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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: Comet C/2011 L4 (PANSTARRS) [Re: pogrzex]
      #5668390 - 02/08/13 06:45 AM

Quote:

There is no chance to see +2 mag object with naked eye so low over the horizon + on so bright sky. To me it's no comparison!




Its called a washout!


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Comet C/2011 L4 (PANSTARRS) [Re: pogrzex]
      #5670398 - 02/09/13 09:48 AM

A new analysis of selected visual photometric data spanning Jan 7 to Feb 9 results in the following magnitude formula:

m1 = 5.6 + 5 log(D) + 6.7 log(r) , cc = 0.93

The fit to the data is excellent and it is noted that these parameters are hardly any different from those derived a week ago, suggesting that these terms are becoming quite certain now. They are also rather similar to those derived for Comet Kohoutek back in 1974. Descriptions using binoculars and small telescopes continue to indicate a bright pseudo nucleus within the coma and a short but obvious and rather dramatically curved dust tail, with the ion tail almost absent.

Considering our viewing geometry in mid to late March the above implies an ever more probable peak magnitude of close to +2.5 with the comet displaying a broad, highly curved tail, perhaps of rather low surface brightness which will have reduced visibility particularly when viewed at low elevation against a twilight backdrop.

BrooksObs

Edited by BrooksObs (02/09/13 09:58 AM)


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Jure Atanackov
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Reged: 05/04/10

Re: Comet C/2011 L4 (PANSTARRS) [Re: BrooksObs]
      #5670516 - 02/09/13 11:08 AM

So, something similar to C/2002 V1 (NEAT) in the second week of Feb 2003...

Mr. Bortle, what do you think about comet Kohoutek's return to a higher n at log(r) < -0.6? Would it be reasonable to expect something similar from PanSTARRS?

http://www.lesia.obspm.fr/perso/nicolas-biver/cometsmagpreperi-2011l4-2012s1.jpg

Clear skies!
Jure

Edited by Jure Atanackov (02/10/13 08:27 AM)


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BrooksObs
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Reged: 12/08/12

Re: Comet C/2011 L4 (PANSTARRS) [Re: Jure Atanackov]
      #5671957 - 02/10/13 08:28 AM

Jure, while a possibility I would tend to think it is more likely that Comet PanSTARRS' post perihelion behavior will be rather different from Kohoutek's by virtue of PanSTARRS' very high dust production rate. In constrast, Kohoutek showed a strong ion tail, which has been so far very weak in PanSTARRS, and only a modest dust tail (except when very near perihelion) for a comet with such a small perihelion distance. This would imply a marked difference in composition, reflected in the gas vs. dust production ratios of the two objects. PanSTARRS so far seems to be a very dusty comet (particularly "heavy" dust at that).

I would also note that, in my opinion, total magnitude determinations when a comet is further than 2.5 a.u. from the Sun should be regarded as increasingly suspect, particularly those made during the post-T interval, for a number of quite valid reasons.
BrooksObs

Edited by BrooksObs (02/10/13 09:41 AM)


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Phillip Creed
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Reged: 07/25/06

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Re: Comet C/2011 L4 (PANSTARRS) [Re: BrooksObs]
      #5674257 - 02/11/13 03:32 PM

One thing the low elevation of C/2011 L4 does remind me of is whether or not the ICQ extinction tables/method needs updating.

There are tables provided for altitude to every 0.5 km to account for Rayleigh scattering and the 0.016-mag/air mass extinction for ozone is essentially "baked into the cake".

It's the aerosol extinction, though, that perhaps needs a makeover. There are three tables to account for various aerosol loads ("average", "winter", and "summer"), with the "average" set at 0.120-mag/air-mass and the winter and summer values set at 70% and 130% of that, respectively.

There are multiple issues I've got with this approach. (1) The "winter" extinction table can be used year-round at desert locations and most of Australia, and even that doesn't account for how clean the air can be in these locations. Likewise (2) some locations with humidity and high industrial emissions rarely have clean enough air to justify the "average" extinction table, and (3) the aerosol extinction is assumed to decrease logarithmically with altitude. Normally that's the case, but there could high-altitude haze (e.g. forest fire smoke, volcanic ash) that might be absent from the lowest 10,000-ft altogether.

Aerosol extinction, in other words, simply is what it is for a particular location and time. The only way to really account for it in your estimate is to compare the brightness of stars low along the horizon with those near overhead and to back-calculate the aerosol (and, thus, total) extinction if--IF--an estimate needs to be corrected for atmospheric extinction. It'd be very cumbersome to put it in tabular form, but a simple spreadsheet program with a few inputs would easily suffice.

Obviously an experienced comet observer should go out of his/her way to seek out suitable comparison stars at similar altitudes as the comet to avoid the use of extinction tables altogether, but some comets take away that option from the get-go. And I'd concede that the methodology of the observer can easily introduce as much error and uncertainty as atmospheric extinction to begin with. It's just something that's bothered me for some time, and the low elevation of C/2011 L4 at its brightest just reminds me of it. This isn't meant to be a slight to Dr. Green's very important and well-researched 1992 ICQ article regarding atmospheric extinction; quite the opposite. Maybe it's time to update to procedure a bit by tailoring the equations to specific conditions and locations.

Clear Skies,
Phil


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Phillip Creed
Idiot Seeking Village
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Reged: 07/25/06

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Re: Comet C/2011 L4 (PANSTARRS) [Re: Phillip Creed]
      #5674272 - 02/11/13 03:39 PM

For those interested in Dr. Green's article on atmospheric extinction, see:

http://www.icq.eps.harvard.edu/ICQExtinct.html

Clear Skies,
Phil


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Comet C/2011 L4 (PANSTARRS) [Re: Phillip Creed]
      #5683038 - 02/16/13 09:29 AM

Updating the comet's photometric parameters to include this week's reported observations from select southern hemisphere observers suggests a possible slight up-tick in the figures. However, it must at the same time be appreciated that the comet can now only be observed at extremely low altitude in the morning sky in brightening nautical twilight. Data spanning 2013 Jan 7 to Feb 16 results in the magnitude formula:

m1 = 5.6 + 5 log(D) + 7.1 log(r) cc = 0.97

If carried to perihelion this implies a peak magnitude of 2.1 as seen from the Earth during mid March.

Reports of the comet's appearance as seen with with small instruments and binoculars this week indicates the head to be extremely condensed and the typical double tails (ion and dust) are now both apparent and photographically of almost equal length.

BrooksObs

Edited by BrooksObs (02/16/13 10:22 AM)


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krp
Vendor. DarkSiteFinder.com


Reged: 11/10/10

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Re: Comet C/2011 L4 (PANSTARRS) [Re: BrooksObs]
      #5684226 - 02/16/13 08:55 PM

What will be the best time to photograph C/2011 L4? Is the tail likely to grow after perihelion as the comet dims? Even though it will be at its brightest 3/9 I'm just wondering if it will be more photogenic after that as it moves away from the horizon and the tail possibly grows.

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SabiaJD
scholastic sledgehammer


Reged: 02/20/05

Loc: Clarks Summit, PA
Re: Comet C/2011 L4 (PANSTARRS) [Re: krp]
      #5685013 - 02/17/13 11:33 AM

You will require a unobstruted western horizon. It will set soon in twilight close to the crescent Moon on 12 March 2013. I am expecting dimming of the tail by moonlight and distance above the horizon.

Comet will be in twilght skies. Would like to see it in darker skies, but appreciate any view I can get of this comet in the March skies. Expecting clouds on the horizon to be a hinderence.


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Comet C/2011 L4 (PANSTARRS) [Re: SabiaJD]
      #5685549 - 02/17/13 05:10 PM

KRP - John Sabia is quite correct in his post. The fact is that there is a distinct possibility there won't be a time when the comet's tail will be easily viewable, or can be photographed to real advantage, before it fades.

The head of Comet PanSTARRS will be seen only very low and in the western twilight well into April for mid northern latitude observers, by which time C/2011 L4 will have faded considerably. At the same time odds are that the comet's bluish ion tail could prove difficult to recognize, particularly visually, in the intense twilight. The brighter and more visually apparent dust tail is likely to initially be directed toward the southeast at a low angle and largely subdued by the bright sky. The latter appendage may likely not be seen to project more-or-less upwards in classic comet fashion until fairly late in March, by which time the moon, then in its gibbous phase, could severely hamper any visibility.

So, astro photographers are likely to be faced with a catch as catch can senario, attempting to trade off comet altitude, favorable tail direction, twilight and moonlight interference to gain any impressive images.

BrooksObs

Edited by BrooksObs (02/17/13 05:15 PM)


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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: Comet C/2011 L4 (PANSTARRS) [Re: BrooksObs]
      #5685660 - 02/17/13 06:07 PM

Quote:

So, astro photographers are likely to be faced with a catch as catch can senario, attempting to trade off comet altitude, favorable tail direction, twilight and moonlight interference to gain any impressive images.




Indeed - with ten years of comet photography under my belt, this one is proving to be a hard plan!


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Special Ed
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Reged: 05/18/03

Loc: Greenbrier Co., WV 38N, 80W
Re: Comet C/2011 L4 (PANSTARRS) [Re: Tonk]
      #5685729 - 02/17/13 06:46 PM

Of course, all this is educated speculation at this point.

Have your cameras and pencils ready, folks.


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Rich (RLTYS)Moderator
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Reged: 12/18/04

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Re: Comet C/2011 L4 (PANSTARRS) [Re: Special Ed]
      #5686515 - 02/18/13 07:44 AM

We can only hope.

Rich (RLTYS)


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Comet C/2011 L4 (PANSTARRS) [Re: Rich (RLTYS)]
      #5686866 - 02/18/13 11:57 AM

Far from mere speculation, I'm afraid, Special Ed. In the case of Comet PanSTARRS the facts conspire to give us the least favorable viewing circumstances for any bright comet in decades.

BrooksObs

Edited by BrooksObs (02/18/13 11:58 AM)


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eps0mu0
member


Reged: 01/16/06

Loc: San Francisco, CA
Re: Comet C/2011 L4 (PANSTARRS) [Re: BrooksObs]
      #5687821 - 02/18/13 09:09 PM

Least favorable viewing circumstances in decades?
At least we'll get a chance.
I seem to remember Kohoutek also had less than optimal viewing circumstances: low in the evening sky right after sunset. I was a fairly young and inexperienced observer, but I was unable to catch it, even though both Venus and Jupiter provided helpful guideposts in the sky.


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Special Ed
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Reged: 05/18/03

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Re: Comet C/2011 L4 (PANSTARRS) [Re: BrooksObs]
      #5687927 - 02/18/13 10:09 PM

Quote:

Far from mere speculation, I'm afraid, Special Ed. In the case of Comet PanSTARRS the facts conspire to give us the least favorable viewing circumstances for any bright comet in decades.

BrooksObs




Hey--not "mere" speculation--I said "educated" speculation. I think you and Tonk and Jure are awesome (and I never use that word).

I'm just hoping that we visual observers and sketchers can see and record something that eludes the AP types (because of exposure issues, etc.).


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Dave M
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Re: Comet C/2011 L4 (PANSTARRS) [Re: Special Ed]
      #5688058 - 02/18/13 11:27 PM

Nice shot of PANSTARRS on spaceweather
http://spaceweather.com/gallery/indiv_upload.php?upload_id=76708


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Rich (RLTYS)Moderator
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Re: Comet C/2011 L4 (PANSTARRS) [Re: Dave M]
      #5688361 - 02/19/13 06:17 AM

WOW! A beautiful image.

Rich (RLTYS)


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Special Ed
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Re: Comet C/2011 L4 (PANSTARRS) [Re: Rich (RLTYS)]
      #5688368 - 02/19/13 06:31 AM

Indeed.

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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Comet C/2011 L4 (PANSTARRS) [Re: eps0mu0]
      #5688501 - 02/19/13 09:05 AM

Quote:

Least favorable viewing circumstances in decades?
At least we'll get a chance.
I seem to remember Kohoutek also had less than optimal viewing circumstances: low in the evening sky right after sunset. I was a fairly young and inexperienced observer, but I was unable to catch it, even though both Venus and Jupiter provided helpful guideposts in the sky.




In point of fact even Comet Kohoutek was far better situated in the sky for observation by northern hemisphere observers than will be PanSTARRS. Kohoutek rose steadily upward from the SW horizon and away from the twilight as January of '74 progressed. PanSTARRS, on the otherhand, will virtually parallel the western horizon from mid March until well into April, never being higher up than 5-10 degrees an hour after sunset in twilight and essentially setting by the time full darkness comes. A nice binocular object in the afterglow of dusk perhaps, but no naked eye spectacle.

Incidentally, looking back over my observing journals, which span 1957 to the present, I cannot find any "bright" comet that was ever so continuously poorly placed for observation from my site following its perihelion as is PanSTARRS!

BrooksObs

Edited by BrooksObs (02/19/13 09:07 AM)


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