The eternal quest for adjustability

The aluminised spot on the null lens works like a charm!  Now we can get good fringes from both the front & back faces of the lens with the 94%-reflective reference mirror we need to use to test M2.

 

 

It turns out we need more coarse adjustability in decentre for the null lens so an extra set of screws was added to the flexures...

 

& it really helps to be able to adjust the interferometer, fold mirror & null lens as a unit once they've all been aligned with each other.  Mods were thus made to the whole M2 test frame to make it adjustable in tip, tilt & decentre.

  

Pity Ockert's found out where the most comfortable parking spot in the clean room is - now we're gonna have to fight over it...

  

A rare week in Cape Town!!

A few members of the team spent this week working in Cape Town as the Faro testing that needed to be done could easily be carried out in the lab at the SAAO.  With the arm set up on its test plate & all the accessories scattered around like our favourite toys, it felt just like home...

 

We wanted to test our extension probes to see if any are capable of delivering the sort of accuracy we'll need for the remaining steps in the repair plan.  The short, standard Faro probe (left in the pic below) defined our best case & then we set about putting the different custom-made probes through their paces.  These included a hollow stainless steel probe (seen here after it'd been tested & had its nose transplanted to its solid stainless steel cousin), as well as hollow & solid carbon fibre probes that underwent various surgeries of their own.

  

Although the probes were given unfairly harsh physicals - the sets of Single Point Accuracy Tests (SPATs) involved the largest amplitude movements possible in all directions - the results were rather disappointing in most cases...

  

Happily though, the solid stainless steel probe performed brilliantly - almost matching the SPAT results for the short probe & so we have a winner :)

  

We also had a chance to check out the titanium base that'll hold the new CGH - seen here (upside-down) sporting its zippy flexures.

 

& we got Malcolm to aluminise a spot on the curved surface of null lens for us.  This was necessary as we have to be able to pick up that surface with the 94%-reflective reference mirror on the interferometer (as that's what the M2 test will be done with), but the uncoated lens only produces useful fringes if we use the 4%-reflective reference mirror.  This cunning work-around's perfect as the central part of the null lens falls within the central obscuration due to the hole in M2 & thus doesn't participate in the M2 test.

 

Aligning the target for the M2 test

Last week we did a rough alignment of the M2 test gear, but now that the new target holder's ready, we hope to refine that initial effort.  Unfortunately the new Faro probe extension isn't up to the job, but we can still try using the new target to mark the optical axis of M2 (while a new probe is designed & manufactured).

 

We still get the tip & tilt of the interferometer with the paddle mirror, but the paddle gets slid out of the way to allow access to the target which is used to set the decentre of the interferometer.

The Faro was used to position the target as close as possible to the optical axis of M2 & then we could adjust the interferometer to get it lined up with the target.

 

The next task was to set the tip & tilt of the target so that the reflection makes it back into the diverger on the interferometer...

The fine adjustment was made easier by having live feedback from the interferometer - shown below as the reference beam (left) & the target reflection (right) begin to overlap & produce interferometric fringes.

So the good news is that the target system works in principle - we just need to do better with the Faro extension so that we can measure everything more accurately...

Trying out new hardware for the M2 test

Upon our return yesterday, we were greeted by one of SALT's resident Rock Kestrels - she didn't seem entirely convinced that we ought to be here...

Our goal for the week is to test the new hardware for the M2 test.  This includes the new solid carbon fibre Faro probe extension which will allow us to reach otherwise impossible places...

  

The other new piece of gear is the target carrier that will locate within the hole in M5 to mark the optical axis of M2 for the null test.  The target carrier consists of a base that seats in the hole via a set of spiral-arm like flexures.

  

The next layer is a translation stage, that also relies on flexures, for adjusting the target in decentre...

  

& lastly, on top - the actual target holder part that includes a tip/tilt adjustment mechanism.

  

Since a change is alleged to be as good as a holiday, I went over to the far side of the plateau to see what Geoff's been up to in the electronics workshop during his standby week... It appears he's been keeping busy building a nifty new rack for SALT!

First fringes off M2!

We don't yet have all the hardware we need for the M2 test, but we can do a rough-&-ready trial run to check that it'll work as planned...

 

Here's the test frame carrying the interferometer, fold mirror & null lens.  A laser beam's sent out by the interferometer & bounced off the fold mirror that directs it through the plano-concave null lens, up to M2.  There the beam's reflected back through the system & returned to the interferometer where, if all goes well, fringes ought to be produced.

  

A flat paddle mirror on the back of M2 (the same one used to align M5 way back when, although now with a hole added to its centre) is used to get M2 squared up in tip & tilt & aligned in decentre with the optics onboard the test frame.

  

After making rough measurements of the axial spacings & adjusting the null lens & interferometer to get the despaces right - success!!  The first fringes from M2 :)  It's too soon to interpret the interferogram as nothing was aligned to anywhere near the proper tolerances - this was, afterall, just the "let's see if we can do this" week...

 

Great job guys!

Gearing up for the M2 test

Summer in the Hex River Valley - grapes Everywhere!  A truly beautiful scene in this magical place...

 

Meanwhile, in the clean room we're getting everything ready for the M2 null test.  Here's the fold mirror going into its cell.

  

The SAC's tilted to the operational angle of 37 degrees for the test & needs to be securely locked down in this position.

  

Before trying to align anything, we made sure all the hardware fits & works - first the fold mirror at the bottom of the test frame...

  

Then the null lens in its cell, attached to its kinematic mount.

 

So far so good - tomorrow we'll set up the interferometer on the frame & have a go at firing a laser through the system!