Re: LONEOS Discovers Asteroid with the Smallest Orbit (2004 JG6)

From: Craig Gullixson (cgullixson_at_nso.edu)
Date: 06/16/04 

Date: Wed, 16 Jun 2004 18:34:07 +0000 (UTC)

In article <40ce10df$1@cfanews.cfa.harvard.edu>, willner@cfa.harvard.edu (Steve Willner) writes:
>In article <ca7a1r$fag$1@noao.edu>,
> cgullixson@nso.edu (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 swillner@cfa.harvard.edu
>Cambridge, MA 02138 USA
>(Please email your reply if you want to be sure I see it; include a
>valid Reply-To address to receive an acknowledgement. Commercial
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________________________________________________________________________
Craig A. Gullixson
Instrument Engineer INTERNET: cgullixson@nso.edu
National Solar Observatory/Sac. Peak PHONE: (505) 434-7065
Sunspot, NM 88349 USA FAX: (505) 434-7029

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