The discussion centers around the clarity of photographs of galaxies, specifically addressing why stars on the far side of a galaxy, such as Andromeda, appear clear despite the long time it takes for their light to reach us. Participants explore the implications of star movement and exposure times in astrophotography.
Participants express differing views on the relevance of star movement and exposure time, with no clear consensus on the implications for image clarity. Some agree on the negligible effect of angular motion during exposure, while others raise questions about the overall clarity of the images based on movement over time.
Limitations include assumptions about exposure times and the effects of star movement, as well as the complexity of factors influencing image clarity that are not fully resolved in the discussion.
To add to Orodruin's comment, if you had an exposure time of, say, 1,000 years then you would get fuzziness. Most photographers aren't willing to wait that long.zuz said:
The stars didn't move much during the exposure, they moved long before it.zuz said:
? Why do you think a single airplane going past the lens for maybe a second would "ruin" a 1,000 year exposure ?zuz said:
I was looking for something once, using google satellite view. As I zoomed in on the subject I noticed a weird fuzzy spot in the image... zoom zoom, sure enough, there was a slightly blurred, half transparent airliner. Not sure how common that is.zuz said:
Looking in the other direction, it is extremely common for an aircraft to fly across the Moon when you live in the UK and spend any time Moon-viewing in a telescope. There are hundreds of those things up there and they subtend a degree of angle or more. They are always leaving streaks (dotted lines) over astrophotography frames but clever stacking progs will eliminate them from a stack of a dozen or more.gmax137 said:
Not sure if that's supposed to be funny or what, but it certainly has nothing to do with why images of galaxies are so clear.castrogiovanni said:
yes they are wrong ... it was disproved many years agocastrogiovanni said:
Star trails due to Earth's rotation are a real bind for the ill-equipped astrophotographer but they can be dealt with with a motorised mount that rotates the camera / telescope about the polar axis (tracking). You can improve on this with a guiding arrangement that will lock onto a brightish star and follow it, allowing you to get sharp images of a more faint object of interest.phinds said:
As already pointed out, "fuzziness" would have to involve really long exposures during which the stars were moving. But this would effect both the near side and far side stars equally as they both would be moving during the exposure.zuz said:
I'm trying to make sense of this idea.The only reason for seeing something different in the layout of the distant stars would (surely?) be if there was significant expansion or some other change but if you are just looking at a uniform wheel, how would the extra delay from the distant stars give a detectable effect? We would basically be looking that a different set of stars occupying any particular angular distance but with the same density. Galaxies are not all that big, compared with their distance so the time differences across their diameter would not be that huge.Janus said:
You will still see a disk, but with stars on the approaching side much brighter - which also means you see more of them if you don't correct for that effect.sophiecentaur said:
Yes - nothing about the perceived motion / streaking /spacing, though. Except perhaps the density of visible stars would be less - if, indeed you could actually distinguish the front from the back. Talking of that, the side that's coming towards us might have different red shift from the other side but how to tell which ones are at the back, I wonder. Perhaps Cepheids would give a clue.mfb said:
