Sept. 15, 2012:  Testing a 20" and a 4.0" m.a. flat

This 20" mirror came in for testing and refiguring.  The departure from a parabolic figure is shown in the error plot below.

Error plot for 20" mirror

This mirror is not too bad in terms of peak-to-valley error, but there are some significant slope errors (referred to as transverse error), particularly on either side of the 80% zone, that will throw light well outside of the ideal airy disk diameter, thus bloating star images and harming the sharpness of stars and other small details seen on planets or deep sky objects.  The figure of revolution of the mirror looked quite good.

The flat was shipped with the primary, and here is a test of it against a reference flat.

Test of 4.0" m.a. flat

As is painfully obvious, the fringes are not straight, parallel and evenly spaced, so this "flat" is nowhere near flat.  In fact, it is at least a full wave concave based on a quick look at the fringe shape.  This curvature would likely have caused significant astigmatism in the views seen through the eyepiece of this telescope.

Both mirrors are now being reworked.

Please check back for future installements of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

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