Bob Wrote:

>>Bill's update of Guide on 6 Jan has a very neat feature that allows one to
>>plot astrometric positions on Guide. It reads the data file that one
>>generates in Charon and plots the position on the Guide display.

At 08:56 PM 1/23/00 +0000, Lawrence Harris wrote:>

>First: how is this done?

Check the updated new features on the Project Pluto web page and load the
Jan 6th or later version of Guide7.

You must change the name of the file that holds the Charon produced MPC.LOG
positions in the MPC_OBS.TDF from "example" to something else. I changed it
to MPC.LOG

Look at: http://www.projectpluto.com/extras.htm#display_mpc_obs For details.

>
>Second: I am a little puzzled about the principle involved here. We use
>Charon to analyse a CCD image via star positions originating from Guide.
>The suggestion is that a new feature in Guide can plot these positions
>on the Guide display. Can someone explain to me the significance of

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>this because it seems that one is asking Charon to calculate a position
>from Guide and then plot the same position back on Guide.

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Not quite. Charon calculates YOUR position of the minor planet as <<imaged
on YOUR CCD>>using the star positions from Guide.( known as "O" for
observed}. Guide then plots these calculated positions on the star chart
along with a trail calculated from the positions of the minor planet using
the orbital elements stored in Guide (known as "C" for calculated). The
residuals displayed in the charon program are the "O-C" values.

By displaying the Observed positions and the calculated trail one can see
how consistent your calculated positions are over time. It is the
consistency over time rather than the actual value of the residuals that is
important.

Last April, Paul Comba gave a paper at the Flagstaff Minor Planet Workshop
that showed rather wild changes in the residuals of a near earth object that
he finally explained as
due to parallax caused by the earth's rotation!

My question about Comet 1999 Y1 came about because the comet data from the
MPC is only updated once a month unlike the minor planet data in MPCORB
which is updated daily. So a newly discovered comet that has a very
preliminary orbit may change significantly before it is updated.

See http://cfa-www.harvard.edu/iau/Ephemerides/Comets/Soft02Cmt.txt
and http://cfa-www.harvard.edu/iau/mpec/J99/J99Y02.html

>
>My query results from comments about my initial astrometry on a few
>asteroids having large residuals. Anything that helps me to reduce
>inaccuracies is good!

This is where the consistency of the residuals come into play. If the
observations show the residuals "all over the place" say +/-2 or more arc
sec then they may be suspect.
for example a set of residuals like 4.3,4.8,5.2,4.7 would be fine since the
scatter is less than 1 arc sec. Usually orbit calculators will toss out any
values that don't agree with the new orbit by larger than 2.0 arc sec. IOW
if the O-C is more than 2.0 arc sec they omit the observation from the
calculation.

I hope some of this helps.

Bob Elliott
750 Fall Creek

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