* Astronomy

A small tip to make some of your different branded eyepieces parfocal, is to manufactures some spacer rings by either gluing together a long strip of paper, or gluing strips of thin plastic that overlap or are laminated. These rings can then be slipped over the silver barrel of the eyepiece.
The width of the spacers can be roughly judged by raising the eyepiece from the eyepiece holder.

A slightly less attractive way is to use a jubilee clip (or similar).

But, commercial Parfocal Rings are available.

The advantage is that once the eyepieces are parfocalised then little or no refocusing adjustments are required when changing the eyepieces.