July 13, 2006, Starsplitters of Wyalusing 1 TPoint What the #@* is Bisque doing now?

July 13, 2006, Starsplitters of WyalusiJuly 13, 2006, Starsplitters of Wyalusingng

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TPointTPoint

What the #@* is Bisque doing What the #@* is Bisque doing now?now?


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How is TPoint used?How is TPoint used?

A reticle eyepiece or CCD camera is used to “CENTER” A reticle eyepiece or CCD camera is used to “CENTER” selected stars. A sequence of stars (20-100) of selected stars. A sequence of stars (20-100) of knownknown coordinates is pointed to and centered (with SkySix software). coordinates is pointed to and centered (with SkySix software).

Once a series has been logged, the ‘Fit Data’ command of Once a series has been logged, the ‘Fit Data’ command of TPoint is used to create a pointing TPoint is used to create a pointing modelmodel for the telescope. for the telescope. Coordinates are adjusted such that repeatable errors are Coordinates are adjusted such that repeatable errors are corrected.corrected.

TPoint automatically calculates the optimum size for each TPoint automatically calculates the optimum size for each term, such that the overall model is the term, such that the overall model is the best possible fitbest possible fit (applying “Analysis of Variance”). (applying “Analysis of Variance”).

Another use of TPoint is to identify opportunities for Another use of TPoint is to identify opportunities for mechanical adjustmentmechanical adjustment. Examples, 1. TPoint enables the polar . Examples, 1. TPoint enables the polar axis of an equatorial mount to be set up accurately even when 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 you cannot see the pole; 2. The mirror’s axis can be made perpendicularperpendicular with the DEC axis (with shims). with the DEC axis (with shims).


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What kind of pointing can I What kind of pointing can I expect using TPoint?expect using TPoint?

The pointing accuracy you are able to achieve with your The pointing accuracy you are able to achieve with your automated telescope is very much dependent on the integrity automated telescope is very much dependent on the integrity of your telescope mount and optical tube assembly. Very small of your telescope mount and optical tube assembly. Very small mechanical errors manifest themselves as pointing errors. mechanical errors manifest themselves as pointing errors.

If the mechanical errors are repeatable, then TPoint will most If the mechanical errors are repeatable, then TPoint will most likely be able to determine the magnitudes of the errors and likely be able to determine the magnitudes of the errors and create a model that corrects for them.create a model that corrects for them.

Non-repeatable errors, such as mirror-flop, focusing shift and Non-repeatable errors, such as mirror-flop, focusing shift and others, fall into the ‘slop’ category and must be dealt with others, fall into the ‘slop’ category and must be dealt with separately.separately.

On most amateur systems, root mean square (RMS) pointing of On most amateur systems, root mean square (RMS) pointing of two arcminutes or better can be achieved without great two arcminutes or better can be achieved without great difficulty. Robust professional systems enjoy pointing to a few difficulty. Robust professional systems enjoy pointing to a few arcseconds arcseconds with the aid of TPoint.with the aid of TPoint.


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The general approach taken by TPoint is that The general approach taken by TPoint is that as much as possible, the telescope model as much as possible, the telescope model should describe real effects (geometrical should describe real effects (geometrical misalignments, well understood flexures,misalignments, well understood flexures,etc.), and empirical functions should be used etc.), and empirical functions should be used only to mop up any remaining systematic only to mop up any remaining systematic errors. errors.

There is a school of thought that advocates There is a school of thought that advocates using empirical functions, such as using empirical functions, such as polynomials and harmonics, for the whole polynomials and harmonics, for the whole job. However, the TPoint approach has job. However, the TPoint approach has advantages.advantages.


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A realistic model of a telescope often A realistic model of a telescope often exposes mechanical deficiencies, exposes mechanical deficiencies, which can then be diagnosed and which can then be diagnosed and cured. cured.

A realistic model is likely to require A realistic model is likely to require fewer terms, and the number of stars fewer terms, and the number of stars observed in a pointing test can be observed in a pointing test can be correspondingly smaller.correspondingly smaller.

Physically based models are less likely Physically based models are less likely to misbehave when extrapolating to misbehave when extrapolating outside the area covered by the outside the area covered by the available test data.available test data.


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Interaction with TheSkySixInteraction with TheSkySix

Gathering pointing data (mapping) Gathering pointing data (mapping) requires physically pointing the requires physically pointing the automatedautomated telescope to a number of telescope to a number of knownknown objects in objects in the sky. the sky.

A graphical database of such objects A graphical database of such objects reduces the time required to accumulate a reduces the time required to accumulate a sufficient number of mapping points. sufficient number of mapping points.

TPoint works with TheSky6 to provide a TPoint works with TheSky6 to provide a set of functions set of functions


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Deleting a TPoint ModelDeleting a TPoint Model

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


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But you can do But you can do something about thissomething about this


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4.3.1 What is Periodic Error?4.3.1 What is Periodic Error? (from page 43 ff of (from page 43 ff of Gemini/LosmandyGemini/Losmandy))

The Gemini servomotor on the RA axis turns a worm that meshes The Gemini servomotor on the RA axis turns a worm that meshes with a worm gear that moves the telescope. Ideally, the interaction with a worm gear that moves the telescope. Ideally, the interaction between the worm and worm gear is such that for a constant speed 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.of the worm, the worm gear also turns at a constant speed.

However, in the real world, this is not the case. Slight imperfections However, in the real world, this is not the case. Slight imperfections in the shape of the worm, manufacturing tolerances in the gears and in the shape of the worm, manufacturing tolerances in the gears and bearings, etc., can all cause slight variations. These variations bearings, etc., can all cause slight variations. These variations generally occur in a pattern that repeats itself every time the worm generally occur in a pattern that repeats itself every time the worm makes a complete revolution, hence the term “periodic error.”makes a complete revolution, hence the term “periodic error.”

Periodic Error Control (or PEC) is meant to handle these Periodic Error Control (or PEC) is meant to handle these imperfections of the worm and worm gear combination of your imperfections of the worm and worm gear combination of your mount. mount.


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4.3.2.5 Drift Correction4.3.2.5 Drift Correction When you train the PEC, it is assumed that the only error When you train the PEC, it is assumed that the only error

being corrected is the periodic error in the worm (‘ad hoc’). being corrected is the periodic error in the worm (‘ad hoc’).

If, however, there was systematic drift of the object caused If, however, there was systematic drift of the object caused by something else (i.e. misalignment, atmospheric by something else (i.e. misalignment, atmospheric refraction, etc.), then the corrections reflect this error also.refraction, etc.), then the corrections reflect this error also.

This is not a problem This is not a problem ifif (1) the object being imaged is the (1) the object being imaged is the same as (or very near) the object used to train the PEC. same as (or very near) the object used to train the PEC. Otherwise you’ve a problem.Otherwise you’ve a problem.

Drift Correction solves this problem by effectively removing Drift Correction solves this problem by effectively removing the systematic drift component from the recorded PEC the systematic drift component from the recorded PEC data. data.

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July 13, 2006, Starsplitters of Wyalusing 1 TPoint What the #@* is Bisque doing now?