RE: [guide-user] Comets mag, how true?

Owen Brazell Oct 28 11:35 AM

I know people like Greg Crinklaw do an analysis of reported magnitudes from Comets OBs and then put some numbers in to match those. That however relies on knowing who you can trust in terms of providing observations. That is the big deal as there are always outliers there which can skew the magnitudes immensely.




From: [] On Behalf Of Bill Gray
Sent: 28 October 2013 3:01 PM
Subject: Re: [guide-user] Comets mag, how true?



Hi Rolf, Gerhard,

>> So it would be a really big improvement if Guide could load this list
>> from the MPC and would have an additional setting for an "observed
>> magnitude limit".

It's an interesting idea... I don't think the MPC list will work very
well, though, because it relies on just the last magnitude observation.
If you look at the photometry reported to MPC, there are astonishing
variations from observer to observer. People use different filters and
apertures; some measure the nuclear magnitude, others the total magnitude.
(And _very_ annoyingly, the MPC format provides no clue as to what
filter was used for comet magnitudes. That is to say, you don't know if
the magnitude is a raw CCD magnitude or a visual one or, less rarely,
something else.)

My expectation would be that the sheer randomness of just using the
most recent observation would cause roughly as much trouble as the
current method, of computing a single absolute magnitude using all of
the photometry.

What we'd actually need would be a list of "current comet magnitudes"
that is based on, say, averaging the last few days or weeks of comet
photometry. (With, on occasion, some tweaking required because an
object is in outburst.) Better still would be if, instead of just
determining a "current magnitude", a "current _absolute_ magnitude"
was determined, i.e., one that takes the distance of the comet from
the earth and sun into account. But does anyone know of such a list?

-- Bill