TPoint

1. TPoint What the #@* is Bisque doing now?

4. How is TPoint used? • A reticle eyepiece or CCD camera is used to “CENTER” selected stars. A sequence of stars (20-100) of known coordinates is pointed to and centered (with SkySix software). • Once a series has been logged, the ‘Fit Data’ command of TPoint is used to create a pointing model for the telescope. Coordinates are adjusted such that repeatable errors are corrected. • TPoint automatically calculates the optimum size for each term, such that the overall model is the best possible fit (applying “Analysis of Variance”). • Another use of TPoint is to identify opportunities for mechanical adjustment. Examples, 1. TPoint enables the polar axis of an equatorial mount to be set up accurately even when you cannot see the pole; 2. The mirror’s axis can be made perpendicular with the DEC axis (with shims).

5. What kind of pointing can I expect using TPoint? • The pointing accuracy you are able to achieve with your automated telescope is very much dependent on the integrity of your telescope mount and optical tube assembly. Very small mechanical errors manifest themselves as pointing errors. • If the mechanical errors are repeatable, then TPoint will most likely be able to determine the magnitudes of the errors and create a model that corrects for them. • Non-repeatable errors, such as mirror-flop, focusing shift and others, fall into the ‘slop’ category and must be dealt with separately. • On most amateur systems, root mean square (RMS) pointing of two arcminutes or better can be achieved without great difficulty. Robust professional systems enjoy pointing to a few arcseconds with the aid of TPoint.

6. The general approach taken by TPoint is that as much as possible, the telescope model should describe real effects (geometrical misalignments, well understood flexures, etc.), and empirical functions should be used only to mop up any remaining systematic errors. • There is a school of thought that advocates using empirical functions, such as polynomials and harmonics, for the whole job. However, the TPoint approach has advantages.

9. A realistic model of a telescope often exposes mechanical deficiencies, which can then be diagnosed and cured. • A realistic model is likely to require fewer terms, and the number of stars observed in a pointing test can be correspondingly smaller. • Physically based models are less likely to misbehave when extrapolating outside the area covered by the available test data.

10. Interaction with TheSkySix • Gathering pointing data (mapping) requires physically pointing the automated telescope to a number of known objects in the sky. • A graphical database of such objects reduces the time required to accumulate a sufficient number of mapping points. • TPoint works with TheSky6 to provide a set of functions

12. Deleting a TPoint Model • When any physical changes are made to the telescope system (such as changing the polar alignment or changing the optical tube assembly) you must perform a new mapping run starting with an empty TPoint model.

14. But you can do something about this

17. WSP

20. 4.3.1 What is Periodic Error? (from page 43 ff of Gemini/Losmandy) • The Gemini servomotor on the RA axis turns a worm that meshes with a worm gear that moves the telescope. Ideally, the interaction between the worm and worm gear is such that for a constant speed of the worm, the worm gear also turns at a constant speed. • However, in the real world, this is not the case. Slight imperfections in the shape of the worm, manufacturing tolerances in the gears and bearings, etc., can all cause slight variations. These variations generally occur in a pattern that repeats itself every time the worm makes a complete revolution, hence the term “periodic error.” • Periodic Error Control (or PEC) is meant to handle these imperfections of the worm and worm gear combination of your mount.

21. 4.3.2.5 Drift Correction • When you train the PEC, it is assumed that the only error being corrected is the periodic error in the worm (‘ad hoc’). • If, however, there was systematic drift of the object caused by something else (i.e. misalignment, atmospheric refraction, etc.), then the corrections reflect this error also. • This is not a problem if (1) the object being imaged is the same as (or very near) the object used to train the PEC. Otherwise you’ve a problem. • Drift Correction solves this problem by effectively removing the systematic drift component from the recorded PEC data.