[BAA Comets] CCD astrometry and photometry

[Nick James]

Hi Roger,

I think that text is a bit negative. It is perfectly possible to get 
good magnitudes for comets with a large coma, you just need to make sure 
that you are using the right instrument! I certainly agree that the 
approach that we use for faint comets is not appropriate for bright 
ones. Things won't extrapolate well if you image a comet with a 30 
arcmin coma and you measure it in a 1 arcmin aperture. You need a big 
enough FoV to get a decent sky reference for one thing.

I see no reason why bright/large comets can't be measured using 
instruments with larger fields of view such as a DSLR or my 72mm 
refractor + SXVR-H18. The method I proposed on this list based on 
getting a zero point using non-offset images and Astrometrica and then 
doing aperture photometry on an offset image works quite well and 
doesn't take long. The main thing is to make sure that the image is deep 
enough and that the background is flat enough for you to extract the 
diameter of the coma, then use that as your measurement aperture.

It is true that there is far less need for electronic photometry of 
bright comets since visual observers produce high quality results in 
that magnitude range.

Nick.



> 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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