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Old 11-07-2017, 08:31 AM
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sil (Steve)
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Large virtual scopes on the cheap?

For years I'm sure most of you have heard of projects to use radio telescopes (and more recently Optical telescope) spread across say 100km wide area to give the imaging power of a virtual 100km telescope.

Apart from using magic to join the data meaningfully. How well does this really work and the obvious question is why aren't people doing this from home? Many of us has multiple scopes and I know many have dual mounted scopes (side by side) so why arent people for example using their ED80 +ED120 side by side to get images from a virtual ED300? Surely the algorithms to combine data are published and someone has knocked up a tool to handle combining the data? Plenty of people are out there with the gear to image side by side? If these massive telescope array projects can give science so much more than surely and two consumer scopes can get the hobbyist better images than any single one?

Or am I too brain damaged to see whats wrong with this question/idea?
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Old 11-07-2017, 09:44 AM
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Quote:
Originally Posted by sil View Post
For years I'm sure most of you have heard of projects to use radio telescopes (and more recently Optical telescope) spread across say 100km wide area to give the imaging power of a virtual 100km telescope.

Apart from using magic to join the data meaningfully. How well does this really work and the obvious question is why aren't people doing this from home? Many of us has multiple scopes and I know many have dual mounted scopes (side by side) so why arent people for example using their ED80 +ED120 side by side to get images from a virtual ED300? Surely the algorithms to combine data are published and someone has knocked up a tool to handle combining the data? Plenty of people are out there with the gear to image side by side? If these massive telescope array projects can give science so much more than surely and two consumer scopes can get the hobbyist better images than any single one?

Or am I too brain damaged to see whats wrong with this question/idea?
No not brain damaged but unfortunately it doesn't work easily with optical frequencies.
What you are talking about is called "synthesis imaging" that relies on combining the signal (or light) from separate telescopes and creating an interference pattern. This requires combining the same wave front from 2 separate scopes. This can be done with radio frequencies relatively easily when the wavelengths are ~20cm but is very hard with light when you have a wavelength of a few nanometers.
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Old 11-07-2017, 10:59 AM
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billdan (Bill)
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I think the Dragonfly telescope array is based on a virtual telescope.

In 2012 they started with 3 lens (Canon 400mm f2.8) and by 2014 went to 8 lens and now they have a 48 lens array. Each lens gathering data of the same subject.

It has been very successful for finding very faint objects, having found previously unknown dwarf galaxies.

The benefits must come from stacking, if you tracked an object for 6 hours, you would stack 6 hours x 48 lens = 288 hours of data to stack.

EDIT: Link with photos here
http://www.dunlap.utoronto.ca/instru...ion/dragonfly/

Cheers
Bill

Last edited by billdan; 11-07-2017 at 11:24 AM.
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Old 11-07-2017, 01:00 PM
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Most of this is good on paper but isn't overly practical in the real world when it comes to a "virtual telescope".

With radio telescopes it is very useful as it allows for a resolution increase where the resolving power of the radio telescope is determined from the distance betweeen detectors. As the wavelengths get longer the larger the telescope that is required to achieve the same resolution.

For optical telescopes, due to atmospheric dispersion anything over 10" is virtually redundant from the standpoint of resolution (always limited to seeing conditions). Radio wavelengths don't have the same issue with atmospheric seeing conditions so larger absolute aperture (using many detectors over a wide area) is a good idea.

With Dragonfly, the resolution is limited by the image scale used, not by optics or seeing. If all 48 are used at F/2.8 it could be said that it is running the same as a virtual single 400mm F/0.404 lens. This is about as accurate as me saying that if I captured 5 hours of data with my Sigma 85mm @ F/2.8 with 120s subs that it has been captured as a single 120s shot with a 85mm F/0.2286 lens.
Or maybe 10x 120s exposures with a 85mm F/0.723 lens.

Unless you're wanting to increase image resolution with radio astronomy, telescope arrays are used because it is either more financially viable to have many smaller telescopes than one bigger one or you want to have a specific FOV and capture a lot of data in a short time (Dragonfly).
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Old 12-07-2017, 06:17 AM
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I got to thinking on this again recently from some podcast news I'd heard about an optical telescope array project in the UK and wondering what they actually meantby the equivalent of a multi-100km diameter scope.

So I guess they are only talking about light gathering/stacking power for a set exposure time over what a single scope can handle. I was hoping they had a way to increase resolution from the technique. I think the project was going after exoplanets and I don't know if parallex is accurate enough with short the baseline distance to help gain accurate exoplanet measurements. Maybe for near earth object detection?

cheers guys for the responses
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Old 12-07-2017, 06:23 AM
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Even 1000km isn't enough for astronomical parallax. From Earth parallax is usually done using the Earths orbit as a baseline; 6 months from the initial baseline photograph. This is ~300,000,000 km
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Old 13-07-2017, 06:45 AM
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It is for local astronomical obs but I doubt for exoplanet ob, but I dont know where the limit is and I am surprised at the precision some observations in science can make. Gravity waves I read were detected based on a movement of the laser of a fraction of the width of an atom. So maybe someone has found a way? No doubt this sort of precision is more to do with statistics than an absolute measurement.
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Old 13-07-2017, 07:17 AM
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The issue is that the Earth moves 1000km every 60-90 seconds.
In space parallax can be calculated out to an incredible distance (small movements) but the atmosphere severely limits the detectable limit even at 300,000,000 km baseline.

Gravitational waves are not affected by atmospheric turbulence
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Old 13-07-2017, 07:31 AM
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Quote:
Originally Posted by sil View Post
It is for local astronomical obs but I doubt for exoplanet ob, but I dont know where the limit is and I am surprised at the precision some observations in science can make. Gravity waves I read were detected based on a movement of the laser of a fraction of the width of an atom. So maybe someone has found a way? No doubt this sort of precision is more to do with statistics than an absolute measurement.
Hello Sil,

From what I understand, I'd suggest that it had more to do with the measurement. Earlier investigations, in one case an earlier announcement of a "discovery", couldn't really discern any signal beyond the background signal from cosmic dust. It was only through subsequent advancements in the sensor and instrumentation tech. that more precise / improved signal to noise measurements were possible and the first discovery made.

The instrumentation is being improved further allowing them to look even further back and these improvements resulted in another discovery earlier this year even further out.

Best
JA
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Old 13-07-2017, 07:05 PM
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Aperture synthesis has been alive and well at optical wavelengths for many years....e.g. Mount Wilson in 1920

Sydney University's Stellar interferometer also comes to mind (I wrote about the opening of this facility for Sky & Space magazine, many, many moons ago)

....so yes, you can combine the light from two smaller apertures separated by a baseline and get superb optical resolution. But you need to be able to look at wavefront changes in real-time, and adjust them accordingly.

Plonking a bunch of telescopes/lenses together (eg dragonfly) and summing the images will only increase the signal gathered over time....unfortunately this does zippo for the system's resolution....but does help in gathering few and far between photons.
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Old 20-07-2017, 09:40 AM
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Thanks Peter!
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