Last updated on
4/11/
2006

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Choosing a CCD Camera
[CCD Parameters]

There are a number of good astronomical CCD cameras available, and choosing the right one can be rather complicated. What you want is a camera with a large CCD chip, small pixels in the chip, and low price.  Unfortunately, chip size and price go together - click on the thumbnail on the left to view a size/price comparison chart for many of the most common CCD chips. 

To make a good astronomical CCD camera choice you will find it helpful to know the following:
a)
You can change the focal length of your scope by using a focal reducer or barlow lens in front of the CCD camera.  This change will of course provide a different field of view (comparable to using different lenses on a terrestrial camera). To photograph planets you need a “telephoto” view, which at a minimum is the prime focus setup on an SCT but probably will be better with a barlow lens.  For deep-sky objects like nebulas you need as wide a “wide-angle lens” as possible, which requires a focal reducer or even better, the Fastar system on Celestron SCTs.
b) The field of view defined by any given optical setup needs to be matched to the size of the pixels of the camera.  If you think about it you’ll realize that the wider the field of view (i.e. the more sky you take in with your scope) the smaller the pixels need to be to capture detail on a specific object.  A common way of describing this is the term Pixel Scale (in units of arc-seconds/pixel), which is (205 x P)/ FL, where P = pixel size in microns and FL = Focal Length in mm. Astro- photographers note that the right Pixel Scale for good photographs is about 2.5 arc-seconds per pixel.Adrian Catterall has a brief but good discussion of this issue in his Morden Observatory Web site (which should also cause you to appreciate my choice of background color for this site <grin>). I have included an explanation of both the pixel scale parameter and the field-of-view parameter here on a CCD parameters page here.  As explained on that page, f or most amateur astrophotographers it is acceptable to have a pixel scale of less than 2.5 but it's better not to go over 2.5. Since CCD chips are only manufactured with certain pixel sizes, to achieve a Pixel Scale of 2.5 or less you need to reach a combination of the chip with the right pixel size plus perhaps an adjustment to your scope’s focal length as noted above. 
c) Once you have the right Pixel Scale, you also want a physically large CCD chip but this depends on your financial situation - the price of a CCD camera (or a consumer digicam for that matter) is proportional to the area of the CCD chip.
d) The physical size of the chip is related to the pixel area times the pixel size. The larger the chip the better but what you want is as large a CCD chip as you can afford that still has the right Pixel Scale for your scope for your intended purpose. See the notes below for more information on this.
e) Some SBIG CCD cameras include a second, small CCD chip that connects to Celestron or Meade LX200 scopes to serve as an autoguider while the main CCD chip takes the photograph.

Good and affordable astronomical CCD cameras are made by: SAC Imaging, Santa Barbara Instrument Group (SBIG), and Starlight Express in the U.K - a U.S. distributor of Starlight Express cameras is Adirondak Video Astronomy, which also has a number of useful accessories for CCD photography. If you want a really large-area CCD camera, Apogee Instruments has the AP-16E with a chip area of  4K by 4K pixels with 9 micron pixcels.  This 16 megapixel chip captures more information than 35mm film (which contains about 10 megapixels of data)!  But note that the price of a CCD chip is proportional to its area, which is the linear dimension squared, so that camera sells for as much as an average new automobile.  But if you’re independently wealthy, it might be the way to go. Apogee also sells other high-end CCD cameras, with prices and specifications more or less starting at the top end of those sold by the vendors listed above. Also, Finger Lakes Instrumentation makes a number of CCD cameras that are well-regarded by many astrophotographers, with prices roughly at the higher end of the “affordable” cameras sold by the vendors listed above.

Here are some of my observations when I compare camera specifications and prices:

1. In the past, the setup that had come the closest to the ideal 2.5 arc-seconds/pixel was an f/3.3 focal reducer, with the CCD chips with the smallest pixels.  The pixel scale for the Fastar system with the TC-237 chip formerly used by SBIG (now replaced by the ST-402ME) wasn’t ideal but it provided a wider field of view which is needed for most deep-sky objects. To get a Pixel Scale of 2.5 with the Fastar system requires a chip with 5 micron pixels and previously no one manufactured such a chip. However, the SAC cameras with 5.6 micron pixel chips provide a Fastar pixel scale of 2.87 (on an 8” SCT), which comes very close to the optimum pixel scale for the Fastar system on a 2000mm scope. Especially considering their reasonable prices, SAC cameras have become a very popular choice for a Celestron 8” scope using the Fastar system. Also, the Starlight Express cameras have an elongated cylindrical form that works well for the Fastar setup since they don’t obstruct the aperture of the scope very much. They are good cameras and have become popular at the higher end of the camera specifications range, for Fastar use.

2. When you compare specifications it would appear that none of the cameras will work well for the prime focus (high-magnification) setup you need for planetary photography.  But it turns out that the camera manufacturers give you the option of electronically “binning” the pixels. Binning combines pixels to effectively make them larger and get the pixel scale up closer to the right range for prime focus photography. You can also put a Barlow lens in front of a CCD camera to narrow the field of view and thus change the pixel scale. So the small pixels in most cameras can work for both deep-sky and planetary photography.

3. Nevertheless, binning is still a compromise since you in effect end up with a smaller chip.  There are at lease two CCD cameras which because of their design will work particularly well for planetary photography with an 8” f/10 SCT. The SBIG ST-9E camera uses a Kodak KAF-0261E CCD with 20x20µ pixels which places its pixel scale at 2.05 at prime focus.  And the SBIG ST-1001E camera uses a Kodak KAF-1001E CCD with 24x24µ pixels which places its pixel scale at 2.46 at prime focus. Either of these would be a better choice if your primary interest is planetary rather than deep-sky photography, and if you can afford either of these cameras (which by themselves cost more than most GoTo telescopes).

4. Many of the Starlight Express cameras have the ability to autoguide the scope (as an additional option) while you are taking pictures, which is a nice feature.  The Starlight Express MX5C, MX7 C and the SXV-H9C units are among the only color CCD cameras available (they use Sony color chips). This eliminates the need to take separate shots through each of three color filters, at the expense of sensitivity (comparable to film speed in a film camera).  Note that a color CCD camera may be worth thinking about if you plan to concentrate in planetary imaging. To capture a color image with a normal (non -color) CCD camera requires three photos in succession, with a red, green, and blue filter respectively, using a color wheel accessory. (You need to add a color filter wheel if you want to take color photographs, because all the astronomical CCD chips are monochrome chips except those noted above.)  Both Mars and Jupiter rotate about their axes quickly. So for Mars you would need to capture all three color filter shots within 5 minutes if the three photos are to register the same view of Mars.  For Jupiter, all three photos would need to be captured within 2 minutes. Astrophotographers do this regularly but a color CCD camera may be more convenient. Look at Bob Holzer's Urban AstroImaging Web site for images with an MX-7C camera - it works well with an 8” Celestron SCT and its price is lower than the SBIG ST-7.

5. The Pictor 208/216 cameras formerly sold by Meade and sometimes available in the used equipment market, had very low prices but the chip they use is very small and there are a lot of deep-sky objects too large to fit onto it. It was regarded as a CCD camera that amateur astronomers outgrew quickly and traded in for a camera with a larger CCD. The Pictor 201 uses the same chip but is not cooled and was not marketed by Meade as a CCD camera, only as a stand-alone autoguider.  I.e. it will autoguide a GoTo scope without requiring a computer but cannot be used for astrophotography. Nevertheless many astrophotographers had good results using it as a secondary camera configured as an autoguider.
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* The actual quality of a CCD camera isn’t an issue of its pixel scale, but has to do with sensitivity, dark current, read noise, antiblooming, and the digital transfer rate of the camera to your computer. But the Pixel Scale is where you need to start, to narrow down your choice of camera.


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