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Sherry D. Byers
Administrative Assistant

Ph:    (540) 464-7225
Fax:  (540) 464-7767
Physics@vmi.edu

320 Mallory Hall
Virginia Military Institute
Lexington, VA  24450

CCD Electronic Imaging

Orion Nebula Electronic imaging has completely revolutionized observational astronomy over the past ten years! In fact, photographic observations, which were critical to astronomy observations for over one hundred years because of the ability to make time exposures, are now banned at some observatories because of the increased efficiency and accuracy of electronic imaging. This section summarizes CCD imaging techniques at the VMI observatory.

The image to the left of the famous Orion Nebula is a 10 second exposure. Far more detail is recorded than with a 30-min exposure on film. CCD images typically record more detail than can be displayed on screen and the dynamic range of the image can be stretched to show the fainter portions of the nebula.

 

Principles of CCD Detectors 

Charge Coupled Device ChipThe VMI Observatory uses a professional quality electronic camera to obtain images of faint stars for photometric measurements. The detector is the charge coupled device (CCD), a computer chip with a window, which is placed at the telescope focus. The image from the telescope is focussed onto the active area area of this chip, and a corresponding (and extremely accurate) pattern of electrons collect on the chip, in rows and columns, during a time exposure. The chip is then "read out" at the end of the exposure, and the resulting pattern of light is recorded as a computer file, which can then be displayed as an image.

These images can then be measured to determine the brightness of stars. The results are more accurate than with conventional photometry, with the added advantage that an entire image is saved. Examples of images obtained with our telescope are shown in this section and the astrophotography page.

The VMI Observatory employs two CCD cameras. For the 20-inch reflecting telescope we use an Apogee Instruments Ap6ep astronomical CCD camera, with large 24-micron pixel size to match the large plate scale of the 20-inch telescope. A link to the Apogee webpage: Apogee Instruments, Inc.

For the 5-inch astrographic refractor, we use an SBIG ST-7 (Santa Barbara Instrument Group). This CCD camera was chosen because of the very small pixel size (9 micron), to match the short focal length and wide field of the 5-inch f/6 astrographic refractor.

Astronomical CCD cameras must be cooled so that time exposures may be obtained. Ordinary digital cameras also use CCDs as detectors, but long exposures (greater than a few seconds) are not possible, because thermal electrons contribute huge amounts of noise to the image.

Both of our astronomical CCD cameras are cooled using Peltier coolers, which hold the chip temperature approximately 40 degrees C below ambient temperature. With the corresponding lack of thermally-generated electrons, time exposures may be made. The incredible sensitivity of CCD cameras (up to 50 times more sensitive than film), combined with time exposures, allow deep sky imaging in relatively short exposures.