Blue-Red Offset Time Series
MOBY was designed to measure underwater radiance, irradiance and surface irradiance.
Underwater radiance at 450 nm is up to 4 orders of magnitude greater than that at 700 nm. This results in
difficulties measuring the blue and red underwater spectrum using one spectrograph. In a single
grating spectrograph covering the visible band between 350 and 900 nm, scattered blue light and
second-order spectra impinging on the red portion of the detector overwhelm the small red signal.
Therefore, two spectrographs and a "water mirror", which rejects blue wavelengths from the red
spectrograph, are used to measure the blue and red underwater spectra.
The blue and red spectrograph measurements overlap from 550 to 640 nm. We find that the blue and red spectra
are offset in the overlap area. Internal reflections and stray light in the spectrographs produce
this offset. The offset at 620 nm between the overlapping spectra, as well as, the fractional difference
are shown below.
