Tutorial:
Gimp for astrophotography

Content:

Calibration of monitor:
• 0.0: Monitor calibration

First steps:
• 0.1: Introduction: why use Gimp ?
• 0.2: How to load 32-bit r,g,b images in Gimp
• 0.3: Set levels of rgb image
• 0.4: Align the red, green, blue layers

Background correction:
• 1.1: Background subtraction, easy case
• 1.2: Background subtraction, difficult case, during work
• 1.3: Background subtraction, comet & bright stars

Vignetting correction:
• 2.1: Correct image for vignetting
• 2.2: Correct for really bad vignetting

Separation of stars and DSO:
• 3.1: Separation of Stars and DSO
• 3.2: Star select
• 3.3: Star and DSO mask
• 3.4: Star image and DSO image

Alternative methods for selecting stars:
• 4.1: Selecting by color
• 4.2: Selecting by threshold
• 4.3: Selecting by difference
• 4.4: Selecting by wavelet (coming soon)

Processing a nebula with masks:
• 5.1: Setup masks
• 5.2: Editing nebula
• 5.3: Editing nebula with holes filled (coming soon)

Note:
I take no responsibility or liability for what are written here, you use the information at your own risk!

0.0: Monitor calibration

Calibration:

Before doing any image editing your monitor has to be calibrated. Without that there is a big risk that the colors are wrong. There are a calibration procedure built into Windows that don't need any calibration instrument. Better than nothing, but with a calibration instrument the monitor calibration can be done with much higher quality.

Screen calibration tutorials:

• https://www.rtings.com/ ...How To Adjust Your Monitor's Settings
• https://www.youtube.com/ ...How to Calibrate your Monitor - 4 Methods

Calibration instrument:

I have an old Spider 4 calibrator from Datacolor. It was old already when I bought it. When Windows 10 was introduced it had some problem and today the software doesn't work anymore. Is it only a trash now? That was what I thought, but then I found that there was a general control software that could use many brands of hardware, including this one.

I have two 24 inch monitors connected to my working computer, very handy. One is a Fujitsu / Siemens and the other is a NEC. Both has the resolution 1920x1200, little bit higher than the normal HD standard. The NEC to the right has a bit better quality I have found when I calibrated them, but not much difference.

The calibration instrument should sit in center of the monitor. I calibrate both monitors and store each calibration file in Windows. Windows use these files by default when Windows start up.

Software:

This is the software that gave my calibration instrument a prolonged life, DisplayCAL. It can be downloaded for free. Follow the instructions how to install and setup, for my Spider 4 I have to download and install some other driver.

The calibration process is slow, it can take an hour to complete. Don't forget to save the new profile when finished.