In article , (Steve Willner) writes:
In article ,
(Craig Gullixson) writes:
Currently, solar telescopes using state of the art adaptive optics systems
are achieving resolutions on the order of 0.15 arc seconds over relatively
small fields of view. The solar telescopes producing the best images have
effective diameters of about 1 meter. The proposed Advanced Technology
Solar Telescope (ATST) is a 4 meter class telescope designed to have a
resolution of 0.03 arc seconds at 550 nm.

Thanks. I hadn't heard about ATST. Amazing!

The contrast of the granular structures on the Sun is a few percent. Many
solar observations are photon limited and signal to noise suffers from a
lack of photons. This is due to a lot of our observations are taken at
very high spectral resolution (spectrographs having a spectral resolution of
delta lambda/lambda 1,000,000 are not uncommon and imaging systems having
a spectral resolution of delta lambda/lambda on the order of 250,000 exist).

For transit observations of an asteroid -- the original topic of this
thread -- presumably one would use a broad bandwidth. Of course that
precludes the telescope's normal program so there would have to be
scientific merit to a transit observation for it to be considered for
scheduling.

Broad band for us tends to be about 1 nm. Remember we are also geared for
high temporal resolution, so that constrains the maximum bandwidths we use.
Broad band in the nighttime sense, 10 to 100 nm, can break optics and cause
flames in this business.


I must admit that for a nighttime astronomer like me, the concept of
solar observations being photon-starved is a bit mind-boggling. (Not
that I doubt you!)

I started out on the dark side, so it suprised me also.


An additional complication is that we also desire very short exposure times
(a few milliseconds) to reduce image blur due to seeing and to properly
sample the changes with time of solar structure.

For transit observations, many minutes or hours of data could be
added together. All in all, it seems the observations would be
limited by the contrast of the solar surface.

You also have scattered light from the atmosphere, scattered light from
the telescope and instrumentation. Even with AO, very good seeing would
be required to achieve 0.03 arcsecond resolution. Finally, the surface
structure of the sun changes with time.


Transits and eclipses are useful to measure scattered light in our optical
systems.

That seems a hard way to do it, although I don't suppose there are
any easy ways.

In any case, I don't think a transit of a body 0.06 arc seconds
in diameter could be seen anytime in the foreseeable future.

Based on what you have written above, observing such a transit seems
trivial for ATST. The asteroid in transit would be fully black for
the equivalent of four spatial resolution elements. Even with
current solar telescopes, the observation looks possible. The
brightness in a single resolution element would be diminished by 16%,
several times the contrast fluctuations, and this location of
diminished brightness would cross the solar disk in a predictable
way. Wouldn't you expect that to be detectable?

I think it would be really tough. If the seeing was good and the atmospheric
scatter was really small and the orbit of the object was known well enough
that one could blind track the object and sum a bunch of images, perhaps it
could be done with ATST.


Mind you, I'm not advocating for or against a potential observation,
just wondering about feasibility.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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__________________________________________________ ______________________
Craig A. Gullixson
Instrument Engineer INTERNET:
National Solar Observatory/Sac. Peak PHONE: (505) 434-7065
Sunspot, NM 88349 USA FAX: (505) 434-7029