File: sigma.txt

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#   Comments can be added freely as lines starting with '#'
#
#   'sigma.txt' is the replacement for 'weights.txt',  and renders it
# obsolete.  It is similar to 'weights.txt',  but extends its capabilities.
#
#   'sigma.txt' allows you to set up rules describing the default
# uncertainties in position,  time,  and magnitude for astrometric data.
# The idea is that you can tell Find_Orb that all observations from a
# given observatory over a given time span and magnitude range have,
# by your estimate,  specific uncertainties in those three quantities.
#
#   The resulting uncertainties are simply default values,  used when
# there's no other way to know the uncertainty.  If you use the
# AstDyS/NEODyS .rwo format,  the data have astrometric uncertainties
# given with each observation.  And Find_Orb provides ways to specify
# uncertainties via special keywords within the observations.  See
#
# https://www.projectpluto.com/find_orb.htm#sigmas
#
#  for details.
#
#   This is a revised version of what used to be 'weight.txt'.  That
# file was quite similar,  except that instead of storing uncertainties,
# weights were stored.  These basically just boiled down to 1/sigma_posn,
# with sigma_posn expressed in arcseconds.  Also,  only sigmas for
# position could be defined;  'sigma.txt' adds sigmas for magnitude
# and time as well;  and allows you to specify a project code as well.
#
#   Find_Orb looks through this file from top to bottom.  Non-comment
# lines give an observatory code,  an optional program code,  and a date
# range and magnitude range.  (The ranges can be blank,  to specify "any
# date" and/or "any magnitude",  or to indicate that there is no lower
# or upper limit on that range.)  The idea is that such a line defines
# what the sigmas are for any observation from that code,  taken within
# that date range,  and within that magnitude range.
#
#   When Find_Orb finds a line describing the observation in question,
# it extracts any non-blank sigmas.  It keeps going,  possibly to
# the bottom of the file,  until sigmas have been found for all
# three quantities. That's why there is a final line that will
# assign sigmas to _any_ observation that hasn't already been handled.
#
#    Comments can be added at the end of each line.  I've just listed
# the station names in that slot.
#
#   Some comments on how to assign sigmas are at the bottom of this text.
# I've noticed that the following stations get unusually good astrometry.
# These uncertainties roughly reflect how well I've seen them do :
#
#Obs P  <--start-> <--end -->           Pos   Mag  Time
#COD C  yyyy mm dd yyyy mm dd mag1 mag2 sig   sig  sig    Comment
 689                                    .1                Flagstaff
 673                                    .1                Table Mountain
 422                                    .3                Loomberah
 413                                    .3                Siding Spring
 E12                                    .25               Siding Spring #2
 568 2                                  .1          .2    David Tholen et. al., Mauna Kea
 T09                                    .1          .2    David Tholen et. al., Subaru
 T12                                    .1          .2    David Tholen et. al., 2.24-m
 T14                                    .1          .2    David Tholen et. al., CFHT
 568 ~                                  .1          .2    TNO survey folks, Mauna Kea
 J95                                    .2          .2    Great Shefford
 J75                                    .2          .1    La Sagra
 Y00                                                .2    SONEAR
#   (J95), (Y00) and Tholen at (568) have demonstrated,  via observations of
# artsats,  that they really have gotten timing right.  (J75) has had
# cause to observe very fast-moving artsats;  their actual time sigma
# is probably quite a bit better than the above .1 second.
#
#    SOHO and STEREO,  though,  were not intended to do astrometry and
# produce noisy data.  (Each has multiple instruments,  and we really
# should take that into account at some point...)
 249                                    10                SOHO
 C49                                    10                STEREO-A
 C50                                    10                STEREO-B
# MPC appears to be weighting (C51) WISE data lightly,  which appears to
# be... well... wise:
 C51                                     1                WISE
# ...but Hipparcos and HST get incredibly good astrometry:
 248                                    .01               Hipparcos
 250                                    .01               HST
# The radar folks don't really have 'uncertainties' in RA/dec or mag;
# they aren't measuring those quantities.  The quantities they _do_
# measure -- round-trip time and Doppler shift -- have sigmas given
# for them,  provided along with the data;  we don't need to specify
# them in 'sigma.txt'.  The only thing we need to specify is the
# uncertainty in the timing.  That sigma really should be effectively
# zero.  We can't actually do that,  but a nanosecond is good enough.
 251                                     7         1e-9   Arecibo
 252                                     7         1e-9   Goldstone DSS 13, Fort Irwin
 253                                     7         1e-9   Goldstone DSS 13, Fort Irwin
 254                                     7         1e-9   Haystack, Westford
 255                                     7         1e-9   Evpatoria
 256                                     7         1e-9   Green Bank
 257                                     7         1e-9   Goldstone DSS 25, Fort Irwin
 259                                     7         1e-9   EISCAT Tromso UHF
#  You MUST have some final "catchall" lines that assign sigmas to all
# observations not otherwise catalogued.  Here,  we'll say that if the
# observation predates 1993,  we'll assume a sigma of 3",  no matter
# which observatory it came from.  (Older observations are usually
# photographic,  and reduced with older catalogues such as the SAO;
# the assumption that they're just a little bit dodgy is usually right.)
                   1993 01 01            3
# ...which will just leave everything after that date,  and we'll
# assume a default sigma for those (more "modern" CCD obs) of a half
# arcsecond,  half a magnitude,  and one second.  This is the "catch-all"
# line mentioned above,  to assign uncertainties to anything not already
# found.  It's also used if you have turned off weighting (i.e.,  all
# observations are to be treated equally,  with identical uncertainties).
                                        .5     .5    1    Default
#
# HOW TO ASSIGN SIGMAS:  I don't really know of a simple way to do this.
# The above numbers reflect what I've observed in the data,  and are
# better than no sigmas at all,  but they are somewhat ad hoc.
#
#   One _can_ analyze data from an individual observatory and get a feel
# for what their RMS residuals are and base the sigmas on that.  It helps
# if the observatory in question gets a _lot_ of data,  to get some
# meaningful statistics.  .rwo files give uncertainties assigned in
# this manner.