Technical data

Chapter 5 Starmap 21
To maintain a consistent point of reference of celestial objects, a fixed reference frame must be established. Currently
there are two different reference systems in use: the Julian reference (FK5) and the Besselian reference (FK4). The
Julian system is based on a year that is exactly 365.25 days in length. The Besselian system is based on the tropical
year, which varies from year to year. In 1984, the International Astronomical Union adopted the Julian system and
established that the new standard reference (Epoch) will be called J2000.0. J2000.0 begins on 2000 January 1, at
12:00 noon Dynamical Time (TD).
To allow you to input coordinates from another epoch, such as B1950.0, that epoch value must be entered into the
Epoch field. To select the Besselian reference frame (FK4) select the FK4 (Fundamental Katalog 4) button, the Epoch
field will then be set to B1950.0. You may change the epoch year value, but the Besselian reference will be used
instead of the Julian reference. Selecting the FK5 (Fundamental Katalog 5) button sets the Epoch to J2000.0. Again,
you can change the year value, but the Julian reference will remain.
If you are entering star coordinates from a catalog other than one referenced to J2000.0, you should enter the
appropriate reference frame, FK4 or FK5 and set the catalog's epoch value. You should also enter the star's proper
motion in both R.A. and Dec. to accurately locate the star.
When the OK button is selected, all of the entered values will automatically be transformed from FK4 to FK5 if
needed, then star's proper motion will be applied, then the coordinates will be precessed from the epoch year to
J2000.0 . If you redisplay the Custom Viewpoint dialog box, you will see the epoch value reset to J2000.0 and the
coordinates will now show their corrected J2000.0 positions.
Object Selection
Displays the Object Selection dialog box. The Object Selection feature allows you to select an object from one of
Autostar Suite's catalogs or from one of the object databases. There are four major catalogs:
1) Messier
2) NGC (New General Catalog)
3) IC (Index Catalog of Nebula)
4) SAO (Smithsonian Astrophysical Observatory)
The Messier catalog was originated by Charles Messier (1730-1817) as objects "to be avoided while seeking comets".
It contains some of the best galaxies, clusters and nebulas. Messier objects are given names such as, M51 or M42.
Many Messier objects are frequently called out by their NGC designations which sometimes leads to confusion.
Autostar Suite translates Messier designations into NGC, with the exception of M24 and M45 (The Pleiades) which do
not exist in either the NGC or IC catalogs.