[BAA Comets] Comet Photometry

[Giannantonio Milani]

Dear firends,
it's my first message  and I thankyou to be invited to join to this 
group. I always read with interest the  messages and in particular I 
find this discussion very interesting to me.
I would like to add some thinkings about this important topic. I 
apologize if I am going too long.

If we take a look to the cometary amateur astronomers techniques we find 
that, on an average, the visual technique is defined in a much more 
rigorous way then the CCD one.
Visual magnitudes refers to a specific photometric system (the visual 
one) and apply well defined methods for the estimates. The CCD approach 
looks sometimes more confused and often we refer to generic magnitudes 
that are de facto instrumental magnitudes.  We forget that a magnitude 
has a real meaning only if referred to a photometric system and to 
specific coupling of filters/sensors with proper calibration procedures. 
Instrumental magnitudes can be useful for some kind of investigation of 
course, but use only part of the digital imaging power.

Concerning our field of interest unfortunately the professional cometary 
photometric system is based on narrowband filters with the goal to get 
gas production rates, a solution not easy to be used by amateur 
astronomers with a  complex observing and reduction procedure. Some 
comercial narrowband filters close to the professonal standards are 
available from some dealers (eg. Edmund).  The gas producion calculation 
is complex, but we should consider that would be already very useful to 
give total magnitudes for instance for the C2 coma, after subtracting 
the continuum contribution. This has some degrees of complexity but some 
advanced observer can master the procedure and perhaps this could be an 
interesting new way to be explored, at least for relatively bright comets.

On the other hand for a general use  a V filter  is close  to the visual 
system and can be used to produce total V magnitudes to complement the 
visual magnitudes. The main problem is what to measure. The method 
proposed by Ignacio Ferrin (the infinite growth method)  is really fine 
for comets that exibit a "pure" coma, with a negligible tail. But the 
magnitude of a comet with a tail will continue to grow increasing the 
aperture. The study performed many years ago by Herman Mikuz on comet 
Hale-Bopp clearly shows that there are systematic differences among 
visual estimate and CCD measurements that vary in time during the 
apparition. CCD total magnitudes required ad hoc measuring window sizes 
to match visual magnitudes. This I think is  due to the strong tail 
component and the changing shape of the comet.  It apperars that our 
brain/eye works in a bit different way than CCDs do and this is a 
trouble. But CCDs can surely help to get accurate magnitudes over a wide 
range of  time and heliocentric distances.
So I think the first step should be to define the way to select the size 
of the coma to be measured to get a standard CCD total magnitude and a 
method to be applied by all the observers on the comets with and without 
a tail.  A first idea could be for instance to measure the coma profile 
at an agle of 90 degrees respect to the radius vector, escluding the 
tail, but other better solutions can be explored of course.

A second point concerns the use of photometric filters, proper reference 
stars, and accurate flat field and dark frame per-processing. A very 
good flat field correction is needed for instance to use the "infinitre 
growth method" of Ignacio Ferrin. This if our goal is to achieve good 
quality photometry. Of course this can be more complex than taking 
simple generic imaging, but I think that photometry require his own 
specific method.

The Afrho approach, we explored more than 10 years ago, was sometimes 
apparently misunderestood I think. It is not an alternative (or 
competitive) method to the  magnitudes, but a different way to 
investigate comets and to get information on the dust. But I don't want 
now expand the discussion around this.  I think it is useful to 
concentrate now on the total V and visual magnitude problem and to 
define a standard method for CCD.  After doing this the problem of the 
format for reporting the data probably will be simpler.

Sorry for my long message

Best regards
Giannantonio Milani


Il 17/09/2012 22:08, denis buczynski ha scritto:
> David, these are exactly the questions we need to pose and to have them answered (by experts in the
> field) who can point us in the right direction. Inso far as I know the two software packages mainly
> in use by comet observing groups are Focas-11 (the Spanish group) and Winafrho (the Italian CARA
> group). The only person I know who uses either of them amongst our group is Roger Dymock, who may
> want to comment. There are probably others in our group who use their own software, but may be able
> to advise on how we can progress.
> Denis
>
> -----Original Message-----
> From: David Storey
> Sent: Monday, September 17, 2012 8:49 PM
> To: 'BAA Comets discussion list'
> Subject: Re: [BAA Comets] Comet Photometry
>
> Hi Denis,
>           What photometry results are we aiming for here. MPC reports
> generated by Astrometrica define N magnitudes. Is this what we need BAA CCD
> observers to report to ICQ. Or do we need to establish a programme where the
> whole coma is taken into account to derive a magnitude as appears to be
> happening on the COBS web site. There is a link to the photometry packages
> they use (DAOPHOT II for PCs, FitsPro ) but my computer will not display
> those pages. The whole area of making CCD photometry of comets needs looking
> at to establish what computer program is needed to get good data. I
> personally don't know of a program that does full photometry of comet coma's
> other than what I have read on the COBS site. Does anyone else of one that
> is readily available?
>
> Best Regards
>
> Dave Storey
>