2026-01-28

For a while now I’ve been troubled by some gaps in the data of my observation challenge lists. The biggest issue is the lack of magnitude values for many targets. This is mostly a problem for my current Open Cluster challenge.

Typically for any gaps in data I would look to a resource like SIMBAD to try to get basic data on the target. But for open clusters I’ve come to find that the Flux V value, which represents visual magnitude, is missing for many open cluster targets.

This last weekend I took some time to better learn SIMBAD and what data it offers. I have since learned that not only does it contain basic data on a huge number of targets, but it also has an index of various catalogues that the target shows up in (usually with a different identifier). While exploring, I also stumbled upon VizieR, which seems to be a collection of various databases with additional details for a particular target respective to a specific field of study. VizieR contains databases for the NGC, IC, WDS catalogs (and more), including a database for variable stars and carbon stars updated regularly from the AAVSO!

This changed my perspective on how useful this tool is. I quickly also found that both SIMBAD and VizieR can be queried using ADQL (a SQL variant) and the database table schema is available online. They both also have an API that supports ADQL queries.

This set me on a path to solve my open cluster magnitude problem. Even if visual magnitudes might not be available for many of these targets I should be able to query the ‘child’ objects of these clusters (being the stars within the cluster) and get a max, min, and average magnitude of those stars. Using the database schema reference document and some trial and error, I came up with the following query (truncated):

SELECT
  b.main_id,
  ROUND(MAX(f.V),1) AS "Max MAG",
  ROUND(MIN(f.V),1) AS "Min MAG",
  ROUND(AVG(f.V),1) AS "Avg MAG"
FROM
  basic AS b LEFT OUTER JOIN
  h_link AS hl on hl.parent = b.oid JOIN
  basic AS c on hl.child = c.oid JOIN
  allfluxes AS f on c.oid = f.oidref
WHERE
  b.main_id IN (
    'Berkeley 2',
    ...
    'Stock 4')
  AND c.otype = 'Star..'
  AND f.V IS NOT NULL
GROUP BY
  b.main_id

Which resulted in:

     main_id      |Max MAG|Min MAG|Avg MAG
------------------|-------|-------|-------
"Cl Ruprecht    3"|13.5   |12.0   |12.9   
"Cl Stock   23"   |13.9   |7.5    |11.9   
"Cl Stock    4"   |14.2   |9.2    |12.4  
...

This covered my magnitude gap for about one-third of my open cluster list. I only have about 10 or so targets remaining that still could not be found. All of these remaining targets seem to be non-true open clusters; clusters of stars that are actually very far apart but only appear to be clustered together due to them overlapping in the sky from our perspective from earth.

With over 90% of my open cluster list now containing some measure of magnitude to help me plan my targets more effectively, I’m going to call this a success.

This got me thinking about my future use of SIMBAD, and now I’m tempted to develop a program that will take a ‘sparse-list’ of targets and query them (using the SIMBAD / VizieR API), then take the results and store them in a sqlite database cache locally. Doing this would take the burden of data management for my targets off of my shoulders as I would no longer have to manually edit and maintain my target lists.