Contents of: V/102/./deficent.txt

The following document lists the file deficent.txt from catalogue V/102.
A plain copy of the file (without headers/trailers) may be downloaded.

The following is a brief explanation of current problems in and shortcomings 
of SKY2000, version 2.0. The user should recognize that these problems exist 
and should use the data accordingly. We are working on these things and plan 
to eventually have all the major problems solved, although we realize that the
catalog will never be "perfect". We group the following material by topic; 
i.e., by type of information, which is essentially the order of the format.

1. Identifications
   a. HD numbers are not complete for stars of the second extension 
      (HD272151 - HD359083) 
   b. Identification discrepancies involving BD supplemental stars 
      (stars with lower case "a" and "b" designations) may still be 
      present. A thorough review of all supplemental stars is planned. 
   c. Component identifications are not complete for multiple systems. Since 
      the WDS does not assign components for two-star systems, we need to do 
      this independently. (The WDS contains pairs of stars in a record, so it 
      is not necessary to assign component identifiers unless there are more 
      than two stars in a system.) We must also check for consistency in the 
      way that we have assigned components. 
   d. A further complication for the assignment of components stems from the 
      fact that we have only one components field when it would be preferable 
      to have separate fields for astrometric and photometric data. Thus, in 
      cases where positions and proper motions represent, say, only the 
      primary, while the photometry represents a blend, we must choose to put 
      either A or AB into the components identifier field following the WDS 
      number. This is usually chosen according to the astrometry, but we have 
      not been entirely consistent. 
   e. There are stars having WDS data for double and multiple stars, but with 
      no WDS identifiers. These are being isolated, analyzed, and corrected at 
      the present time.
   f. Variable-star names are not complete. We do not know why, but some 
      problem in a past version of the catalog, probably in the software, 
      caused some of the information in the GCVS to be missed. We have 
      discovered this to be mainly the names of variables. 

2. Astrometric Data 
   a. The Tycho Reference Catalogue (TRC) has not yet been incorporated into 
      SKY2000. Since the Tycho subsets of ACT and the TRC are somewhat 
      different, there are quite a few thousand stars for which we now 
      include Tycho data that can be replaced with the considerably better 
      TRC positions and (especially) proper motions. Users who need the 
      highest accuracy proper motions and find Tycho data in SKY2000 should 
      check the TRC and use TRC data when they exist. A few TRC data (source 
      80) have been used in Version 2.0 during problem solving, but full 
      processing is planned. 
   b. Parallaxes from HIP are as good as or better than the best ground-based 
      data (Gatewood 1998, AJ, 116, 1501), but the short baseline and lower 
      accuracy methods of Tycho make its results much less reliable. Where 
      Tycho parallaxes exist in the Yale Parallax Catalogue (van Altena, Lee, 
      and Hoffleit 1995), we suggest using the latter. 

3. Photometric Data 
   a. Magnitude and color data in the photoelectric fields from sources 94 and
      95 are problematical and should not be trusted. They were expected to be 
      replaced with source 20 data and were not, which indicates some kind of 
      problem such as misidentification, discrepant data, etc. We intend to 
      look at these cases and replace or confirm the quality of the data, then
      change the source(s). 
   b. The Bright Star Catalogue (BSC) was a high-priority source for certain 
      data in SKY2000, but was not intended to be a higher priority source 
      than 20 (Mermilliod's UBV catalog). Unfortunately, the BSC data were put 
      into SKY2000 with assigned errors (there are no errors in the BSC), which
      is not considered acceptable for data evaluation. We intend to extract 
      UBV data from source 20 for the BSC stars, but as an interim step, we 
      have replaced the BSC data with Tycho BV photometry. Since the latter 
      have a rather short time scale, the errors are often artificially low. 
      We also lost U-B data with Tycho, so we intend to process source 20 
      again to retrieve high-quality ground-based data for the bright stars. 
   c. Another problem with the Tycho photometry is that blended flags ("b") 
      have not been assigned because that information is not available in TYC. 
      We are in the process of adding flags to SKY2000 data when it can be 
      determined that Tycho observed more than one component, but it is not 
      always easy to identify those cases because we can only compare TYC V 
      data with visual magnitudes for the components given in the WDS. 
   d. Photometric data from source 27 (the Russian WBVR catalog) were assigned 
      errors because there were no errors given in the Northern Hemisphere 
      Catalog of Red Magnitudes (Warren 1994). However, the original WBVR 
      catalog (Kornilov et al. 1991) does have error estimates based on 
      numbers of observations that can be used in SKY2000. For source 27 BVR 
      data, users are encouraged to look at the original catalog for error 
   e. We intend to replace all RI data on the Johnson system with Cousins 
      system data, since the latter is a superior system. Users should 
      substitute Cousins data for the KY2000 Johnson data when they exist. 
   f. Large numbers of ptv and ptg data are missing from SKY2000. These are 
      useful for consistency checks for photoelectric data, even though they 
      are often blends for close companion stars. We intend to reprocess some 
      of the original source catalogs (e.g., SAO) to put these data back into 
      SKY2000 (they were removed earlier when a few errors in the original SAO
      were found, even though those had been corrected in source 29). 

4. Spectroscopic Data 
   a. Many stars that currently have MK types do not presently have their types
      in the catalog. This is because many of the types that were in the old 
      SKYMAP catalog have been removed because of unreliability. The uncertain 
      information came from not knowing for sure if the MK types had been 
      "derived" from one-dimensional spectral types and color data. The vulgar 
      practice of deriving MK types has been discontinued and we want no such 
      types in the SKY2000 catalog. Although most of the types from SKYMAP 
      were legitimate, the original sources had been lost anyway, so we plan to
      replace all of those data with newly ingested MK data. 
   b. MK types with sources in the nineties should be used with great caution 
      because the original sources are not identified in SKY2000. This is not 
      what was intended when SKYMAP spectral types were transferred to the new 
      catalog. Unfortunately, the SKYMAP source replaced the originals when 
      the MK types were transferred, so we must return to the original sources
      to re-retrieve the types. Perhaps this is all for the best because many 
      more types are now available and errors in the original sources have 
      sometimes been corrected. It also allows us to consider all the available
      types for each star, although choosing among those available is often too 
      daunting a task to perform on large numbers of objects. A helpful source 
      is the Selected MK types catalog of M. Jaschek (1978), but the age and 
      frequent uncertainties of the types chosen are often a problem. 
   c. The MK types for carbon stars are not all on a consistent classification 
      system and must be checked for accuracy. It has even been found that 
      some MK types for C stars are completely missing from SKY2000, even 
      though they sometimes appear in the MK catalogs that were used as the 
      original sources for SKYMAP. 

5. Radial Velocities 
      The only radial-velocity data in SKY2000 had originally come from Wilson
     (1953), with a few data from the BSC. The Hipparcos mission added a lot of
      new data not in the catalog. When we created the equinox and epoch 
      J2000.0 version of the HIP for SKY2000 usage, we transferred RV data 
      from the Hipparcos Input Catalogue (HIC) to our version to make it 
      easier to ingest new RV data into SKY2000. When the HIP J2000.0 was 
      subsequently processed, the radial velocities went into SKY2000 as coming
      from source 66 (HIP) instead of 33 (HIC). This has already been fixed in
      our latest proprietary version, but is still present in version 2.0.

      New RV catalogs have been published by Barbier-Brossat and Petit. These 
      will be processed and ingested into SKY2000. In the meantime, users 
      should consult other catalogs if there is no RV in SKY2000. 

6. Double and Multiple Stars 
   a. As mentioned under item 1, component identifications are far from 
      complete in Version 2.0 of SKY2000, and there are double stars in the WDS
      whose identifiers are missing from the WDS field. 
   b. Orbital data for visual binaries having published orbits are not yet 
      included in SKY2000. Actually, we have room only for periods of orbital 
      pairs at the present time. If we wish to predict what a spacecraft star 
      tracker will see at any epoch, ephemerides must be computed. This 
      requires orbital elements, which we may include in a future version of 
      SKY2000. We intend, however, to include period data before that so that 
      users can at least have a handle on the motions of orbital pairs. 
   c. A thorough analysis of bright, orbiting pairs should probably be 
      undertaken because these can cause significant problems for star 
      trackers when they are incorrect. Although we have found and corrected 
      many of the problems, without a complete review (mostly by machine), we 
      can be quite sure that some problems remain. We do know of 
      inconsistencies for close pairs that are both in the main HIP and in the
      components catalogs. These are subarcsecond pairs that have blended 
      photometry and the position of only one component from HIPc (because 
      that was the highest priority source). Thus, the photometric data are 
      inconsistent with the astrometric information. For those cases, the 
      blended position from HIP (coded * in duplicity column) should have been
      used. An example of this situation is HR 266 (SKY2000 
      J005646.97+602146.0), which we have treated manually to use the blended 
      position and motions. 

7. Variable Stars 
    a. As discussed earlier, variable-star names are missing for many GCVS 
       stars. This situation will be corrected by reprocessing the latest 
       edition of the GCVS. 
    b. Some data from the NSV have been put into SKY2000 for the wrong stars, 
       probably as a result of some software bug in the distant past. This is 
       not the case for a large percentage of stars, but some have been 
    c. As a result of the GCVS and NSV catalogs being higher priority sources 
       than the UBV catalog (source 20) and because the former were processed 
       later, V magnitudes from the GCVS and NSV replaced those given in 
       source 20. For stars that are variable, this procedure is invalid 
       because the magnitudes and colors come from different sources and were 
       observed at different times (epochs). To fix this problem, V magnitudes
       from the same sources as the colors should be put back into SKY2000 and
       then the U and B magnitudes recomputed.