Re: beamwidth vs diameter

[T3D Brian Reynolds <reynolds@xxxxxxxxx>]

John R wrote:
> 
> >Date: Tue, 10 Jun 1997 12:12:25 -0500
> >From: T3D Bob Wier <wier@xxxxxxxxxxxxxxx>
> >Subject: beamwidth vs diameter
> 
> >This question has come up in a satellite tv list, but the optical
> >principals are the same, so I thought I'd ask here...
> 
> >...The question has come up - suppose you set up say four 18" dbs
> >dishes and install c-band electronics taking the combined signal
> >from all of them additively. Would that work?
> 
[snip]
>
> >But then, I remembered that multiple small optical (and I presume
> >radio, as in the VLA) units can be used to get effectively a very
> >narrow beamwidth, although the actual physical surface area is very
> >small compared to effective surface area (ie, diameter) of the
> >virtual mirror/dish.
> 
> The signals are combined *interferometrically*, to get greater effective
> aperture (and directionality) than you would get with a single small
> dish. Note that the overall gain is not necessarily as great as it would
> be with a single large dish at the "effective aperture". Another nifty
> trick is phased arrays, where there are multiple antennas, not all the
> same distance from the source.
> 
> As you say, similar tricks can be used in optics. The Multiple
> Mirror Telescope (MMT) was built with six parabolic mirrors (and the
> signals were optically combined. (I think they later replaced those
> six smaller mirrors with one large mirror, to improve
> performance). The Keck observatory in Hawaii was designed for
> optical combination of signals from two identical large telescopes
> (via and underground tunnel) for extremely high resolution in one
> dimension.
> 

There are some differences between VLA (and other radio astronomy long
baseline interferometry) and what MMT (and either of the individual
Kecks) does.  VLA uses separate radio telescopes and a very accurate
time standard to combine the signals at a later time for
interferometric study.  A lot of computing goes into combining the
signal from each of the radio telescopes into a single data set.

MMT and the Keck telescopes use multiple mirrors (at least until they
replaced MMT's primary) to form one optical instrument.  To a close
approximation (hopefully close enough to make it worth their effort)
the multiple segments appear as a single mirror.

Taken together the two Keck telescopes are attempting to form a long
baseline (well the size of the mountain top at least) optical
wavelength (as opposed to radio) interferometer.  Because optical
wavelengths are much shorter than radio waves the trick is to have a
very good time standard for combining the two signals.  I haven't
heard if they've actually gotten this working yet, but I haven't
really followed it too closely over the last couple of years.

By the way, to help answer the original question, resolution is
dependent on aperture and wave length.  The reason for using multiple
telescopes is to increase resolution without having to build a bigger
telescope.  With radio telescope you get get an aperture approximately
the diameter of the earth.  You might want to ask an amateur radio
astronomer (try sci.astro.amateur) what he thinks the answer to your
question is.

-- 
Brian Reynolds                  | "Humans explore the Universe with five
reynolds@xxxxxxxxx              |   senses and call the adventure science."
http://www.panix.com/~reynolds/ | - Edwin P. Hubble


------------------------------

End of TECH-3D Digest 173
*************************
*************************
 Trouble? Send e-mail to 
 wier@xxxxxxxxxxxxxxx 
 To unsubscribe select one of the following,
 place it in the BODY of a message and send it to:
 listserv@xxxxxxxxxxxxxxx 
   unsubscribe photo-3d
   unsubscribe sell-3d
   unsubscribe mc68hc11
   unsubscribe overland-trails
   unsubscribe icom
 *************************