SCT Collimation

For the optics experts out there,

I have noticed on my Meade 12" LX200, which stays on a permanent pier mount in my observatory, that the apparent collimation seems to change with temperature conditions, or more precisely with the guide tube lube viscosity. My scope does not have any more more slop than any other sct I've seen and actually less than most.

The question I have for the optics experts is, "At what rate does the primary mirror tilt cause image degradation?"

I have not taken my scope a part to take some measurements but by making some assumptions:

1) .00015in. clearance between the slide tube and mirror carrier
2) the tempature expansion coefficient for the tube and the carrier is the same
3} the effect of the slide tube grease on clearance is zero, which I know is incorrect but I am only trying to simplify the problem definition
4) the mirror o.d. is 12in. and the slide tube od. is 3in.

I have concluded that the .00015 clearance on the tube, which probably is on the tight side, would yield a total tilt envelope of +- .0024" at the outer mirror edges. This is about double what we can manual resolve the focus to on our scopes and with an electric focuser we can probably resolve the focus increments to .0005" or less.

It seems to me +-.0024" is a lot of probable error compared with the focusing movement resolution and that this would have to have a huge effect on image quality. All of the formulas I have come across, always assume an optical system in perfect collimation.

If what I have observed is true, is this why the larger scopes don't tend to perform on average to the smaller counter parts and especially the refractors, central obstruction issues aside?

thanks in advance for any insights and answers

I have concluded that the .00015 clearance on the tube, which probably is on the tight side, would yield a total tilt envelope of +- .0024" at the outer mirror edges.
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0.00015 inch is tight enough that it would probably seize on a regular basis.

jon

Tilt of the primary mirror in an SCT
changes its relative position in regard to both
corrector (aperture stop) and secondary mirror.
Relative tilting of the corrector has no appreciable
effects, but tilting of the fast primary does result
in axial coma, which is partially compenseted by that on the secondary, which has identical amount of opposite tilt relative to the primary, and is also now decenterd relative to it. If I got it right, for the typical configuration with an f/2 primary and spherical secondary at about 3/4 primary's f.l. from it, coma resulting from the primary tilt is more than double that created at the secondary. Final axial coma (full blur length) comes to ~460tD in Airy disc diameters (t=tilt angle in radians, and D=aperture diameter in inches).

In this case, t=0.0004 and D=12, giving axial coma
of ~2.2 Airy disc diameter, which is bellow 0.80 Strehl (the RMS is 2.2/20.9=0.105, and the Strehl
0.64). Since the coma is propotional to the tilt angle and aperture, a "diffraction-limited" tilt
level for the primary would be given by 0.0034/D
in radians, for D in inches. Of course, the less the better.

Vlad