I've done some tests using a 16" or 18" Dob as a collimator. In effect a pinhole is mounted at the focus of the Dob, which produces parallel light and sends it into the C14. There is some change in the collimation of the C14, but it is fairly minor. One can do foucault, ronchi, star tests, etc. in this manner. Many errors -- spherical, astigmatism, zones -- are easily seen. Of course, it also shows up any defects in the Dob optics, so you want one with good optics in it.
Coma is more difficult to probe, since there is degeneracy between collimation errors in the C14 and in the Dob, but its not really a problem. Of course, you want the Dob well collimated. But any residual collimation errors in the Dob just mean you need to tweak the C14 collimation a little when you get is out under the real stars.
Attached star test (in focus) was done in this way with a C14 and an 18" Dob as the light source. Theres an Airy disk, some rings. Slight trace of coma -- collimation not quite perfect (yet). Also some hints of astigmatism and trefoil indicated by break-up of the first ring.
I think the best test would be with the C14 pointed vertically in a double-pass test. Use a large flat over the front aperture. Probably there would need to be some careful consideration to how the flat is supported, and what effects its deformation might have. But I don't have a large flat, only a couple Dobs.
Coma is more difficult to probe, since there is degeneracy between collimation errors in the C14 and in the Dob, but its not really a problem. Of course, you want the Dob well collimated. But any residual collimation errors in the Dob just mean you need to tweak the C14 collimation a little when you get is out under the real stars.
Attached star test (in focus) was done in this way with a C14 and an 18" Dob as the light source. Theres an Airy disk, some rings. Slight trace of coma -- collimation not quite perfect (yet). Also some hints of astigmatism and trefoil indicated by break-up of the first ring.
I think the best test would be with the C14 pointed vertically in a double-pass test. Use a large flat over the front aperture. Probably there would need to be some careful consideration to how the flat is supported, and what effects its deformation might have. But I don't have a large flat, only a couple Dobs.
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