Section 4 – Camera Hardware
Page 49
where size is the pixel dimension or CCD dimension in millimeters and the focal
length is the focal length of the telescope or lens. Also remember that 1° = 3600
arcseconds.
Read Noise - The readout noise of a CCD camera affects the graininess of short exposure
images. For example, a CCD camera with a readout noise of 30 electrons will
give images of objects producing 100 photoelectrons (very dim!) with a Signal to
Noise (S/N) of approximately 3 whereas a perfect camera with no readout noise
would give a Signal to Noise of 10. Again, this is only important for short
exposures or extremely dim objects. As the exposure is increased you rapidly get
into a region where the signal to noise of the final image is due solely to the
exposure interval. In the previous example increasing the exposure to 1000
photoelectrons results in a S/N of roughly 20 on the camera with 30 electrons
readout noise and a S/N of 30 on the noiseless camera. It is also important to
note that with the SBIG CCD cameras the noise due to the sky background will
exceed the readout noise in 15 to 60 seconds on the typical amateur telescopes.
Even the $30,000 priced CCD cameras with 10 electrons of readout noise will not
produce a better image after a minute of exposure!
Full Well Capacity - The full well capacity of the CCD is the number of electrons each pixel can
hold before it starts to loose charge or bleed into adjacent pixels. Larger pixels
hold more electrons. This gives an indication of the dynamic range the camera is
capable of when compared to the readout noise, but for most astronomers this
figure of merit is not all that important. You will rarely takes images that fill the
pixels to the maximum level except for stars in the field of view. Low level
nebulosity will almost always be well below saturation. While integrating longer
would cause more build up of charge, the signal to noise of images like these is
proportional to the square-root of the total number of electrons. To get twice the
signal to noise you would have to increase the exposure 4 times. An ST-5C with
its relatively low full well capacity of 50,000e
-
could produce an image with a
S/N in excess of 200!
Antiblooming - Most SBIG CCD cameras have antiblooming protection. The TI CCDs used in
the ST-5C, ST-237, ST-237A, TC-237 autoguider and TC-211 autoguider have
antiblooming built into the CCDs. The Kodak CCDs used in the ST-7XE and ST-
8XE have Antiblooming versions of the CCDs available and the CCD used in the
ST-2000XM only comes with antiblooming. Blooming is a phenomenon that
occurs when pixels fill up. As charge continues to be generated in a full pixel, it
has to go somewhere. In CCDs without antiblooming protection the charge
spills into neighboring pixels, causing bright streaks in the image. With the
CCDs used in the SBIG cameras the excess charge can be drained off saturated
pixels by applying clocking to the CCD during integration. This protection
allows overexposures of 100-fold without blooming. The trade off is sensitivity.
Antiblooming CCDs are less sensitive than non-antiblooming CCDs. In the case
of the ST-7XE and ST-8XE, for example, the non-antiblooming versions are very
roughly twice as sensitive.
The CCDs used in the ST-9XE, ST-10XE and ST-10XME and ST-1001E
detectors do not come in an antiblooming version.
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