Now here's a new idea

[John Schutkeker]

richard schumacher <no-spam@invalid.com> wrote in news:no-spam-
28240E.21555118032005@news.isp.giganews.com:

Never mind transmitting the information: I wonder what he imagines will
be used to detect the optical phase at each point in the image plane.

DOH!

 

[Martin Brown]

John Schutkeker wrote:

Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote in news:d1e4mt$tfj
$1@news8.svr.pol.co.uk:

Why? What makes you think you would *ever* see any fringes?

Twisted pair is radially symmetrical. If that's too demanding, I can
relax the constraints to shielded-twisted pair or coax.

I was referring more to baseline length. The Earth's atmosphere
scrambles phases so this is a *lot* harder to do than it sounds.

The baseline lengths that Earth based optical interferometers are likely
to be useful at are sufficiently short that you can afford to use any
material you like to connect them up to the correlator!

However, I'm
following this idea on someone else's word that fiber optic can do it.
Was that guy telling me the truth?

Fibre optics are used for signal transfer in several major
interferometer systems but you would need to look carefully to see how
they overcame all the practical difficulties. Merlin is moving to fibre
optics with a claimed total working bandwidth of 30GB/s.

http://www.jb.man.ac.uk/~mkargo/articles/emerlin.html

BTW A single interferometer pair cannot form an image. So your claimed
objective is impossible or an oxymoron. You need a several different
baselines and a range of PA angles to stand any chance of forming a
primitive image from interferometer data.

If each baseline is cheap enough, you can afford to install several
detectors.

It is a lot more tricky than that. At optical frequencies you need a
minimum of 3 baselines in a closed loop to get one good observable phase
free from atmospheric distortion.

State of the art research optical interferometers like COAST use
relatively modest path lengths, devious path compensation measures and
climate controlled bunkers.

I'm gambling that by the time I finish this, an enabling technology for
phase preserving delay lines will emerge. Hopefully somebody smart is
working on that, but if they're not, then I have yet another hard
problem to work on.

Some of the best engineers and scientists on the planet have just about
managed to get these optical systems to work with about two decades of
work. To say it is distinctly non-trivial underplays the enormous
technical acheivement of getting path compensating correlators to work
for optical signals.

At least look at IEEE Transactions and IAU proceeding in Indirect
Imaging to avoid making some of the more obvious mistakes.

I am unsure what the longest optical interferometry
baseline to see useful fringes is to date but I would be
surprised if it was more than 500m (the longest working optical
baseline I know of is 330m at CHARA).

Let me know if you can find any hard numbers about this. For proof of
concept, we just need to match the state of the art.

OLBIN provides the latest news of successes and set backs.

ADS abstracts has some of the technical details online. Try the JPL
OLBIN coordinating site for an explanation of what you are up against:

I guess my homework this weekend is to read up on ADS, OLBIN, COAST and
aperture synthesis.

Researching the literature before jumping in feet first will save you a
lot of heartache.

Regards,
Martin Brown

 

[Larry G]

On Fri, 18 Feb 2005 16:31:06 GMT, John Schutkeker
<jschutkeker@sbcglobal.net.nospam> wrote:

Somebody on this group recently told me that scientists are building an
optical interferometer, with two sensors linked by fiber optic cable.
Does anybody have any thoughts on how to step-down the EM frequency so
that the signal could be transmitted by twisted pair, without losing
synchronization between the two signal sources?

It is unlikely to work, even if you could sufficiently down-convert
the original signal to much lower frequencies. Optical interferometers
rely heavily on signal *phase* information, not just amplitude.
While such phase info might be converted as well, the necessary precision
probably would not be acheived, without sending some reference wave as
well. Most lasers loose coherence after a few feet, at best.

Cheers,
larry g.

 

[John Schutkeker]

"R.Lewis" <h.lewis@connect-2.co.uk> wrote in
news:39uhtmF63us08U1@individual.net:

I wanted to build an optical imaging interferometer that worked over
a phone line.

Why didn't you?

I'm already a two or three years behind schedule on my main project, and
since I don't have my PhD yet, I'd never be able to get anybody to take me
seriously in a grant application. I'm still in the process of thinking it
through, to see if it makes sense to try, and I'll give it a priority bump
if I can get a paper out of my proto-thesis.

Besides, I'm just a humble mathematician, and I really don't know as much
about hardware as I'd like.

 

[John Schutkeker]

greg.hennessy@tantalus.cox.net (Greg Hennessy) wrote in news:d1d401$7ns$1
@tantalus.no-ip.org:

In article <Xns961CAF8E9E119johnschutkeker@151.164.30.48>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> wrote:
If someone can repeat the question (I only see replies, not the head
article) I can try to answer the question. I am an employee of USNO.

I wanted to build an optical imaging interferometer that worked over a
phone line.

How were you planning on downconverting the terahertz frequency
optical frequencies to a bandwidth a phone line could handle?

I'm not a heterodyne savant, but somebody told me that lasers might do it,
although I'm still open to suggestions. Perhaps you could have a staged
system, where you step it down once or twice by several dB using different
methods. HeNe's would be great, because they're also dirt cheap.

I'm willing to sacrifice bandwidth for twisted pair (or shielded twisted
pair) because the heart of the idea is to gain distance (which translates
to aperture size) cheaply.