Forum Replies Created
-
Posts
-
HNY everyone!
The closest thing I have to an objective measurement of night-sky conditions in Chelmsford through the year is a plot of the total number of sporadic meteors detected by my two cameras here. This shows much less variation than the total number of meteors since that is biased by the conditions at the time of the major shower peaks. On this criterion 2018 was slightly worse than previous years but the variation is not significant.
Using recent astrometry from iTelescope Siding Spring (obtained by myself and Martin Mobberley) the orbit is starting to settle down a bit although it is still very uncertain. The 1 sigma position errors at closest approach in mid February are still around 10 degrees. I don’t think this comet will be particularly bright.
Perihelion 2019 Feb 5.343663 +/- 1.72 TT; Constraint: e=1
Epoch 2018 Dec 23.0 TT = JDT 2458475.5 Earth MOID: 0.2740 Find_Orb
q 1.262 +/- 0.0247 (J2000 ecliptic)
Peri. 357.57 +/- 0.53
Node 146.43 +/- 1.0
e 1.0000 +/- 0 Incl. 160.48 +/- 0.05423 of 25 observations 2018 Dec. 18-23; mean residual 0″.74
My main telescope can’t get that low so this was using a portable mount set up at the other end of the garden which has a pretty good southern horizon. This plot shows the brightness of the sky background in the frames of this field. I started imaging just before 05:50 when the comet was about 7 deg aove the horizon and there was some cloud low down. The sky darkened a bit as the comet rose and the cloud cleared and then started to brighten rapidly at 06:30 as dawn arrived. My optimum time was between 06:10 and 06:30 when the sky was at its darkest and the comet was around 10 deg up. Astro twilight started here at 05:57 but my sky low down is so bright from other sources that I didn’t see the effect of the Sun until around 30 mins later. By nautical twilight at 06:39 the sky was brightening rapidly.
Robin, That’s a wonderful description of a very rare observation made by Chris from somewhere called Chelmsford. This is the JBAA report referenced in the S&T article.
At a declination of -24deg and only 10 deg above the horizon at dawn C/2018 Y1 (Iwamoto) is a challenge from the UK. Here’s my image from this morning from Chelmsford. It is currently very slow moving which makes it difficult to get a good orbit. Prospects for it should become clearer over the next few weeks.
Many congratulations to Masayuki Iwamoto. The comet is currently low in the morning sky on the border of Hydra and Libra slowly moving north and crossing into Virgo in mid-January. As Jonathan says the orbit is currently very uncertain. As soon as we get a bit more astrometry I’ll post some charts here.
As a challenge it may be currently observable from the UK. From Chelmsford it is currrently around 10 degrees elevation in the SSE at the start of nautical twilight (around 0640 UT). This is 10 deg lower and a bit to the right of where Venus is.
Robin, Yes you’re right about the nucleus diameter (I was thinking it was around the same as Rosetta’s actual target, 67P, but it is quite a bit smaller. Your calculation for the bare nucleus mag is pretty good. JPL Horizons gives a nuclear mag on Dec 9 of 12.85. I get mags of 11.91V, 11.30R in an 11 arcsec diameter aperture and the R profile is definitely broader than G close in (R is mainly dust, G is mainly gas). I think you are seeing the inner dense dust cloud around the nucleus which is probably around 1.5 mags brighter than the nucleus on its own.
This plot was obtained using a widefield system with a pixel size of 2.77 arcsec.
Great work from all the spectroscopists on this. The Comet Section Director is obviously going to have to learn a bit more chemistry!
Robin. I think the bare nucleus of 46P is only around 4km in diameter so you won’t be seeing anything from that. Even though most people are imaging the green Swan emission lines from gas in the coma there will be a considerable amount of dust close in. The surface brightness profile of the coma is attached. The dust will be significant in the inner part of the coma.
Yes, the “official” recording won’t be much different to that. We’ll just chop out all of the interval stuff and upload the talks as individual items.
Lars. Glad you enjoyed it and found it useful. It’s certainly something that we want to do again when we can.
Hi Ronan. Glad you enjoyed the meeting watching from the west of Ireland. Yes I remember the wonderfully dark skies in Mayo and I hope you’re getting a great view of 46P when the weather cooperates.
We’ll certainly try to do this again when we have the opportunity. I’ve just watched a few clips from the meeting on Youtube and the quality is very good. This was a learning experience for us but it worked well and we had around 30 people watching the livestream.
We could livestream this meeting because that the IoP had the facilities and expertise to provide a very good user experience. This is not the case at our other venues. Hopefully we’ll be back at the IoP soon so that we can do this again and, hopefully, some of the other venues we use would be able to offer this in the future.
Thanks again to the IoP for hosting the meeting and to Hazel Collett for the excellent organisation.
Happy Christmas and look out for that comet (and those meteors and variable stars).
Nick.
Are you using background from PSF or background from aperture? I think you should always use the latter since the PSF background estimator needs a much biffer photometric aperture to work properly. They often give very different magnitude zero points.
George. Congrats on another nice discovery. I’m getting 17.4 unfiltered (UCAC-4 R band) tonight (see attached). I think the photometric apertures that you are using in Astrometrica may be a bit small. This may lead to some problems with it estimating the sky background.
Recent magnitude estimates indicate that this comet is not brightening very quickly and latest estimates put it around mag 9. It will get increasingly difficult to observe as its elongation decreases but please try to get more images and estimates as it heads in to perihelion. There is a good set of images in the Comet Section archive.
Lars, The ephemeris from the MPC that Peter gave is about as good as it gets. Any differences in the 1981 ephemeris will be due to a different epoch (B1950 vs J2000) and fewer observations. I’ve attached an ephemeris from JPL Horizons wich is very similar to the MPC one. I’ve also attached a scan of BAA Circular 615 from 1981 March 4 which includes an ephemeris based on only 12 positions over a short arc so you can compare.
