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QSI 500 SERIES USER GUIDE

draining off excess electrons before they exceed the capacity of the pixel. This can incre

the dyn

ase

amic rage of the CCD by as much as 300 times or more. This increase in dynamic

range greatly reduces the difficulty of imaging bright objects.

at hits the photosen

sitive portion of the CCD. Most CCDs are

g 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 500

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 three CCDs available in the QSI 516.

ed with CCD cameras by taking three (or more) images through

Anti-blo

oming 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 th

“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 circuit

that make a CCD work. Light strikin

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

capturing more of the light striking the

CCD.

KAF-1602LE has roughly half the QE of

the non-anti-blooming KAF-1603E. A

higher QE allows shorter exposures by

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 produc

red, green and blue filters. The resulting images are then combined using computer image

processing programs into a final color image.

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