[BAA Comets] CCD astrometry and photometry

[Richard Miles]

Yes - to back up Nick's contributions to this discussion - I've been using 
the approach he describes for quite a few years now and one's ultimate 
accuracy can be as good as 0.01-0.02 mag.

The methodology differs markedly from the way Astrometrica works since that 
piece of software is designed for astrometry of comets and more especially 
astrometry and photometry of asteroids / point sources. The methodology Nick 
describes is necessarily more labour intensive but the improvement in 
photometric accuracy makes it worthwhile.

The key issue with bright comets is to ensure that the optics used yield a 
FOV that extends well beyond the coma. That has been said by by both Roger 
and Nick but I too would like to echo this point. Obtaining an accurate sky 
background reference level is crucial to measuring coma brightness.

Clear skies,
Richard


----- Original Message ----- 
From: "Nick James" <ndj at nickdjames.com>
To: "BAA Comets discussion list" <comets-disc at britastro.org>
Sent: Thursday, January 22, 2015 7:33 PM
Subject: Re: [BAA Comets] CCD astrometry and photometry


> 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/