[BAA Comets] C/2017 S3 PanSTARRS

[Andrew Robertson]

Nick,

As you point out, it's clearly getting brighter since this 2nd outburst. I had another visual last night and was surprised to see it as well as I did. Whilst my skies were mag 5 overhead they were very damp, humid & misty such that Mirfak and Capella were dimmed considerably. For this observation I was using my 7" STF Mak-Cass in parallel with my 72mm ED. With hindsight I wished I'd set my 24" dob up a few days ago but it's a lot of work for a brief questionable view, I tend to wait until the start of astro dark under a new moon before getting this out.

But despite the claggy skies low down I could see it readily in the 72mm ED with detail apparent in the 7". As before, a stellar nucleus, brighter inner core with a fainter slightly asymmetrical outer halo. At x60 the Swan bad filter enhanced the contrast and made the outer coma look larger. The filter didn't help quite as much at x90 no doubt due to those humid skies. I didn't attempt a magnitude estimate believing it would be futile and inaccurate in those damp skies.

I made a brief sketch more for the record of my observation and its position in the middle of that rhombus of stars looking somewhat like Delphinus. I observed it again an hour after my sketch and noted it had moved directly between the two closer stars. Of course darker now but my skies had got even mistier so the earlier view better.

As a visual observer I reckon I have until Sunday night then that's it as that gibbous moon will be in the way. Hopefully if we do get a thunderstorm it will clear out these skies!

Andrew Robertson

-----Original Message-----
From: Comets-disc [mailto:comets-disc-bounces at lists.britastro.org] On Behalf Of Nick James
Sent: Friday, July 20, 2018 7:55 AM
To: BAA Comets discussion list <comets-disc at britastro.org>
Subject: [BAA Comets] C/2017 S3 PanSTARRS

C/2017 S3 PanSTARRS continues to brighten and it doesn't show any sign of imminent disruption so this does have the potential to become a fairly bright object over the next few weeks as it heads towards perihelion (q=0.21) on August 15. The current COBS lightcurve is here:

https://cobs.si/analysis2?plot_type=0&end_date=2018/09/30%2000:00&col=comet_id&fit_curve=1&perihelion=1&obs_type=3&id=1690&start_date=2017/09/01%2000:00

The brightening does appear to have been sustained although you should take the predicted magnitude fit with a large grain of salt since this is a dynamically new comet and we don't really know what it will do as it approaches the Sun.

It is really important to keep this object under observation for as long as possible. The observing circumstances are getting more difficult as the elongation reduces so you will need a good northeastern horizon but if it does get bright binoculars or a portable imaging setup should be sufficient to get good data.

Observing circumstances after perihelion are poor but Worachate Boonplod has pointed out that it should be in the SOHO C3 FoV from August 14. It should also be visible in the STEREO HI1-A FoV from July 31.

Nick James, Director.

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