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

[Nick James]

Roger,

For visual equivalent magnitude estimation of large/bright comets 
extrapolation is only needed if you don't have the entire coma in the 
field of view. I've just done a quick direct measurement using some 
images obtained on Jan 15. These are widefield and so I have the full 
coma and plenty of sky to use to estimate the background.

What I did was as follows:

1 - I generated two stacks, one aligned on the comet and the other 
aligned on the stars.

2 - I used Astrometrica to measure the stack aligned on stars using 
CMC-14 to get the magnitude zero point. This was 23.01 from 720 stars 
with a dmag of 0.08.

3 - I then used a median estimator to get the sky background and 
stretched the image around this point to show the size of the coma.

4 - Using this as I guide I did aperture photometry on the coma using an 
aperture radius of 300 pixels (12.9 arcmin). This is the inner circle in 
the image on the left at the link below. This gave a magnitude of 4.1.

You can see a screenshot of this here, IRIS on the left monitor, 
Astrometrica on the right:

http://www.nickdjames.com/Comets/2015/20150115_magestimate.jpg

COBS shows visual mags are currently around 4.0 so this seems OK.

http://www.cobs.si/images/latest-lightcurve.png

Although I would be the first to admit that it is a hell of a lot more 
effort than going out with a pair of binoculars and a star atlas! CCD 
mags are only really worth doing for fainter objects I think.

Nick.


On 16/01/2015 15:19, Roger Dymock wrote:
> 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/
>>