by Dave Skitt
AAAP member Tom Swords, in consultation with John Church and with my assistance, is currently working to improve the connection of the focuser for the Hastings-Byrne refractor to the tube assembly. We have become aware that the focuser attachment screws would loosen over time, allowing the heavy focuser to wobble. While the views through this F/15 refractor are still wonderful and a de-focused star test shows no noticeable distortion of the airy disk, misalignment of the focuser to the optical path is typically not considered ideal. We believe our efforts will not only correct the attachment issue, but also improve the optical alignment and future ability to collimate the telescope.
The first task, after removing the focuser, was to square up the end of the steel tube. As John recounted, he and Gene Ramsey were reluctantly forced to shorten the tube when the original, worn out, focuser was replaced with the current one some years ago. In the process of shortening the tube, a slight deviation in the cut was inadvertently introduced. Tom and I identified the deviation and Tom gingerly removed the protruding metal with a small grinding wheel (Photo 1).
The preliminary result has been to move the optical center of the focuser closer to the center of the lens cell. In Photo 2, Tom’s finger is pointing to where the center point was falling prior to squaring, and the red laser dot is where it is now pointing with the focuser only temporarily attached in the newly squared tube (Photo 3). Further refinements, described below, will likely bring the final alignment even closer to the true center of the lens cell.
Since the historic steel tube is known to have a very slight taper from front to back, a small gap was likely introduced between the new focuser’s outer diameter and the tube’s inner diameter when the tube was shortened. Tom is currently in the process of experimenting with shim material of different sorts, widths and attachment methods to find the best match to remove the wobble the focuser was experiencing. After temporary placement for final adjustments, the existing holes in the focuser will be drilled and tapped for larger screws with increased gripping surfaces.
While the cold weather has delayed further work on the project at the observatory, Tom has used the time while the focuser has been in his shop to perform some additional maintenance. Photos 4 shows the freshly cleaned and polished coarse focus rollers and new rubber tension spring washers. Photo 5 shows the freshly cleaned and lubricated fine focus assembly. And finally, Photo 6 shows the freshly painted interior surface of the drawtube.
We hope to have the Hasting-Byrne refractor back in operable condition in the next few weeks. If you have any questions or comments about our work, please feel free to reach out to Tom, John or myself.