<blockquote>
<p>What is the orientation of the optical telescope assembly with respect to the direction of motion of the Hubble Space Telescope?</p>
</blockquote>
<p>It could be just about anything from parallel to perpendicular to antiparallel.</p>
<p>In principle Hubble can point in any direction it wants. Its orbital motion direction is not related at all to the pointing direction.</p>
<p>But the ground controllers are very careful not to let it point anywhere near the direction of the Sun because the results would likely be catastrophic.</p>
<p>Since it orbits the Earth every ~90 minutes the Earth tends to get in the way a lot, it takes up pretty much the whole &quot;down&quot; hemisphere. If you are luck and looking in a direction perpendicular to the orbit that wasn't near the Sun, you could look for weeks at a time I think. But if you wanted to look in a direction that was closer to the plane of the orbit you get roughly 40 minutes of observing time followed by 50 minutes of looking in a different direction for someone else's observation because your target is behind the Earth.</p>

CC BY-SA 4.0
2022-07-19T11:01:22.380Z
2022-07-19T21:12:12.967Z
2022-07-19T21:12:12.967Z
uhoh
<p>The telescope points towards the object that Hubble is imaging.  That direction could be forwards, backwards, sideways or whatever.</p>
<p>When you talk about &quot;direction&quot; in space, you need to say what you are measuring it relative to.  Hubble is moving in a curved orbit relative to Earth at 7km/s (much, much faster than a bullet from a rifle) but it is also moving at 30km/s around the sun.  So at the same time it might be pointing &quot;forward&quot; relative to the Earth but &quot;backward&quot; relative to the sun.</p>
<p>Unlike a car or a plane, there is no need for objects in space to face &quot;forwards&quot;.  So most of the time, Hubble will point &quot;sideways&quot; relative to its motion around the Earth.</p>
<p>And since its motion is curved, it may start an observation looking forwards, but be sideways by the end.  It has to very carefully track the object it is observing.  If it doesn't, the image will become blurry streaks.</p>
<p>The main mirror faces the object that is being observed.  It has the shape of a ring, with a hole in the centre.  There is a secondary mirror that catches the reflected light and sends it back through the hole to the focal point, where the image is formed and detected.  Hubble has a Cassegrain telescope design.</p>

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2022-07-19T07:12:45.387Z
It will actually often do all of the above. With an orbit in ~500km height it will orbit Earth in ~2h so any pointing backward or forward relative to the orbital movement may occur if it looks roughly in Earth's orbital plane. The only directions it cannot look is directly towards the sun and Earth, so it will preferrably look out of the plane.
planetmaker
CC BY-SA 4.0
2022-07-19T06:58:42.350Z
2022-07-19T06:58:42.350Z
James K
<p>What is the orientation of the optical telescope assembly with respect to the direction of motion of the Hubble Space Telescope? If it happens to be in the direction of motion, does the primary mirror face the direction of motion or is it facing backwards, similar to a rear view mirror?</p>

<observational-astronomy><space-telescope><hubble-telescope>
Orientation of the optical telescope assembly in the Hubble Space Telescope
JKrsl
# Orientation of the optical telescope assembly in the Hubble Space Telescope

(Asked by: username_0 on 2022-07-19)

What is the orientation of the optical telescope assembly with respect to the direction of motion of the Hubble Space Telescope? If it happens to be in the direction of motion, does the primary mirror face the direction of motion or is it facing backwards, similar to a rear view mirror?

**Tags:** observational-astronomy, space-telescope, hubble-telescope

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## 2 answers

### Answer by username_1 on 2022-07-19. Score: 2

> What is the orientation of the optical telescope assembly with respect to the direction of motion of the Hubble Space Telescope?

It could be just about anything from parallel to perpendicular to antiparallel.

In principle Hubble can point in any direction it wants. Its orbital motion direction is not related at all to the pointing direction.

But the ground controllers are very careful not to let it point anywhere near the direction of the Sun because the results would likely be catastrophic.

Since it orbits the Earth every ~90 minutes the Earth tends to get in the way a lot, it takes up pretty much the whole "down" hemisphere. If you are luck and looking in a direction perpendicular to the orbit that wasn't near the Sun, you could look for weeks at a time I think. But if you wanted to look in a direction that was closer to the plane of the orbit you get roughly 40 minutes of observing time followed by 50 minutes of looking in a different direction for someone else's observation because your target is behind the Earth.


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### Answer by username_2 on 2022-07-19. Score: 1

The telescope points towards the object that Hubble is imaging. That direction could be forwards, backwards, sideways or whatever.

When you talk about "direction" in space, you need to say what you are measuring it relative to. Hubble is moving in a curved orbit relative to Earth at 7km/s (much, much faster than a bullet from a rifle) but it is also moving at 30km/s around the sun. So at the same time it might be pointing "forward" relative to the Earth but "backward" relative to the sun.

Unlike a car or a plane, there is no need for objects in space to face "forwards". So most of the time, Hubble will point "sideways" relative to its motion around the Earth.

And since its motion is curved, it may start an observation looking forwards, but be sideways by the end. It has to very carefully track the object it is observing. If it doesn't, the image will become blurry streaks.

The main mirror faces the object that is being observed. It has the shape of a ring, with a hole in the centre. There is a secondary mirror that catches the reflected light and sends it back through the hole to the focal point, where the image is formed and detected. Hubble has a Cassegrain telescope design.

> **Comments:**
> **username_3**: It will actually often do all of the above. With an orbit in ~500km height it will orbit Earth in ~2h so any pointing backward or forward relative to the orbital movement may occur if it looks roughly in Earth's orbital plane. The only directions it cannot look is directly towards the sun and Earth, so it will preferrably look out of the plane.  

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