[BAA Comets] Comet C/2014 Q2 (Lovejoy)

Fri Jan 16 15:19:08 GMT 2015

Good afternoon all,

My maths not being up to Nick's standard I took a different approach.

1) Plot FoCAs mags against apertures using EXCEL.

                                   10x10  20x20  30x30  40x40  50x50  60x60   SNR   SB   COD
COMET         UTC                    +/-    +/-    +/-    +/-    +/-    +/-     N  FWHM  CAT
------------  -------------------  -----  -----  -----  -----  -----  -----  ----  ----  ---
C/2014 Q2     14/01/2015 04:03:04   9.65   8.71   8.19   7.82   7.55   7.33  50.5  16.6  G68
C/2014 Q2     14/01/2015 04:03:04*  0.01   0.01   0.02   0.01   0.01   0.01     5   3.0  CMC

The box sizes were translated into their equivalent diameters of apertures of the same area ie;
11.3, 22.6, 33.9, 45.1, 56.4, 67.7 arc secs.

2) Fit a power trend line extending out to the coma diameter I measured using AIP4WIN - 704 arc
secs. The equation of the trend line being y=9.6703x-0.1535. Log and Exponential trend lines were
tried but give magnitudes which are much too bright.

3) Calculate the magnitude for the measured coma diameter using the above equation which turns out
to be 3.5. The visual mags on the COBS data base immediately adjacent to mine are 3.8 so a better
result.

Kphot may not be able to cope with comets of this nature as it gives a coma diameter of only 3.6 arc
mins. I seem to remember exchanging emails with Uwe Pilz on this subject so I should do so again as
I can't remember what the outcome was. The CCD Astrometry and Photometry procedure does not use
kphot calculated coma measurements anyway.

Regards

Roger Dymock
Email: roger.dymock at ntlworld.com
Tel: 023 92647986
Skype: rogerdymock67 or
Skype: rjvdymock
Project Alcock http://www.britastro.org/projectalcock/

-----Original Message-----
From: Comets-disc [mailto:comets-disc-bounces at britastro.org] On Behalf Of Nick James
Sent: 15 January 2015 20:52
To: BAA Comets discussion list
Subject: Re: [BAA Comets] Comet C/2014 Q2 (Lovejoy)

Jonathan/Roger,

I think you are correct about many electronic observers not including a 
large part of the coma. What diameter do you estimate visually?

To show the effect I've taken a north-south cut through this image from 
2015-01-13:

http://www.britastro.org/cometobs/2014q2/2014q2_20150113_2339_ndj.html

And produced this plot:

http://www.nickdjames.com/Comets/2015/2014Q2_20150113/logscale.png

The y axis scale is logarithmic and the coma is still detectable above 
the sky background out to a radius of 20 arcmin. You can fit the 
brightness of the coma quite nicely to a curve corresponding to:

	I(r) = exp(9.7-2.1*r^(-0.5))

where r is the distance from the photocentre in arcmin. This is shown as 
the green line on the plot.

The count in an aperture of radius R is then given by the integral of 
the product of I(r) and the area of an annulus at r:

	I(r) * 2 * pi * r * dr

from 0 to R. This is plotted in:

http://www.nickdjames.com/Comets/2015/2014Q2_20150113/integrated%20counts.png

Turning this into magnitude loss you get:

http://www.nickdjames.com/Comets/2015/2014Q2_20150113/magloss.png

The bottom line is that you lose a magnitude for an aperture diameter of 
4.5 arcmin and 2 mags for an aperture diameter of 2 arcmin. To lose less 
than 0.5 mag you need a diameter of 8 arcmin.

Nick.

> I think as noted in The Comet's Tale, the electronic observations are not capturing the entire
coma and hence will report magnitudes that are too faint.  This is one reason why it is important to
use the ICQ format, which includes an estimate of the coma diameter.  Although I haven't done it
yet, it would be possible in theory to correct for under-estimates of the coma size, just as it is
possible to correct for aperture (which probably has the same effect of reducing the apparent coma
size).
>
> Regards,
>
> Jonathan Shanklin
> ________________________________________
> From: Roger Dymock [roger.dymock at ntlworld.com]
> Sent: 15 January 2015 14:48
> To: comets-disc at britastro.org
> Subject: [BAA Comets] Comet C/2014 Q2 (Lovejoy)
>
> Good afternoon all,
>
> It would appear that this comet is not playing ball as far as equivalency of visual equivalent
> magnitudes from CCD images and visual magnitudes are concerned. The magnitude chart on COBS
> indicates that these started to diverge when the comet brightened past mag 8 and are now somewhere
> between 1.5 and 2 mags different - the visual mags being the brighter.
>
> The same tends to be true for the coma diameter. These are widely spread for both visual and CCD
obs
> - 10 to 40 arc mins in diameter - with the CCD obs being at the lower end.
>
> I doubt all those who submit observations to COBS are using our method so at least there is some
> consistency. Thoughts anyone?
>
> Regards
>
> Roger Dymock
> Email: roger.dymock at ntlworld.com
> Tel: 023 92647986
> Skype: rogerdymock67 or
> Skype: rjvdymock
> Project Alcock http://www.britastro.org/projectalcock/
>
>
>
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