White Light Solar Image Processing in GIMP 2.6

[David Smith]

I started solar imaging back a few months now and have been using Bizibilders excellent tutorial as a guide:-

http://stargazerslou...g/#entry1550177

Not being a Windows user I do not have access to Photoshop or the like and so use GIMP for image processing. It's felt like a bit of an uphill struggle using GIMP but I think I am getting some where now. I have translated the Photoshop process in the tutorial into "GIMP speak" below.

Colour > Auto > Stretch Contrast

Colour > Levels Move the mid slider to 0.5

Colour > Curves fix the centre of the line (click on it but don’t move it) then click “half way” to the bottom and set that point at 64/55. Click half way to the top and set that point to 187/197. (Note: first number is input value/second number is output value)

Colour > Colour Balance Move the sliders (Midtones) Cyan / Red to +50 and Yellow / Blue to -70

Colour > Hue-Saturation Set the Saturation to 10

Filter > Sharpen > Unsharp Mask Set Radius to 120 and amount to 0.10

Filter > Sharpen > Unsharp Mask (i.e. Do a second unsharp mask). Set Radius to 1.5 and amount to 0.50

Filter > Blur > Gaussian Blur Set the radius to 0.4px

There are a couple of steps that either I cannot translate or did not work well for me in GIMP and I have omitted:-

"Image > Adjustments > Exposure Set the Gamma Correction to 0.75"

I have not been able to translate this yet. Closest I can get is Colour > Levels move the middle slider to 0.75 but this does not work well for me and I do not use it.

"Image > Adjustments > Photofilter Set the colour to R=220, G=170, B=30 and move the density slider to 50%"

Again, this can be done in Colour > Levels by selecting the appropriate channel and then adjusting the slider to the required value. However each time I have done this it gives a really odd colour cast to my image.