Tutorial:
Gimp for astrophotography

Content:

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!

4.4: Selecting stars by wavelet

Wavelet is a sort of Fourier transformation. It separate the images in different spatial frequencies. At high frequencies you find the stars and you also find noise there. At low frequencies, slowly spatial frequencies variation, you found the soft DSO, i.e. nebulas. This advanced technique can be used to do some interesting things with.

More information about Wavelet transform:

• https://en.wikipedia.org/ wiki/ Wavelet transform

Preparing the NGC 7000 image for star separation:

The test image I use is NGC 7000, the California Nebula. The camera is a Nikon D800 DSLR and the lens a Pentax 645 FA 300 mm f/4 ED lens. Exposure 24x240 seconds at ISO 400. This photo is taken in a Bortle Class 4 area. It's in 32 bit floating point format. Flat and photometric color calibrated. The edges has been cropped. In Gimp I have increased the saturation, crank up the gamma a bit and set its base level, that's all.

To be continued.