…continuedWide-Field Imaging with CCD Cameras
Arrival of the CCDs
During the 1990s CCD cameras entered the world of amateur astro imaging. Intrigued with the CCD's potential, I ordered an ST-6 camera from Santa Barbara Instrument Group (SBIG). I quickly learned that the small size of the camera’s silicon chip (6.5 by 8.6 mm) and the large size of its pixels (23 by 27 microns) weren't ideal for imaging large swaths of the sky. The camera worked fine with long-focal-length instruments, such as the 14-inch Celestron telescope that I had in my backyard observatory, but I needed a camera with either a larger chip or smaller pixels to match the short-focal-length lenses I wanted to shoot with.
|Sky Coverage* of SBIG Cameras|
|Lens focal length||ST-7E||ST-8E||ST-10E|
|50 mm||5.3° × 7.9°||10.5° × 15.8°||11.5° × 17.1°|
|85 mm||3.1° × 4.7°||6.2° × 9.3°||6.7° × 10.0°|
|105 mm||2.5° × 3.8°||5.0° × 7.5°||5.5° × 8.1°|
|135 mm||2.0° × 2.9°||3.9° × 5.9°||4.2° × 6.3°|
|180 mm||1.5° × 2.2°||2.9° × 4.4°||3.2° × 4.7°|
|200 mm||1.3° × 2.0°||2.6° × 4.0°||2.9° × 4.3°|
|300 mm||0.9° × 1.3°||1.8° × 2.6°||1.9° × 2.8°|
|*The formula given on page 4 of this article for calculating a lens's plate scale does not work well with focal lengths shorter than about 50 mm.|
In 1995 SBIG came out with its ST-7 and ST-8 models and, six years later, the ST-10. The physical size of the chips increased slightly, but the pixel size shrank to 9 microns square (for the ST-7 and ST-8) and 6.8 microns square (ST-10). Now fitted with Kodak's new E chips, these cameras’ sensitivity at blue wavelengths improved substantially, greatly increasing their potential for tricolor imaging (defined on the next page). The combination of smaller pixels and short-focal-length lenses worked fine. Although the pixel scales are coarser than people are used to using for telescopic work, I was able to obtain good results.