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BrooksObs
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Reged: 12/08/12

Re: Comet C/2011 L4 (PANSTARRS) new [Re: RolandosCY]
      #5742515 - 03/19/13 09:50 AM

Quote:

Quote:

On another front, I'm hope that tomorrow will see the comet's quoted magnitude on Spaceweather revised. While therein it is stated as currently +0.2 , in fact I obtained +2.6 this evening, which would be much more in line with PanSTARRS proving to be a difficult object to spot without some form of optical aid in recent days.

BrooksObs



Dear John, having observed this comet finally without clouds on the sky, I wonder about this 2.6 magnitude. I am not experienced in judging comet magnitudes, but certainly, from 34.5 N under totally clear and transparent skies it seemed brighter than 2. On Sunday evening we could easily locate it naked eye with direct vision as a fuzzy star, and at times we could detect a tiny tail with averted vision. No equally bright nearby stars to compare, but it seemed to be much brighter than Algenib or alpha and beta Aries (mag 2 to 2.8). I estimated it around 1, and a friend around 0.5. Now, as I said I don't have much experience, and certainly nothing to compare with your experience. But wouldn't an extended fuzzy object of 2.6 seem fainter than a 2.7 star? Just curious...




Rolando - I must admit that the degree of scatter that I am seeing in regard to Comet PanSTARRS' reported magnitude since it became visible to Northern Hemisphere observers, currently amounting to something like 3.5 magnitudes(!), has stunned me. I've not seen such uncertainty in a comet's brightness in several decades and that it should occur in conjunction with such a relatively faint comet (as compared with others seen earlier) is rather perplexing.

I would note that drastically less scatter is evident in the magnitude data posted by among the more reliable comet observers from the Southern Hemisphere prior to the comet's perihelion. When plotted (see below), these data form a very tight lightcurve which extends from the first days of January until just a couple of days prior to perihelion. The photometric parameters derived from these data have a coefficient of correlation amounting to 0.99 , i.e. a very high accuracy. Further, my observation placing the comet at +2.6 on March 19.0UT corresponds almost precisely to that predicted by the formula for that date with the comet outbound from the Sun.



At the same time, judging by a comparison of similarly exposed photos of the comet from down south when PanSTARRS was being reported as definitely about 2nd magnitude, the physical appearance and brightness when it was last seen from that hemisphere is absolutely no different from the recent examples from up north. This clearly infers that no physical changes in either brightness, or the comet's appearance and visibility, have occurred between the final southern and early northern sightings.

You ask would not the diffuse nature of the coma of a comet make it appear fainter than a star of the same brightness? Indeed it normally would were the coma to subtend a significant size to the unaided eye. In PanSTARRS' case, however, with a diameter of only about 3'-4' and its very high degree of condensation, the comet's head appears essentially stellar to the unaided eye and can be compared directly with little, if any, resulting error.

Now there have been a great many reports from observers that PanSTARRS has been near, or even below, the level of detection with the naked eye, even under rather favorable sky conditions. I've observed a number of magnitude zero to +1.0 comets in twilight over the years and let me give you an impression of how they normally looked. Comet Bennett, in 1970, was a fine example. Shortly following its perihelion, when it was just becoming visible from the Northern Hemisphere in morning twilight as an object of +0.5 magnitude, my observing log includes a passage that indicates my initial impressions. One early morning while awaiting the best time to begin observation of the comet I sat down to watch a little TV beforehand. The room was modestly illuminated by the light of the TV and a small lamp. In spite of this illumination I commenced to notice a ghostly shaft of light out the east-facing window. After a time, very low over the eastern horizon in the breaking dawn I could see a fully developed comet rising, with a bright head and perhaps up to 5-degrees of tail visible through the nearby tree branches...as seen from inside the house through the window in the lighted room.

Thus, the impact to the unaided eye of a truly magnitude +0.5 comet is rather jaw-dropping, even in early/late twilight, and certainly not perceived as no more than a vague wisp in the fading evening light. Believe me, the next actual zero magnitude comet to come along will reveal to observers just how faint and insignificant PanSTARRS turned out to be.

BrooksObs

Edited by BrooksObs (03/19/13 01:46 PM)


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Unknownastron
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Reged: 04/06/05

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Re: Comet C/2011 L4 (PANSTARRS) new [Re: johnpd]
      #5743134 - 03/19/13 02:47 PM

I saw it again last night, observed first time with my biggest binocular, 20x125. I saw no indications of any breakup in the coma or disconnections in the tail. But I was limited to 20X. The tail looked more symmetrical, almost a perfect 'V' shape with the northern part of the V slightly brighter. WIth the large aperture I estimated the tail length at 35--40 minutes or arc. Small image but still a very beautiful comet.
Clear skies and clean glass.
Mike


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Phillip Creed
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Re: Comet C/2011 L4 (PANSTARRS) new [Re: BrooksObs]
      #5743374 - 03/19/13 04:26 PM

Quote:

Rolando - I must admit that the degree of scatter that I am seeing in regard to Comet PanSTARRS' reported magnitude since it became visible to Northern Hemisphere observers, currently amounting to something like 3.5 magnitudes(!), has stunned me. I've not seen such uncertainty in a comet's brightness in several decades and that it should occur in conjunction with such a relatively faint comet (as compared with others seen earlier) is rather perplexing.

I would note that drastically less scatter is evident in the magnitude data posted by among the more reliable comet observers from the Southern Hemisphere prior to the comet's perihelion. When plotted (see below), these data form a very tight lightcurve which extends from the first days of January until just a couple of days prior to perihelion. The photometric parameters derived from these data have a coefficient of correlation amounting to 0.99 , i.e. a very high accuracy. Further, my observation placing the comet at +2.6 on March 19.0UT corresponds almost precisely to that predicted by the formula for that date with the comet outbound from the Sun.



At the same time, judging by a comparison of similarly exposed photos of the comet from down south when PanSTARRS was being reported as definitely about 2nd magnitude, the physical appearance and brightness when it was last seen from that hemisphere is absolutely no different from the recent examples from up north. This clearly infers that no physical changes in either brightness, or the comet's appearance and visibility, have occurred between the final southern and early northern sightings.




John,

I can think of one big reason the Northern Hemisphere estimates are going all over the place.

The Southern Hemisphere in general, and Australia in particular, is blessed with much, much cleaner air than the Northern Hemisphere. Only 10% of the world's population lives below the equator and industrial emissions are far, far less.

The ICQ extinction tables account for aerosol extinction of 0.084-mag/air-mass ("winter"), 0.120-mag/air-mass ("average") and 0.156-mag/air-mass ("summer").

If you examine aerosol data from Australia:

http://aeronet.gsfc.nasa.gov/cgi-bin/climo_menu_v2_new

you'll find that Aerosol Optical Thicknesses @ 500-nm (close to the 510-nm peak of scotopic vision) often run 0.10 or below in Australia, where most Southern Hemisphere observers are located. (For any wavelength, AOT x 1.086 = aerosol mag/air-mass extinction.) Much of Australia could get by with the ICQ's winter extinction table year-round.

I just think the Southern Hemisphere observers can, in general, provide more reliable estimates of low-hanging comets because less extinction = more precision when applying an extinction correction.

Clear Skies,
Phil


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nytecam
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Re: Comet C/2011 L4 (PANSTARRS) new [Re: Phillip Creed]
      #5743402 - 03/19/13 04:32 PM Attachment (28 downloads)

Cloud for last week but clear at dusk tonight for shot below via scope in yard obsy with comet running into trees - the 2 stars are mag 9 - hint of bright 'horns' from back of nucleus

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Dave Mitsky
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Re: Comet C/2011 L4 (PANSTARRS) new [Re: nytecam]
      #5743420 - 03/19/13 04:44 PM Attachment (26 downloads)

Here's a photo of some of the ASH members who observed Comet PanSTARRS from the Naylor Observatory on the evening of March 14th. A few other photos can be seen at http://www.facebook.com/media/set/?set=a.600273573333854.1073741826.100000536...

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RolandosCY
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Re: Comet C/2011 L4 (PANSTARRS) new [Re: BrooksObs]
      #5743622 - 03/19/13 06:06 PM

Dear John, thanks for your informative reply! I am lucky enough to have seen Hayutake and hale-Bopp, but at the time astronomy was not the major part of my life that it is now! I do recall though seeing Hale-Bopp easily while driving in twilight and that was really bright! The brightest comet I have seen since was Holmes, and that was an altogether different story. It is great for us to be able to build on the experience of observers like you!

I re-observed the comet earlier tonight, but through some horizon haziness and streaks of thin clouds. Its appearance was not at all similar to the grand appearance of Saturday and sunday. Tonight it was not a naked-eye object, and certainly it did look much fainter than nearby Algenib.


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Tonk
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Re: Comet C/2011 L4 (PANSTARRS) [Re: RolandosCY]
      #5743968 - 03/19/13 08:46 PM

Quote:

hint of bright 'horns' from back of nucleus




Maurice - I believe that is an effect caused by a cone shadow cast back through the tail from the denser muck around the nucleue.

Compare to this image from Spaceweather.com front page 16th March

http://spaceweather.com/images2013/16mar13/split_strip.jpg


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magic612
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Reged: 09/30/08

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Re: Comet C/2011 L4 (PANSTARRS) [Re: Tonk]
      #5744088 - 03/19/13 09:54 PM

I got some more pictures tonight - below is the best one. The weather was quite brutal; 30 degrees when I left, but a stiff 15 to 20 mph wind! True, that only converts to a 20F wind chill, but it felt much colder and my fingers were almost numb when I got home!

I searched for quite a while at the 30 minutes past sunset mark - partly outside, partly from my car. Did not finally spot it until almost 60 minutes past sunset. Well, I didn't exactly "spot" it; the camera captured it. My fingers were too cold at that point to try and work the binoculars, and the wind was causing my eyes to tear up when I tried using them anyway. So rather than waste precious time, I fired off as many shots as I could. Most were bad, due to the terrible wind conditions and the need to use 1/2 second exposures at that point, but a managed a few usable ones.

The comet is less than 2 degrees from (I believe) 5.0 magnitude 47 Piscium in this picture (that same point is in several other pictures I took, and doesn't move); I suppose one can use that to estimate magnitude from there. I'm better at estimating star magnitudes, not so much extended objects, so I'll leave the comet magnitude estimations to others.



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waso29
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Re: Comet C/2011 L4 (PANSTARRS) [Re: magic612]
      #5744132 - 03/19/13 10:15 PM



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magic612
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Re: Comet C/2011 L4 (PANSTARRS) [Re: waso29]
      #5744134 - 03/19/13 10:17 PM

Thanks Bill! Did you catch it tonight at all? I saw your picture from last week.

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MDB
super member


Reged: 06/06/12

Loc: Idaho
Re: Comet C/2011 L4 (PANSTARRS) [Re: magic612]
      #5744266 - 03/19/13 11:29 PM

Dave, thank you for the PanSTARRS info on Eyes On The Sky, I have successfully observed PanSTARRS 3 nights in a row. I appreciate your effort.

Mike


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nytecam
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Reged: 08/20/05

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Re: Comet C/2011 L4 (PANSTARRS) [Re: Tonk]
      #5744500 - 03/20/13 04:37 AM

Quote:

Quote:

hint of bright 'horns' from back of nucleus


Maurice - I believe that is an effect caused by a cone shadow cast back through the tail from the denser muck around the nucleue.Compare to this image from Spaceweather.com front page 16th March
http://spaceweather.com/images2013/16mar13/split_strip.jpg


Thanks Tony - that sounds very plausible

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magic612
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Re: Comet C/2011 L4 (PANSTARRS) [Re: MDB]
      #5744717 - 03/20/13 09:19 AM

Quote:

Dave, thank you for the PanSTARRS info on Eyes On The Sky, I have successfully observed PanSTARRS 3 nights in a row. I appreciate your effort.

Mike




That's great to hear, Mike! I'm glad to have helped.


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Phillip Creed
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Re: Comet C/2011 L4 (PANSTARRS) [Re: magic612]
      #5744726 - 03/20/13 09:22 AM

I don't think John Bortle's 2.6-mag estimate for March 19.00 UTC was that far off the mark at all.

I'm on cub duty (my affectionate term for looking after our children) in the evenings, so any observing has to be extremely brief. I got a look at C/2011 L4 last night at 8:36 p.m. local time (March 20.03 UT). The comet was 7.0 deg above the horizon, with 2.1-mag Alpheratz at 9.7-deg above the horizon, and the sun at -12 deg (end of nautical twilight). The coma appeared very condensed; DC ~ 8, in 15x70 binoculars, with a bright, fan-shaped 0.6-deg tail visible. The comet was not visible to the naked eye.

As far as the mag estimate goes, there's only one star I used (again, this was a very brief observation), and that was Alpheratz. I defocused the comet in the binoculars just enough to have its light be uniform, then swung over to Alpheratz and defocused it to the same diameter. The defocused Alpheratz was obviously brighter than the slightly-defocused comet.

So the next thing I did (disclaimer--this is NOT recommended practice for a quality mag estimate, but it's the best I could do given a very limited time span! Always use multiple comparison stars whenever possible.) was to defocus Alpheratz until its surface brightness matched the surface brightness of the slightly-defocused comet. I found that I had to defocus Alpheratz until it was about twice the diameter of the defocused comet.

Why do this? If the comet had the same surface-brightness as Alpheratz, but was only 1/4 the area, it would stand to reason that the comet's integrated brightness is 2.05 (mag of Alpheratz) + 2.5*log (4), or mag 3.55. But we also have to account for the extinction difference, because Alpheratz was higher in the sky. Using Rozenberg's Equation, the difference between 9.7 and 7.0 degrees is the difference between looking through 5.80 air masses and 7.79 air masses, or about 2.0 air masses. Transparency was excellent, so total extinction--including aerosols, Rayleigh scattering for my 1,100-ft MSL observing site and ozone absorption--is about 0.24-mag/air mass if I apply the ICQ's winter extinction table.

Thus, I come up with a mag estimate of about 3.1 for the comet, a significant fading from last week.

Again, this was a very rough estimate. If the amount of defocusing of Alpheratz was, say, 1.8X the diameter of the defocused comet vs. 2X, my mag estimate would be 2.9 instead of 3.1, and a similar deviation if it were 2.2X. Again, a very crude method under a time crunch.

The comet will be roughly the same elevation as Alpheratz in the next several days, so this should help bolster the reliability of the comet's mag estimates from comet observers.

Clear Skies,
Phil


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Edward E
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Re: Comet C/2011 L4 (PANSTARRS) [Re: nytecam]
      #5744873 - 03/20/13 11:01 AM

Very nice shot of the comet.

Southern AZ has been under a persistent band of ciriform clouds since last Friday so comet viewing has been on hold here. It's nice to be able to follow the comets progress from your images and all the rest of the post on this forum.


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bremms
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Re: Comet C/2011 L4 (PANSTARRS) [Re: Edward E]
      #5744947 - 03/20/13 11:39 AM

Nice shot of the comet. I tried to find it last night with my 7x50's with no luck. There was some haze in the sky and my 3 year old was with me and was getting antsy. Probably my last shot for a while due to weather.

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Dave Mitsky
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Re: Comet C/2011 L4 (PANSTARRS) new [Re: bremms]
      #5745260 - 03/20/13 02:23 PM

I was able to observe Comet PanSTARRS again on Tuesday evening from the ASH Naylor Observatory using 7x50 and 15x70 binoculars, a 5" f/5 achromatic refractor, and a 17" f/15 classical Cassegrain at 118 and 162x. The comet was positioned farther to the west and seemed to be noticeably fainter than on March the 14th.

The interior temperature of the French Dome was about 41 degrees initially but it was so windy that it was rather uncomfortable, even inside the dome. The 17" actually moved from time to time when the wind gusted.

Afterwards, I observed Jupiter but the seeing was so bad that I was unable to see the transit of Europa's shadow that was underway. I kept the magnification relatively low (118, 144, and 162x) and stopped the 17" down to 12" with an aperture mask but was still unable to detect the shadow. I did catch Io coming out of eclipse, however.

I also observed the just-past First Quarter Moon for a bit, garnering fairly good views of craters such as Alphonsus, Archimedes, Aristillus, Arzachel, Autolycus, Cassini, Ptolemaeus, and Purbach, the Montes Alpes and Montes Caucasus mountain ranges, and Vallis Alpes (the Alpine Valley).

I stayed until Ganymede reemerged from eclipse. At that point, clouds began to roll in rather rapidly. By the time I was almost home, a brief snow flurry began.

[url=http://www.astrohbg.org/ASH-i/Image_Gallery/Pages/Naylor_Observatory_Tour.html#11"]http://www.astrohbg.org/ASH-i/Image_Gallery/Pages/Naylor_Observatory_Tour.htm...[/url]

Dave Mitsky


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Dave Mitsky
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Re: Comet C/2011 L4 (PANSTARRS) new [Re: Dave Mitsky]
      #5745643 - 03/20/13 05:00 PM Attachment (39 downloads)

Here's a photo that I took from inside the French Dome last night.

Dave Mitsky


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Dave Mitsky
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Re: Comet C/2011 L4 (PANSTARRS) new [Re: Dave Mitsky]
      #5745832 - 03/20/13 06:05 PM Attachment (36 downloads)

Here's a shot of the other side of the 17" classical Cassegrain.

Dave Mitsky


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Azawashkomaengun
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Re: Comet C/2011 L4 (PANSTARRS) [Re: johnpd]
      #5747797 - 03/21/13 03:01 PM

Odd object left of Panstarrs taken in Calgary, ALberta

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