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Tutorial:
DSLR cameras RGB multipliers


RGB Multipliers

  1. Introduction
  2. Download DCRaw and copy to a folder
  3. Open Command Window
  4. Change directory in Command Window
  5. DCRaw parameters
  6. Get cameras RGB multipliers or white balance values

1: Introduction

When working with a DSLR color camera it could be a bit frustration to have correct colors on the final astroimage. If you know what your DLSR camera think is a natural looking image it would be easier. With the tool DCRaw you can get information from your camera's multipliers it use to the three color channels.

Why this? The sensor in the camera doesn't have equal sensitiveness to different color or wave lengths. And the white balance also depends of the lighting conditions. In the case of stars and astrophoto the daylight setting could be close for a unmodified DSLR camera.

I normally have the camera set to daylight white balance. With that setting you will get close to our Sun's color as a light source. Sun is a star so maybe don't too stupid.

Important:
The White Balance doesn't alter the raw image, but the RGB multiplier (scaling) values are written in the header to let the image program know how to color balance the image. It's those values we are looking for.

Note:
It only works for an unmodified camera! Or maybe for a factory built astrocamera like a Canon 20Da, I don't know for sure.

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2: Download DCRaw and copy to a folder

Google for a compiled version of DCRaw that fits your computer system. I have a Windows 10 64 bit computer and this tutorial will be for that. New cameras needs the latest version.

After when you have downloaded the DCRaw file, place it in some directory where you want to have it. I think you can have it under the Windows directory and then it could be accessed from all directories, I don't use it that way.

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3: Open Command Window

To use the DCRaw tool you have to access it from Windows Command window. In Windows 10 you can use the Windows key + X key to open this window below, sorry in Swedish. You shall click on the "Kör" or in English "Run". There are others way to access the Command Window, search how to open it for your operation system.

Windows + X key

After clicking on "Kör" or "Run" you get this new window:

Run CMD

Type cmd (Command) and then click "OK" to start Command Window.

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4: Change directory in Command Window

If you succeed with the above you shall have the Command window open now like this (without the last typing):

Change directory in Command Window

Now, move to the correct directory. It's DOS commands.

  • CD
  • Change Directory

  • CD..
  • Go up one directory, in this example I gave this command twice.

  • CD dcraw
  • Means open DCRaw directory, if you have another name of it, type that name.

  • DIR
  • List all files in directory. We see here the dcraw.exe file and my raw camera file img_7137.cr2.

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5: DRaw parameters

Type dcraw and you will get a list of all commands you can send to dcraw:

DCRaw parameters

In this tutorial we use the command -v (Show Metadata) and -w (Camera White Balance).

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6: Get cameras RGB multipliers or white balance values

Now we are ready to get information from the raw file.

Cameras RGB Multipliers or White balance parameters

Type dcraw -v -w filename. In this case: "dcraw -v -w img_7137.cr2".

We can now see that Canon 6D has the multipliers:

  • Red channel: x1.961914
  • Green channel: x1.0
  • Blue channel: x1.632813

This is the gain factor for the red and blue color channels relative the green channel. Multiply the values with this and then you get what Canon thinks is a natural color for a daylight scene. You can set the gain factors in your astro image editing program, like DSS (Deep Sky Stacker), Fitswork or others program you use. Most of them shall have a multiply function, or you can set the values already when it read the raw files. I do prefer to do it later to have more control of it. This is a rough estimation, but you come much closer and make it easier to get a natural look.

Note:
You must at least bias calibrate your image if you apply this scaling after that you have the raw files read in. After the scaling has been done the first look will normally be a very red image, that's because of light pollution. You shall now subtract (not multiply) away the background (light pollution).

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