CCDciel

The “measure cone error” tool measures the slew error after a meridian flip. This slew error will be twice the cone error.

Cone error is a misalignment of the telescope axis away from or towards the plane of the mount puck. The front/light inlet of the telescope is either pointing slightly inwards towards the puck or outwards away from the puck.

The cone error results in less pointing accuracy of the telescope and continuous drift during guiding. It is typical to have a cone error up to 30 arc minutes but it is better reduce it to a lower value with some shims, sheet-metal ring or a few layers of thick aluminum foil.

With a significant cone error, polar alignment using the drift method will not work.

This tool will only report accurately if mount modelling is switched off or not available.

The “measure cone error” slews the telescope to two points high in the sky east and west of the meridian at the same declination position. While crossing the meridian the telescope will flip. At both positions an image is taken and solved. The distance between these two solved positions is compared with the distance between the two mount positions as reported by the mount. The difference is reported as slew error and will be twice the cone error. To assure no other effects are measured it is possible to measure first east then west or the other way around. The reported slew error values should be very equal.

If the reported value is positive then the situation is as above sketch. To correct the positive error the telescope front has to move to the puck. If the reported value is negative then the telescope front has to move away from the puck.

Exposure and binning settings are taken from the preview menu.

The cone error is explained in this video

When using multiple camera in CCDciel an additional prompt ask the camera to use for the measurement.

This allow to first correct the cone error on the main instrument, then to repeat the measurement with another camera to adjust the guider or finder so they point at the same location relative to the main instrument on both side of the meridian.