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View Full Version : Is there a limit to the resolution of telescopes?



CosmicUnderstanding
2010-Oct-13, 07:12 PM
I hope that title makes sense. What I'm wondering is, will it ever be possible for us to achieve such a high resolution of optics that will allow us to 'zoom in' on distant stars or planets to see details on their surface?

Think of a digital camera. Most (if not all) have the ability to use digital zoom. Digital zoom isn't a true optical zoom but instead it just enlarges what is already there. If the resolution of the camera is good enough, one can use digital zoom and still retain a fair amount of detail even at the highest level of digital zoom. Can this same type of technology eventually be used on telescopes so that we could practically spot formations on the surface of distant planets? I understand this question might not be answerable at this time due to our technological constraints, but that is why I asked in the first place because I simply do not know. I sincerely hope a day will come when we can take a snapshot of a distant planet and then zoom in to capture the finer details of the surface, etc.

cjl
2010-Oct-13, 07:41 PM
There is a limit, and it's related to the wavelength of the light and the diameter of the telescope. Cameras are subject to it as well. It's known as the diffraction limit. Because of this, to see any detail on the level you are talking about, you'd need an enormous telescope, far larger than you could build anywhere on earth. Space-based arrays of telescopes could help with this, but those have problems as well (such as keeping the components of the arrays in perfect alignment with each other).

CosmicUnderstanding
2010-Oct-13, 10:55 PM
It just so happens I came across diffraction on wiki earlier but I had no clue that it tied in with my original question. Many thanks!

Trebuchet
2010-Oct-14, 01:51 AM
The diffraction limit is the answer to the frequent Moon Hoax believer question of "Why don't they just point the Hubble at the moon and show us the landing site?" It just can't resolve that fine. Of course the HB'ers can't understand that. It's a conspiracy, I tell you!

Jens
2010-Oct-14, 04:16 AM
I think the answer is already pretty clear, but just to make it simple, I think there is no theoretical limit, but just a practical limit. Seeing further requires a bigger telescope, and there are practical limits on how big a telescope can be, in terms of budget, engineering, etc. Eventually you'd start getting problems like the delay in travel time of light between the different parts of the telescope and the central point.

Trakar
2010-Oct-14, 04:33 AM
I think the answer is already pretty clear, but just to make it simple, I think there is no theoretical limit, but just a practical limit. Seeing further requires a bigger telescope, and there are practical limits on how big a telescope can be, in terms of budget, engineering, etc. Eventually you'd start getting problems like the delay in travel time of light between the different parts of the telescope and the central point.

With distant planets there is also the issue of atmospheric distortion (where applicable). We can somewhat compensate for this in real-time when the atmosphere is close enough for us to quantify the distortion with a reference signal and calibrate this into an adaptive optics system, but on distant targets by the time we could assess any reference signal, the distortion would have changed.

dgavin
2010-Oct-14, 06:00 AM
Also with imaging planets, you have the issue of a lack of photons. They only relect about a billionth of the light thier parent star gives off, so the star completely over powers the planet. This can also be compensated for, but only goes so far. To actualy image a planet you would need 1. and inferometer scope with a seperation of the orbit of saturn (making it very difficult to control remotely. 2. A lot of patience, even if you can block all the parent stars light, you'll need exposure in the hundreds of hours to get enough photons for a visable picture. Because of that you would only get blurry images.

One possible way around is only speculative. Called a Gravitational Lens space scope. It uses a heavy dense ring of material to gravitationally to focus light. Hypotetially it shouldn't suffer from diffraction, but you rougly need the mass of say a small moon, and the focal point is about a million miles distance from the ring. This makes syncing the focal dector and the ring together a major proplem.

Hornblower
2010-Oct-14, 06:32 PM
Also with imaging planets, you have the issue of a lack of photons. They only relect about a billionth of the light thier parent star gives off, so the star completely over powers the planet. This can also be compensated for, but only goes so far. To actualy image a planet you would need 1. and inferometer scope with a seperation of the orbit of saturn (making it very difficult to control remotely. 2. A lot of patience, even if you can block all the parent stars light, you'll need exposure in the hundreds of hours to get enough photons for a visable picture. Because of that you would only get blurry images.One possible way around is only speculative. Called a Gravitational Lens space scope. It uses a heavy dense ring of material to gravitationally to focus light. Hypotetially it shouldn't suffer from diffraction, but you rougly need the mass of say a small moon, and the focal point is about a million miles distance from the ring. This makes syncing the focal dector and the ring together a major proplem.

My bold for reference. If I am not mistaken, low photon flux alone should not be to blame for a blurry image. If all else is perfect, then a long exposure time should make just as sharp an image as that of a bright object for which the image was made with the same total photon count in a snapshot. This is in theory, assuming no atmospheric blurring or guiding errors.

loglo
2010-Oct-15, 02:09 PM
A theoretical way of getting past the diffraction limit is by using a metamaterial (http://en.wikipedia.org/wiki/Negative_index_metamaterials) with a negative refractive index (http://en.wikipedia.org/wiki/Perfect_lens). It seems to be a technology that may never be practical.. but that has been said of a lot of things so you never know.

CosmicUnderstanding
2010-Oct-15, 11:18 PM
Thanks for all the replies. Very interesting indeed. I hope you are right about that loglo. It would be great to find a work around to the problems outlined by the previous posts by coming up with some different technology.

novaderrik
2010-Oct-16, 05:29 AM
why can't they just use the same technology that they use on those tv shows to enhanced blurry surveillance camera footage?

HenrikOlsen
2010-Oct-16, 09:13 AM
why can't they just use the same technology that they use on those tv shows to enhanced blurry surveillance camera footage?
In case this wasn't sarcastic, the answer is that that's because those techniques use magic and the real world supply of pixie dust is too limited to be used for that purpose.

peter eldergill
2010-Oct-16, 01:31 PM
I'm pretty sure it was sarcastic :)

Pete

ravens_cry
2010-Oct-16, 04:08 PM
The trouble is that reliably detecting sarcasm on the Internet also requires pixies dust, and as you said it is in short supply.

Ara Pacis
2010-Oct-19, 03:03 PM
I've never known a pixie that wasn't short.

tashirosgt
2010-Oct-19, 03:44 PM
Whatever novaderrik's intent, he does raise an interesting technicality. Algorithms that enhance images can be interpreted as an attempts to infer the actual image from the distorted image that is received. The definition of "resolution" in optics only allows certain algorithms to be performed on the signal. They have to be the ones that nature performs using mirrors and lenses, don't they?