Saturday, January 19, 2013

Messier 42 with D800 and Astrotrac

Messier 42 aka M42 or NGC 1976 or Great Orion nebula is the first photographed deep sky (object outside the solar system) subject. First time it was photographed in 1880 by Henry Draper and the picture can be seen at Wikipedia. So, 133 years later I was in the path of Mr Draper, only with much more modern equipment that is operable by a single man.

M42 & M43
(The faint nebula above M42/M43 pair is the triplet NGC 1973, 1975 and 1977. It's an emission nebula divided in three by a dark nebula.)

Due to the immense brightness (relatively speaking) of the center of the nebula this was photographed at ISO 400. Bigger sensitivities would only lead to the center being overexposed and offering no advantage noise-wise. The dynamic range of a camera is always biggest at the lower ISOs as you can see in these graphs. Canon sensors seems to make an exception there though.

If there was less light pollution (white snow reflects terrestrial light sources back to the skies too well) and the D800 would bit more sensitive to deep red h-alpha light, the nebula would indeed appear a bit bigger and some more details would be recorded. See example image here.

Twenty images were stacked together from 16bit TIFF files made by Raw Therapee. Images were aligned in Hugin and stacked with Enfuse. Final histogram stretching was made also in Raw Therapee. Histogram stretching is the step that brings out the details in astrophotography imagery; black level is limited by light pollution and the white point is limited by camera's sensor and sensitivity. It is not uncommon to clip out almost 80 percent of the histogram, mostly from the dark section of the image.

By the way, if you ever wonder if a bright spot on the night sky of light is star or a planet, you can tell them apart very easily. Just look for twinkling - planets will not twinkle as they are disks and not point sources of light. Below you can see comparison picture with two stars: Betelgeuse and Sirius. They are the bayerian alpha stars of Orion and Canis Major - respectively. At the right is the yellowy light of Jupiter.

How to differentiate between planets and stars

Note how the stars have changed color during the exposure. It is the scintillation effect of the turbulent air between observer and the star that causes these rainbow-colored streaks. Since the planet is so big (again, relatively speaking), it has drowned the twinkling effects in its big size.

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