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Section 2 - Introduction to CCD Cameras
Page 11
people think that smaller pixels are a plus, you pay the price in sensitivity due to the fact that
smaller pixels capture less light. For example, the ST-9E with its large 20 x 20 micron pixels
captures five times as much light as the ST-7E and ST-8E's 9 micron square pixels. For this
reason we provide 2x2 or 3x3 binning of pixels on most SBIG cameras. With the ST-7 and ST-8,
for instance, the cameras may be configured for 18 or 27-micron square pixels. Binning is
selected using the Camera Setup Command. It is referred to as resolution (High = 9µ
2
pixels,
Medium = 18µ
2
pixels, Low = 27µ
2
pixels). When binning is selected the electronic charge from
groups of 2x2 or 3x3 pixels is electronically summed in the CCD before readout. This process
adds no noise and may be particularly useful on the ST-10E with its very small 6.8 micron
pixels. Binning should be used if you find that your stellar images have a halfwidth of more
than 3 pixels. If you do not bin, you are wasting sensitivity without benefit. Binning also
shortens the download time.
The halfwidth of a stellar image can be determined using the crosshairs mode. Find the
peak value of a relatively bright star image and then find the pixels on either side of the peak
where the value drops to 50% of the peak value (taking the background into account, if the star
is not too bright). The difference between these pixel values gives the stellar halfwidth.
Sometimes you need to interpolate if the halfwidth is not a discrete number of pixels.
Another important consideration is the field of view of the camera. For comparison, the
diagonal measurement of a frame of 35mm film is approximately 43mm, whereas the diagonal
dimension of the ST-7E chip is approximately 8 mm. The relative CCD sizes for all of the SBIG
cameras and their corresponding field of view in an 8" f/10 telescope are given below:
Camera Array Dimensions Diagonal Field of View at 8" f/10
Tracking CCD 2.64 x 2.64 mm 3.73 mm 4.5 x 4.5 arcminutes
ST-5C 3.20 x 2.40 mm 4.00 mm 5.6 x 4.2 arcminutes
ST-237 4.74 x 3.55 mm 5.92 mm 8.2 x 6.1 arcminutes
STV 4.74 x 2.96 mm 5.58 mm 8.2 x 5.1 arcminutes
ST-6 8.63 x 6.53 mm 10.8 mm 14.6 x 11 arcminutes
ST-7E 6.89 x 4.59 mm 8.28 mm 11.9 x 7.9 arcminutes
ST-8E 13.8 x 9.18 mm 16.6 mm 23.8 x 15.8 arcminutes
ST-9E 10.2 x 10.2 mm 14.4 mm 17.6 x 17.6 arcminutes
ST-10E 14.9 x 10.0 mm 17.9 mm 25.1 x 16.9 arcminutes
ST-1001E 24.6 x 24.6 mm 34.8 mm 41.5 x 41.5 arcminutes
35mm 36 x 24 mm 43 mm 62 x 42 arcminutes
Table 2.2 - CCD Array Dimensions
2.4.6. Guiding
Any time you are taking exposures longer than several seconds, whether you are using a film
camera or a CCD camera, the telescope needs to be guided to prevent streaking. While modern
telescope drives are excellent with PEC or PPEC, they will not produce streak-free images
without adjustment every 30 to 60 seconds. The ST-7E, ST-8E, ST-9E and ST-10E allow
simultaneous guiding and imaging, called self-guiding (US Patent 5,525,793). This is possible
because of the unique design employing 2 CCDs. One CCD guides the telescope while the
other takes the image. This resolves the conflicting requirements of short exposures for
guiding accuracy and long exposures for dim objects to be met, something that is impossible
with single CCD cameras. Up to now the user either had to set up a separate guider or use