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A Discussion of Some CCD Parameters

There are two basic parameters of CCD cameras that you need to understand, in order to choose the right camera for your scope - the pixel scale and the field of view of the camera. For a given camera, both are influenced by the parameters of the optical system.

The pixel scale is used to determine whether an object that is a small round spot covers enough pixels that it appears as a nice round spot in the resulting image. To put that in more technical language, scopes display a star (which is a point source) as an airy disk due to the wave nature of light and the resulting interference patterns that an optical system creates.  In real viewing situations the size of this disk is larger than its theoretical minimum size because of atmospheric dispersion - i.e. the quality of the seeing at your location. What you want is for the airy disk to cover many pixels but not too many.

If the airy disk covers too few pixels - "undersampling" - in the resulting image the disk has pronounced stair-step edges because the CCD's pixels are square (or sometimes rectangular but usually close to square).  This is shown here on the right, and is the same result that you get when you print a curved line (such as a very large font) on a printer with too small a resolution.  Try printing the letter "C" in a very large font (say 100 point) on a printer set at a resolution of 300 dpi and look closely at the edges of the letter. Then print it at 600 dpi and then at 1200 dpi and observe the difference.

If the airy disk covers too many pixels - "oversampling" - the edges of the image are nice and smooth, and this is what you want (as shown here on the right).  But if you�ve set up an optical system where you oversample too much, you've set up a situation where you've reduced the field of view so much that you've limited the size of the largest object that will fit on the CCD. Nevertheless, if you must make a choice it's better to oversample than undersample, because at least with oversampling the images you get of the objects that will fit on the CCD chip, are good images. (For more info on all this go to Apogee's Web site and click on the "CCD University" button. This is the place from which I shamelessly copied the two images above.)

The parameter of achieving a pixel scale of 2.5, is a somewhat empirical goal that considers the typical seeing conditions and optical systems for amateur astrophotographers.  (If you live on a mountain top with superb seeing, the best pixel scale will be different for you.) For most amateur astrophotographers, it is acceptable to have a pixel scale of less than 2.5 (oversampling) but it's better not to go over 2.5 (undersampling). Note that you can adjust the pixel scale by adding a focal reducer or a barlow lens in front of the CCD camera on a given scope.

After achieving the right pixel scale for your scope (a combination of choosing a CCD camera with the right pixel size and adjusting the optics with a focal reducer or barlow lens), you can only get more field of view by purchasing a CCD camera with a larger CCD chip. Unfortunately that isn�t very practical for anyone with a finite budget. So for most folks, for a given scope the CCD camera with the combination of the right pixel size and an affordable price, determines the largest deep-sky object that will fit on the camera�s CCD chip. To check the field of view of a particular combination of scope and CCD camera, look at the bottom of the Field of View Calculator page on this site for free software that allows you to check this. 

In any given scope, all things being equal planets are much smaller than most deep-sky objects.  If you set up your scope and CCD camera for the proper pixel scale, typically you�ll find that planets don�t fill much of the CCD chip.  For planetary work you can place a barlow lens in front of the CCD camera and/or experiment with binning the CCD chip. I'd probably want to use a good barlow for astrophotography - the Celestron Ultima at least (which fortunately isn't very expensive).

The attributes that make a CCD camera a good one, include a number of issues such as sensitivity, dark current, read noise, antiblooming, and the digital transfer rate of the camera to your computer.  But the Pixel Scale is a good place to start, for chosing an appropriate camera for any given scope.

Back to A Comparison of CCD Cameras


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