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On 2017 October 5 a number of observers successfully recorded the occultation of a magnitude 12.7 star by Neptune’s largest moon, Triton. Tim Haymes and I wrote a provisional report for the 2017 December BAA Journal which included a profile of Triton displaying the UK observers’ chords.Eric Frappa (FR) has added a more comprehensive shape profile to the Euraster website using data from 17 stations (ranging from Yorkshire to Algeria) submitted to PlanoccultBecause Triton has a significant atmosphere, producing gradual rather than instantaneous occultations, most observers reported D and R timings at the 50% flux level. The resultant estimated diameter of Triton (2,850 km) is within about 5% of the official figure (2,707 km).Although it’s pleasing, this result is of no great scientific value. The real interest is in the light curve data that will be analysed by the professional researchers of Bruno Sicardy and his Lucky Star Project team (Paris) to monitor any changes in Triton’s atmosphere, 20 years after an earlier study.Clear skies,Alex.
Many thanks for the update on this fascinating piece of work, Alex. Do keep us posted on what Bruno Sicardy finds out about the atmosphere.
Jeremy
Jeremy, in due course all contributing observers should receive a copy of the Lucky Star team’s paper, perhaps a preprint. Depending on where it’s published we’ll have to see if general readers have to negotiate a paywall, although the key results should wend their way into the public domain.
Ray, I’m just summarising the results of many observers. The real science will come from the work of Bruno’s team, hopefully revealing any changes in Triton’s atmosphere since 1989.
Cheers,
Alex.
The preprint of the formal paper by Joana Marques Oliveira et al has been published on ArXiV
https://arxiv.org/abs/2201.10450
The analysis and paper were the subject of her PhD thesis under the supervision of Prof Bruno Sicardy (Observatoire de Paris). Here’s a video of Joana’s defence of her thesis
Soutenance de thèse – Joana Marques Oliveira – YouTube
A number of UK observers recorded the occultation and contributed to this work.
Alex.
Interesting paper Alex.
From a quick read, I note that some results were not used as they had:
‘insufficient signal-to-noise ratios (S/N) to provide relevant contribution to that fit’.
What would we do differently for a similar future event?
Regards & Thanks,
Michael.
Hi Michael,
I’m sure the S/N of the light curves would have been good enough to give an astrometric position and shape profile of an asteroid / TNO, but in this paper they’re studying Triton’s atmosphere, so their modelling has more demanding criteria. They rejected data from a 50cm scope because of light contamination from Neptune.
Bruno once explained – In the case of modelling the atmospheres of solar system bodies, lower quality data provides astrometry, defining the position and shape of the body (the chords across Triton), and higher quality data are used to model its atmospheric density, temperature gradient, winds, etc.
For a future campaign we need an image scale providing good separation between planet and moon, a S/N of ~10 (although magnitude of target and duration of occultation limits our choices), and a 12-bit sensor (or better) to capture more detail in the light curve.
It’s quite a challenge to get good data. The Lucky Star team were delighted to have 90 light curves from this pro-am campaign.
Cheers,
Alex.
Hi Michael,
In addition to this paper modelling Triton’s atmosphere
Occult – Minor Planet occultation observations – Summary lists
gives the diameter and astrometric position of Triton derived from chord timings submitted by observers.
Cheers,
Alex.
Alex. thank for explaining some of the concepts.
If S/N was important, with hind-sight i would not have used a red filter. Of all the observations i count 5 instances of the use of an R filter. My data seemed OK (the READING light curve), but might have been better without the filter. The logic was to improve the seeing and so the resolution planet/moon.I made some rough notes which were useful to me at the time, but some of links are now unavailable. On the whole i believe we followed most of the good advice that was on offer.
Hi Tim,
We have a lot of experience in recording various types of occultations, and as you say we followed the guidelines for this event and promoted the campaign amongst BAA observers. Many obtained good light curves. Looking at the figures in the paper, their criterion for selecting light curves (for the atmospheric modelling) even rejected the data from the 1-metre at Pic du Midi, no less. They had 90 light curves, so they could be selective.
Note: “…the first group of figures corresponds to light curves that had sufficient S/N to be used in Triton’s atmospheric fit, while the second group is for light curves with lower S/N that were not used in the fit. It should be noted, however, that the best synthetic models expected for those light curves (plotted in grey in Figs. C.6-C.9) are fully consistent with the observations, in the limit of the noise level.”
They received lots of good data and I’m sure we’ll contribute even better light curves next time.
Cheers,
Alex.
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