Q S I 5 0 0 S E R I E S U S E R G U I D E
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Anti-blooming is a feature available on many full-frame and most interline transfer CCDs
Anti-blooming technology limits the number of electrons that can accumulate in a pixel b
drainin
.
y
g off excess electrons before they exceed the capacity of the pixel. This can increase
the dynamic rage of the CCD by as much as 300 times or more. This increase in dynamic
lty of imaging bright objects.
-
re
rface 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.
microlenses which focus more of the light
otosensitive area away from the circuits.
n circuits is one factor in determining the quantum
e of how efficiently the CCD converts photons
any given pixel. QE varies by type of CCD and by
s to a front-illuminated CCD will raise the
ak QE values for the CCDs used in QSI 500
r 80%. Microlens models tend to have the
els tend to have the lowest QE. Here is a graph
e in the QSI 516.
Note that the KAF-1603ME (with microlenses)
has the highest QE peaking around 650nm in
the red band of visible light. The KAF-1603
without microlenses has a slightly lower QE, but
a similar curve, while the anti-blooming KAF-
1602LE has roughly half the QE of the non-anti-
processing programs into a final color image.
range greatly reduces the difficu
Anti-blooming CCDs make astrophotography more convenient, but with tradeoffs 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 a
“front illuminated” meaning that the light strikes the top surface of the integrated circuit
forming the CCD. A portion of the su
The surface of some CCDs is covered with
striking the surface of the CCD onto the ph
The amount of the CCD surface covered i
efficiency (QE) of the CCD. QE is a measur
striking the CCD into electrons stored in
the wavelength of light. Adding microlense
quantum efficiency of the CCD. Typical pe
Series cameras range from 35% up to ove
highest QE, while anti-blooming gate mod
showing the QE of the three CCDs availabl
blooming KAF-1603E. A higher QE allows
shorter exposures by capturing more of the light
striking the CCD.
Single-shot color 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. Color
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