Download Digital Image Processing: Getting the Color Right!

Tips for Processing LRGB and HaRGB
Images: Getting the Color Right!
By Chris Schur
Introduction
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With Digital Imaging as the mainstay for astrophotograpy today,
color balance remains an elusive target.
Here we will discuss key points on locking in accurate color
balance for Deep Sky Images that use LRGB methodology.
Our topics will include:
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Setting a reference for our gray scale
Understanding the LRGB hue and saturation shift
Correcting the shift in LRGB images
Understanding the Hydrogen Alpha contribution to an image
The role of Hydrogen Beta in the final color of nebula
Tactfully combining Hydrogen and RGB data for balanced images
How the Eye Perceives Color
Setting the Reference: The Sun
Kitt Peak Solar Observatory
An actual G2V Calibration
Sample G2V Calibrated Image: M3
Sample G2V Calibrated Image: M81
Ha/L RGB Hue Shifts
LRGB Images: The Color Shift
Problem: Salmon Pinks
Lab image details
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Some math details – Why the shift?
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Conversion to Lab starts with RGB -> CIE XYZ
space then to Lab.
Conversion math reveals a coupling of G and L
“Salmon Pink” is light red + a bit of green added
Increasing L in value in conversion elevates G
So by adding additional L, we shift reds to salmon
Correcting the Hue Shift
Next, Correct the Saturation
The Final Comparison
The Final Result:
Adding H-Alpha Data to the Image:
-Why?
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For Emission Nebula can be 85% of photons
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But - H-alpha is very close to the near IR band = 70% total
AND H-Beta is very close to the center of the Blue band = 15%
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Hydrogen enhanced images are Simply Stunning,
and provide additional Significant Structure.
-How?
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We then add it to the closest wavelengths: RED, BLUE.
Emission Nebula:
The Process of adding Hydrogen Data – How
NOT to do it…
Star Color distortion
By adding H-alpha data we change the
G2V balance of the star images
 We can maintain the balance by
combining Hydrogen data frame with
the original R & B frame using Lighten.
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Restoring G2V balance with Lighten
Second Example: M16 in Serpens
Final Discussion
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Understanding how adding additional Luminance
data will affect the entire Lab Color Scheme
Tastefully adding Hydrogen data into RGB images
while retaining G2V balance is possible