large rotating rings

Anyone aware of a manufacturer who can make rotating rings in the size range of say, 25-28" ID for observatory mounted newtonian equatorial soon under construction? Would prefer to avoid rotatable upper cage assembly due to collimation difficulties, if possible.

Parallax Instruments makes standard rings for up to 26.4" tube diameters and rotating for up to 19". You need to contact them for larger sizes.

http://www.parallaxinstruments.com/ring.htm#TOA130


Robert W.

Our club used 2 focusers on their 20" F/5. Each was 180 degrees apart on the tube. The secondary mirror rotates to the needed focuser simply by rotating on the shaft that attaches to the center spider lug. The focusers are not exacly 180 degrees apart, so one focuser is well collimated and the other is slighly off. The very large 100 percent illiminated field on this scope slightly makes up for the off collimation of the 2nd focuser. The system works well and was fabricated by a guy who has made alot of telescopes. This is just another way to do it.

Is precision pointing of this scope important? If so, then the optical axis of the scope must coincide with the rotation axis of the rings. Otherwise, the aim will shift whenever the OTA is rotated.

I use Parallax rings with my 8" Newtonian. The rings are slightly oversized (by about 0.2") so I can adjust the mounting screws to aim the scope within the rings to compensate for differences between the optical and mechanical axes of my scope. Of course, the collimation mavens will insist that the axes can be aligned, but I find it easier to have this extra degree of freedom.

Allen