[BAA Comets] Imaging and photometry of C/2011 L4 (PANSTARRS)

Thu Apr 4 11:25:56 BST 2013

Morning Roger,

The webpage you have created is an excellent initiative as it is a start at 
putting together all the various aspects of CCD imaging and photometry. It 
is something which can be built on and fine-tuned in the near future as we 
learn more.

A few of thoughts at this stage:

1. We need to encourage more people to use wideband filters (especially 
Johnson V, SDSS-r' and/or Cousins R) and those people who take colour RGB 
images to utilise the 'G' (Green) image for photometry, and image analysis 
for Coma Diameter, Tail Length/tail p.a.

2. I wonder re. FoCAs if Julio Castellano has considered updating his 
software changing from square aperture boxes to circular ones?

3. Kphot and FoCAs are all well and good when dealing with more distant 
comets subtending relatively small fioelds of view, but when comets become 
brighter and start developing significant tails then it is likely their 
results will diverge more from that obtained by the visual observer.

4. John Bortle reminded us in a post yesterday that the visual observer must 
remember that "the extra-focal image of the comparison stars must equal that 
of the coma's in-focus diameter as measured across its latus rectum". We 
therefore also need to bear this in mind and so have to come up with a more 
robust mathematical method for obtaining the Coma Diameter from CCD images - 
we can't rely on eyeballing the image on the screen since the stretch 
applied and the background sky noise are big factors affecting the result. 
We can then read off Total Magnitude from the photometric growth curve at 
the point where it reaches the same size as the Coma Diameter. That I 
suggest would be a systematic way of generating a CCD-derived Total 
Magnitude which could then be correlated with the Visual Magnitude.

5. The discussion about visually-derived Degree of Condensation (DC) is also 
relevant.  CCD observers need to determine an equivalent quantity, which 
again can be correlated with DC. So far all I have seen is using the 
magnitude difference of 10" and 30" boxes or similar approach. In practice 
what might be better is to normalise the growth curve of the comet by 
ratioing it against the equivalent growth curve for stars in the same field. 
By this approach, a comet which has a DC=9, i.e. appears stellar, would have 
a slope of 0.00 if you plotted the ratioed magnitude versus the log of the 
aperture diameter. As the comet becomes more diffuse then the slope of the 
plot begins to become increasingly negative reaching -1.00 for a coma 
exhibiting the normal inverse power law (DC=3 ?) and an even more negative 
slope (-2.17 max.?) as we approach a DC=0 value. For the record the growth 
curve of C/2011 L4 has a slope of about -0.83.

Thought I'd get these down here as I am unable to attend the BAA Comet 
Section meeting on May 18 next owing to a prior engagement.
Cheers,
Richard

----- Original Message ----- 
From: "Roger Dymock" <roger.dymock at ntlworld.com>
To: "'BAA Comets discussion list'" <comets-disc at britastro.org>; 
<comets-ml at yahoogroups.com>
Sent: Thursday, April 04, 2013 8:55 AM
Subject: Re: [BAA Comets] Imaging and photometry of C/2011 L4 (PANSTARRS)

> Good morning Richard,
>
> As to merging CCD and visual data take a look at
> http://www.britastro.org/projectalcock/CCD%20Astrometry%20and%20Photometry.htm
>
> It seems to work in that mags derived using this procedure are similar to 
> visual mags
> (TA Mag and COBS) and mags derived form CCD images using a different 
> procedure
> (COBS).
>
> Knowing your expertise in this area your comments would be very welcome.
>
> Regards
>
> Roger Dymock
> Email: roger.dymock at ntlworld.com
> Tel: 023 92647986
> Skype: rjvdymock
> MPC Observatory Code: 940
> Project Alcock http://www.britastro.org/projectalcock/
>
>
> -----Original Message-----
> From: comets-disc-bounces at britastro.org 
> [mailto:comets-disc-bounces at britastro.org] On
> Behalf Of Richard Miles
> Sent: 03 April 2013 23:40
> To: Comets-disc; comets-ml at yahoogroups.com
> Subject: [BAA Comets] Imaging and photometry of C/2011 L4 (PANSTARRS)
>
> Here are URLs of two deep images of this bright comet in the same field as
> Messier 31 taken from Dorset, UK and stacked at sidereal rate.
>
> April 1st, V-filter image (1.9 Mb) with a logarithmic stretch applied:
> http://britastro.org/~rmiles/Images/C2011L4_M31_20130401_RMb.jpg
>
> April 2nd, V-filter image (2.0 Mb) with a linear stretch applied:
> http://britastro.org/~rmiles/Images/C2011L4_M31_20130402_RMiles.jpg
>
>
> V photometry has now been performed at Golden Hill Observatory on 5 epochs
> between March 14 and April 2. Results to date are shown at the following
> URLs:
>
> Photometric growth curves (0.3 Mb):
> http://britastro.org/~rmiles/Documents/C2011L4_Mar14-Apr02_phot_curves_RMiles.jpg
>
> V magnitude vs. time of the 1-arcmin diameter coma (0.16 Mb):
> http://britastro.org/~rmiles/Documents/C2011L4_Mar14-Apr02_Vmag_1arcmin_RMiles.jpg
>
>
> The growth curve at all five epochs are well fitted by a power law which 
> is
> gradually diminishing with time, probably tending towards zero as the coma
> activity declines and its shape takes on a more spherical appearance. For
> the last weblink, a simple 2nd-order polynomial has been fitted to the V
> magnitude plot, which models the observed magnitudes to a precision of 
> about
> 0.02 mag.
>
> As can be seen, using CCD photometry it is possible to derive very 
> accurate
> magnitudes. The big question is how can the CCD methodology be employed so
> that it is well correlated with mean visual magnitude data. The BAA Comet
> Section is putting together some plans to quantify this correlation using
> the apparition of Comet 2P/Encke later this year (from about September 01 
> to
> perihelion on November 21). Since Comet Encke is a fairly 'standard' comet
> and will brighten from about 15th mag to 5th mag during this time interval
> we stand a good chance of nailing the correlation. All participants both
> visual and CCD (equipped with V- or Green filters) are of course welcome 
> to
> participate in this!
>
>>From the trend in brightness over time, Comet PANSTARRS appears to be a 
>>very
> well-behaved object exhibiting a gradual fall-off in brightness without 
> any
> fragmentation or minor outbursts as yet. However, with good photometric 
> data
> any slight change in activity can be picked up clearly. If we follow
> PANSTARRS closely over the next few months both visually and with 
> V-filtered
> CCDs then we also stand a chance of correlating CCD and visual studies of
> this relatively bright comet in time for the BIG one, Comet ISON!
>
> Richard Miles
> BAA
>
>
>
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