Quote:
Originally Posted by croweater
I don't know why either Allan. I certainly don't claim to be an expert. I'm just doing what I've always done and the stars test good so that's good enough to me. I wonder if its imaging scopes or high end maybe that do it that way so there is no tilt in the system that would show up under very close scrutiny. Hopefully someone better qualified than me will chime in.
Cheers to you, Richard
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Thanks Richard,
you have been very helpful.
This link sort of answers it too:
https://catseyecollimation.com/mccluney.html
Offset Away from the Focuser -
Unless the spider assembly or secondary mirror holder is specifically designed to include it, offsetting in this direction can be difficult. For a typical 4-vane spider the mounting holes may be drilled slightly shifted in the tube wall in order to accommodate the offset. It may also be possible to offset the spider using the spider leg mounting hardware by loosening the leg(s) nearest the focuser and tightening the farthest leg(s). This solution is less desirable as it may tend to increase the width of diffraction spikes seen around bright objects, or even make each existing spike 'branch' into two spikes. The primary reason for offsetting the diagonal away from the focuser is to keep the optical centerline and the telescope tube centerline coincident and prevent vignetting at the front entrance of the telescope. If this could be a problem, or if the most perfectly possible aligned system is desired, then include this offset dimension. When offset in this direction is not included, the optical centerline will be reflected by the secondary mirror by slightly more than 90 degrees. This will be compensated for by primary and secondary mirror tilt with no detriment to the telescope's performance.
And here as Richard pointed out:
And here where there is no offset outwards from the focuser so it's done via tilt:
https://skyandtelesc...condary-mirror/
But what if you want the fully illuminated field centered in the eyepiece,
but must leave the secondary mirror centered in the telescope tube?
It can be done, as shown in diagram C, by slightly adjusting the tilt of both mirrors. Now the optical axis is slightly tilted within the telescope tube. In practice, this is not a problem because the tilt is never more than a small fraction of a degree. Since the secondary is offset down the tube,
this is known as partially offset collimation. It is no doubt the most common situation, even among telescope owners who may not even realize that their scope's secondary is offset at all.