Wednesday, March 17, 2010

Even better than we'd hoped for!

Wow!  Contrary to all expectations - we managed to obtain a better wavefront for M2 than SAGEM did :)  The interferogram below still contains some spherical aberration due to the daily temperature variations in the clean room, but this is easily removed by adjusting the null lens in piston.

The wavefront map shows some trefoil due to the mounting of the mirror, but nothing serious...

& with the interferometer's reference mirror tilted to produce enough fringes to allow good fits to the wavefront, we obtained a RMS of 0.048 waves (compared to SAGEM's 0.056 waves), a peak-to-valley of 0.28 waves (compared to SAGEM's 0.55 waves) & a Strehl Ratio of 0.91!

So - M2 is Not a problem at all! :)

Friday, March 5, 2010

Tidying up our M2 interferograms

Having the Faro set up within reach of all the M2 test gear has made it possible to significantly improve our alignment of the null lens & interferometer with respect to M2. 

This has led to cleaner interferograms &, consequently, a growing sense that this just might work - rather well in fact if the preliminary Durango (interferometer software) results are anything to go by...

The interferograms below show the effects of various adjustments made to the null lens & interferometer (including its reference mirror).  Our best M2 wavefront so far - similar to that shown in the right-hand image below - had a RMS of 0.18 waves.


The original SAGEM interferogram for M2 was good to nearly 1/20th of a wave RMS.  We hope to achieve a similar result, but this may be overly optimistic given that M2 was still a "free range" mirror at that stage - it had yet to be mounted in the SAC cage...


Wednesday, March 3, 2010

Zooming in with the Faro

So far our approach in carrying out the M2 test has been the clean room equivalent of zooming in on a destination with Google Earth...  We started out with Africa filling the screen & this week we're hoping to close in on the small Northern Cape town of Sutherland by bringing in the Faro Arm.  This will allow us to refer directly to the CAD & optical models, enabling us to steer all the components much closer to where they need to be. 

At this stage we're simply "jumping" the Faro from its high perch on the NRS simulator to a lower base that's within reach of the M2 test gear.  The Faro software performs the coordinate transformation after a specific set of points has been measured from the two base positions.


Having taken the leap, we could establish & adjust the tip/tilt & despace of the null lens by measuring a plane on its retaining ring, while the decentre was determined by measuring a circle on the cell's outer diameter.

Similarly, we could adjust the interferometer using the Faro to measure the position of the point source focus.