[g__day (Matthew)]

My thoughts are how this works are as follows.

The threaded focus shaft to the mirror is not well designed to ensure it is secured so the shaft can only turn - not shift laterally up or down. Such shifting - even if only very minor would be a big worry. Push the focuser up or down while not rotating it an see stars move significantly. Only a 1/2" diammeter bearing restricts lateral movement. Here's a simply confirming test - focus in then out half a turn of the focuser. When you change direction focusing you may see stars jump 5 - 10 arc minutes, that's shaft shift!

The Featerher focuser is bolted at three points to a thicker steel plate, and the shaft passes through a tighter fitting hole in this plate, and the shaft fits along way into a narrow and very rigid focuser - making the shaft much more secure and greatly restricting stray lateral movement. When I first tried to attach this focuser to the shaft I was worried the hole in the attachment back plate wasn't machined in the right spot. Unscrewing it all and gently pushing the threaded focusing shaft upwards caused the shaft to suddenly move more that 5mm upwards - definitely changing mirror position!

By simply re-designing attachment points to the focusing shaft or inserting one of these microfocusers - it appear one can greatly eliminate lateral focusing shaft movement - mirror stability is greatly improved - meaning all of polar alignment, tracking pointing benefit substantially too!

I'm delighted!