[BAA Comets] CCD astrometry and photometry

[Shanklin, Jonathan D.]

Hi Roger,

I would say the problem is threefold - Visual observers using large apertures will record fainter magnitudes than those using small apertures.  The effect can be substantial if one observer is using a 40cm and another is using 7x50B.  In general using the smallest aperture that clearly shows the comet is recommended.  A similar argument probably applies to the electronic measurements, so using a DSLR with a wide-angle lens is probably the best option for 2014 Q2 at the moment.

Regards,

Jonathan Shanklin
________________________________________
From: Roger Dymock [roger.dymock at ntlworld.com]
Sent: 22 January 2015 10:11
To: comets-disc at britastro.org
Subject: [BAA Comets] CCD astrometry and photometry

Good morning all,

I am planning to add the following paragraph to the CCD Astrometry and Photometry procedure at
http://www.britastro.org/projectalcock/CCD%20Astrometry%20and%20Photometry.htm

Limitations

Very large comets such as C/2014 Q2 (Lovejoy) may give total magnitude (m1) values that are fainter
than visual measurements. The light curve below was copied from the COBS database and shows that the
visual and CCD mags started to diverge from approximately mag 8. The problem is twofold - CCD images
rarely capture the whole of the coma and the software used here cannot accurately extrapolate from
nuclear magnitudes measured using apertures from 10-60 arc secs to comae measuring tens of arc mins.


There aren't too many very bright comets (unfortunately) so maybe it is best to avoid using this
procedure for any brighter than mag 8. As Nick James suggests - put away the computer and go outside
with a pair of binoculars and make a visual estimate!!!

Any thoughts before I do so?

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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