by MarkBour » Fri Aug 12, 2022 4:39 pm
johnnydeep wrote: ↑Fri Aug 12, 2022 2:02 pm
Thanks! That
almost makes sense to me but I can't quite convince myself how far away things would look to an off axis observer looking at the mirror from a short distance as in this APOD. I'd think that you would see a more continuously varying stretching of images across the surface than what we see here, which seems to be a band of extreme stretching surrounded by almost no stretching across most of the rest of the mirror. I wish I had a small parabolic mirror to experiment with.
By the way, I should apologize for sounding so sure of myself in my explanation. I think it's right, but as you think about it, it's good to be skeptical of it. In particular, I don't know how much, or in what ways this depends on our position relative to the mirror. Also, I am thinking that a fast exposure would not have shown so obvious of an effect, but once you get a long exposure with star tracks, even if they were only short tracks, then the mirror effect can exaggerate them. I don't think it can exaggerate perfect points of light, though it would still spread the distances between them (?)
Want a parabolic mirror to experiment with? I thought you'd never ask! The one in my previous image was stolen from an advertisement, and the owner was selling that mirror for $399. Or, a smaller one is quite economical. Here's one that's about 12" in diameter and going for about $35 on Amazon. I'm wondering if it is true, high-quality, parabolic, though.
Of course, any amateur's reflecting telescope will have such a mirror, but I don't want to rip apart mine to get a look at it off-center. I'd never get it back together just right, I think. And the only way to look at it in the scope is straight on.
I'd love to take a trip to La Palma to see these MAGIC scopes.
- capture2.jpg (26.59 KiB) Viewed 3226 times
[quote=johnnydeep post_id=325015 time=1660312939 user_id=132061]
Thanks! That [b][i]almost[/i][/b] makes sense to me but I can't quite convince myself how far away things would look to an off axis observer looking at the mirror from a short distance as in this APOD. I'd think that you would see a more continuously varying stretching of images across the surface than what we see here, which seems to be a band of extreme stretching surrounded by almost no stretching across most of the rest of the mirror. I wish I had a small parabolic mirror to experiment with.
[/quote]
By the way, I should apologize for sounding so sure of myself in my explanation. I think it's right, but as you think about it, it's good to be skeptical of it. In particular, I don't know how much, or in what ways this depends on our position relative to the mirror. Also, I am thinking that a fast exposure would not have shown so obvious of an effect, but once you get a long exposure with star tracks, even if they were only short tracks, then the mirror effect can exaggerate them. I don't think it can exaggerate perfect points of light, though it would still spread the distances between them (?)
[float=left][attachment=1]Capture.JPG[/attachment][/float]
Want a parabolic mirror to experiment with? I thought you'd never ask! The one in my previous image was stolen from an advertisement, and the owner was selling that mirror for $399. Or, a smaller one is quite economical. Here's one that's about 12" in diameter and going for about $35 on Amazon. I'm wondering if it is true, high-quality, parabolic, though.
:-)
Of course, any amateur's reflecting telescope will have such a mirror, but I don't want to rip apart mine to get a look at it off-center. I'd never get it back together just right, I think. And the only way to look at it in the scope is straight on.
I'd love to take a trip to La Palma to see these MAGIC scopes.
[float=right][attachment=0]capture2.jpg[/attachment][/float]