30
Anti-blooming CCDs make astrophotography more convenient, but with trade-offs in quantum
efficiency (QE) and linearity. Anti-blooming protection requires additional circuitry on the surface
of the CCD, reducing the physical size and consequently the light gathering area of each pixel.
Anti-blooming CCDs also have a non-linear response to light. This non- linearity becomes
significant as a pixel fills beyond 50%. The closer a pixel gets to full-well capacity, the greater
the rate of electron drainage in order to prevent blooming. This generally isn’t a problem if your
goal is producing great-looking pictures of the night sky, but anti-blooming CCDs are generally
not appropriate for photometric and other scientific use where accurately recording the relative
brightness of objects is important.
Microlenses
CCDs only record the light that hits the photosensitive portion of the CCD. Most CCDs are “front
illuminated” meaning that the light strikes the top surface of the integrated circuit forming the
CCD. A portion of the surface of the CCD is covered with the electronic circuits that make a
CCD work. Light striking a part of the CCD covered by a circuit will not get recorded by
the CCD.
The surface of some CCDs is covered with microlenses which focus more of the light striking
the surface of the CCD onto the photosensitive area away from the circuits.
The amount of the CCD surface covered in circuits is one factor in determining the quantum
efficiency (QE) of the CCD. QE is a measure of how efficiently the CCD converts photons
striking the CCD into electrons stored in any given pixel. QE varies by type of CCD and by the
wavelength of light. Adding microlenses to a front-illuminated CCD will raise the quantum
efficiency of the CCD. Typical peak QE values for the CCDs used in QSI 600 Series cameras
range from 35% to over 80%. Microlens models tend to have the highest QE, while anti-
blooming gate models tend to have the lowest QE. Here is a graph showing the QE of the CCDs
available in QSI 600 Series cameras at the time of printing.
Note
that the non-anti-blooming, full frame
KAF-3200 and KAF-1603 have the highest QE,
peaking toward the red end of the spectrum around
650nm. The anti-blooming, interline transfer KAI-
2020 and KAI-04022 have the lowest QE, peaking
toward the blue end of the visible spectrum around
450nm.
Single-shot colour CCDs
CCDs are inherently monochrome devices with varying response to different frequencies of
light. That varying response can be seen in the quantum efficiency graph above. Colour images
are normally produced with CCD cameras by taking three (or more) images through red, green
and blue filters. The resulting images are then combined using computer image processing
programs into a final colour image.
