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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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DesertRat
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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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