Three Fathoms Observatory
Stewart Yeung

 Current Special Feature



Problem of Adding a Rotator

My aim is to facilitate the selection of suitable guide stars.

First, I have tried using the SBIG external guider with a small refractor attached to the main scope. It didn’t work at all due to a number of reasons, the main ones being flexure of the off-axis setup, mismatch of image scale (my main scope, LX200R 14” has a long focal length of 3550mm), difficulty in firmly attaching the small refractor to the external guider,…

A rotator can achieve the same purpose by facilitating the selection of guide stars along the circumference of the main imager. A Pyxis 2" rotator was installed.

This implementation encountered a number of difficulties :

  1. the skewed weight of the CFW-10 : this is remedied by a) the attachment of balance weight to the CFW-10 and b) reinstalling the AO-8 in an inverted orientation (see photos in "Observatory and Equipment" section).
  2. setting the mechanical center of the rotation to the optical axis – the is tackled by trial and error aided by the adding shim to the interface between the microfocuser and the rotator; a really difficult job and even now the center of the optical image is still slightly off-set from the mechanical center :

The rotator was rotated while taking this image.The bright star was placed exactly at the center but moved in a small circle due to residue displacement of the mechanical center from the optical axis.

3. flexure of the lengthened image train – it is overcome by tightening the connections between the various components in the image train;

4. the sheer weight of the loading handled by the 2” Pyxis – the loading of the rotated assembly exceeds the designed loading of the 2” Pyxis, but I hesitated to use the 3” Pyxis due to its increased weight and backfocus issue. My 2” Pyxis setup improvised by the balanced weight/inverted Ao-8 orientation is working all right when aiming at targets at reasonably high altitude. Near horizontal position, the torque of the loading exceeds the capability of the 2” Pyxis and it would slip.

5. the problem of introducing arc-shaped artifacts in the flat image : see "Artifacts in Flat Image" in the previous special feature.

A new T-Point model for the whole setup was run and the subsequent pointing/tracking are as accurate as before the add-on. I can guide with the internal guider for as long as two hours (in most part of the sky) when shooting color series.

This setup works well and it always enables me to find suitable guide stars. The rotator also serves a vital purpose in my setup : it can be operated fully remotely which is what I am doing.



Thanks for advice and tips from Mike Siniscalchi, Terry Tuggle and Wolfgang Renz.