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# Interferometers

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OK--not about the physics of space per say--but the physics of optics.  It is well understood that if 2 mirrors (or lenses?) are positioned some distance apart and view (image?) a particular object--say the Moon, the resolution is defined by the 2 lenses AND THE SPACE BETWEEN THEM.  That is an extraordinary fact that begs the question--can we amateurs set up an interferometer with our small scopes.  Many of us do have 2 (but 2 mounts are needed as well).  The possibilities are intriguing.  2 12 inch scopes separated by 50 feet--that would be serious resolution.  There are equations and limitations of course.  50 feet may be too far--or not far enough for 12 inch scopes, or 5 inch scopes--whatever the case may be.    But there is some distance where it would work.  The question is--how hard would it be?

Rodd

Edited by Rodd
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You could in theory, but I believe the increase in resolution would be only on the axis of the two mirrors.

Chris

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42 minutes ago, Rodd said:

OK--not about the physics of space per say--but the physics of optics.  It is well understood that if 2 mirrors (or lenses?) are positioned some distance apart and view (image?) a particular object--say the Moon, the resolution is defined by the 2 lenses AND THE SPACE BETWEEN THEM.  That is an extraordinary fact that begs the question--can we amateurs set up an interferometer with our small scopes.  Many of us do have 2 (but 2 mounts are needed as well).  The possibilities are intriguing.  2 12 inch scopes separated by 50 feet--that would be serious resolution.  There are equations and limitations of course.  50 feet may be too far--or not far enough for 12 inch scopes, or 5 inch scopes--whatever the case may be.    But there is some distance where it would work.  The question is--how hard would it be?

Well, we could do it, but you have to get the focal planes lined up to a tolerance of 45 nanometers (λ/10)! That's only just larger than the hepatitis B virus! The reason for this is that interferometry relies on the waves being in phase, and thus forming constructive interference at the receiving end. The other way of doing it would be to (somehow) create a camera sensor that maintained the wave-property of the light it collected and aligning the waves via computer and clock later. However, storing that much data is impossible at this time (storing a sound file sampled for 20KHz sound waves has a sample rate of 48KHz, imagine how much space would be taken up for every second of 600THz stored!)

However, amateur radio astronomers could probably achieve interferometry simply by starting their telescopes recording at the same time or by utilizing specially lengthed coaxial cable (so the waves arrive in sync - much easer at wavelengths of centimeters or meters than at wavelengths in nanometers!)

EDIT: I don't, however, understand exactly *why* interferometry works, only roughly what it does and roughly what you can do to achieve it

Edited by pipnina

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3 minutes ago, pipnina said:

Well, we could do it, but you have to get the focal planes lined up to a tolerance of 45 nanometers (λ/10)! That's only just larger than the hepatitis B virus! The reason for this is that interferometry relies on the waves being in phase, and thus forming constructive interference at the receiving end. The other way of doing it would be to (somehow) create a camera sensor that maintained the wave-property of the light it collected and aligning the waves via computer and clock later. However, storing that much data is impossible at this time (storing a sound file sampled for 20KHz sound waves has a sample rate of 48KHz, imagine how much space would be taken up for every second of 600THz stored!)

However, amateur radio astronomers could probably achieve interferometry simply by starting their telescopes recording at the same time or by utilizing specially lengthed coaxial cable (so the waves arrive in sync - much easer at wavelengths of centimeters or meters than at wavelengths in nanometers!)

Darn it.  Possible but EXPENSIVE it sounds like.

Rodd

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2 minutes ago, Rodd said:

Darn it.  Possible but EXPENSIVE it sounds like.

Rodd

Scott Manley did a good video where he briefly mentioned how the VLT achieves it:

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I could be wrong but I have in the back of my mind seeing someone who did manage to produce interference fringes between two scopes (on the same mount) but as stated, the tolerances in the two light paths is extremely small, and  you also need to continuously alter the relative path lengths to keep them in phase as you track the sky.  Even then you still run into the problem of seeing though. (pros interferometers observe from good sites, use adaptive optics and tend to  work in the IR where the seeing and path length tolerances are greater).  There is an application of interferometry called speckle interferometry that some double star observers use though which can improve resolution in larger telescopes in seeing limited conditions.

There are several examples of amateur radio interferometers though where the tolerances are much higher

Robin

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15 minutes ago, robin_astro said:

I could be wrong but I have in the back of my mind seeing someone who did manage to produce interference fringes between two scopes (on the same mount) but as stated, the tolerances in the two light paths is extremely small, and  you also need to continuously alter the relative path lengths to keep them in phase as you track the sky.  Even then you still run into the problem of seeing though. (pros interferometers observe from good sites, use adaptive optics and tend to  work in the IR where the seeing and path length tolerances are greater).  There is an application of interferometry called speckle interferometry that some double star observers use though which can improve resolution in larger telescopes in seeing limited conditions.

There are several examples of amateur radio interferometers though where the tolerances are much higher

Robin

The next generation mount from AP or 10 micron, or SB......integrated so alignment is controlled by computer.

Rodd

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3 minutes ago, Rodd said:

The next generation mount from AP or 10 micron, or SB......integrated so alignment is controlled by computer.

Rodd

It is not the tracking capability of the mount which is the limiting factor. It is the requirement to maintain the relative optical path length in the light beams from the two telescopes to a fraction of a wavelength prior to combining them.

Robin

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You could try out the principle (and thus highlight the practical difficulties) but putting a two hole Hartmann mask on a large telescope thus removing the path length issues to a large degree.

I would predict the seeing will wash out any fringes unless you have a perfect night.

Regards Andrew

Edited by andrew s

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1 minute ago, andrew s said:

I would predict the seeing will wash out any fringes unless you have a perfect night.

I suppose, if we're going to see turbulence anyway, we might as well make the turbulence look as crisp as possible!

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1 hour ago, robin_astro said:

It is not the tracking capability of the mount which is the limiting factor. It is the requirement to maintain the relative optical path length in the light beams from the two telescopes to a fraction of a wavelength prior to combining them.

Robin

But that could be accomplished with a sophisticated control of movement of the mount plates, no?

Rodd

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44 minutes ago, andrew s said:

You could try out the principle (and thus highlight the practical difficulties) but putting a two hole Hartmann mask on a large telescope thus removing the path length issues to a large degree.

I would predict the seeing will wash out any fringes unless you have a perfect night.

Regards Andrew

What I seem o be hearing is the benefit of the Interferometer is small--negligible in fact (equivalent to seeing).  I thought the benefit was big.  If seeing can negate the benefit--what's the point.

Rodd

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Poor seeing amounts to phase differences across the illuminated aperture(s) . For an interferometer to work you need to compensate for these phase differences.

That's why they can be used to control adaptive optics systems.

Regards Andrew

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When we talk about a mirror being figured to 1/5 wavelength, that means that the incident light wavefront from everywhere across the full aperture of the mirror is brought to the primary focus having travelled the same distance, to that level of precision. This also means the 'signal' received from each part of the mirror surface reaches the primary image with exactly same delay - i.e.  to a precision of 0.2 x wavelength of light x speed of light -> about 0.3 x10^-15 seconds for 500nm light.

If you are aiming to combine the light from multiple telescopes, you would need to correlate the signals with that sort of precision.

The precision of time needed is inversely related to the wavelength of the signal, which is why interferometry has long been achievable for radio astronomy, but is harder with optical designs.

However, the four individual telescopes that constitute the VLT at the ESO in Chile can be used interferometrically to give a resolution of 0.001 arc-second, 20x that of the individual units

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4 hours ago, Gfamily said:

However, the four individual telescopes that constitute the VLT at the ESO in Chile can be used interferometrically to give a resolution of 0.001 arc-second, 20x that of the individual units

That's where I heard about this.  I guess its not really feasible.  I did not realize that it was more difficult optically.

Rodd

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