Tips for Processing LRGB and HaRGB Images: Getting the Color Right!

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Tips for Processing LRGB and HaRGB Images
Getting the Color Right!

• By Chris Schur

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Introduction

• 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
• 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

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How the Eye Perceives Color
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Setting the Reference The Sun
Kitt Peak Solar Observatory
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An actual G2V Calibration
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Sample G2V Calibrated Image M3
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Sample G2V Calibrated Image M81
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Ha/L RGB Hue Shifts
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LRGB Images The Color Shift Problem Salmon
Pinks
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Lab image details

• Some math details Why the shift?
• Conversion to Lab starts with RGB -gt 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

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Correcting the Hue Shift
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Next, Correct the Saturation
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The Final Comparison
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The Final Result
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Adding H-Alpha Data to the Image-Why?

• For Emission Nebula can be 85 of photons
• 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
• Hydrogen enhanced images are Simply Stunning,
• and provide additional Significant Structure.
• -How?
• We then add it to the closest wavelengths RED,
  BLUE.

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Emission NebulaThe Process of adding Hydrogen
Data How NOT to do it
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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
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Second Example M16 in Serpens
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Final Discussion

• 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