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Darren Drake
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Resolved disk of Ceres
      #5623926 - 01/15/13 07:43 AM

Last night I noticed that Ceres' disk is about 0.7 arcseconds across on Skysafari Pro. I've routinely resolved the disk of Titan which has about the same angular diameter so I thought I'd give Ceres a try. It was no problem. The disk at 410X was clearly non-stellar. I did this once before when Vesta was right next to a star of similar brightness and saw it had a larger disk than the star. Anyone else here ever resolve the disk of an asteroid?

Edited by Darren Drake (01/15/13 07:43 AM)


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Eddgie
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Re: Resolved disk of Ceres new [Re: Darren Drake]
      #5624053 - 01/15/13 09:29 AM

I have not. Something I might be interested in trying some time though.

Interesting. Nice challange.

Thanks for the report.


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David Gray
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Re: Resolved disk of Ceres new [Re: Darren Drake]
      #5624055 - 01/15/13 09:31 AM

Hi Darren,

You can find an observation of mine here http://alpo-j.asahikawa-med.ac.jp/kk09/o090302z.htm This is a computerised compilation from my notes and sketches.

Back in 2010 March got something similar with Vesta: some nights getting a definite disk and others where it was virtually stellar - possibly variously catching it end-on/broadside-on. Will have to seek out my notes/sketches and do something similar as with Ceres.

You can see my 'scope etc here: http://alpo-j.asahikawa-med.ac.jp/kk12/c121213z.htm and some hi-res Jupiter sats. here: http://alpo-j.asahikawa-med.ac.jp/kk13/j130104z.htm
you will need to scroll to bottom with this one!

Regards,
David.


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Darren Drake
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Re: Resolved disk of Ceres new [Re: David Gray]
      #5624066 - 01/15/13 09:38 AM

I just calculated that the exact angular diameter of Ceres last night was 0.66 arcseconds. That is the same is observing a dime 3.5 miles away. BTW this was done with the 18 inch. A small scope couldn't do this.

Edited by Darren Drake (01/15/13 10:35 AM)


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reiner
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Re: Resolved disk of Ceres new [Re: Darren Drake]
      #5624145 - 01/15/13 10:34 AM

Hi Darren,

A few days ago, I had posted precisely the same in our German forum here

http://www.astrotreff.de/topic.asp?TOPIC_ID=144934

I had observed Ceres and Vesta a week ago. Disk diameter of Ceres was .77" and .49 for Vesta according to JPL Horizon.

I observed with my 22" Dob under good seeing (hardly any blurring, only rapid image motion). The disk of Ceres was easily visible with my scope and also with the C9 of a friend who actually told me to try to resolve the disk.

Vesta as well looked very different than nearby stars. The diffraction patterns of stars had more fringes, while those of Vesta were more like a small bumping disk.


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Darren Drake
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Re: Resolved disk of Ceres new [Re: reiner]
      #5624184 - 01/15/13 11:02 AM

Using the wikepedia diameters instead of skysafari diameters (they were not quite the same) I have calculated the angular diameters of both bodies as of last night. Ceres was 0.72 arcseconds and Vesta was 0.40 arcseconds. I would think that the C9 would be at the very limit or incapable of distinguishing Vesta from a star. BTW at theses angular diameter looking at Ceres would be the same as a dime 3.2 miles away and Vesta at 5.8 miles. Kinda puts things in perspective....

Edited by Darren Drake (01/15/13 11:40 AM)


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Asbytec
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Re: Resolved disk of Ceres new [Re: Darren Drake]
      #5624282 - 01/15/13 12:06 PM

That's a great catch, Darren. I can only imagine your excitement with such an achievement.

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reiner
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Re: Resolved disk of Ceres new [Re: Darren Drake]
      #5624490 - 01/15/13 02:12 PM

Quote:

I would think that the C9 would be at the very limit or incapable of distinguishing Vesta from a star.




Very likely! Vesta was with my 22". And with Vesta, the difference as compared with nearby stars was not the size of the spot but rather the noticeably reduced significance of diffraction effects.

I still have no clearcut explanation where this quite obvious difference in the diffraction pattern comes from for Vesta. I would therefore not say that we really resolved the disk of Vesta, it's just the diffraction patterns that were very different.


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Darren Drake
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Re: Resolved disk of Ceres new [Re: reiner]
      #5624515 - 01/15/13 02:24 PM

Oh a 22 inch absolutely would do it. The airy disk pattern would definately be smaller than Vesta and if there had been a nearby star of similar brightness I have no doubt you would have seen that if seeing allowed. BTW there is no better time than right now to attempt to resolve an asteroid. Both Ceres and Vesta being near oposition gives us a better than usual chance espeically with them so high in the sky...

Edited by Darren Drake (01/15/13 03:48 PM)


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azure1961p
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Re: Resolved disk of Ceres new [Re: Darren Drake]
      #5624695 - 01/15/13 03:53 PM

Gary Nowak of Vermont AS had mention in sky and tel some years ago of resolving Ceres as a disc with a 10" tri-schiefspiegler. Prior to that accomplishment it was Titan with the same scope - an easier achievement I did with my 8".

My thinking on resolving a disc used to be that if it was the size of the aperture Dawes limit than it was resolved. I believe that was unrealistic now however and a little more angle is needed to call it resolved. Norme smith had the value on it - something like 1.17 x the Dawes limit or some such. I forget the exact number but it seemed more accurate.

Pete

Edited by azure1961p (01/15/13 03:53 PM)


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David Gray
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Re: Resolved disk of Ceres new [Re: reiner]
      #5624754 - 01/15/13 04:29 PM

It might be of some interest to compare the appearance of suitably bright asteroids to similar stars in the field, event though they may well be beyond resolution 9 Metis perhaps currently.

Back in 1996 October I observed 4197 1982 TA during its very close approach. It was a night of very good seeing and it passed several similarly bright stars in its rapid motion. Though not seeing a bona fide disk, at no time could I see it truly matching the Airy disks of those stars; and even started to wonder about the presence of a coma!! I recall it had a fine golden hue.

David.


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Rick Woods
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Re: Resolved disk of Ceres new [Re: David Gray]
      #5625223 - 01/15/13 09:24 PM

This is a very exciting time!

First we have several reports of detail being seen on Ganymede; then cloud bands observed on Uranus; and now, Ceres resolved as a disk.
There's a fearless new breed of amateur planetary observer that is really pushing the boundaries. You guys are on the edge.


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Asbytec
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Re: Resolved disk of Ceres new [Re: David Gray]
      #5625234 - 01/15/13 09:31 PM

Actually, anything about 1/4th the Airy disc diameter will begin to produce a PSF larger than a point source. The Airy disc for a C9 is ~ 1.1" arc in diameter, so anything above 0.3" arc will begin to show a broader PSF. At half the Airy diameter (~0.5" arc), the PSF is about 8% larger (This roughly corresponds to the Raleigh limit.) At about .7" arc it's PSF should be =/>10% larger in the C9. Comparing to nearby stars might just show something with careful observation. Of course, the larger the aperture, the (technically) easier it becomes as long as seeing permits. So, seeing a disc on anything in the sub arc second range is quite a feat requiring good seeing and careful observation.



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Special Ed
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Re: Resolved disk of Ceres new [Re: Asbytec]
      #5625266 - 01/15/13 09:49 PM

I made a series of observations of Vesta back in 2007 when it was making a close approach using 15x70 binoculars (no chance of resolving a disk there), but I did notice that Vesta appeared different from the stars surrounding it. It was a difference in the intensity and quality of the light reflected by it as opposed to the appearance of the light being generated by the background stars. Anyone else ever notice this?

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azure1961p
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Re: Resolved disk of Ceres new [Re: Asbytec]
      #5625267 - 01/15/13 09:50 PM

What's the airy disc size for an 8" Norme?

Pete


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Asbytec
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Re: Resolved disk of Ceres new [Re: azure1961p]
      #5625580 - 01/16/13 02:19 AM

Michael, I cannot say I have ever looked at Ceres. Maybe it will be challenging thing to do and look different.

Pete, 138/200 ~ .69 (corrected to about 0.66" arc with a 0.2D CO.) So, Ceres is about half the Airy disc diameter and will be a slightly larger (~8%) PSF than a star. So, if you can see that tiny difference, then maybe it will appear non stellar. That's pretty tight, though, but technically it will be an extended object in an 8".

In the 10" you mentioned, half the Airy disc is 0.54" arc, so that's nearly 80% of the actual disc size with a significant PSF. It's possible, I think, to start to see something in a 10". Same thing happens with Io at 1.2" arc in 150mm obstructed CAT (1.68" arc diameter.) Io is nearly 75% the Airy disc diameter and /should/ be non stellar, disc-like in appearance. But no actual disc. In a 6", Ceres is so close to stellar PSF in size as to be very difficult if not impossible.

I don't fully understand how the image behaves as it transits from a point source to enlarged PSF to extended object. I believe as the actual radius is a fraction of the Airy disc (image) radius, the PSF merely expands until the disc size equals (100%) the Airy disc size. At this point and beyond, the actual disc and the image size are the same as in the 18" scope (comprised of point source Airy discs including any diffraction affects.)

I forgot to mention, it seems the 18" actually comes close to resolving the actual disc at 138/457 ~ 0.3" arc (0.3 * 2 ~ 0.6" arc diameter.) With CO diffraction effects, it might just resolve it. So, it seems the difference in smaller apertures is a non stellar appearance while an 18" seems capable of true resolution (even if it is so small as to appear non stellar, too.) Interestingly, as 1/4 Airy disc for an 18" scope is well smaller than the 0.7" arc angular size of Ceres, then it's collection of 0.15" arc "point sources" might actually resolve some surface detail given sufficient contrast and very good seeing. (That's what happens for a 6" scope's 0.42" arc "point sources" scattered over Ganymede or Io.)

But, those are the numbers. So many other things will affect success, including the math being correct.


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Astrojensen
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Re: Resolved disk of Ceres new [Re: Rick Woods]
      #5625596 - 01/16/13 02:57 AM

Quote:

First we have several reports of detail being seen on Ganymede; then cloud bands observed on Uranus; and now, Ceres resolved as a disk.
There's a fearless new breed of amateur planetary observer that is really pushing the boundaries.




What? All of this was already done visually decades ago. Oh yes, by professionals, you say. Yes, but many of todays amateur has equipment equal to or better than what most professional visual astronomers ever had access to in those days.

And THAT is really exciting!


Clear skies!
Thomas, Denmark


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Rick Woods
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Re: Resolved disk of Ceres new [Re: Astrojensen]
      #5626153 - 01/16/13 12:55 PM

Exactly!

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Mike Phillips
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Re: Resolved disk of Ceres new [Re: Rick Woods]
      #5626370 - 01/16/13 03:13 PM

I have not tried visually, but should. Photographically I've done it a couple of times. Most recently in a 14" with poor collimation. Have a look.

http://astromaphilli14.blogspot.com/2013/01/high-resolution-asteroids-ceres-a...

Mike


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Re: Resolved disk of Ceres new [Re: Mike Phillips]
      #5626451 - 01/16/13 03:57 PM

I've observed the disk of Ceres with a C8 many years ago in exceptional seeing. For verification its important to compare it to a similar magnitude star nearby. Also I recall the color of Ceres was unlike a star, being somewhat like a miniature Mars. Later I ran across a reference, now misplaced, that reported a B-V magnitude measure which contradicted the color I saw. I think in a 10" scope and above the disk of Ceres should be noticeable at opposition in good seeing.

Bart Declercq imaged the disk of Vesta with a 12" about 3 years ago. You can search the archives for that in the imaging forum. At first I was sceptical but then became a believer.

Glenn


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Rick Woods
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Re: Resolved disk of Ceres new [Re: DesertRat]
      #5626543 - 01/16/13 04:34 PM

Quote:

Also I recall the color of Ceres was unlike a star, being somewhat like a miniature Mars. Later I ran across a reference, now misplaced, that reported a B-V magnitude measure which contradicted the color I saw.




Glenn,

If it's any consolation, the book "The Planets" by Giles Sparrow has, on page 132, four photos of Ceres in natural color made by the HST; and it does indeed look like a miniature Mars.


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azure1961p
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Re: Resolved disk of Ceres new [Re: DesertRat]
      #5626749 - 01/16/13 06:31 PM

Mike,

Bravo on the challenging imaging. Clearly there is a marked difference in the star image versus the asteroids. I'd hunk exposure time or gain is important in keeping the same sensitive ties so the apparent sizes are more reflective of whats had independent of image swelling etc . This is great stuff Mr. Phillips!!!!!

And Im seriously glad you got your collimation issues addressed.

Pete


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azure1961p
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Re: Resolved disk of Ceres new [Re: azure1961p]
      #5626761 - 01/16/13 06:35 PM

Okay here's the link to the work Bart did. Brilliant!!!
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3636823/page...

Pete


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Mike Phillips
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Re: Resolved disk of Ceres new [Re: azure1961p]
      #5626990 - 01/16/13 08:45 PM

Thanks Pete! I mean to try again with my tighter optical train, with less slag between the catseye and cfw, powermate, cam, but weather and conditions have not been in my favor!

Mike


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azure1961p
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Re: Resolved disk of Ceres new [Re: Mike Phillips]
      #5627037 - 01/16/13 09:18 PM

You know what, from Texas to Maine I think everyone's had lousy weather - cept for Florida.

Pete


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DesertRat
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Re: Resolved disk of Ceres new [Re: azure1961p]
      #5627373 - 01/17/13 02:45 AM

Anyone interested in the visual investigation of asteroids might be interested in the work of van de Bos and W.S. Finsen at Johannesburg Observatory during a close approach of Eros in 1931. Using the 26" refractor there they were able to confirm the elongated shape of the asteroid. You can read about it here: http://adsabs.harvard.edu/full/1931AN....241..329V

I don't have the reference before me but I recall reading that W. Herschel measured Ceres and Pallas. His estimates of diameters were off the mark, but its clear he was resolving the disks, recognizing they were more than simple points of light.

Glenn


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Re: Resolved disk of Ceres new [Re: Asbytec]
      #5627746 - 01/17/13 10:14 AM

Quote:

Actually, anything about 1/4th the Airy disc diameter will begin to produce a PSF larger than a point source.




Classic MTF. According to MTF theory, a white line on a black background appears wider than it really is. This is diffraction at work. By the same logic, small disk on a black sky appears larger than it really is.

Every point on the circumference of the disk will act as the center of a "Airy disk" so that what you wind up with is esentially an almost but not quite stellar object.

For example, if a telescope produced an Airy Disk that was 1 arc second in size and you looked at a disk that was .75 arc seconds in size, the image would be a point that was bigger than the .75 arc second target. It would in fact appear to be about 1.75 arc seconds across (75% larger) than the 1 arc second that a similar brightness star would produce.

It would not look stellar.

I think when the source gets to be about 30% of the size of an Airy disk size for the instrument, it starts to show more of a classic diffraction effect with something that looks like a fat, soft first diffraction ring, but not quite like a fat, soft diffraction ring.

Bottom line.. Diffraction makes it possible for an aperture to "Resolve" a disk smaller than the diffraction pattern the instrument creates.

Another example is a Sparrow split. Two point sources so close to one another that they produce an elongated Airy Disk.

In fact, this is a great case of how an apture "Resolves" targets that are less than half the radius of the Airy Disk. As long as the resulting image is bigger in diameter or in any direction than an Airy Disk for a star of the same magnitude, we can say that we have detected (Norme would say "Resolved") some detail. Now the human eye starts to struggle (Sparrow splits were almost all photographic) because at some point the eccentric image is to spherical for the eye to really see as elongated, but precision instruments can measure the elongation. We haven't really "Resolved" the split because we cannot see a dip in the valley, but we can infer that it is there if other evidence shows the source to be to distant to be non-stellar. So while we can't see the Sparrow split as a true binary star, we can infer that it is one because of the elongation of the Airy Disk.

The exact same thing is happening when a disk is smaller than the Airy Pattern of a star in an instrument. The disk "Expands" the size of the "Airy Disk" by the diameter of the source. Agian, the theory for image formation of extended targets is that it is a composite of an infinite number of overlapping Airy Disks being formed by every point on the surface of the source.

Diffraction explains all of this easily.

The difference is that in a much larger aperture, the disk will have progressivly harder, more distinct limbs so that at some point, it is decidely not stellar, while in the smaller instrument, it will be expanded angularly by the diffraction, but the center of the spot will be broader and flatter than it would for an Airy disk, but the limb itself could not be resolved because it falls inside of the diamater of an Airy Disk size for the instrument. In the smaller aperture, we see no distinct limb, but a soft sided ball of light. The important point though is that it will not appear stellar because the circumference will be too large and the central "Spike" will be very broad.

And this is what MTF is all about. It describes the contrast transfer. The contrast of the limb is lost in the smaller aperture. But careful measurment (or a skilled observer) would easily see that in the case above, the target is not stellar. And if it appears to be slightly larger than a star with no defined first diffraction ring, then the most logical explination is that it is not a point source, but rather a disk that is the source of the light. It will indeed look more like a disk than a star.


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Asbytec
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Re: Resolved disk of Ceres new [Re: Eddgie]
      #5627909 - 01/17/13 11:41 AM

I say resolved because the surface, of Ganymede for example, is composed of numerous point sources. As I understand it, each point source is, for extended objects near the Airy disc size, is contributing to an overall Airy disc nearly doubled in size. The diffraction ring resides outside this enlarged Airy disc and not around each point source that comprise it. It has to do with the OPD of each point source being constrained within the central disc diameter and the phase at various radii (which change as the object exceeds point source.) Together, these tiny extended objects contribute to one overall Airy pattern. This is why we see a dim, fat diffraction ring around Ganymede. Only when extended objects are large enough to contain a series of full Airy disc sized diffraction patterns is planetary detail reduced.

Now, say there is a less bright area on Ganymede's limb. The collection of point sources that comprise it will be dimmer (contributing less intensity to the enlarged PSF) than the surrounding brighter collection of point sources. On Ganymede, at about the size of a 6" aperture Airy disc, one can easily fit one line pair - which is resolution at the Raleigh limit (which really does not apply, I think, in the pure sense of two Airy discs.) In fact, one can include more than one line pair at the Dawes limit. So, depending on the contrast level, Ganymede is plenty large to separate features of sufficient contrast. The same should be true for Ceres in an 18" scope, provided sufficient contrast and great seeing, of course.

"...according to the optical theory, a point-source image has to be less than 1/4 of the Airy disc in diameter; larger image enlarges the central disc, and alters energy distribution in the area of rings (at the image size of ~0.25 Airy disc diameter, the FWHM is enlarged ~2%, at twice that size it is about 8% larger, and with the image equaling the Airy disc in diameter the FWHM is nearly doubled, and the ring structure greatly suppressed)."

http://www.telescope-optics.net/diffraction_image.htm

"Diffraction image of a point source on the surface of most extended objects could be detected only if separated from the rest of surface, not because it is small and relatively faint, but because it is typically of much lower intensity than that of the surface. "

"Consequently, diffraction image of an extended surface can be evaluated as a product of surface dots not larger than 1/4 of the Airy disc diameter..."

"Surface of an extended object can be decomposed on point-sources, that overlap and grow into a larger diffraction image of it. Any distinctive area on such surface also can be decomposed on its effective point-sources. Whether such an area - a surface detail - will be visible in the telescope image depends on the multiple factors: its size, brightness and contrast and, if colors are present, hue specificity and saturation."

http://www.telescope-optics.net/telescope_resolution.htm

Again, I don't yet fully understand how object resolution transitions from point source to extended object. But, this is how I read it.


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Darren Drake
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Re: Resolved disk of Ceres new [Re: Asbytec]
      #5628101 - 01/17/13 01:34 PM

This is good stuff and very educational. Thanks.

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