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

[Nick James]

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/
>
>
>
> _______________________________________________
> Comets-disc mailing list
> Comets-disc at britastro.org
> http://lists.britastro.org/mailman/listinfo/comets-disc
>
> This message (and any attachments) is for the recipient only. NERC is subject to the Freedom of Information Act 2000 and the contents of this email and any reply you make may be disclosed by NERC unless it is exempt from release under the Act. Any material supplied to NERC may be stored in an electronic records management system.
> _______________________________________________
> Comets-disc mailing list
> Comets-disc at britastro.org
> http://lists.britastro.org/mailman/listinfo/comets-disc
>