Truss-Tube Project

Update 01/10/06

The donated 16" mirror for the Truss-Tube was shipped to Galaxy optics back in November for refiguring and coating.  This Monday we received an evaluation of the optics and an update on refiguring.

Optical testing, evaluation and refiguring was completed last week by John Hudek.  The mirror originally had some rather severe defects; it was severely "under corrected", meaning that the curve was closer to a sphere than the ideal parabola and there was also some astigmatism probably caused by stress release from a chip that developed near the back of the mirror.  Most of the problems were in the outer 2" of the surface.

John explained that the mirror has an excellent figure now after correcting the major lobe of astigmatism and the under correction problem, but is not perfect due to the nature of the problems with the mirror.  The surface could be improved, but that would require re-polishing the entire surface from scratch, which would add considerable cost to the project.  With the work that John has done thus far, the surface is well within the ¼λ limit and will produce an excellent all around scope.  So, to remain within our budget, John is going to coat the optics without further work.

John also tested the secondary and there are problems.  The test actually revealed that the secondary had a bit of "power" rather than being absolutely flat.  This could show up as a bit of astigmatism at the eyepiece.  John also commented that the diagonal size is actually larger than it has to be for this scope and that the larger size contributes to less crispness in the image.  John suggested that we test the scope with our original diagonal, and if we detect any astigmatism in the image, we should consider replacing the diagonal.  If the diagonal is replaced, we should downsize from a 4" to a 3.1" (at a cost of about $200).  This move would remove any remaining astigmatism in the image and sharpen the images from the reduced diagonal size.

John is going to coat both the primary and secondary this week and we should have them in a week or two.

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Last Updated Tuesday, April 17, 2007