Sunday, November 29, 2009

A new way to sort out M4's decentre

Yesterday's Z6 grid measurements produced a reasonably decent looking saddle, but there are still some spikes out near the edge.  The image below shows the theoretical surface on the left & our measured version on the right.  

Epic amounts of effort went into getting this far & the whole routine's wearing painfully thin so we decided it was time to try a different approach!  Just couldn't stomach yet another grid scan...


Instead, Darragh came up with a nifty new technique.  This involves decentring M4 by some amount, moving the pinhole to compensate for the resulting coma & then stepping across the field with the pinhole & wavefront camera to measure Z6 at 5 positions (0, +/- 1 mm & +/- 0.75 mm).  Each such iteration produces a curve as shown below.

The difference between the Z6 values at opposing off-axis positions can then be plotted & the cross-over point marks where M4's decentre is zero.  The turquoise curve above is the most symmetrical, indicating that the astigmatism's well balanced at that position.  We've only done this in the Y direction so far, but at least the procedure works so we'll get back to this when we return next week!

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