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This is a brief description of the STC parameters recognized by Celestia v1.4.0 pre1. STC functionality has undergone a substantial improvement when compared to the previous versions of Celestia, v1.3.2 and earlier.
Some of these may change before the final release of v1.4.0.
An STC catalog file can be used to describe either Stars or Barycenters around which Stars, Barycenters and SSC objects can orbit.
A Barycenter is described only by a location. You can't see it. It's the center of mass of a collection of bodies. A Barycenter can orbit around another Barycenter or around a Star.
A Star is a glowing object described by a location plus visual parameters like SpectralType, AppMag or AbsMag and optional Mesh, Radius, SemiAxes and Texture declarations. A Star can orbit around another Star or around a Barycenter.
There are three optional fields used to introduce an STC object. They specify the type of object it is and provide unique identifiers for it. In their required order of appearence, they are
Examples:
Barycenter 100000 "My Barycenter:My System" { ... description ... }
"My Star" { ... description ... }
All of the other parameters used to describe a particular STC object are enclosed within curly-brackets: { }
For example:
orRA 299.0 # in fractional degrees, 0-360 Dec -10.5 # in fractional degrees, -90 to + 90 Distance 30 # in light years
OrbitBarycenter "My Star"
Notes:
The orbit of a Star or Barycenter is specified using the same EllipticalOrbit declaration as is used in SSC files. (A list of parameters is omitted: see SSC documentation. Celestia v1.5.0 Frame declarations are not yet supported in STC catalogs.)
A Star's or Barycenter's elliptical orbit is defined relative to the object's nominal location, which is specified using either an RA, Dec, Distance triplet or by OrbitBarycenter.
(Specifying RA,Dec,Distance is slightly more efficient than OrbitBarycenter, but only at load time, when Celestia has to lookup the position of the Barycenter.)
SemiAxes specifies the relative sizes of a Star's radii in three dimensions:
SemiAxes [1 0.85 1]
A Mesh may be a 3DS model, or one of Celestia's proprietary model types, either CMOD or CMS, which are described elsewhere.
Also, AltSurface and Location declarations are not (yet?) supported.
This example is taken from Grant Hutchison's star catalog file
Celetia/data/nearstars.stc
. It's one of the catalogs
which are included with Celestia v1.4.0pre5 and later.
Barycenter "Rigel Kentaurus:Toliman:ALF Cen:Gliese 559" { RA 219.917516 # mass ratio 1.09:0.92 Dec -60.837128 # Distance 4.365 } 71683 # ALF Cen A { OrbitBarycenter "ALF Cen" SpectralType "G2V" AppMag 0.01 EllipticalOrbit { Period 79.914 SemiMajorAxis 10.765 # mass ratio 1.09:0.92 Eccentricity 0.5179 Inclination 82.980 AscendingNode 67.708 ArgOfPericenter 3.774 MeanAnomaly 200.119 } } 71681 # ALF cen B { OrbitBarycenter "ALF Cen" SpectralType "K0V" AppMag 1.34 EllipticalOrbit { Period 79.914 SemiMajorAxis 12.755 # mass ratio 1.09:0.92 Eccentricity 0.5179 Inclination 82.980 AscendingNode 67.708 ArgOfPericenter 183.774 MeanAnomaly 200.119 } }
The following procedure can be used to change the names associated with a Star or Barycenter in Celestia.
For example, this STC file (betel.stc) renames Betelgeuse:
27989 "Betelgeuse:Al Mankib:Almerzamo nnagied:Gula:Besn:ALF Ori:58 Ori" { RA 88.792958 Dec 7.407064 Distance 419.13 AppMag 0.58 SpectralType "M1Iab" }
The alternative names above are from "Star Names, Their Lore and Meaning" by Richard Allen, Dover, 1963. The location and spectral information are from the Simbad catalog server.
In view of various different standards used to characterize (apparent) magnitudes, it might be worth specifying what Celestia actually wants:
In Celestia, AppMag corresponds to the so-called V-magnitude in the Johnson UBV photometric system!
Confusion may arise, since Celestia uses data from the Hipparcos catalog and that has introduced its own visual filter profile. The Hipparcos "visual" magnitude is called Hp.
This profile is much wider in wavelength than the "standard" Johnson one.
Moreover, Tycho data again use different photometry, designated with V_T and B_T for the visual and blue magnitudes, respectively.
There exist detailed mutual transformation algorithms and|or lookup tables. The relations are quite non-trivial in general and thus it is useful to be informed here...
For interested people, here is a good summary of transformations etc. http://www.tass-survey.org/tass/catalogs/tycho.html
Celestia uses Hipparcos catalog numbers as the primary identifier for its Stars. It does not any stars from the Tycho catalogs. However, several Addons are available which use Tycho catalog numbers to identify their Stars.
The Hipparcos catalog was developed to identify stars that had their positions and distances measured by the Hipparcos satellite. The Tycho Catalogue and Tycho 2 Catalogue of more than 2,000,000 stars were developed from other position measurements by that satellite. For information about the Hipparcos Space Astronomety Mission, see http://www.rssd.esa.int/index.php?project=HIPPARCOS
Hipparcos catalog numbers, often identified in documents with the prefix HIP, range from 1 to 118218. In Celestia, Tycho catalog numbers have to be "encoded" into fake Hipparcos numbers. They are encoded into a 10 digit number with a value greater than 1 000 000 000. To do this, the three numeric fields of a Tycho catalog number are reversed and concatenated into a single number:
TYC aaaa-bbbbb-c becomes cbbbbbaaaa
E.g. TYC 123-45-6 becomes 6000450123.
(Note, however, that this number does not correspond to an
actual Tycho catalog entry. It is only used to show how the
numeric fields are rearranged.)
Celestia rearranges these numbers to display the correct Tycho catalog number on the screen.
The following STC catalog entry defines the star TYC 123-45-6 named "TycStar"
6000450123 "TycStar" { RA 123.45 Dec 45.67 Distance 89 SpectralType "G1V" AppMag 8.7 }
If you don't tell me that something's missing, unclear or wrong, I can't improve it.