various drive method for large telescopes

 

 The typical telescope pointing errors consist of the following items:
   1. Azimuth and altitude misalignments to the celestial pole.
   2. Mechanical and optical non-perpendicularities.
   3. Encoder scale factors, offsets, and eccentricities.
   4. Flexures of the telescope mechanical structure.
   5. Flexures of the optical mounts.

The pointing accuracy should be sufficient to place the object within the field of view of the guider system open loop (without optical feedback). Small telescopes tend to have a relatively large field of view while large telescopes have a very small field of view so the pointing needs to be better for large telescopes. Typically, it is desirable to have a pointing accuracy (RMS) of about 1/4 of the field of view of the guider system. This level of pointing is considered "Good".

The guider system will often have a field of view of several arc minutes so the pointing needs to be a fraction of an arc minute. For moderate size telescopes (0.5 to 1.5 meters) a pointing accuracy of 30 arc seconds RMS or better is sufficient. ( Don't get confused with guiding accuracy)


The DFM Engineering data sheets provide very conservative pointing accuracy with typical values of 30 arc seconds RMS or better stated.

One of the best example is Dickinson College DFM 24-inch RC telescope, which has pointing accuracy of 9.5 arc seconds RMS, from horizon to horizon

 

 

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