<p>This is incorrect in any operational sense, and no astronomer will tell you that:</p>

<blockquote>
  <p>Any astronomer will tell you focus changes are needed between stars, stars farther away, planets, and the Moon. ... If focused on the Moon, you need to refocus slightly when looking at stars. When looking at Jupiter you need to refocus if the target is the Moon. Close stars vs far away stars, the same thing, a need to refocus. ... To the tune of: Jupiter and it's moons, which are close to Jupiter , need focus adjustment between them. Either Jupiter or it's moons will be in focus. Ask any astronomer. That's a difference and it's well past the supposed "infinity focus" point.</p>
</blockquote>

<p>It's true that Jupiter and the Moon are at different distances and thus geometrical optics says that the focus needs to be adjusted, but this is <em>only</em> true in mathematically in an idealize instrument. In any real instrument, it is not possible to discern any difference between the various focus points.  They are all at infinity.</p>

<p>So one size really does fit all. </p>

<p>With exceptional quality optics, it's possible that a distant streetlight might require a very, very slightly different focus than the Moon or Jupiter. Nearby streetlights, sure.</p>

CC BY-SA 4.0
2018-05-22T16:02:05.187Z
It might be (and I'm giving that anonymous commenter the benefit of the doubt) that the original intent of that statement was that, if you were to change the *magnification* when switching from, say, the moon to Jupiter, that minor refocussing would be required simply because of the change in the overall optical train.
Carl Witthoft
CC BY-SA 4.0
2018-05-23T20:55:13.087Z
@Carl Witthoft -- No, the anonymous commenter was clearly implying that differences in distances require different focuses ("Close stars vs far away stars, the same thing, a need to refocus"). You're correct in that changing the optics can require a change in focus, but that applies to things like changing the filter while viewing the same object, not just to changing eyepieces.
Peter Erwin
CC BY-SA 4.0
2018-05-22T01:46:34.347Z
2018-05-22T01:46:34.347Z
Mark Olson
<p>My rudimentary experience of photography and telescopes suggests that with commonly affordable optics any object beyond the range of hundreds to thousands of feet will be in adequate focus at the proper "infinity" position. For example the advice here for overcoming difficulties in seeing the focus of a star on a DLSR screen to instead focus on a brighter and distant city light: <a href="https://photo.stackexchange.com/a/23973">https://photo.stackexchange.com/a/23973</a>. </p>

<p>However an interlocutor recently offered this advice (collated from a series of comments):</p>

<p><em>Apparently you haven't actually used a camera or a telescope. The focus adjustment is there for a reason. Any astronomer will tell you focus changes are needed between stars, stars farther away,  planets, and the Moon. ... If focused on the Moon, you need to refocus slightly when looking at stars. When looking at Jupiter you need to refocus if the target is the Moon. Close stars vs far away stars, the same thing, a need to refocus. ...  To the tune of: Jupiter and it's moons, which are close to Jupiter , need focus adjustment between them. Either Jupiter or it's moons will be in focus. Ask any astronomer. That's a difference and it's well past the supposed "infinity focus" point.</em></p>

<p>I wouldn't be wholly surprised if retargeting a telescope could require a focus tweak to correct for slight changes of the mirror distances (etc), or from thermal shrinkage across a cooling evening, but such effects would probably be invariant across objects, whereas as described in that quote targetting back on the original object would presumably require the focus to be restored to the original setting. </p>

<p>Is there a distance-dependent component for focus in astronomical observation, and if so what sort of equipment does it affect in practice?</p>

<photography>
Does the focus of terrestial telecopes need adjustment for the distance of astronomical objects?
Lappan Sommer
# Does the focus of terrestial telecopes need adjustment for the distance of astronomical objects?

(Asked by: username_0 on 2018-05-22)

My rudimentary experience of photography and telescopes suggests that with commonly affordable optics any object beyond the range of hundreds to thousands of feet will be in adequate focus at the proper "infinity" position. For example the advice here for overcoming difficulties in seeing the focus of a star on a DLSR screen to instead focus on a brighter and distant city light: <https://photo.stackexchange.com/a/23973>.

However an interlocutor recently offered this advice (collated from a series of comments):

*Apparently you haven't actually used a camera or a telescope. The focus adjustment is there for a reason. Any astronomer will tell you focus changes are needed between stars, stars farther away, planets, and the Moon. ... If focused on the Moon, you need to refocus slightly when looking at stars. When looking at Jupiter you need to refocus if the target is the Moon. Close stars vs far away stars, the same thing, a need to refocus. ... To the tune of: Jupiter and it's moons, which are close to Jupiter , need focus adjustment between them. Either Jupiter or it's moons will be in focus. Ask any astronomer. That's a difference and it's well past the supposed "infinity focus" point.*

I wouldn't be wholly surprised if retargeting a telescope could require a focus tweak to correct for slight changes of the mirror distances (etc), or from thermal shrinkage across a cooling evening, but such effects would probably be invariant across objects, whereas as described in that quote targetting back on the original object would presumably require the focus to be restored to the original setting.

Is there a distance-dependent component for focus in astronomical observation, and if so what sort of equipment does it affect in practice?

**Tags:** photography


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## 1 answers

### Answer by username_1 on 2018-05-22. Score: 4

This is incorrect in any operational sense, and no astronomer will tell you that:

> Any astronomer will tell you focus changes are needed between stars, stars farther away, planets, and the Moon. ... If focused on the Moon, you need to refocus slightly when looking at stars. When looking at Jupiter you need to refocus if the target is the Moon. Close stars vs far away stars, the same thing, a need to refocus. ... To the tune of: Jupiter and it's moons, which are close to Jupiter , need focus adjustment between them. Either Jupiter or it's moons will be in focus. Ask any astronomer. That's a difference and it's well past the supposed "infinity focus" point.

It's true that Jupiter and the Moon are at different distances and thus geometrical optics says that the focus needs to be adjusted, but this is *only* true in mathematically in an idealize instrument. In any real instrument, it is not possible to discern any difference between the various focus points. They are all at infinity.

So one size really does fit all.

With exceptional quality optics, it's possible that a distant streetlight might require a very, very slightly different focus than the Moon or Jupiter. Nearby streetlights, sure.

> **Comments:**
> **username_2**: It might be (and I'm giving that anonymous commenter the benefit of the doubt) that the original intent of that statement was that, if you were to change the *magnification* when switching from, say, the moon to Jupiter, that minor refocussing would be required simply because of the change in the overall optical train.  
> **username_3**: @username_2 -- No, the anonymous commenter was clearly implying that differences in distances require different focuses ("Close stars vs far away stars, the same thing, a need to refocus"). You're correct in that changing the optics can require a change in focus, but that applies to things like changing the filter while viewing the same object, not just to changing eyepieces.  

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