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SOHO comet astrometry text files


The Procedure

Astrometry 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 Files

The 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.

Annotated example of a (C2) astrometry file

(Click to enlarge)


  • # Observations: This shows the complete numbers of observations (positions) recorded for the comet. It also shows the year of the comet discovery.
  • DATE TIME (hrs): This is the date and time of the observation.
  • S/C-X (AU) S/C-Y (AU) S/C-Y (AU): This is the position of the SOHO satellite from Earth (in AU).
  • # OBJECTS: This is the number of objects in this observation. This number includes the object (comet) and stars recorded.
  • SAO: This is the HD Catalog number (incorrectly labelled as SAO in the reports: now corrected) of the stars recorded. A zero implies the position is that of the comet.
  • x-pixel y-pixel: This is the x,y position of the object (comet or star). The origin (0,0) is always in the lower-left of the image. The image size is 1024x1024 for the vast majority of the files.
  • RA(HIC) DEC(HIC) / KNOW / DATE TIME (UT) / EXP: The "RA" and "DEC" are position elements of the recorded stars. "Know" refers to whether the object is a known (eg a star = 1) or unknown (eg a comet = 0) object. "Date" and "time" are the image date and time. "Exp" is the exposure time of the image
    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 information

    The 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.


    Links to the astrometry files

    The measurements contained within these files were taken by D.Biesecker, C.St.Cyr, D.Hammer, K.McGleam and K.Battams. If you use these files, please check the corresponding MPEC/IAUC and give credit to the appropriate person(s) who made the measurements. The MPEC/IAUCs are available via the 'SOHO Comet Discoveries:' links.

    Kreutz comets (by year):

    Non-Kreutz comets:


    Please contact the webmaster if you find an astrometry file missing from this archive.