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!

Jacarandas & Z6 surfaces

Add another striking colour to the Hex River Valley's palette: Jacaranda trees in full bloom!

 

So what are we trying to achieve with this M4 alignment?  Darragh's encyclopaedic collection of Zemax simulations includes an amazing grid of Z6 (one of the two astigmatism Zernikes) surface plots that shows the effect of M4 tilt & decentre.  The perfect saddle-shaped surface (in the centre of the grid) that indicates no tilt or decentre is what we're after...  The astigmatism curves mentioned in recent posts refer to sets of wavefront measurements that slice through the Z6 surface in the X & Y directions.

Below's a zoomed in version of the central part of the grid.  Note that the vertical scale's exaggerated in the middle plot to show the saddle more clearly.

At the moment we're struggling with erratic wavefront measurements, which result in vicious spikes all over our Z6 saddle...  Naturally this is far from ideal so we've been trying Everything to track down the source of the noise.  One thing that proved marginally useful was wrapping the wavefront camera & its USB cable in aluminium foil to shield it from interference. 

 

Besides, it contributes to the spacey look we're going for ;)

Still adjusting M4!

Sadly not much excitement to report - we're still chugging away on the M4 alignment...  A recurring theme in this project is the thrill that goes with working out how to do something, followed by the slow torture of actually grinding out the process to the required tolerances!

 

Producing those beautiful astigmatism curves raved about in an earlier post is now part of our daily slog & not all the results generate streams of poetry...

Charl's been particularly insensitive to our misery during this phase.  Each time we make our way to/from the clean room we have to walk past him happily beavering away (making visible progress - constantly!) on the new electronics for the calibration system!

 

Cynthia appears to have the right idea: Retirement!!  The hostel staff put on a wonderful surprise farewell tea for her this afternoon (with the help of Linda & Glenda who hauled her up here on false pretences!).  After 44 years of service, it's the end of an era at the SAAO - all the best, Cynthia!

 

1 fierce rodent, great wavefronts & a big bottle!

Ardisha found this little guy in her kitchen & brought him up to SALT before releasing him into the veld.  It's a Spectacled Dormouse, probably the same one that's been known to visit the 1.0-m dome from time to time..   Really cute but Extremely aggro - made the most amazing noises while trapped in the bucket!

Aah... free at last! Now where the hell am I??

 

Meanwhile, back in the clean room we were getting M4 aligned to M5 & along the way obtained our best wavefront to date: RMS = 0.05 waves!

 

By the end of the day we all needed to let off some steam so it was off to Eben's place for one of Ockert's potjiekos dinners (pronounced "poy-kee-kos": a traditional South African stew that's cooked in a cast iron pot over a bed of coals).

As the sun sank, the temperature plummeted & we had to get serious about survival.  Our host to the rescue with a bottle of wine to share between the 10 of us...  While that may sound conservative, the bottle was of course a Jeroboam :)

The little corkscrew that could!

 

Not exactly the sort of weather you'd expect in November in the southern hemisphere - apologies to our visitors from the north!

 

It really helps to be Eben & be absolutely immune to cold...

 

Thanks guys - a great evening had by all :)

What happens in...

Upon returning to the clean room, we found this up on the door! :)

The modifications to the tilt-adjustment mechanism on the M4 mounting ring have been made so the whole lot could be re-installed...

M4 back in the SAC, ready to be aligned - really this time! ;)