Globular Cluster DSC Creator

Here you find pointers to utilities that help you create addons for Celestia.
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LukeCEL
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Globular Cluster DSC Creator

Post #1by LukeCEL » 15.04.2020, 15:40

Hi everyone,

I've created a little tool that helps with the creation of globular cluster .dsc files. To use, simply make a copy of my Google Sheet, then go to the "Calculator" tab (bottom left), and start editing. In the green area, simply enter in the appropriate values and below, a definition for a globular cluster will be written for you.

The first section is the place to input the coordinates. You have the choice of entering the right ascension and declination in sexagesimal (hh:mm:ss, dd:mm:ss), or in decimal degrees. For the distance, enter the distance in light-years or parsecs.

The second section is the place to input designations. Enter in as many of the globular cluster's designations as you need, starting with the preferred names in the box that says "Name 1", and so on. They will automatically be joined together with colons as a delimiter. "Name 1" is also used for the "InfoURL" parameter, which links to SIMBAD.

The third section is the place to input parameters. For the "AbsMag" parameter, you need to enter an apparent magnitude. Enter this in the box that says "app. mag." The magnitude should be in the Johnsons V band. If not available, a wavelength of 555 nm should suffice, as it is close to the V band's peak.

The "KingConcentration" and "CoreRadius" both refer to King models, which is a method of modelling globular clusters. In a King model, a globular cluster has a core radius, the radius of its core region, and the tidal radius, the radius that contains stars that are gravitationally bound to the globular cluster. The tidal radius is typically much larger than the core radius; this difference is conveniently characterized by the King concentration parameter (c), which is the log (base 10) of the tidal radius divided by the core radius. They can often be found in papers about globular clusters; look for the keywords "surface brightness profiles" or "structural parameters".

The box that says "c (log(rt/rc))" is preferred for the "KingConcentration" parameter. If you have a value, then you don't need to specify a tidal radius, but if lacking, then the tidal radius becomes necessary in order to calculate the value.

Be very careful about the units. This calculator asks for the tidal radius and core radius, but two units can be used: the angular radius, in arcseconds, and the actual (physical) radius, in parsecs. However, in Celestia arcminutes are used for the "CoreRadius" parameter. Always check the units: a single prime (′) refers to arcminutes while a double prime (″) refers to arcseconds.

The "Radius" parameter, as defined in Celestia, is the radius at which the surface brightness of the globular cluster is 25 magnitudes per square arcsecond. This can be reportedly found in some astronomical catalogs. However, in the case that it can't be found, it can be calculated from the King concentration parameter and the central surface brightness, µ(0). The equation is from Fridger Schrempp (t00fri)'s globulars.pl, which was used to generate the original globulars.dsc catalog.

The "Axis" and "Angle" parameters are not included here. Axis and Angle values will presumably be useful in a later version of Celestia, when the ellipticity of globular clusters is implemented. Currently, however, globulars in Celestia are spherically symmetrical and therefore there is no need to specify the globular cluster's orientation.

Once you finish entering in all of the parameters, a full globular definition will be written. Copy the cells in A14 through B24, and paste them into a .dsc file. You will inevitably paste some extra tabs after the brackets and after the designations; this is unavoidable. I recommend deleting those extra tabs.

Enjoy: https://docs.google.com/spreadsheets/d/1ScEdMWIHD ... Dv1toyik9uhnw/edit?usp=sharing

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