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These two images were
created with RGB ratios of 1,1,1. Run your mouse over each image to see
a view with eXcalibrator ratios.
For the first image, eXcalibrator calculated RGB ratios of 1.00 1.155
and 1.46.
For the second image eXcalibrator calculated RGB ratios of 1.00 1.305
and 1.533.
In all cases, pixel math was applied to the individual red, green and
blue images to remove color bias. The eXcalibrator ratios were identical
before and after removing the bias.
After creating the RGB images, the ones using the eXcalibrator ratios
had a strong cyan bias. Pixel math was again used to remove this before
stretching the images.
There is a significant difference in the eXcalibrator results.
Unfortunately, the Sloan Digital Sky Survey star data were not
available. The ratios were calculated with the NOMAD database. These
data are sometimes inconsistent and unreliable. Removing statistical
outliers left only nine stars for the color ratio calculations. And
still, the standard deviation was relatively high. With the NOMAD data,
more stars are needed to average errors.
The eXcalibrator color in the first image looks good. Without this color
adjustment, the nebula is mostly red with almost no color variance. The
eXcalibrator version shows some bluish-red areas. We often see this with
emission nebula. Also, the bright star left of center correctly has a
blue cast.
With the second image, the eXcalibrator ratios appear too extreme.
Something about halfway between the non-corrected and eXcalibrator view
is probably about right. |
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