# 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. I am also informed by Joe Masiero that the mag # data from WISE (which is an R magnitude guesstimated from W1 and W2) # is very approximate. C51 1 3 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 # Gaia data will carry its own sigmas, except for time. We'll set that # to be effectively zero, same as for radar. 258 7 1e-9 Gaia # Occultations give extremely precise positions. (Unfortunately, # the 80-column astrometric format won't give us all of that.) # I assume accuracy in time to a video frame = 1/60 second. 244 .001 .017 Geocentric occultation # 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.