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

By Chris SchurBy Chris Schur

IntroductionIntroduction

With Digital Imaging as the mainstay for astrophotograpy today, With Digital Imaging as the mainstay for astrophotograpy today, color balance remains an elusive target.color balance remains an elusive target.

Here we will discuss key points on locking in accurate color Here we will discuss key points on locking in accurate color balance for Deep Sky Images that use LRGB methodology.balance for Deep Sky Images that use LRGB methodology.

Our topics will include:Our topics will include: Setting a reference for our gray scaleSetting a reference for our gray scale Understanding the LRGB hue and saturation shiftUnderstanding the LRGB hue and saturation shift Correcting the shift in LRGB imagesCorrecting the shift in LRGB images Understanding the Hydrogen Alpha contribution to an imageUnderstanding the Hydrogen Alpha contribution to an image The role of Hydrogen Beta in the final color of nebulaThe role of Hydrogen Beta in the final color of nebula Tactfully combining Hydrogen and RGB data for balanced imagesTactfully combining Hydrogen and RGB data for balanced images

How the Eye Perceives ColorHow the Eye Perceives Color

Setting the Reference: The Setting the Reference: The SunSun

Kitt Peak Solar Observatory

An actual G2V CalibrationAn actual G2V Calibration

Sample G2V Calibrated Image: M3Sample G2V Calibrated Image: M3

Sample G2V Calibrated Image: M81 Sample G2V Calibrated Image: M81

Ha/L RGB Hue ShiftsHa/L RGB Hue Shifts

LRGB Images: The Color Shift LRGB Images: The Color Shift Problem: Salmon PinksProblem: Salmon Pinks

Lab image detailsLab image details Some math details – Why the shift?Some math details – Why the shift?

Conversion to Lab starts with RGB -> CIE XYZ Conversion to Lab starts with RGB -> CIE XYZ space then to Lab.space then to Lab.

Conversion math reveals a coupling of G and LConversion math reveals a coupling of G and L ““Salmon Pink” is light red + a bit of green addedSalmon Pink” is light red + a bit of green added Increasing L in value in conversion elevates GIncreasing L in value in conversion elevates G So by adding additional L, we shift reds to salmonSo by adding additional L, we shift reds to salmon

Correcting the Hue ShiftCorrecting the Hue Shift

Next, Correct the SaturationNext, Correct the Saturation

The Final ComparisonThe Final Comparison

The Final Result:The Final Result:

Adding H-Alpha Data to the Image:Adding H-Alpha Data to the Image:-Why?-Why?

For Emission Nebula can be 85% of photonsFor Emission Nebula can be 85% of photons But - But - H-alphaH-alpha is is veryvery close to the near IR band = 70% total close to the near IR band = 70% total AND AND H-BetaH-Beta is very close to the center of the Blue band = 15% is very close to the center of the Blue band = 15%

Hydrogen Hydrogen enhanced images are enhanced images are Simply StunningSimply Stunning, ,

and provide additional and provide additional Significant StructureSignificant Structure..

-How?-How? We then add it to the closest wavelengths: We then add it to the closest wavelengths: RED, RED, BLUE.BLUE.

Emission Nebula:Emission Nebula:The Process of adding The Process of adding HydroHydrogengen Data – Data – How NOT to do it…How NOT to do it…

Star Color distortionStar Color distortion

By adding By adding H-alphaH-alpha data we change the data we change the G2V balance of the star imagesG2V balance of the star images

We can We can maintainmaintain the balance by the balance by combining combining HydroHydrogengen data frame with data frame with the original the original R R && BB frame using frame using LightenLighten..

Restoring G2V balance with LightenRestoring G2V balance with Lighten

Second Example: M16 in SerpensSecond Example: M16 in Serpens

Final DiscussionFinal Discussion Understanding how adding additional Luminance Understanding how adding additional Luminance

data will affect the entire Lab Color Schemedata will affect the entire Lab Color Scheme Tastefully adding Hydrogen data into RGB images Tastefully adding Hydrogen data into RGB images

while retaining G2V balance is possiblewhile retaining G2V balance is possible