Occultation of Chariklo 12th May

[tonybarry]

The celebrated asteroid 10199 Chariklo (yes, the one with the rings) is due for an occultation on the 12th May, visible from Australia (if the clouds co-operate).

The link for the prediction can be found here:-

http://devel2.linea.gov.br/~braga.ribas/campaigns/2015-05-12-17.982_NIMA3_Chariklo.jpg

Note that the prediction shows the path as being in a line from Perth to Brisbane. This should properly be seen as a prediction rather than a guarantee; it is entirely possible that a person in Alice Springs (or Melbourne) may see the occultation, as the uncertainties are substantial.

The star to be occulted is mag +14.2 so a telescope of reasonable size (8" or better) and an integrating video camera, which can integrate to a couple of seconds, will be needed.

10199 Chariklo is a Centaur, orbiting between Saturn and Uranus. It is the first minor planet known to have rings, which were discovered by occultation methods also.

http://en.wikipedia.org/wiki/10199_Chariklo

For further info, talk to your local astro club's occultation person, visit the RASNZ's web page, or discuss here.

http://occultations.org.nz/

Regards,
Tony Barry
WSAAG

[Blue Skies]

Has this event been put in Occultwatcher yet? I know there were problems with the Rio predictions but I thought it had been sorted and its not coming up in my feed.

[tonybarry]

Hi Jacquie,

I will have a chat with Dave Gault who knows everything about occultations.

He will know about the OW feed or lack of it. I got the prediction from John Talbot of RASNZ.

Regards,
Tony Barry

[AstroJunk]

I can see it - I think it needs an OW update...

[Blue Skies]

Edit: Found it. Limits weren't far enough and also looking at wrong day!

[DaveGee]

I've just updated the Occult Watcher feed.

For those of you not Occult Watcher wise :sadeyes: the updated prediction can be found here...
http://www.kuriwaobservatory.com/RIO_TNO_Events/20150512_50325h_summary.html

Occult Watcher is available (for free) :thumbsup: here...
http://www.occultwatcher.net/

[Blue Skies]

Well, I don't think I'm going to be seeing this one! Hail and snow in the forecast overnight. Hope you guys further north have clearer skies.

[DaveGee]

Hi Jacquie,

Snow for Melbourne! brrrrr!

It's looking dismal in The Blue Mountains.:shrug:

The fellas in Brisvegas appear to have the best of the sky offerings.

Chalk another up to Bozo.

[tonybarry]

The forecast for Western Sydney is not good.

Wind, cloud.

Oh well. Hopefully Jonathan and John B. have some success.

Regards,
Tony Barry
WSAAG

[Moon]

occulted by clouds here too :-(

[AstroJunk]

The target star is a good magnitude fainter than reported. It's a marginal event for me at best...

[tonybarry]

Well done Jonathan. First Eris, now Chariklo.

It would be nice to see the light curve if you are free to publish it here.

Regards,
Tony Barry
WSAAG

[Blue Skies]

Looks like it shifted north back to the original path? Well done for getting it, Jonathan. Had no hope of seeing it down here.

[AstroJunk]

Thanks team!

It was a marginal event. Clarity was poor and the seeing had dreadful spells. However, the main event was clear at 8 frame integration.

I've marked where the rings 'may' have been - one certainly wouldn't have discovered them with this observation! More interesting is the dip later on which is in a period of perfect seeing and very obvious visually.

Sadly I think I'm the only one to observe it unless some pro's are out here, so we may not have got to help with the size :(

[tonybarry]

Jonathan, love your work, but you do need to come south of the border to truly appreciate dreadful seeing. :-)

Your dip does look interesting. It will be good to know what the pro's think of it.

Well done mate. You built your house in the right place !

Regards,
Tony Barry
WSAAG

[AstroJunk]

My final light curve!

Just this once, limove did a better job. So what do we think - are those rings or artefacts...

We'll just have to see what the pro's say :)

[Blue Skies]

Nice! Those dips do look suspicious, but I saw Bruno's comment this morning via email CC and he seems positive about it as well. I'll have to make a note of what he said about timing. 6km width doesn't leave you with any room for wobbles in the seeing. I suppose we'll have to hope for some brighter stars in the future.

[deanm]

Folks: I find this fascinating - and it clearly involves considerable effort.

I'm a scientist, and (like you), we like to measure things.

Repeated or replicate measurements afford statistical analysis ( and confidence intervals).

Correct me if I'm wrong (which my wife tells me I always am!), but you seem to be making astrometric measurements at around 1 second intervals.

Is it possible to up this rate by (say) 5-10 fold. That way, instead of individual points apparently indicative of rings, you would have significantly more measurements over event times.

I understand that this would generate 5-10 times the amount of data - but digital storage is super cheap nowadays - and (for an authentic ring occultation) you would see 5-10 points on your time (aka frame number) versus flux graphs.

So doing would also greatly enhance spatial/chronological resolution of these occultation events.

Surely you generally use video-format type data capture (which clicks along faster than that: 30 fps is considered 'low-end', apparently).

Can your software handle such?

Dean

[AstroJunk]

Hi Dean.

You are quite right. using standard AV equipment, we can get results down to 20ms using PAL format equipment. That's one fiftieth of a second.

PAL records at 25fps and each frame can be separated into 2 fields (the format scans all the odd numbered lines then all the even numbered lines (or vide-versa))

Of course NTSC is 30/60 fps.

Our biggest issue is simply the brightness of the target! That's where aperture and location rules. So on a good night you can recoded events like this: https://www.youtube.com/watch?v=lSyMu0l75zQ at max rate. In this case, the non-immediate disappearance suggests that the star may be a contact binary. You never know what you will discover during an occultation and that's what keeps us going :)

It is possible to get even higher frame rates with other video devices, but getting a good timing signal becomes the main issue. Thankfully we bow down to Tony Barry for being our resident electronics expert who has made so much of this possible in recent years :bowdown:

[tonybarry]

Thank you for the kudos, Jonathan. Not sure how much I deserve. If Geoff H had not ceased production of KIWI-OSD I'd never have gone down the rabbit hole with IOTA-VTI. That is for sure. And Dave G has provided enormous amounts of quality assurance which was horribly irritating at the time but it ended up Making Things Better. So it is a team effort.

Dean, the paper which first described the rings of Chariklo can be found here:-

http://arxiv.org/abs/1409.7259

The telescope which provided most of the confidence that the rings existed was a 1m device ... with unfortunately poor timing (unexplained offsets of 1.6 sec, or about 35km at Chariklo). However, it would appear this was known, and a smaller (0.4m) telescope in the same facility (but with known good timekeeping ability) also was used to observe the occultation. They were able to normalise absolute times by blending the observations together, and the high temporal resolution of the Big Scope produced a wonderful light curve which nailed down the rings pretty well.

The moral of the story is that more observers for a given event produce a very much more confident result.

Jonathan's scope is a 350mm f/2 SCT, which was working at the limits of observability given the star's magnitude and the poor seeing ... but even so appears to have nailed the rings pretty well (the professionals will make pronouncement on this, but it looks fairly suspicious).

John Broughton in South Queensland observed a miss, but even that was of great use in establishing that the path of Chariklo was well north of the prediction.

So the replication of results (to provide greater certainty) in this case is not often done by finer time-slices, which requires much bigger scopes. It is done by more observers looking at the same event from different perspectives.

It is also a real blast to help out the professional astronomers. I have learned a lot from such associations, and I am sure that others have too.

If you'd like to know more about occultations, have a read of Blueskies" (Jacquie M) excellent Occultation Handbook.

Regards,
Tony Barry

[deanm]

Many thanks, Tony & Jonathan - your comments are appreciated.

I guess someone could come up with an algorithm (an App!) that optimises sampling rate versus magnitude for a given aperture + stellar magnitude.

Cheers!

Dean

[tonybarry]

Hi Dean,

The aperture of the scope and the magnitude of the star are important. Also the "seeing" or the transparency of the sky; all these things go into the determination of frame rate.

The optimisation also has to use signal to noise ratio as an input, because sometimes for an occultation the target star is much brighter than the occulting body, so it's just an on / off signal; but other times it's a step change in brightness (because the two objects have similar brightness levels).

In the case of PHEMU observations (mutual occultations by Jupiter's satellites) the light curve is a very gradual diminution over tens of seconds to minutes, rather than a step change.

The gain of the camera can be an influencer - at high gain, the light curve is noisier than at lower gain (pretty obvious), so dropping gain, and reducing frame rate may provide a cleaner output, with a reduction in temporal accuracy as a trade off.

The last thing in the mix is the tiredness of the operator. I've found that at 3am my mind is working at about 50%, and my rig has to be **very** easy to use, and hard to get wrong. So anything that gets done easily is good.

Regards,
Tony Barry
WSAAG

[Blue Skies]

I've thought about something similar, a graphical representation, for several years, being keeper of the observing manual, and I've had someone else suggest similar to me as well. But as Tony has explained, it just ain't that easy. I've kept a log of my settings for each event I've monitored and conditions vary greatly from night to night. Sometimes I can get a mag 12 star with only x4 integration, other times I've needed x24 on a night of bad seeing and the Full Moon within 5 degrees of the target star! Nonetheless I think there some "rules of thumb" that could be formulated, such as the timing needed to clearly detect the passage of the rings of Chariklo, and I'll ruminate on them a bit more for next update of the manual, I think. The best thing at the moment is to observe frequently and get to know how your current set-up performs, and what it can and can't do - and write it down so you don't forget!