f/2.8 then shoot at f/4, etc.  Also, a good quality lens can’t be beat for minimizing coma and chromatic aberration.  Because of the inherent chromatic aberration in most camera lenses though, some type of IR blocking filter is desirable when shooting with a CCD camera otherwise, bloated stars may result.

Focusing requires some effort because the "cone of focus" becomes smaller and smaller as the lens f-ratio gets faster.  In other words, when shooting at f/2.8 or f/4, focus is very critical, down to thousandths of an inch.  I soon tired of doing this with my big fingers; it took a long time to find the “sweet spot”.

To solve this problem I modified a RoboFocus, made by from Technical Innovations, by attaching it to a bracket modified to work with my system. I then ran a long, endless rubber timing belt from the gear on the motor shaft around the focus barrel of the lens.  I adjusted the bracket so that I could provide enough pressure to move the focusing mechanism of the lens but not distort it in any way.  This system allows me to focus the lens to within 1/1000 of an inch and do it all from my computer by using the software supplied with the RoboFocus.  (There are details on how to make this simple device on my web site, as well as a “Wide-Field Gallery” and provide other information on wide- field CCD imaging.)

The data is then gathered just as with any other CCD camera image.  You can use the LRGB or RRGB technique, as Rob Gendler has done to produce the excellent color images you see here. Or, shoot with the hydrogen-alpha filter to obtain wide-field images in hydrogen light as done by John Gleason and myself.

Wide field CCD imaging is fun, relatively easy and opens up, dare I say it, a wide new field for those using CCD cameras.
 
 

Steve Mandel, is the CEO of a management training company by day and an avid CCD imager by night.  He can be reached through his web site at www.galaxyimages.com.