How was the green/blue crossover point
selected? It seems biased toward the green.
The blue/green crossover point was selected
with special consideration of the prominent
nebular emission lines of oxygen (OIII)
which are very close to 500nm. Two
additional system factors were considered:
- Human eye response to OIII emission lines
(i.e. human eye blue/green crossover)
- CCD sensor blue vs. green sensitivity
Since typical CCD's require a longer
blue-channel exposure time for proper balance
of broad-band (blackbody) sources, a slight bias
toward green in the blue/green crossover points
in the filter response was added
so that the final image would have properly
balanced color for both blackbody sources (i.e.
stars) and OIII emission line sources (i.e.
oxygen-rich nebulosity.
The net response of
the filters combined with the Kodak KAF3200ME
spectral response curve are shown in figure 1
(type Z filters) and figure 2 (type 4 filters).
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Figure 1. Type-Z / Kodak KAF3200ME curves,
including 1.4X exposure time for blue channel.
Click on the chart for a
detailed view.
Figure 1. Type-4 / Kodak KAF3200ME curves,
including 1.3X exposure time for blue channel.
Click on the chart for a
detailed view.
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