Official Comet Hunting Guide
Report a SOHO Object
Report a STEREO Object
Basics (via NASA)
LASCO C3 Transits
STEREO L4/L5 Campaign
STEREO JAVA Orbit Tool
C2/C3 Comet Tracks
NASA Citizen Science
SOHO Real-Time Images
Comet McNaught in 2007
15-years of SOHO Comets
SOHO comet astrometry text files
The ProcedureAstrometry measurements are made on the SOHO (LASCO only) images via fairly simple 'point-and-click' software. All of the images in which the comet is visible are loaded, and in each image the comet is identified and marked. The software checks a star catalog to see which stars should be visible in that particular image, and their approximate x,y location. The software then tries to find all of the stars brighter than ~8.5mag. Typically there can be anything between 180 and 300 stars in the LASCO C3 field of view, and 6 to 30 in the C2 field of view. The software then records the position of the brightest ~25 C3 stars (or all C2 stars if there are less than 25 visible). Also recorded is SOHO's position in space at the time of each image. All of these results then get combined into one report: the astrometry (IAU) file.
The FilesThe astrometry files all have an identical format. At the top of each file there is standard header which helps identify the parameters in the main body of the file.
Below is an extract from of one of these files with the key parts highlighted. Below the image is an explanation of each of these parts.
The astrometry files contain positional information on the spacecraft (SOHO), the object (comet) and a selection of bright stars from the image, as well as the time and date. All of this information is used to determine the orbit of the object. For each image, there is a 'block' of data. The first line of this block is the date/time and positional information of SOHO. The second line (marked below with an arrow) is the position information of the object (comet). The remaining lines of each block are the HD catalog names (incorrectly labelled SAO) and positional information of the bright stars selected form the image.
The links to the files are at the bottom of this page.
Note: There is a minor change in IAU files later than SOHO-819. In the first line, along-side #Observations you will see the SOHO# (format: SOHO-xxx), camera (eg C2, C3, SWAN, etc) and discoverer(s) name(s).
Important informationThe positions contained within these files are, where possible, the complete set of measurements taken for that particular object. For many comets submitted to the MPC, especially the non-Kreutz comets, some positions given in the astrometry file are rejected due to a poor fit to the orbit. For the sake of completeness, no corrections have been made to these data files to remove positions not included in the final orbit determination. It is therefore strongly recommended that users of this data cross-reference the files with the published orbit for each comet to determine which positions, if any, were omitted from the final orbit solution.
Specific questions relating to the SOHO astrometry data and procedures should be directed to the sungrazer webmaster via the "contact" link at the foot of the page.
Kreutz comets (by year):
Please contact the webmaster if you find an astrometry file missing from this archive.