Section 2 - Introduction to CCD Cameras
Page 31
2.4.5. Pixels vs. Film Grains
Resolution of detail is determined, to a certain degree, by the size of the pixel in the detector
used to gather the image, much like the grain size in film. The pixel size of the detector in the
ST-10XE is 6.8 x 6.8 microns (1 micron = 0.001mm, 0.04 thousandths of an inch). In the ST-7XE
and ST-8XE it is 9 x 9 microns, in the ST-9XE it's 20 x 20 microns and in the ST-2000XM it is 7.4 x
7.4 microns. However, the effects of seeing are usually the limiting factor in any good
photograph or electronic image. On a perfect night with excellent optics an observer might
hope to achieve sub-arcsecond seeing in short exposures, where wind vibration and tracking
error are minimal. With the average night sky and good optics, you will be doing well to
achieve stellar images in a long exposure of 3 to 6 arcseconds halfwidth. This will still result in
an attractive image, though.
Using an ST-7XE or ST-8XE camera with their 9 micron pixels, an 8" f/10 telescope will
produce a single pixel angular subtense of 0.9 arcsecond. An 8" f/4 telescope will produce
images of 2.5 arcseconds per pixel. If seeing affects the image by limiting resolution to 6
arcseconds, you would be hard pressed to see any resolution difference between the two focal
lengths as you are mostly limited by the sky conditions. However, the f/4 image would have a
larger field of view and more faint detail due to the faster optic. The ST-9XE, with its 20 micron
pixels would have the same relationship at roughly twice the focal length or a 16 inch f/10
telescope. See table 4.4 for further information.
A related effect is that, at the same focal length, larger pixels collect more light from
nebular regions than small ones, reducing the noise at the expense of resolution. While many
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-9XE with its large 20 x 20 micron pixels
captures five times as much light as the ST-7XE and ST-8XE's 9 micron square pixels. For this
reason we provide 2x2 or 3x3 binning of pixels on most SBIG cameras. With the ST-7XE and ST-
8XE, 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-10XE 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-7XE 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:
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