[BAA Comets] Comet Photometry

[Roger Dymock]

An interesting website wrt obtaining total comet mag from CCD images can be found at
http://webdelprofesor.ula.ve/ciencias/ferrin/teaching/curveofgrowth.pdf In simple
terms it uses mags generated by using concentric, increasing apertures as mentioned
by Richard Miles. Such data can be gleaned from both CARA and the Spanish groups
methods. I have given it a brief try and it isn't difficult to do but it would be
useful to compare such results with visual obs.

I would agree with Denis in that we do need to establish an observing program or
programs however the procedures/software/etc available to us may, to some extent,
slant those programs in one direction or another. 

Actually having an open discussion on such matters is a great step forward and we
mustn't drop this particular ball. 

Regards

Roger Dymock
Email: roger.dymock at ntlworld.com
Tel: 023 92647986
Skype: rjvdymock
MPC Observatory Code: 940

-----Original Message-----
From: comets-disc-bounces at britastro.org [mailto:comets-disc-bounces at britastro.org] On
Behalf Of Richard Miles
Sent: 17 September 2012 21:31
To: BAA Comets discussion list
Cc: Jure Zakrajsek
Subject: Re: [BAA Comets] Comet Photometry

Denis,

I think this is an excellent suggestion to use COBS to capture observations 
made by BAA Comet Section members.

Back in May 2010 I met Jure Zakrajek and signed up to COBS mainly as a means 
of accessing the data rather than submitting my own.  The software is well 
tried and tested and so it would make a lot of sense to use it rather than 
to 'reinvent the wheel'.

We need to pay close attention to the details of the ICQ format and try and 
standardise on a few things common to CCD observations of comets.  The first 
thing I would say is that the ICQ reporting aims to generate photometry that 
is comparable with visual observers.  In other words, the "total V 
magnitude" and this is reported to the nearest 0.1 magnitudes.  ICQ state 
that they "do not want estimates of the nuclear condensation alone". 
However, some observers using COBS have routinely reported for a given 
epoch, several magnitude measurements corresponding to different size 
photometric apertures, which is therefore a measure of the (spherically 
symmetrical) distribution of light within the coma.  Obtaining the total V 
magnitude of a comet is non-trivial and relates directly to the issue of the 
discrepancy between CCD and visual estimates partly caused by different 
effective photometric aperture sizes / telescope focal ratio, visual 
magnification, etc.

So imagine you have a stacked high SNR image of a comet, by reporting 
magnitudes of concentric apertures centred on the nucleus it may allow one 
to extrapolate the measures to a "pseudo V magnitude" which corresponds to 
the equivalent coma diameter reported by the visual observer.  I think 
that's why the COBS reports often contain several magnitude reports for a 
particular comet at a particular epoch date/time.

Direct measurements of the "total coma" are almost certainly doomed to fail 
especially for brighter comets since the coma extends further out than is 
obvious from looking at the CCD image and in any case field stars 
increasingly contaminate the coma the further out you try to make a 
measurement such that attempts to subtract out the field stars become 
increasingly inaccurate.  However, we may find a methodology which can 
successfully equate the two sets of data for some comets and this is where 
COBS becomes a valuable resource.

The last thing I would throw into the mix at this stage is that once the 
discussion has been properly aired and we have homed in on some specifics, 
why not hold an observers' workshop say at the BAA in London where people 
can be tutored / trained as to how to use COBS and the ICQ format including 
some 'hands-on' work on their laptops?

Richard Miles


----- Original Message ----- 
From: "denis buczynski" <buczynski8166 at btinternet.com>
To: "BAA Comets discussion list" <comets-disc at britastro.org>
Sent: Monday, September 17, 2012 7:14 PM
Subject: Re: [BAA Comets] Comet Photometry


> The reporting of photometric data to Jon or COBS is the procedure at the 
> very end of the process,
> and hopefully the COBS web based interface which finally compiles the data 
> into ICQ format will
> solve the problem for Jon. That is why I pointed out the link in my 
> initial post on this subject.
> However before we get to the reporting we need to have a discussion about 
> how the Section and it's
> members are advised to make those photometric measurements. In essence we 
> need to define a well
> thought out Section Programme with details as to how we want members to 
> participate. My concern is
> that we are starting to discuss this procedure from the end rather than 
> the beginning. Let us
> firstly decide the nuts and bolts of the observing programme and then move 
> onto the reporting of
> results when we are content with operation of the observing programme. We 
> need a discussion about
> the merits of alternative programmes and then choose a route which we all 
> support and can offer to
> our members. Once the observations and measurements are made and the 
> Section programme and
> procedures are followed, then the reporting will normally be undertaken in 
> line with the
> recommendations of the programme. When observers use Astrometrica for the 
> astrometry they submit to
> MPC they report in the format of the programme, they don't then go off and 
> use a different format.
> This should be the case for the BAA Comet Photometry Programme(whatever 
> that might be), this is what
> we need to establish firstly.
> Denis Buczynski
>
>
> -----Original Message----- 
> From: Roger Dymock
> Sent: Monday, September 17, 2012 7:44 PM
> To: 'BAA Comets discussion list'
> Subject: Re: [BAA Comets] Comet Photometry
>
> The MPC format is for reporting both astrometry and photometry (of 
> asteroids and
> comets). In recent times they have got quite hot on the accuracy of 
> photometrc data.
>
>
> Asteroid lightcurve data does use a different, ALCDEF, format - see
> http://minorplanetcenter.net/light_curve
>
> Agree on the need for a web interface to make life easier for the Director 
> and anyone
> else who wishes to access and analyse the data so, as mentioned, will 
> investigate
> COBS.
>
> Regards
>
> Roger Dymock
> Email: roger.dymock at ntlworld.com
> Tel: 023 92647986
> Skype: rjvdymock
> MPC Observatory Code: 940
>
> -----Original Message-----
> From: comets-disc-bounces at britastro.org 
> [mailto:comets-disc-bounces at britastro.org] On
> Behalf Of Shanklin, Jonathan D.
> Sent: 17 September 2012 18:27
> To: BAA Comets discussion list
> Subject: Re: [BAA Comets] Comet Photometry
>
> The MPC format is for reporting astrometry, and the magnitudes reported 
> there are
> approximate with no metadata.  If you report photometry different 
> information is
> required.  As an analogy you wouldn't report the position of a new 
> variable star in
> the same format that you submitted a magnitude estimate.
>
> I agree it would be helpful to have only one format for reporting visual
> observations, and would be quite happy if only the ICQ format was used 
> (Roger didn't
> mention many of the other formats - eg Morris, CBAT etc).  The BAA format 
> is only
> marginally different to the ICQ one, but many reports received don't quite 
> match
> either format and so need a lot of editing.  A decent web interface that 
> allows quick
> and simple input and thence transmission to the Director would be a great 
> help.
>
> Regards, Jon
>
> Director Comet Section, British Astronomical Association
> http://www.ast.cam.ac.uk/~jds
>
>
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