Am I missing ANYTHING by not using a Cheshire?
Posted 02 April 2013 - 12:17 PM
Posted 02 April 2013 - 02:29 PM
Semantically, a cheshire is a tool used only for primary mirror alignment. It is a bright circle with a dark center, viewed in reflection through a peep hole in the middle of the tool.
I do like the cheshire for the secondary mirror alignment and its crucial when I collimate my bird-jones scope but to each his own.
If you refer to a tool that combines this with a sight tube (peep-hole plus crosshairs), it is called a "combination tool", or "sight tube + cheshire".
Collimating a Jones-Bird scope with the lens in place is an exercise in frustration. They are much easier to collimate by removing the lens, collimating, then replacing the lens.[In that case, simple newtonian collimation techniques are used] Some, though, have the lens glued in place in the focuser or mounted in front of the secondary mirror. These are best collimated by removing the secondary, aligning the primary with the front of the tube, then replacing the secondary and using the eye to collimate the secondary (for that, a sight tube helps, but the crosshairs don't help much at all--it's mostly making everything except the shadow of the secondary concentric).
A version of the barlowed laser collimation technique can be used once the primary is properly aligned with the tube center. Then the centermark shadow can help align the secondary. Because of the double pass through the lens (if the lens is mounted in front of the secondary mirror), a bright laser is needed. If the lens is in the focuser, then the standard barlowed technique can be used, but to align the tilt of the secondary instead of the primary. In both cases, the "barlow" used is the internal lens--no external barlow is used.
As for aligning the primary mirror by itself, this can be easily done with a transparent or translucent tube cap with a 1/4" hole in the center. Or, the central hole of a spider with the secondary removed. You align the center mark on the primary with the reflection of the opening of the tube cap (your pupil fills the hole). This was a standard technique back in the '50s and '60s. It assumes the primary is centered in the tube, so check that first.
Posted 02 April 2013 - 03:57 PM
Posted 02 April 2013 - 04:16 PM
If you can *just* see the clips, then the secondary is only adequately sized enough to illuminate the very center of the field to 100%, and this small a secondary will significantly vignette the field in longer focal length eyepieces.
What is true is that the secondary, if properly centered under the focuser and collimated, will have the outer edge of the reflective surface concentric with the outer edge of the reflected primary. But that presupposes the centering of the secondary under the focuser, and the best tool for doing that is the sight tube.