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A Simple Cone Error Correction Procedure


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A Simple Cone Error Correction Procedure

By Ian Thompson

Sometimes referred to as the Collimation Error, Cone Error refers to a condition with a German Equatorial mount (GEM) where the telescope is not truly at a right-angles to the Declination Axis (Dec.), i.e. is not parallel to the Right Ascension Axis (RA).   For visual observation, Cone Error Correction (CEC) is not necessary, but for astrophotography and setup of GoTo mounts, CEC may be advisable.

Most of the procedures I have read utilise Polaris as a reference star and the method cannot be generalised.   Also, such procedures are not applicable to the Southern Hemisphere, where there is no fixed star at the celestial pole.  The following alternative method works well and can be applied equally well in both hemispheres:

  1. Make sure that your GEM is level and drift aligned. An excellent description of the latter is available on http://www.andysshotglass.com/DriftAlignment.html.

  2. Select a star near the equator and meridian.   Turn the telescope to the eastern side of the pier/ tripod and centre the star in the field of view (FOV).   Once satisfied, immediately note the time and RA reading on the RA setting circle or hand controller readout.   Turn the telescope to the western side of the pier and centre the same star in the FOV.   Again, note the time and RA reading.   If the difference between the two RA readings, minus 12 hours, differs from the time interval between the observations, the telescope is not truly at right-angles to the Dec. Axis.

  3. If the difference is greater than the time interval between the successive readings, then, with the telescope E of the pier, the objective end of the scope must be moved east, i.e. away from the pier.  If your scope is mounted on a dovetail bar that has adjustment screws at either end as shown in the photograph, it is a simple matter to loosen the central bolt (A) holding the scope rings to the dovetail bar, and wind the cone error adjustment screws (B, C) a little (same amount each one) before re-tightening the mounting bolt (A).   In the event that you do not have an adjustable dovetail bar, you can place thin shims under the front tube ring mount block and re-tighten bolt (A).   Thin red fibre washers or folded aluminium foil make good shims, or you can use whatever you have to hand, as long as it is largely incompressible and durable.

  4. Repeat (2) and (3) above and by trial and error, determine the amount of adjustment required to give satisfactory agreement between the two RA readings and times.

  5. If the difference is less than the time interval between the successive readings, then, with the telescope E of the pier, the focuser end of the scope must be moved east, i.e. away from the pier.

  6. Carry our (2) and (3) above by adjusting the rear scope ring until there is satisfactory agreement between the two RA readings and times.

  7. Re-check drift alignment and adjust as necessary.


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