<p>Depressions are ideal for extremely large single-dish telescopes like Arecibo and FAST for several reasons, but the single greatest advantage is structural. These instruments are several hundred meters in diameter and therefore require delicate support systems to maintain their shape and avoid collapse. You could certainly try and build a freestanding dish, but it would be even harder than it already is. The karst depression FAST was built in, Dawodang, is close to spherical, like the telescope, making building the support structure comparatively easy (<a href="https://core.ac.uk/download/pdf/216959906.pdf" rel="noreferrer">Zhu et al. 2018</a>).</p>
<p>Besides the structural benefits unique for these large dishes, building a telescope in such a depression provides natural shielding from radio frequency interference. Some radio telescopes built on flat ground, like those at the Green Bank Observatory, are similarly protected by the surrounding hills (as well as, in Green Bank's case, a large radio quiet zone). Both Arecibo and FAST benefit from walls of their depressions -- one reason Arecibo's current site is considered promising for the telescope's possible successor, regardless of configuration (<a href="https://arxiv.org/abs/2103.01367" rel="noreferrer">Anish Roshi et al. 2021</a>).</p>
<p>Finally, depressions provide drainage. Water can flow to the center of the natural bowl and, particularly in karst topography, subsequently seep into the ground -- one advantage they have over other types of depressions, like craters or mine pits. While karst is permeable, it isn't fragile, so you don't have to sacrifice stability for the sake of drainage.</p>
<p>Basically, the structural advantages of depressions are applicable just to these extremely large telescopes, but there are side benefits, albeit ones which other, smaller radio telescopes not located in depressions can achieve in other ways.</p>

CC BY-SA 4.0
2023-08-29T16:34:17.437Z
Exactly the answer I hoped for, thank you for the citations as well.
joeyfb
CC BY-SA 4.0
2023-08-29T16:36:50.200Z
@joeyfb You're welcome! Though I'd suggest waiting to accept this answer -- it might discourage other folks from answering as well, and they could add helpful information.
HDE 226868
CC BY-SA 4.0
2023-08-30T02:08:24.203Z
"The karst depression FAST was built in, Dawodang, is close to spherical, like the telescope" So the "resting" shape is spherical, but it is deformed into a paraboloid when in use?
Acccumulation
CC BY-SA 4.0
2023-08-30T07:24:49.183Z
what about protection from wind?
gelonida
CC BY-SA 4.0
2023-08-30T07:58:45.100Z
I assume that the "karst isn't fragile" claim was meant in a _structural_ sense.  The underground environment is very sensitive to pollution, so care must be taken with run-off from the man-made stuff.
Toby Speight
CC BY-SA 4.0
2023-08-30T11:56:24.810Z
@Acccumulation The off-axis aberration of a paraboloid is bad, so if you're going to steer the telescope by moving the feed around, a spherical shape is better. You can design the feed to compensate for the resulting spherical aberration.
John Doty
CC BY-SA 4.0
2023-08-30T13:33:06.527Z
@Acccumulation The telescope is spherical when not observing; when it's being used, a ~300-meter-diameter portion of it is deformed into a paraboloid.
HDE 226868
CC BY-SA 4.0
2023-08-30T13:33:18.313Z
@TobySpeight Yep, I meant it purely structurally.
HDE 226868
CC BY-SA 4.0
2023-08-30T13:35:21.600Z
@gelonida That's an interesting question, and I don't know the answer! The dishes themselves are probably at less inherent risk from wind than large fully-steerable telescopes, but it's possible that the receiver cabin might be at risk; I don't know how much protection the depression provides.
HDE 226868
CC BY-SA 4.0
2023-08-30T13:35:38.383Z
Because low EM frequencies aren't as sensitive to atmospheric disturbances, there's also less need to be high up on top of mountains compared to optical.  So one can prioritize the structural benefits to save cost without hurting the science capabilities.
Paul T.
CC BY-SA 4.0
2023-08-30T21:02:07.203Z
I'm not sure about the drainage aspect.  The ground being permeable is of course helpful but putting a building in a depression isn't helpful for drainage, it's a problem.
JimmyJames
CC BY-SA 4.0
2023-08-29T15:53:42.173Z
2023-08-29T15:53:42.173Z
HDE 226868
<p>Radio telescopes are often built into sink holes or natural depressions in the landscape.* This is opposed to optical telescopes which are usually placed on mountains to avoid as much atmosphere as they can.</p>
<p>So why natural depressions? Is it easier to build them in a hole, less interference from local emissions, or something else?</p>
<p>*Or at least the well known ones like former Arecibo Observatory and Five-hundred-meter Aperture Spherical Telescope</p>

CC BY-SA 4.0
2023-08-31T13:02:10.890Z
You save the work of creating an artificial depression ;-).
Peter - Reinstate Monica
<radio-telescope>
Why are radio telescopes often built into natural depressions?
joeyfb
# Why are radio telescopes often built into natural depressions?

(Asked by: username_0 on 2023-08-29)

Radio telescopes are often built into sink holes or natural depressions in the landscape.\* This is opposed to optical telescopes which are usually placed on mountains to avoid as much atmosphere as they can.

So why natural depressions? Is it easier to build them in a hole, less interference from local emissions, or something else?

\*Or at least the well known ones like former Arecibo Observatory and Five-hundred-meter Aperture Spherical Telescope

**Tags:** radio-telescope

> **Comments:**
> **username_1**: You save the work of creating an artificial depression ;-).  

---

## 1 answers

### Answer by username_2 on 2023-08-29. Score: 39 | Accepted answer

Depressions are ideal for extremely large single-dish telescopes like Arecibo and FAST for several reasons, but the single greatest advantage is structural. These instruments are several hundred meters in diameter and therefore require delicate support systems to maintain their shape and avoid collapse. You could certainly try and build a freestanding dish, but it would be even harder than it already is. The karst depression FAST was built in, Dawodang, is close to spherical, like the telescope, making building the support structure comparatively easy ([Zhu et al. 2018](https://core.ac.uk/download/pdf/216959906.pdf)).

Besides the structural benefits unique for these large dishes, building a telescope in such a depression provides natural shielding from radio frequency interference. Some radio telescopes built on flat ground, like those at the Green Bank Observatory, are similarly protected by the surrounding hills (as well as, in Green Bank's case, a large radio quiet zone). Both Arecibo and FAST benefit from walls of their depressions -- one reason Arecibo's current site is considered promising for the telescope's possible successor, regardless of configuration ([Anish Roshi et al. 2021](https://arxiv.org/abs/2103.01367)).

Finally, depressions provide drainage. Water can flow to the center of the natural bowl and, particularly in karst topography, subsequently seep into the ground -- one advantage they have over other types of depressions, like craters or mine pits. While karst is permeable, it isn't fragile, so you don't have to sacrifice stability for the sake of drainage.

Basically, the structural advantages of depressions are applicable just to these extremely large telescopes, but there are side benefits, albeit ones which other, smaller radio telescopes not located in depressions can achieve in other ways.

> **Comments:**
> **username_0**: Exactly the answer I hoped for, thank you for the citations as well.  
> **username_2**: @username_0 You're welcome! Though I'd suggest waiting to accept this answer -- it might discourage other folks from answering as well, and they could add helpful information.  
> **username_3**: "The karst depression FAST was built in, Dawodang, is close to spherical, like the telescope" So the "resting" shape is spherical, but it is deformed into a paraboloid when in use?  
> **username_4**: what about protection from wind?  
> **username_5**: I assume that the "karst isn't fragile" claim was meant in a _structural_ sense.  The underground environment is very sensitive to pollution, so care must be taken with run-off from the man-made stuff.  
> **username_6**: @username_3 The off-axis aberration of a paraboloid is bad, so if you're going to steer the telescope by moving the feed around, a spherical shape is better. You can design the feed to compensate for the resulting spherical aberration.  
> **username_2**: @username_3 The telescope is spherical when not observing; when it's being used, a ~300-meter-diameter portion of it is deformed into a paraboloid.  
> **username_2**: @username_5 Yep, I meant it purely structurally.  
> **username_2**: @username_4 That's an interesting question, and I don't know the answer! The dishes themselves are probably at less inherent risk from wind than large fully-steerable telescopes, but it's possible that the receiver cabin might be at risk; I don't know how much protection the depression provides.  
> **username_7**: Because low EM frequencies aren't as sensitive to atmospheric disturbances, there's also less need to be high up on top of mountains compared to optical.  So one can prioritize the structural benefits to save cost without hurting the science capabilities.  
> **username_8**: I'm not sure about the drainage aspect.  The ground being permeable is of course helpful but putting a building in a depression isn't helpful for drainage, it's a problem.  

---

