Introduction to TPoint

TPoint is an interactive telescope pointing analysis system. By logging the telescope control system coordinates when the telescope is centered at known equatorial coordinates, much can be learned about the geometry of the telescope/mount system. The process of gathering this data is called mapping.

Mapping a telescope involves pointing the telescope at a series of evenly distributed stars across the sky. When the telescope is centered on a star, both the star coordinates and the telescope control system coordinates are logged. The two coordinates differ by the pointing error or residual. Once these errors are logged for enough points (usually from 10-300), TPoint creates a set of values that are used in various mathematical formulae to compensate for the errors. Each time new coordinates are issued to the telescope they are adjusted by the computed error amount. The mathematical formulas for the various errors are called terms. A set of terms working together to improve telescope performance is called a model.

Once a model is established for a telescope, it is saved and can be used from night to night. As long as no adjustments are made to the telescope, the model will remain valid. In situations where a telescope must be arbitrarily synchronized by pointing at a star each session, only a few of the terms must be re-established each night.

The magic of TPoint is in determining the values of the various terms from a set of mapped points, that is, fitting the data. If each term could be dealt with one at a time, the fit might not be difficult. When zero-points, polar alignment errors, non-perpendicular axes and various other pointing errors are all mixed together, untangling them becomes a daunting task. The Fit command in TPoint automatically computes a new fit each time terms are added to or removed from the model.

TPoint can digest data from either equatorial or alt-azimuth mounts. TPoint models have been applied to many telescopes, including IR, mm and radio, and is routinely used at the AAO, UKST, UKIRT, LPO and many other observatories.

TPoint is unusual in that (a) the telescope model can be changed during the interactive session, and (b) it contains a variety of graphical displays. These two features allow for rapid exploration of the pointing possibilities of any given telescope and lead to the best model for that system.

TPoint for Windows is a Multiple Document Interface (MDI) application that works in concert with TheSky Astronomy Software. Pointing test data are generated by performing a mapping run with your telescope using TheSky. Several Sky Databases are included that provide either a) an evenly distributed set of altitude/azimuth points on the Virtual Sky for use as a guide in pointing or b) sets of equatorial star data evenly distributed for actual pointing targets.