Description of the Asteroid Lightcurve Data Exchange Format (ALCDEF) bundle 1.0 ===================================================================================== Bundle Generation Date: 2021-09-09 Peer Review: 2021 Asteroid Review Discipline node: Small Bodies Node Content description for the Asteroid Lightcurve Data Exchange Format (ALCDEF) bundle ==================================================================================== The Asteroid Lightcurve Data Exchange Format (ALCDEF) database contains metadata and data produced by asteroid time-series photometry by amateurs and professionals and submitted to the database using a format that follows ALCDEF standards definition. There are three related data files: metadata, lightcurve data, and - optionally - comparison stars data. The ALCDEF structure is based on the concept of "lightcurve blocks." Each block contains two (optionally, three) sections: metadata, compstars (optional), and lcdata. Because of the very large number of observations (9944193) for 23847 distinct objects, there are multiple files for the metadata, compstars, and lcdata sections. Each compstars and lcdata file covers the same objects that are in a given metadata file. For example, if the metadata file covers objects numbered 1 to 100, then the corresponding compstars and lcdata files will contain data for those objects only. The ordering of the records in a metadata file is based on the object's number with its name used as the first tie-breaker for unnumbered objects, i.e., when number = 0. For lightcurve blocks of the same object, the SessionDateTime, in ascending order, provides the second tie-breaker. If necessary, the Filter is used for the third tie-breaker. There are 222 data files in the archive, which is comprised of sets of three files (metadata, compstars, and lcdata) with each set having the same base name. The metadata files are split by ObjectNumber into groups, each in its own subdirectory under the root\data directory, containing no more than 100 objects for those numbered between 1 and 999, not more than 1,000 for those numbered between 1,000 and 9,999, and no more than 10,000 for those numbers greater than 10,000. Unnumbered asteroids are grouped into a single file. N.B. Since compstars are not required, it's possible that an entire set of metadata records, e.g., 5400000-550000, will have no compstars at all. The current PDS4 standard does not allow for 0 records in a file so, in this case, a single record is added to the compstars-xxx-xxx.csv that gives the default for a missing entry, e.g., -9 for an integer and '-' for an empty string. Each record in an alcdef_metadata_XXX file includes, among others, the object number and/or name and/or designation, the mid-date (UT) of the data associated with the given lightcurve block, the person submitting the data, contact information for the submitter, equipment used, the filter and magnitude band (e.g., Johnson V) used for the observations, and any corrections applied to the original, raw data (e.g., reduction to unity distances or transformed to a photometric standard such as Johnson-Cousins or SDSS). Each record in an alcdef_lcdata_XXX file gives the JD and magnitude (magnitude error, optional) for a single observation (data point) along with an ID number that ties the observation to a specific metadata record. Each record in an alcdef_compstars_XXX file provides details on one of the comparison stars used during the observations along with an ID number that ties the comp star data to a specific metadata record. Each record includes, among others, the name, RA/DEC (J2000.0), magnitude, and - if used - the color index of each star. Up to 10 comp stars are allowed for each metadata record. The archive includes an alcdef_standard.pdf file that provides extensive details about the ALCDEF standard such as keywords, appropriate values, and cross-checks run during submission to avoid having incomplete data. For example, if the magnitudes have been reduced to unity distances, whether or not a fixed value (at mid-time) was used or point-by-point. The file includes bookmarks for easy navigation to specific sections. Caveats to the data user ======================== The data have been submitted without verification of accuracy. Unlike astrometry, where checks can be run to see if the reported position is reasonable against current orbital parameters, the ALCDEF data should be taken "as-is" and so it is up to the end user to determine which individual lightcurve blocks are suitable for his purposes. The ALCDEF standard and software used to generate ALCDEF files have evolved since the format was introduced in 2010. Therefore, a number of fields that would normally have data in a recent entry will have the default value for "missing" or NULL data. Also, when magnitudes are simple differentials, e.g., +0.758, early data entry did not provide for the zero point that led to the differential value and so the actual "sky magnitude" for the data point is unknown. Also of concern for early submissions is the naming of individual comp stars. Initially, the software often used by amateurs used the X/Y coordinates of the star on a reference image for the name. Afterwards, that software used the Right Ascension and Declination (J2000.0) for the name, e.g., "102310.58 +213512.6". This was preferred over using the number/name from the reference star catalog, which was often Zone:Number and not always fixed depending of the catalog and/or its version.