SBIG
S
ANTA
B
ARBARA
I
NSTRUMENT
G
ROUP
SBIG CFW-9
Page 10
combination while watching the wheel to make sure it positions
properly. To verify the correct filter is in place, look through the
filter with a flashlight. The reflection off the filter is a different color!
Selection of Color Filters
SBIG color filters include an infrared blocking coating so that a
separate IR blocking filter is not needed. The filters used in the
SBIG filter wheel have been chosen to optimize the accuracy of the
colors produced by the tri-color software. The passbands of each
filter are summarized in the table below:
Color Filter Passbands
Filter Passband
(nm)
Red 612-670
Green 488-574
Blue 392-508
The filters are custom filters designed specifically for SBIG
cameras. Dichroic filters were chosen over absorption filters for
three reasons. First of all, they have high transmission in the
passband region. Second, a sharp transition can be obtained in the
region around 496 nm. And last, they are all deposited on a sheet of
glass of the same thickness, eliminating focus shifts due to varying
filter thickness. An infrared blocking coating is applied to these
filters to eliminate spectral leaks in the infrared where the CCD has
high sensitivity. These bands would be severely contaminated if
these leaks were not blocked. Filters designed for visual use and
colored plastic filters also have severe red leaks beyond the visible
spectral range, so one should particularly avoid those.
The sharp transition between the blue and green region is
necessary for proper color rendition of emission sources. The 500nm
