Why is the limb brighter than the center in some images of the sun, and the other way around in other images? I’m guessing either wavelength dependancy or post-processing artistic choices, but I don’t know which and the article made me actually aware of my ignorance.
In that link it is due to the wavelength used (extreme UV, which shows only active regions as bright, and the prominence layer surrounding the limb.)
However, in many amateur pictures taken in the H-alpha wavelength (like in TFA), the artistic choices come into play, as you said. In the Simon's first image the prominence layer (just outside the limb) is shown as-is (positive), while the disc interior is shown inverted (a negative; note e.g. how the sunspots became white). This is done to get a somewhat more "3D look". In a normal, non-inverted image, the mottles and filaments show up as dark features on a bright background, while inverting may make them look more "natural" (to some) - sometimes downright "casting shadows" (a solar filament often has a brightening around its base - which becomes dark after inversion).
Source: I'm the ImPPG's author (and enjoy solar astrophotography myself).
Just a side note, the opposide effect is can be achieved with a phenomenon called Subsurface Scattering (also used in 3d-cg). This has nothing to do with stars, though - unless a star exists that is translucent.
The moon is also fun to photograph. The lockscreen of my phone is a photo of the moon I took with an entry level DSLR and 250mm lens. It’s reasonably sharp and detailed.
I didn’t realize before that just how quick the damn thing is.
It of course depends on the focal length of the lens, but with a tele lens in the 200-400mm region (also depending on your sensor size), I would estimate that the moon crosses the image in the order of a minute. There is plenty of time to frame and focus the shot, but you have to readjust your camera every couple of shots.
This motion is just the earth rotation and the same for the moon, the sun and all the stars. The rotation of the moon around the earth becomes noticeable only from day to day, as the moon roughly moves 13 degrees/day. Tracking mounts have a separate speed setting though, which does take the moons movement into account for greater precision.
Ignoring the (rather slow) orbital motion of the Moon, consider: the sky (and the Moon) appears to rotate 360°/24 h. This amounts to 360/(24*3600) = 0.004167 °/s. The apparent size of the lunar disc is about 0.5°, so it takes about 120 s for the Moon to move by its own diameter in the sky. Plenty of time.
Exposure time was 1/640 of a second[1]. Must say I’m surprised it was that fast, but you fight both blurriness from the motion and the quite intense light reflected by a full moon.
So first manually focus, then I aimed a bit ahead along its path and waited a few seconds. Remote trigger and tripod is necessary to avoid any camera movement.
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[ 830 ms ] story [ 1272 ms ] threade.g. https://en.wikipedia.org/wiki/File:Sun_STEREO_4dec2006_lrg.j... vs. https://en.wikipedia.org/wiki/File:Sun_white.jpg
However, in many amateur pictures taken in the H-alpha wavelength (like in TFA), the artistic choices come into play, as you said. In the Simon's first image the prominence layer (just outside the limb) is shown as-is (positive), while the disc interior is shown inverted (a negative; note e.g. how the sunspots became white). This is done to get a somewhat more "3D look". In a normal, non-inverted image, the mottles and filaments show up as dark features on a bright background, while inverting may make them look more "natural" (to some) - sometimes downright "casting shadows" (a solar filament often has a brightening around its base - which becomes dark after inversion).
Source: I'm the ImPPG's author (and enjoy solar astrophotography myself).
https://www.sciencealert.com/there-could-be-transparent-star...
https://en.wikipedia.org/wiki/Exotic_star
* https://drive.google.com/drive/folders/1M0EpmTs6iU97J05846UA...
And his Instagram:
* https://www.instagram.com/simon2940/
I didn’t realize before that just how quick the damn thing is.
So first manually focus, then I aimed a bit ahead along its path and waited a few seconds. Remote trigger and tripod is necessary to avoid any camera movement.
[1] f/9, Iso 400, -3 step