Orion’s belt and sword, processing with open source tools

Capturing photos

To do processing one needs to take few photos first. Minimal hardware is DSLR with 50mm lens, remote shutter release for DSLR and any kind of cheap telescope with equatorial mount. It is not necessary to buy 50mm lens, kit lens usually 18-55mm will do, if your DSLR have live view and you can set focus. Manually setting focus on kit lens without live view is quite difficult. But it will be nice to have one 50mm F1.8 lens which is much faster and sharper than kit lens. If you have longer focal length lens it is even better, 100mm or even up to 200mm, over it tracking becomes big problem. Why remote shutter release? Camera supports up to 30 seconds exposure and after that BULB, you should press and hold shutter button what will cause lot of shaking and ruin photo. For that reason remote shutter release is used, we can have long exposure without shaking and strain. Telescope is used so that we can piggyback camera on it and achieve long exposure without trailing stars. How to attach camera to scope, there are piggyback brackets, piggyback camera mounts, you can make them on your own using hose clamp. If your telescope got RA motor drive, do polar alignment and switch it on, shorter focal length of the lens is more tolerant towards tracking errors. If there is no RA motor drive you will have to do manual tracking. Telescope with alt-azimuth mount won’t do it must be with equatorial mount. There are specialized devices like Vixen Polarie or AstroTrac which can be used instead, but they are more expensive than small 5" Newtonians with RA motor drive. Why one shouldn’t just do few hundreds shots from tripod and later stack that in Deep Sky Stacker? Because 5 seconds would be longest acceptable exposure for 50mm lens and to get to 5 minutes you need 3600 photos. Way to go is increasing exposure time.
If you are not sure where is Orion or M42 inside Orion install Stellarium http://www.stellarium.org/ it is open source and works on all major operating systems.

Processing

So I went out under my light polluted suburban sky and managed to get few decent photos between clouds. I did the same night before and now I have 13 frames 55mm F5.6 ISO 800 where exposure is from one minute to two minutes. When I decided to crop them and stack them together align_image_stack from Hugin did poor job. Was that too narrow cropping or something else I do not know, for that reason new strategy was stack them by night and later stack final results. So, imported RAWs into darktable, applied chromatic aberration and lens correction and exported them to 16 bit TIF. Should do hot pixels removal but I forgot to do it and did it later in G’MIC, like this:

gmic IMG_0451.tiff -remove_hotpixels 3,10 -c 0,65536 -type ushort -output IMG_0451hp.tiff

Now I aligned stack:

align_image_stack -a tif  *.tiff

and averaged them two by two, saving output:

gmic tif0000.tif tif0001.tif -div 256 -gimp_compose_average 1,0 -mul 256 -c 0,65536 -type ushort -output step1.tiff

Later I combined those steps, two by two, until final result. To stack those two images I need to rotate one and to crop both of them:

gmic m4291.tiff -rotate -66 -crop 1400,2500,3500,4700 -c 0,65536 -type ushort -output m4291c.tiff
gmic m4281.tiff -crop 1450,767,3550,2967 -c 0,65536 -type ushort -output m4281c.tiff

To find out how much to rotate and crop I used GIMP. Now fine alignment of those two with align_image_stack:

align_image_stack -a tif *.tiff

and final blending for16 bit output:

gmic tif0000.tif tif0001.tif -div 256 -gimp_compose_average 1,0 -mul 256 -c 0,65536 -type ushort -output m4289.tiff

and also one 8 bit for GIMP:

gmic tif0000.tif tif0001.tif -div 256 -gimp_compose_average 1,0 -output m4289.jpg

Light pollution and clouds contributed to final result.

It is too bright, too much orange in it, but M42 is visible and Flame nebula is just barely visible. If we look at histogram, this is where we are and where we want to be:

From GIMP menu we select Color->Levels. In Adjust Color Levels we set Channel to Red and move upper slide from the bottom towards middle.

We do the same for green, looking at histogram set to RGB and picture itself. After this image is still too bright and Flame nebula is invisible. To remedy that I will rise contrast, Colors->Brightness-Contrast and I set contrast on 40. Now LAB decompose color boost, described in one of previous tutorials and some more contrast stretching via Colors->Auto->White Balance and after that Edit->Fade, we want histogram stretched but not that much, again checking image and histogram to find out how much.
Here is the final result:

Conclusion I need lens and light pollution filter ;-)