You are correct that the magnitudes given are far, far from actual magnitudes.
These are magnitudes that were derived from the basic sets of orbital elements that
Guide (and other programs, including the MPCORB) uses to extrapolate the magnitude
given the parameters of the orbit.
Rarely are the comets anywhere close to what these provide; there are many comets
listed that are not even visible, much less as bright as given and others are
somewhat brighter than the prediction suggests.

On comets, never go by what is listed by others. Always determine for yourself using
comparison stars, either visually or photometrically via CCD.
An excellent article that sums up all of the methods that are accepted is given at:
http://www.skyandtelescope.com/observing/objects/comets/3304211.html

Best of luck on your comet observing.

Dr. Clay
_____
Arkansas Sky Observatories
MPC H45 - Petit Jean Mountain South
MPC H41 - Petit Jean Mountain
MPC H43 - Conway West
http://www.arksky.org/

----- Original Message -----
From: "Rolf Stadelmaier" <rostm@...>
To: <guide-user@yahoogroups.com>
Sent: Sunday, October 27, 2013 8:25 AM
Subject: [guide-user] Comets mag, how true?


> Hi,
>
> since long I'm dealing with the question: How reliable are the given
> informations by Guide concerning the magnitudes of comets? I always
> thought, they must be very actual after downloading the newest data
> by using MPCORB.
>
> But: The latest pictures of ISON captured by amateurs are showing a far less
> shining comet than Guide suggests. Three days ago I for my own took the
> chance, to acquire a picture of this most promising comet to diagnose a very
> very great difference between Guide's information and reality. Although my
> local conditions for imaging had been very poor (foggy cloudy sky, gibbous
> wanning moon causing reflections, optics covered with damp) the result is
> obviously: ISON's magnitude compared to the magnitudes of stars in the
> imaged field is far beyond the value given by Guide of mag 8.8. It seems to
> correspond more to the values of stars with mag 13.4 or 14.2.
> Can anyone shed some light on this confusing behavior - Bill?
> Thoughts and explanations would bee much appreciated.
>
> Thanks,
> Rolf Stadelmaier
>
> PS: To support my experience I uploded a picture to the Photo section.
> Please have a look.
> http://groups.yahoo.com/neo/groups/guide-user/photos/albums/172052281
>
>
>
>
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