estartab

This is a file in plan 9 that contains star informa tion in a tabular format. I haven’t found much information about this file in the internet. And there doesn’t seem to be a man page for it. Just a reference from the astro man page. Here is what I was able to understand from reading the source of the plan-9 program astro.

It is part of the SAO (Smithsonian Astrophysical Observatory) Star Catalog.^{1 2 3} The file is composed of many entries in a tabular format, here is a sample row:

190556      30359 21 38 58.207 -.0003 -20 02 04.87 -0.007 .00  7.06  K0

This corresponds to this star, cross-referencing the line above with the values in simbad I see the estartab uses the fk4 coordinate system with (ep=B1950 eq=1950) [I’m still unsure what this means, epoch is B1950 and the equinox is 1950, but unsure what equinox=1950 means).

From what I understand of this, an aepoch (astronomy) is a particular point in time that serves as reference. In this case the epoch is B1950, which is the besselian-year 1950.

Here’s my attempt at understanding all of these parameters:

1. 190556 is the SAO catalog ID.
2. 30359 General catalog⁴ number. Seems it’s not used by astro.
3. 21 is called the rah internally, it is the hour part of the right ascension.
4. 38 is called the ram internally, it is the minute part of the right ascension.
5. 58.207 is called the ras internally, it is the seconds part of the right ascension.
6. -.0003 is called the da, it seems some kind of correction to the ascension.
7. -20 is called the dday, it’s part of the declination. Normally would be called degrees ⁵
8. 02 is called the dmin, it’s part of the declination. Normally minutes.
9. 04.87 is called the dsec, it’s part of the declination. Normally seconds.
10. -0.007 is called the dd. It is used in astro/star.c. It seems some kind of correction to the declination.
11. .00 is the px. It doesn’t seem to be used. Maybe related to the positional error as described in ³
12. 7.06 is the mag. The star magnitude.

In this row the right-ascension is $21\degree 38' 58.207"$ and the declination is $-20\degree 02' 04.87"$.

Here is the SAO catalogue translated to J2000: https://heasarc.gsfc.nasa.gov/FTP/heasarc/dbase/dump/heasarc_sao.tdat.gz

This is being used in the astronomicon star finder program I’m building.

Footnotes

1. https://en.wikipedia.org/wiki/Smithsonian_Astrophysical_Observatory_Star_Catalog ↩

2. http://tdc-www.harvard.edu/catalogs/sao.html ↩

3. https://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/sao.html ↩ ↩²

4. https://en.wikipedia.org/wiki/Catalogue_of_Nebulae_and_Clusters_of_Stars ↩

5. Not sure why they call it day here, just normal degrees of declination. ↩