Using a CCD Camera & Tracking a Celestial Object

The sensitivity of a CCD camera is much greater than the sensitivity of a camera using ordinary film. This means that to produce comparable images of a given object, the exposure times using a CCD camera is much less than the exposure times using a camera with film. CCD exposure times vary from fractions of a second for bright objects to several minute for dim objects. The field of view for our telescope/CCD system is approximately 20 minutes of arc. Due to the Earth's rotation, a stationary telescope pointed toward the celestial equator, would show stars moving across the field of view in a little over one minute of time. A time exposure of one minute for this situation would show star trails across the picture. To produce point star images requires the system to have a motorized tracking system that keeps the telescope pointed in the same direction of space as the Earth rotates. The Paramount ME 1100 GT mount can accurately track objects for several minutes without noticeable star streaking. For very dim objects it is usually best to take a series of relatively short exposures and combine the images, rather than to take one long exposure.

Constructing a Color Image

The CCD chips found in digital cameras uses the same technology as the CCD chips for our camera. They differ in the way they produce color images, however. Consider a typical 3 megapixel digital camera. Each pixel will have either a microscopic red, green or blue filter in front of it. These microscopic filters are uniformly spread throughout the chip. The number of electrons accumulated at each pixel (proportional to the light intensity) is proportional to the intensity of either red, green or blue light at that particular spot on the chip. Software in the camera takes this information to produce a color image. Notice that to get a color image, the spatial resolution of the chip is reduced to that of a one megapixel chip. Astronomers are interested in getting the most spatial resolution out of a CCD camera as possible. To do this and also get color images, three separate images are taken of a given object, each through a different color filter (red, green or blue). This diagram shows the light transmittance through a commercial filter set made by Astronomik. A red object will appear bright when viewed through the red filter and much dimmer when viewed through the green and blue filters. In practice, the sensitivity of the CCD chip varies with the wavelength of light (the sensitivity at 400 nm is about half that at 580 nm, for example). This means that exposure times for images taken through the green and blue filters need to be longer than images taken through the red filter. Astronomers often combine the color image with a fourth image taken through a so-called luminance filter, which supplies much of the spatial resolution in the final image.. This filter blocks the near infrared light that
brightens the sky.

These transmittance curves show the wavelengths of light passed by the various filters used for CCD photography.   The black curve shows the transmittance for the clear “luminance” filter, which is effective in blocking the near infrared. The efficiency of CCD cameras decreases as the wavelength decreases, so progressively longer exposures are needed as you move from the red to the green to the blue filter.
Notice that the image of the Sculptor Galaxy appears different through the different filters. These images show the Sculptor Galaxy taken through red, green, blue and luminance filters. This is typical of a spiral galaxy where older, cooler (red) stars are found near the galactic center, and younger, hotter (blue) stars are found out in the spiral arms. You can see the difference in light intensity at the different parts of the galaxy when you compare he images taken through the different filters. The final combined image appears in color.
The center of the galaxy is dominated by many old, cool (red) stars; hence the center appears brighter through the red filter than through the blue filter. The opposite effect is seen in the outer portions of the galaxy, which is dominated by hot, blue stars.