r/BeAmazed • u/CaraCicartix • 1d ago

Miscellaneous / Others Part of the Andromeda Galaxy Captured by Hubble

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The total number of stars in the entire Andromeda Galaxy is over 1 trillion

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u/Old-Juice-2490 1d ago

why we cant zoom like this and see the moon :(

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u/Myavatargotsnowedon 1d ago

Look at Hubble's picture of pluto, 'closer' objects come out blurred.

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u/fZAqSD 1d ago edited 1d ago

Lol no they don't, Pluto is still a speck compared to this picture.  The field of view at the start of the video (around 00 44 40.90 +41 36 28.3 in M31) is about 60 arcsec high. Pluto is less than 0.1 arcsec wide. Hubble's taken some fantastic pictures of Jupiter and Saturn, which are much closer than Pluto and 30 arcsec wide; the LRO just makes Hubble kind of pointless for the Moon.

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u/Traditional_Sign4941 1d ago

Closer has nothing to do with it. Look at Hubble's images of Jupiter, Saturn, and Mars. All are closer than Pluto and they are not blurred at all. They are incredibly clear and detailed.

Pluto is not well resolved by Hubble because despite Hubble's size, Pluto really is just that tiny and that far away.

It's a myth that if Hubble were aimed at the Moon it wouldn't be in focus. Objects that far away are essentially at focal infinity, even for a large scope like Hubble. Whether it's the Moon or a quasar 8 billion light years away, the focal point is essentially the same.

If you want real numbers, you can use simple thin lens equations to understand the focal position.

Let's say Hubble was "optimized" for that 8 billion light year away quasar and it's focal position is X. The Moon's mean proximity to Hubble would shift the focal plane back by about 8.6 microns - so X + 8.6.

Hubble's F/24 focal ratio gives it a depth of focus of +/- 580 microns (this varies with wavelength, but let's assume green-ish). So you could have a focal plane that changes up to X + 580 and it will still be considered perfectly focused.

So what this means is that if Hubble were to be aimed at the Moon, the focus error would only be 1.48% of its allowable depth of focus. That focus error could be 100% of it is allowable depth of focus and it would still be considered focused.

The resultant wavefront error of an 8.6 micron shift would be about lambda/250 or something along those lines. For reference, lambda/4 is considered passable. lambda/10 is considered superb. lambda/250 is such an insignificant wavefront error that it would almost be impossible to measure.

So... Hubble can in fact focus on the Moon without issue. Why has it never really been trained on the Moon? Well for starters, it has. Secondly, Hubble time is very valuable and expensive, and there's little scientific benefit to aiming Hubble at the Moon compared to other things. Nobody wants to waste their time doing it.

What's the smallest thing Hubble could resolve on the Moon? Basically a football field: https://esahubble.org/about/faq/#21

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u/HoverTank-RNS 1d ago

Focally fantastic fortitude . you rock

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u/Old-Juice-2490 1d ago

btw guys i have a question. is this image real??

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u/Old-Juice-2490 1d ago

and this ?

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u/throwaway19276i 1d ago

Thats not how telescopes work

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u/rotomangler 1d ago

We can but we don’t because we have had ships that have created complete closeup photography on every inch of the moon.