How do you build an optical array telescope?

[vincentnm]

Can I keep my C9.25 and C8 say 5 metres apart and get a effective aperture of 5 metres?

I know, photon gathering capacity will not be equivalent to 5 metre aperture, but will I atleast get the angular resolution equivalent to a 5 metre mirror? If this true, it will work wonders for planetary imaging, where targets are already bright, but are too small for details to be resolved.

Does the light need to be optically combined? - prisms, opticfibres,..

Or can it be done electronically?

Cant seem to find much on google. Will be a really interesting research project. Anyone familiar with this ?

Thanks,

Vincent.

[johninderby]

Have you got a few million quid (probably tens of millions) available to set it up? Seriously the precision required and complexity of actually implementing this type of system means that only a couple of major observatories worldwide can afford it.

This link will lead you to more info:

http://en.wikipedia.org/wiki/Very_Large_Telescope#Interferometry_and_the_VLTI

You might be able to get this book from your local library.

The Very Large Telescope Interferometer - Challenges for the Future

A JENAM 2002 Workshop, Porto, Portugal, 3-5 September 2002

Garcia, P.J.V.; Glindemann, A.; Henning, Th.; Malbet, F. (Eds.)

Reprinted from ASTROPHYSICS AND SPACE SCIENCE, 286:1-22004, 332 p., Hardcover

ISBN: 978-1-4020-1518-2

John

[Merlin66]

John's said it all!

They spent a FORTUNE on the John Hopkins MMT multimirror scope ( combining the light from six 60" mirrors in the one tube) never really worked! They went back to a "standard" large mirror.....Hmmm wonder what they did with the old ones??????

[Dangerous-Dave]

Nice topic Vincent. This is one thing that crossed my mind too, wrt planetary imaging. Obviously it's a non-starter in real time for financial issues described above, but is it feasible it could be done after the event, eg processing two simultaneously captured webcam movies?

[vincentnm]

Thanks for your replies guys. That answers the first part of my question. To do it optically - is out of the amateur's league.

What about electronically. As Dave said - two simultaneous webcam movies captured by two scopes. Someone just needs to come up with a piece of software for doing this.

I have great faith in amateurs innovating out of bare necessity. Remember how Registax revolutionised planetary imaging, and resulted in amateurs like Damian Peach obtaining images that rivalled professionals, who were still snapping single frames.

Drawing a parallel – If the professionals are combining light optically and spending a fortune doing it, is it possible that an amateur will come up with a novel algorithm to do it in software?

[Milamber]

It wouldn't work for local/planetary - unless you were all close to each other, has been done many times for dso's though, with collaborating imagers across Europe combining images taken at the same time.

Arthur

[Psychobilly]

There have been a number of colaborative images here on SGL - Normally TJ puts them together - Registar is a great tool for combining data from just about any camera/CCD/Scope combo...

Peter...

[adamsp123]

I am not too sure but I think the idea of having 2 or more scopes at a distance to effectively work as one giant scope relies on interference patterns (interferometry?) which works fine at radio frequencies as the wave length is so large you can get the interference patterns relatively easily, but at optical wave lengths the task would well nigh be impossible due to the accuracy that would be necessary.

Just think how hard it is to get a single Mirror or lens good enough for optical work and you would need the same accuracy between the 2 separate optical telescopes ie an alignment of less than 380 nm

Pizza

[vincentnm]

Now I get it. Did not realise that this concept was based on extracting information from interference patterns. I cant dream of aligning my scopes within a few nanometres. Busted!

Thanks,

Vincent.

[The Warthog]

It wasn't a wasted discussion, though. We all learned something there.

[Sci]

Hi, I joined this forum because this is the only discussion I could find via google about amateur array building.

I'm curious whether it might be possibly to build a sort of optical "buffer" to make up for the actual positioning of optical telescopes being infeasible? Perhaps something like two parallel mirrored plates with an adjustable gap between them? Bounce the light back and forth between them like a "W" and create a variable delay to get the inputs in-sync. Though it opens up more chances for distortion, as well as adjustments would change the position of the light output from the buffer so the output lenses would have to adjust to compensate.

Just checked wikipedia, and found that's called an "Optical Delay Line". Wonder if any are maybe augmented by filling with fluids with slower light transmission speeds?

Array radio telescopes may be a lot more feasible in the short term with the recent developments in chip-scale atomic clocks which could provide the necessary synchronisation.

To use the clock method in an optical system you'd surely need a imaging unit with spectacular response times. A photon counter would probably have the response time, but you only get 1 pixel resolution IIRC.

I hope some of my assumptions aren't insultingly inaccurate. This is by no means my field of expertise.

[The Warthog]

Let us know how you make out. I don't think optical array telescopes or any sort of construction that corrects for atmospheric interference is within the reach of an amateur budget. Nice to talk about over a beer, though.

[michael.h.f.wilkinson]

So what are the chances of building an amateur radio array. A phased-array antenna design such as used in LOFAR is all about cheap and simple. I have some students who did some nice work on beamforming software. Could it be done?

[The Warthog]

I don't know how, but people have been building amateur radio telescopes for sixty years and more. Can definitely be done.

[michael.h.f.wilkinson]

I know that, but I have yet to hear of phased arrays. I will dig around.

[ngc2403]

Hello, Sci,

that sounds interesting about the optical delay line. i must say i don't know enough about it to say if it would be a good idea or not. I can say that Optical Interferometry is far from being possible in the current state. Photon counters are not quick enough to measure the time of photon arrively as it takes femto second read outs to get the time data required for the interferometric results of aperture synthesis. So i was told by one of the detector development lectures. I was asking him about the idea of optical interfermetry.

it looks like it's going to have to wait a while

[michael.h.f.wilkinson]

No harm in dreaming of course. I remember many professional astronomers being skeptical about the possibility of optical arrays, fortunately, some people did not listen.

[ngc2403]

The VLTI is a total waste of money to me. it has 99% light loss and sees objects down to mag 1.5. it just can't do much science so it's a lot of money to find out that optical interferometer is very hard.

it's just not possible at the moment sad as that is

[FraserClarke]

The VLTI is a total waste of money to me. it has 99% light loss and sees objects down to mag 1.5. it just can't do much science so it's a lot of money to find out that optical interferometer is very hard.

it's just not possible at the moment sad as that is

I'm sorry, but that is simply not true.

VLTI currently has a limiting magnitude limiting magnitude of H~7.5 (with AMBER + UTs). And that is low resolution spectroscopy, not imaging. The next instrument to come along (PRIMA) will push that down to ~13th magnitude.

I agree with you, however, that VLTI does not produce a huge amount of science (but it does produce some truly unique results). I think that is more to do with the lack of experience of the science community than the facility though. Observing with interferometers is very different to observing with a single telescope; most astronomers simply aren't aware of what or how to do their science with an interferometer.

On the thoughts of doing interferometry electronically. I'm afraid it's not possible at optical wavelengths. You need to detect and record the phase of incoming wavefront, and then combine them later. This is how radio interferometers work - but there aren't any detectors that can do this at optical wavelengths (by that I mean anything short of about 100 microns wavelength). That is why optical interferometers are so complex -- you physically need to bring the photons together and get them in phase.

Having said all that, there is a bit of a work around -- you could quite feasibly build an intensity interferometer I reckon. It can't image, and is massively insensitive (i.e. not sure you would actually be able to detect *anything*), but you would be combining the light from two telescopes...

Intensity interferometer - Wikipedia, the free encyclopedia

[adamsp123]

So what are the chances of building an amateur radio array. A phased-array antenna design such as used in LOFAR is all about cheap and simple. I have some students who did some nice work on beamforming software. Could it be done?

See no reason why not, during physics A level we had a simple microwave interferometer set up to demonstrate the principle. Trouble is the wavelength of the radio waves means the size of it to work, I would think, would be enormous.

[michael.h.f.wilkinson]

See no reason why not, during physics A level we had a simple microwave interferometer set up to demonstrate the principle. Trouble is the wavelength of the radio waves means the size of it to work, I would think, would be enormous.

All radio arrays are serious pieces of kit, size-wise. Apart from all other issues, getting the thing calibrated (where are all the antennae with respect to each other, to within a fraction of the wavelength) is going to be a real pain. Think of collimating a Newtonian by having to adjust several thousands of screws, not just a handful

It would still beat building a steerable dish several meters across.

Michael

[FraserClarke]

Apart from all other issues, getting the thing calibrated (where are all the antennae with respect to each other, to within a fraction of the wavelength) is going to be a real pain. Think of collimating a Newtonian by having to adjust several thousands of screws, not just a handful

Ah, but with a radio telescope, because you are detecting the phase and recombining it in electronics/software, you don't need to position the 'dishes' accurately. As long as you can *measure* where they are accurately, you can compensate in the software/electronics.

That's what makes them easier than optical interferometers...

[Peter Drew]

On a similar tack, solar observers often use off-axis masks with solar filters on , say, a 12" aperture scope. This results in a satisfactorily bright image but with lesser resolution than the main aperture. If one used two diametrically opposed off-axis apertures would this increase the resolution to nearer the full available aperture?. The images would be from the same optical surface so would be aligned and presumably in phase.

[FraserClarke]

Yes, you're absolutely right. Having your telescopes on the same mounting removes all of the large path length differences you'd otherwise have to correct with optical delay lines.

That would work as an interferometer, if the two sub-apertures were in phase. However, if the two sub-apertures were always in phase, it's likely the whole aperture would be in phase, so you wouldn't get any advantage from using two sub-apertures... If you can see a nice diffraction pattern through your telescope ('airy rings'), you're already seeing it working as an interferometer effectively. The classical airy diffraction pattern is just the product of all the points in a circular aperture interfering with one-another... Above about 3--6 inch separation however (for a typical 2--3 arcsecond seeing site), the atmosphere will usually be taking the sub-apertures out of phase and destroying the coherence of the wavefront. That is why big telescopes are in practice limited by seeing, not the diffraction limit...

However, there is a way around this using sub-aperture masks as you suggested. If the wavefront across the sub-aperture is flat, and you can take images quickly enough that the phase doesn't significantly change during the image (>100Hz, typically), you can reconstruct the phase differences between the sub-apertures post-priori in software. This is analogous to 'lucky imaging' lots of people do with webcams; they are just taking the best 5% of images, when the wavefront happens to be flattest across the whole aperture. If you use sub-apertures, your hit rate will increase and you can work with larger separations (though the data is significantly more complex to understand, as you're not just seeing an image any more...).

That does raise another point on optical interferometers; each of your telescopes needs to be producing a flat wavefront. i.e. it needs to be ~diffraction limited, i.e. if you want to use anything larger than ~3--6" telescope (depending on how good your site is), you'll need to be running an adaptive optics system on each of your telescopes...

[Peter Drew]

Thanks TeaDwarf, I should have known that it wouldn't be that straightforward!