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KBO encounters
AJAW
post Feb 4 2019, 10:52 PM
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Would that still be true if multiple observers saw the same occultation?
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JRehling
post Feb 5 2019, 02:12 AM
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Multiple observers wouldn't (necessarily) have the same pixels. They'd both see a darkening move against the background, but I think the signal to noise ratio would make this hard or impossible.

The nice thing about an occultation is that it makes the event, locally, very boolean. You see the star or you don't. And the star can be quite bright comparatively. But when you have a bright background and you see a pixel dim, but not completely, it's potentially impossible to distinguish an occultation from some other dimming, even in the noise.
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ngunn
post Feb 5 2019, 08:03 AM
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QUOTE (JRehling @ Feb 4 2019, 10:31 PM) *
a given observer could see an occultation begin as one star is occulted, then deepen as the second is, then slack off as the first ceases to be occulted.


I'm sorry but I'm still having trouble with this. It is difficult to imagine an object only a few kilometres across at that distance occulting more than one star at a time. The KBO is effectively probing the stellar background at much higher resolution than the observing instrument since it subtends such a small angle. I take your point though that there may be more than one star in a pixel so we may not see that pixel go completely dark when an occultation occurs.
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Gerald
post Feb 5 2019, 03:55 PM
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QUOTE (ngunn @ Feb 4 2019, 07:59 PM) *
Can you elaborate on the statement 'Individual stars co-align'? Do you mean that we see mutual eclipses of random pairs of stars in these star clouds? I was not aware of this and would find it very surprising if true. ...

In rare cases, stars are so perfectly co-aligned (sysygy), that you get gravitational microlensing effects, like Einstein rings.
But I think, that's rare enough to be neglected here.
Other, almost co-aligned stars usually can be distinguished by their spectra, if sufficiently bright.
There exist also spectrometers for amateur astronomy. But I think, that it would be very hard to resolve any spectra of very faint stars in a real time scenario. In the aftermath, it might be possible to resolve ambiguities.

---

Back to JRehling's scenario: A dark foreground object crossing a background galaxy would be an extreme case of occulting many stars at a time. But I doubt, that it would be noticed at all due to the almost constant mean brightness. A globular cluster in the Milky Way may be a better example of a finite number of stars that can be occulted at a time, but with discrete jumps of brightness within a pixel.
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AJAW
post Feb 6 2019, 09:30 PM
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One aspect of signal extraction from the noise that may help a little, is that we have some idea what to look for. It reminds me in some ways of LIGO's extraction of gravitational wave signals from their raw data. They had modelled (if I recall correctly) thousands of possible waveforms that they could look for, and they knew that a similar waveform would appear within a fairly narrow time window at their second detector. This helped detection a lot.

If a KBO passes over a star, there will be a probably quite small change in brightness in the detector pixels that are influenced by that star, but it will be abrupt, and it will be followed (probably, and dependant on typical shadow paths) by changes due to other stars in predictable positions in relation to the first star, being occulted. (I'm assuming here that we have a good star catalogue that allows simulation of such events.) As the trailing edge of the shadow passes, we get a predictable reversal of the changes. In neighbouring telescopes, we get related changes, which will not be exactly the same, but will have some spatial and temporal correlations to the changes seen by the first telescope. My hope is that the combination of multiple correlated small changes in meaningful patterns may be detectable, in situations where any one change on its own would not be.
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ngunn
post Feb 6 2019, 11:06 PM
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QUOTE (AJAW @ Feb 6 2019, 09:30 PM) *
it will be followed (probably, and dependant on typical shadow paths) by changes due to other stars in predictable positions in relation to the first star, being occulted.


The probability of one occultation is small and it yields no directional information. The probability of a single small KBO occulting more than one star on its traverse of the Milky Way is negligible. The space between the stars is just too great. Even if it were to occur, likely years apart, there would be no way to ascribe the two events to the same KBO.
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AJAW
post Feb 7 2019, 08:46 PM
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Got it. I was confused by an earlier post that I now realise was to do with background galaxies. And I understand that an occultation from one spot can only give you position, not direction. But if you observed the same occultation from two places, and the second one happened, say, 20 seconds later, because the places were far apart, don't you get two position estimates that reflect the distance the KBO travelled in that time? I guess that would be only about 100kms, and I'm not sure what accuracy your position estimates would have, but at least in principle that would give you directional information. Or am I missing something?
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AJAW
post Feb 7 2019, 09:26 PM
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There might be a problem with the logic that led me to believe this was a promising line of thought. I was inspired by the detection of Ultima Thule, which is one of a very numerous type of object, and by the later occultation observations that pinned down its position and shape. I figured that meant that occultations by such objects must be visible using fairly cheap telescopes very frequently... and that even a fixed line of scopes would have occasional occultations visible from their location. That still sounds right. Crucially, though, observers of Ultima Thule knew WHICH star to look at for that occultation, thanks to Hubble. To monitor lots of stars at once, you might well need better telescopes.

However, I'm intrigued by a recent report in Nature Astronomy of the detection of a 1km KBO using two cheap telescopes - so perhaps I'm again confused! Unfortunately I cannot read the paper, but if anyone is able to share the gist of it that would be great. Many thanks for the patient clarifications I've had here, by the way.
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ngunn
post Feb 7 2019, 11:26 PM
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QUOTE (AJAW @ Feb 7 2019, 09:26 PM) *
Unfortunately I cannot read the paper


Unfortunately neither can I. I am with you there. There needs to be a complete rethink about public access to scientific papers.
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elakdawalla
post Feb 7 2019, 11:33 PM
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A polite and brief email to the corresponding author of an article requesting a PDF will virtually always get you a copy. It takes no more effort than posting here.


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My blog - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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alan
post Feb 8 2019, 12:20 AM
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A trick for those to shy to ask the author for a copy of something in Nature: Use google to find news articles about the paper, click on the link, sometimes that will go to a free copy of the paper. For example the one in this story worked for me: https://motherboard.vice.com/en_us/article/...er-solar-system
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AJAW
post Feb 8 2019, 11:12 AM
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Thanks for the tip, Alan, I've just tried it.
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peikojose
post Mar 19 2019, 11:14 AM
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Hi,

When can we expect information about PN70 (PT3), Alan Stern said weeks ago that in mid-march they would point NH towards it to take a glimpse.

Jose
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