Friday, October 26, 2012

Splitting event in comet 168P/Hergenrother

Over the past few weeks, comet 168P/Hergenrother has been under intense scrutiny due to its strange behaviour, namely a 6 magnitude surge in its brightness in a matter  of several nights. Simultaneously the central condensation became markedly brighter and  sharper, whilst the coma was also seen to grow in size. This has triggered the attention of amateur and professional astronomers alike. Most of the time these brightening events (comets are among the most  unpredictable astronomical objects from themselves) originate from some activity within, or associated with, the nucleus of the  comet. There were strong assumptions in the astronomical community on the possible emergence of some visible fragments in the coma, the potential consequences of a break-up of in the comet's nucleus.

There have even been a few claims relating to a sighting within the comet hunting community, however these turned out to be most likely false alarms, due to the unfavourable combination of seeing/instrumental  resolution within the amateur community. Our team have been monitoring the evolution of this comet with a variety of  instruments including the research grade Faulkes Telescopes, with sub arcsecond imaging capability.

Today...we found the fragment...

Our team performed follow-up observations of comet 168P/Hergenrother on 2012, Oct. 26.4, remotely through the Faulkes Telescope North (Haleakala) under good seeing conditions, and a scale of 0.3"/px.

Stacking of 13 R-filtered exposures, 30-sec each, obtained remotely, from the Haleakala-Faulkes Telescope North on 2012, Oct. 26.4, through a 2.0-m f/10.0 Ritchey-Chretien + CCD, under good seeing conditions, shows the presence of a secondary nucleus, or fragment, placed about 2" in PA 188 with respect to the main central condensation of comet 168P.

The magnitude of this fragment was measured to be R about 17, and it shows a diffuse coma nearly 2" in diameter. Click on the image below for a bigger version.


Below you can see an animation obtained by means of the same frames (here East is to the left, North is to the bottom). The animation covers about 20 minutes in total. Click here for a bigger version.


Below you can find the astrometry lines (0168P: main nucleus of comet Hergenrother, 168Pb the new fragment we discovered today):

0168P        KC2012 10 26.42126 23 41 31.60 +34 07 36.5          15.6 N      F65
0168P        KC2012 10 26.42747 23 41 31.37 +34 07 44.6          14.3 N      F65
0168P        KC2012 10 26.42901 23 41 31.31 +34 07 46.6          15.7 N      F65
     168Pb   KC2012 10 26.42126 23 41 31.57 +34 07 34.5          17.2 R      F65
     168Pb   KC2012 10 26.42747 23 41 31.32 +34 07 42.3          16.9 R      F65
     168Pb   KC2012 10 26.42901 23 41 31.28 +34 07 44.3          17.4 R      F65

There was no evidence of this fragment in our previus follow-up images, obtained through the Faulkes Telescopes 2.0-m f/10.0 Ritchey-Chretien + CCD on 2012, Oct. 22.4 and Oct. 3.6, to limiting magnitude about 19.. See our previous post about comet 168P here and here.

UPDATE - September 30, 2014

Paper by Z. Sekanina (JPL) about comet 168P fragmentation has been published on Arxiv with acknowledgement to our discovery and mention to this blog:




To read and download the full paper: http://arxiv.org/abs/1409.7641

by Giovanni Sostero, Nick Howes & Ernesto Guido

Tuesday, October 23, 2012

New Comet: C/2012 U1 (PANSTARRS)

Cbet nr. 3264, issued on 2012, October 22, announces the discovery of a new comet (discovery magnitude 21.0) by Pan-STARRS Survey on images obtained with the 1.8-m Pan-STARRS1 telescope at Haleakala  on October 18.3. The new comet has been designated C/2012 U1 (PANSTARRS).

We performed some follow-up measurements of this object, while it was still on the neocp. Stacking of 5 R-filtered exposures, 120-sec each,obtained remotely, from Haleakala-Faulkes Telescope North on 2012, Oct. 22.45, through a 2.0-m f/10.0 Ritchey-Chretien + CCD under good seeing conditions, shows that this object is a comet: diffuse coma nearly 5" in diameter.

Our confirmation image (click on the image for a bigger version):
  

M.P.E.C. 2012-U66 (including prediscovery Mount Lemmon Survey observations from Oct. 17.4) assigns the following preliminary parabolic orbital elements to comet C/2012 U1: T 2014 June 30.96; e= 1.0; Peri. = 58.03; q = 6.31 AU; Incl.= 66.73.

by Ernesto Guido, Nick Howes, Alison Tripp & Giovanni Sostero

Monday, October 22, 2012

Update on Comet 168P/Hergenrother

Our team performed follow-up observations of comet 168P/Hergenrother on 2012, Oct. 22.4, remotely through the 2m, f/10 Ritchey-Chretien + CCD of Faulkes Telescope North (Haleakala) under good seeing conditions, and a scale of 0.3"/px. Comet 168P has recently undergone an outburst with its magnitude increasing from ~14-15 to magnituide ~9.5. For more info about the recent outburst of this comet, see our previous post here.

Recent observations posted on comet-images ml were showing a "cloud" of material trailing the nucleus in the anti-solar direction. In our image (stacking of 9 x 30-second exposures) is visible an unresolved and diffuse trail about 6" long and 3" wide in PA145. Click on the image for a bigger version.


Below you can see a graph showing recent magnitude estimates of comet 168P.  Click on the image for a bigger version.

Credit: Seiichi Yoshida

by Giovanni Sostero, Nick Howes, Alison Tripp & Ernesto Guido

Thursday, October 18, 2012

New Comet: P/2012 T7 (VOROBJOV)

Cbet nr. 3260, issued on 2012, October 18, announces the discovery of a new comet (discovery magnitude 20.1) by Tomas Vorobjov on three 120-s images that he took on October 15 remotely with Alexander Kostin (Houston, TX, U.S.A.) using a 0.81-m f/7 Ritchey-Chretien reflector located at the Mt. Lemmon SkyCenter via the Sierra Stars Observatory Network. The new comet has been designated P/2012 T7 (VOROBJOV).

We performed some follow-up measurements of this object, while it was still on the neocp. Stacking of 3 R-filtered exposures, 120-sec each,obtained remotely,from Haleakala-Faulkes Telescope North on 2012, Oct. 18.43, through a 2.0-m f/10.0 Ritchey-Chretien + CCD under good seeing conditions, shows that this object is a comet: narrow tail nearly 15" long in PA 270 elongated coma 6"x4" in the same direction
 
Our confirmation image (click on the image for a bigger version):


M.P.E.C. 2012-U40 assigns the following preliminary elliptical orbital elements to comet P/2012 T7: T 2012 June 16.58; e= 0.33; Peri. = 174.76; q = 3.78 AU; Incl.= 13.55.

by Nick Howes, Giovanni Sostero & Ernesto Guido.

New Comet: C/2012 T6 (KOWALSKI)

Cbet nr. 3259, issued on 2012, October 18, announces the discovery of a new comet (discovery magnitude 17.6) by R. A. Kowalski on Catalina Sky Survey images obtained with the 0.68-m Schmidt telescope on October 15.4. The new comet has been designated C/2012 T6 (KOWALSKI).

We performed some follow-up measurements of this object, while it was still on the neocp. Stacking of 7 R-filtered exposures, 30-sec each, obtained remotely,from the Siding Spring-Faulkes Telescope South on 2012, Oct. 16.63, through a 2.0-m f/10.0 Ritchey-Chretien + CCD under good seeing conditions,shows that this object is a comet: narrow tail nearly 13" long in PA 290 and a wide, fan-shaped tail about 20" long toward the North-northwest sharp coma about 7" in diameter.

Our confirmation image (click on the image for a bigger version):


M.P.E.C. 2012-U39 assigns the following preliminary parabolic orbital elements to comet C/2012 T6: T 2012 Aug. 25.08; e= 1.0; Peri. = 196.47; q = 1.79 AU; Incl.= 34.28.

by Ernesto Guido, Alison Tripp, Giovanni Sostero & Nick Howes

New Comet: C/2012 T5 (BRESSI)

Cbet nr. 3258, issued on 2012, October 18, announces the discovery of a new comet (discovery magnitude 18.6) by T. H. Bressi on CCD mosaic images taken with the Spacewatch 0.9-m f/3 reflector at Kitt Peak on October 14.4. The new comet has been designated C/2012 T5 (BRESSI).

According to its preliminary parabolic orbital, comet C/2012 T5 (BRESSI) has it perihelion on T 2013 Feb. 23 with q = 0.31 and it might reach the peak  magnitude ~8. (graph generated using the software Orbitas - click on the image for a bigger version).


We performed some follow-up measurements of this object, while it was still on the neocp. Stacking of 7 R-filtered exposures, 120-sec each,obtained remotely,from H06 (ITelescope network near Mayhill, NM) on 2012, Oct. 18.33, through a 0.43-m f/6.8 astrograph + CCD + f/4.5 focal reducer shows that this object is a comet: compact coma about 5" in diameter and tail nearly 12" long in PA 270.

Our confirmation image (click on the image for a bigger version):

 
M.P.E.C. 2012-U38 assigns the following preliminary parabolic orbital elements to comet C/2012 T5: T 2013 Feb. 23.55; e= 1.0; Peri. = 318.27; q = 0.32 AU; Incl.= 71.87.

by Giovanni Sostero, Nick Howes & Ernesto Guido

Tuesday, October 16, 2012

New Comet: P/2012 T1 (PANSTARRS)

Cbet nr. 3252, issued on 2012, October 11, announces the discovery of a new comet (discovery magnitude 20.7) by Pan-STARRS survey on CCD images obtained  with the 1.8-m Ritchey-Chretien + CCD on October 06. Prediscovery Pan-STARRS observations from 2011 July 28 were identified by G. V. Williams. The new comet has been designated P/2012 T1 (PANSTARRS).

After posting on the Minor Planet Center's NEOCP webpage, other CCD astrometrists have also commented on the object's cometary appearance. We performed some follow-up measurements of this object on 2012, October 11.63. Stacking of 8 R-filtered exposures, 60-sec each, obtained remotely, from the Haleakala-Faulkes Telescope North through a 2.0-m f/10.0 Ritchey-Chretien + CCD, shows that this object is a comet with a faint coma nearly 5" in diameter.

Our image (click on the image for a bigger version):



M.P.E.C. 2012-T55 assigns the following elliptical orbital elements to comet P/2012 T1: T 2012 Nov. 21.89; e= 0.20; Peri. = 323.21; q = 2.41 AU; Incl.= 11.4. According to his orbital characteristics of asteroids (with a Tisserand parameter T_jup = 3.176 ) and the physical characteristics of comets this object seems to be a new member of Main Belt Comets (MBCs). (see also our previous post about 596 Scheila)

Below is shown the distribution of the mass-losing asteroids, including their relation to the asteroids (orange dots) and classical comets (blue dots). In this graph the position of newly discovered P/2012 T1 (PANSTARRS) is just nearby 300163 (2006 VW139). Click on the image for a bigger version.

Credit: David Jewitt

by Ernesto Guido, Robert Shirley, Nick Howes & Giovanni Sostero

Thursday, October 11, 2012

Deep South

While talking with people at star parties, or giving talks to schools, we are frequently asked: "How far can you see through your telescope?”. For people not really involved in astronomy, the usual answer will be in light years perhaps, highlighting objects like a bright quasar such as 3C273, or possibly by explaining the concept of magnitude, and relating it to telescopes like the Hubble. The explanation of aperture rather than magnification is one that can initially be complex for beginners.

However, even professional research scientists need to know how far you can push the limit of the equipment for the work they do. This is an interesting question which we have recently tackled, as a result of a project Nick was invited to take part in with the Lowell Observatory in imaging and refining the orbits of Kuiper Belt objects, and one which the rest of us here at Remanzacco including Nick are now working on.

This concept of measuring the telescope´s capability in terms of how deep an image we could take, in what time, under what conditions, would give us confidence in some projects involving long period cometary bodies, TNO´s and assisting the research work of professional observatories, many of which have much larger telescopes even that the one we were testing. Recently, courtesy of our involvement in the Lowell project and also some very deep images taken of comet 67P in support of the European Space Agency Rosetta mission, the Faulkes telescope team gave us the opportunity to test the capabilities of their professional-class research grade instrument, namely the  2-m Faulkes Telescopes South at Siding Spring.

The idea was to check if, through this instrument with its sensitive cooled CCD, the C/1995 O1 (Hale-Bopp) was still visible. Comet Hale-Bopp was notable as one of the brightest and interesting comets observed in the modern era, and it's still a subject of interest for astronomers. (Nick had also recently met Thomas Bopp at the Tucson Space Fest).

According to a couple of recent works by G. Szabò et Al. (http://arxiv.org/abs/1104.4351 & http://arxiv.org/abs/1210.2785) comet Hale-Bopp was observed in 2010 on Dec. 4 through the 2.2- reflector at the La Silla Observatory at around 30.7 AU solar distance (and receding), and afterwards, on 2011, Oct. 23 at 32 AU. In their papers, the authors discussed a variety of interesting scientific aims with their detection of this comet at magnitude R=23.3 on 2010, Dec. 4, and R=23.7 on 2011, Oct. 23, that have some significant consequences on our understanding of Hale-Bopp, and comets in general.

Our interest on C/1995 O1 was also raised by a report posted in the Yahoo Group - Comets-ml which alluded to the possible detection of this comet at magnitude 21.8 on 2012, Aug. 07 using a more modest telescope. Languishing now at over 33.4 AU, we knew this was going to be an exceptionally tough target. However, we decided to attempt its observation through a professional class instrument, suitably placed in the southern hemisphere (currently Hale-Bopp is located near the south celestial pole, in the constellation Octans, at 87 deg declination south). We planned a systematic follow-up campaign over the course of several nights, in order to test not only if we could image the comet, but also the instrument capabilities through different observing condition (moonlight, seeing, sky transparency, etc.).

On each night we performed a series of 180 second Bessel R-filtered exposures that were subsequently stacked through dedicated software (Astrometrica by Herbert Raab and Maxim DL by Cyanogen). With Astrometrica we stacked along the expected proper motion of the comet (since Hale-Bopp was moving at a rate of 0.07"/min, this exposure time length would not trail the comet)

We estimated that if we were to catch this extremely faint object *(The research paper cited showed a possible light curve indicating in excess of magnitude 24) we knew we would need rather long total integration times, so we asked the collaboration of several colleagues working with the Faulkes scopes, including a work experience student, along with slots which had not been taken up on FTS. Faulkes telescope team members, including a work experience student, Cerys Roche, also assisted with the imaging test.

We accumulated a total of five nights of follow-up from 2012, Sept.25 till Oct.9. With observing runs typically from 30 minutes up to 90 minutes in duration, depending on the scope´s availability and the local weather conditions. We carefully analyzed the resulting stacks in search of comet Hale-Bopp, whilst also measuring sky conditions, limiting magnitude and looking for other moving objects in the field.

This is the log of our observations (the reported negative detection refers to an area wide about 2-arcmin, that's several times the expected error box on Hale-Bopp ephemerides):

2012, Sept.25.5, stacking of 20 exposures, 180 seconds each, comet not seen to limiting magnitude R= 22.5.

2012, Sept.26.5, stacking of 15 exposures, 180 seconds each, comet not seen to limiting magnitude R= 21.5

2012, Oct. 4.6, stacking of 25 exposures, 180 seconds each, comet not seen to limiting magnitude R= 22.5

2012, Oct. 8.6, stacking of 20 exposures, 180 seconds each, comet not seen to limiting magnitude R= 23.0

2012, Oct. 9.6, stacking of 20 exposures, 180 seconds each, comet not seen to limiting magnitude R= 23.5


(Note. On a night, stacking some set of images, we had the impression of an possible positive detection at the very threshold limit of our exposures, however we cannot conclusively state is the comet, due to its extremely low S/N. Whilst it may indeed be Hale-Bopp, without a secure second night of detection, there is a possibility it could just be an anomaly or noise)

In this following figure we report some representative results of our image stacks , where the red square overlay indicates the expected position of comet Hale-Bopp on each of the observing sessions. Click on the image for a bigger version.



It is our conclusion that, whilst on a few instances we may possibly have detected something, we have to state that cannot claim to have a positive and verifiable detection of C/1995 O1 even under our best observing conditions, down to limiting magnitude R about 23.5 with sub arcsecond seeing. In spite of the negative detection of the comet (which has some meaningful scientific consequences based upon the La Silla observations anyway), this was a vital experiment for us, since we have been able to ascertained how far we can go with the Faulkes scopes,  with respect to seeing and other associated conditions. We now know that we can detect moving targets on good nights down to in excess of magnitude 23.

We also understand far more how we can process our data right at the noise level limit. All this will be very helpful in our projects like that with the Lowell Observatory, aiming at the follow-up of faint Trans Neptunian Objects (TNOs) in need of follow-up to refine the knowledge of their orbital parameters, and also with the ESA SSA project, where detected objects can rapidly fade to limiting magnitudes which are beyond the 1m class instrument their team have.

We are grateful to those who kindly assisted us in this exciting hunt, and in particular to: P. Roche, C. Roche, A. Tripp, S. Roberts and K. Rochowicz.

by Nick Howes, Giovanni Sostero and Ernesto Guido

Tuesday, October 9, 2012

Close Approach of Asteroid 2012 TC4

M.P.E.C. 2012-T18, issued on 2012 Oct. 7, reports the discovery of the asteroid 2012 TC4 (discovery magnitude 20.1) by F51 Pan-STARRS 1, Haleakala on images taken on October 04.4 with a 1.8-m Ritchey-Chretien + CCD.

2012 TC4 has an estimated size of 13 m - 29 m (H=26.5) and it will have a close approach with Earth at about 0.25 LD (Lunar Distances = ~384,000 kilometers) or 0.0006 AU (1 AU = ~150 million kilometers) at 0531 UT on 12 Oct. 2012. This asteroid will reach the magnitude 13.7 on October 12 around 02 UT. 2012 TC4 is a potential radar target later this week and astrometry is requested!

We performed some follow-up measurements of this object, from the H06 ITelescope network (near Mayhill, NM) on  2012, Oct. 09.3, through a 0.25-m f/3.4 reflector + CCD. Below you can see our image, stack of 30x60-second exposures, taken with the asteroid at magnitude ~17.2 and moving at 5.33"/min. At the moment of the close approach on October 12, 2012 TC4 will move at ~ 900"/min. Click on the image below to see a bigger version.


Below you can see a short animation showing the movement of 2012 TC4. Click on the thumbnail for a bigger version:
 


The map below (calculated on the Armagh Observatory website) shows all know asteroids in the neighbourhood of the Earth updated to October 08, 2012. The red oval surrounding the earth represents 3.84 million kilometres (projected onto the plane). This is a distance equal to 10 Lunar distances. Any object currently inside this distance will be highlighted in red, in this case 2012 TV & 2012 TC4. Click on the image for a bigger version.



UPDATE - October 16, 2012

Below you can see our image (single 180-second exposure) of 2012 TC4 obtained on October 11 at 21UT, few hours before its close approach. At the moment of the image, the asteroid was of magnitude ~14.4 and moving at ~185"/min. Click on the image for a bigger version.



by Ernesto Guido, Giovanni Sostero, Nick Howes

Sunday, October 7, 2012

Close Approach of Asteroid 2012 TV

M.P.E.C. 2012-T16, issued on 2012 Oct. 6, reports the discovery of the asteroid 2012 TV (discovery magnitude 17.6) by Tenagra II Observatory on images taken on October 05.3 with a 0.41-m f/3.75 astrograph + CCD. 

2012 TV has an estimated size of 24 m - 55 m (H=25.1) and it will have a close approach with Earth at about 0.66 LD (Lunar Distances = ~384,000 kilometers) or 0.0017 AU (1 AU = ~150 million kilometers) at 1504 UT on 7 Oct. 2012. This asteroid will reach the magnitude 13.5 on October 07 around 13-14UT.

We performed some follow-up measurements of this object, from the H06 ITelescope network (near Mayhill, NM) on  2012, Oct. 06.3, through a 0.25-m f/3.4 reflector + CCD.

Below you can see our image, stack of 40x15-second exposures, taken with the asteroid at magnitude ~15.6 and moving at 14.30"/min. At the moment of the close approach on October 07, 2012 TV will move at ~ 815"/min. Click on the image below to see a bigger version.


Below you can see a short animation showing the movement of 2012 TV (each frame is a 15-second exposure). Click on the thumbnail for a bigger version:


UPDATE - October 10, 2012

Below you can find the resulting lightcurve for asteroid 2012 TV using data from two observing sessions on this asteroid obtained separately by Lorenzo Franco and our team. The lightcurve data appears to show a period of 0.0525 h +/- 0.0001 and an amplitude of 0.51 mag.  Click on the image for a bigger version.



UPDATE - December 21, 2012

Our photometry paper has been published on  the latest Minor Planet Bulletin (MPB 40-1) on page 44. You can download it on MPB website or here.


by Ernesto Guido, Giovanni Sostero, Nick Howes & Luca Donato

Friday, October 5, 2012

Comet C/2012 S1 (ISON) - Update 2012, Oct. 4

We obtained further follow-up on C/2012 S1 (ISON) on 2012, Oct. 4.6, again  through the 2.0-m f/10.0 Ritchey-Chretien + CCD + Bessel R filter of Faulkes  Telescope North (F65) Haleakala.

Stacking of 9 exposures, 120 seconds each, produced an image where  a well developed and elongated coma measuring nearly 6"x9" is now visible, extended toward PA  280 deg. Click on the image below for a bigger version.



From this stacking, the Afrho parameter was determined to be about 1530  +/- 150 cm (see panel below). The total magnitude has been measured as R= 17.7. Click on the image below for a bigger version. 



For more info about comet C/2012 S1 (ISON) please see our previous post: 



by Nick Howes, Giovanni Sostero and Ernesto Guido

Thursday, October 4, 2012

Follow-up on 168P/Hergenrother bright phase

According to reports issued by a number of observers to several astro-forums, comet 168P/Hergenrother is currently experiencing a bright phase: over the course of several nights, it increased in brightness by several magnitudes, reaching a total visual magnitude of approximately 8. We performed some follow-up on it remotely, on 2012 Sept. 26 and Oct. 3, through the 2.0-m f/10.0 Ritchey-Chretien + CCD + Bessel R filter of the Faulkes Telescope South, at Siding Spring (click on the image below for a bigger version).


Inspecting our stacked images obtained on Sept. 26, the comet shows an obvious  central condensation, measuring nearly 3" across; the total coma was traced to a  diameter of about 1.7'. On Oct. 3, the central condensation grew to 8" and the  total coma diameter was nearly 3'. It's interesting to notice how, apart the growing of the central condensation  size, also its appearance changed, appearing pretty sharp on Sept. 26, and a bit "fluffy" on Oct. 3.

Subtracting the two images (after normalization of the stackings) of Sept. 26  from Oct. 3, the evolution of the central condensation became obvious. To some  extent, the difference might be due to a slight change in the perspective angles  (e.g. its phase angle passed from 10 to 13 deg), however we consider that a part  of what we see in the panel, is a genuine evolution of the central condensation  due to its active phase (click on the image below for a bigger version).


The photometric data supports the observed evolution: our afrho data on Sept. 26  shows a peak of about 670 +/- 100 cm at 2230 km from the photocenter, while on  Oct. 3 we measured an afrho of 1210 +/- 150 cm peaking at a radius of about 3000 km from the photocenter.  This seems to indicate a two-fold increase of the afrho  activity within the central condensation in a matter of a week, as well the  apparent recession of the activity peak from the central condensation. A change in the photometric profile of the coma is also obvious, from the comparison of the two panels, with the Oct. 3 graph significantly wider and less  steep, compared to that of Sept. 23.

  
By clicking on the thumbnail below you can see a short animation of comet 168P/Hergenrother on our images of 2012 Sept. 26:


 
So our follow-up confirms and adds scientific data to the previous reports: currently comet 168P/Hergenrother is alive and rather active.

Giovanni Sostero, Nick Howes, Alison Trip and Ernesto Guido

Monday, October 1, 2012

Updated data for comet C/2012 S1 (ISON)

Our team performed follow-up observations of comet C/2012 S1 (ISON) on 2012, Sept. 28.6, remotely through the 2m, f/10 Ritchey-Chretien + CCD of Faulkes Telescope North (Haleakala) under good seeing conditions, and a scale of 0.3"/px. After stacking 13 R-filtered exposures, 120-seconds each, comet ISON appears as a pale blob of light, slightly elongated toward the south-west (this is particularly obvious looking at the azimuthal median subtraction rendition). Click on the image below to see a bigger version.


The Afrho (proxy of dust abundance within the coma) calculation we performed on this dataset, using a few Tycho reference stars having colour indexes close to that of the Sun, provided rather puzzling results: in short, we found a significant variation of the Afrho amount, according to the dimension of the measurement window (something pretty different from the steady state coma model).

We measured an Afrho maximum value of 1365 +/- 200 cm, for a corresponding aperture of nearly 16,000 km in diameter at the comet distance. After this peak, the Afrho amount decreases, reaching a minimum of about 900 +/- 200cm for apertures of 50,000 to 60,000 km. Similar afrho values has been found analyzing CCD images obtained with the 1.5-m f/8 reflector at the Majdanak observatory on Sept. 21.99 (courtesy of A. Novichonok, scale 0.42"/px).

Afrho variation along the coma of C/2012 S1

This behaviour seems to indicate a steep photometric variation along the coma's profile, with a significant increase of the afrho values close to the central condensation (i.e. within about 6" to 8" of the nucleus), which would not be detected using average amateur telescope, for reasons of resolution and seeing conditions.

Indeed, our previous Afrho measurements, performed on Sept. 22.5 through the 0-25, f/3.4 reflector + CCD (scale 1.7"/px), provided an Afrho amount of about 600 +/- 150 cm, for a corresponding aperture of nearly 46,000 km at the current comet distance (taking into account the respective error bars, this compares rather well with the values found through the bigger scopes at similar distances from the nucleus).

From this data it seems that, at the current distance of comet ISON from the Sun, small amateur telescopes will have difficulty accounting for its near-nucleus activity, due to their unfavourable scale factor, and seeing conditions (however they still can play a role monitoring the evolution and general trend of this comet).

The punch line : Comet C/2012 S1 appears to be even more active than we first thought...watch this space!

Giovanni Sostero, Nick Howes and Ernesto Guido