compression ring accessories in refractors

Hi,
I was just wondering, since a compressionring diag or other accessory shifts the ep over a bit why doesn't that push off the collimation of the scope to the ep. Or when one is using a cheshire how can one rely on results achieved if the cheshire was shifted to one side due to the comprression ring?

itz marcus said:

Hi,
I was just wondering, since a compressionring diag or other accessory shifts the ep over a bit why doesn't that push off the collimation of the scope to the ep. Or when one is using a cheshire how can one rely on results achieved if the cheshire was shifted to one side due to the comprression ring?

Itz:

Here's a few thoughts/comments..

- The Bore/hole that the eyepiece or accessory fits in has relatively tight fit, for a 2 inch eyepiece, this is normally around 2.002", a clearance of about 0.002-0.004"= .05-0.10mm. This about the thickness of a human hair, not much to say the least.

- The compression ring actually centers the eyepiece/accessory or collimation device because it moves around in the groove and compresses from all sides.

- The eyepiece or accessory will be subject to the same offset as the collimation tool if the compression ring is not moved.

- Eyepieces are quite tolerant of off and non-parallel rays, the sorts of shifts we are talking about here are inconsequential in terms of what you will be seeing in the eyepiece. Consider that your eye is not centered to this perfection.

A quick calculation shows that an offset of 0.004" in an scope with a 750mm focal length is the equivalent of about 30 arc-seconds off axis, again very little...

Bottom line: I think the mechanical alignment of the optics using decent quality compression ring components does not affect the quality of the view.

Jon

I have both set screw and compression TV everbrites and notice no difference with either type of diagonal. Since a set screw applies force on one side of the eyepiece, I'll assume you are not limiting the discussion to compression rings only.