Locking everything up!!

Another bit of house-keeping to be done was to replace the feeler gauges used to shim M5 with more permanent fixtures.

The feeler gauges were used to make little caps that fit snugly over the bolt heads under the spherical bearings.  Seen here from the top...

& from underneath.

M5 was lifted, the old shims removed & the new caps put in place.  Interferograms were obtained before & after each shim was replaced to make sure our precious alignment was not compromised along the way.

Then the locked M5 tie rods were epoxied...

& ditto for M3.

Then the M3 shims were trimmed down & epoxied in place as well.  

So, That's It - NO More Adjustments!!!  This is the SAC we'll go into battle with pretty soon...

Checking the final alignment with the small CGH

The SALT gang decided to get together for a group photo out in the snow today...  Yes, it really was COLD out there!

Then back in the clean room, it was time to put in the small CGH to check that the M3 alignment survived the removal of the big CGH.  A few quick puffs of air to blow off any dust...

Then the SAC was tilted over to 37 degrees & the whole CGH assembly was inserted into the hole in M3.

With the tube in place, all the actuators were connected & the cables secured so that they don't pull on the CGH.

Seen from above, through the hole in M5, one can see the cross hair strung across the CGH.

After using the alignment telescope to set the CGH up on the new invar coat hanger, the CGH & interferometer were adjusted to null the fringes.

Hardly any tip & tilt adjustment of the CGH was necessary - even when the SAC was tilted over & when the temperature was lowered to 10 C.  This is great news as it means that M3 needs no further adjustment!

Here's our final fantastic wavefront measurement: peak-to-valley = 0.82 & RMS = 0.17 waves :)

SAC reference hardware

The steel coat hanger (see the May 4 post), used to mark the optical axis of the corrector for the CGH tests, has now been replaced with an invar one.  This niftier looking model ought to be more stable due to invar's much lower thermal expansion coefficient.

Once the SAC's back up on the telescope we'll need an accessible reference to check its overall alignment. The SAC reference mirror is set up to locate repeatably on the back of M4 & mark the optical axis.

In go the aperture stops

We hope to clean up some more stray light within the system by masking off shiny parts of M2 & M3 that do not form part of the mirrors' clear apertures.  The aperture stops are made of a non-reflective black material & those at the outer edges of the mirrors are attached via the SAC cage.

The M2 stop is the black ring just below the mirror in the next two images.  It is seen here from inside the SAC while M3 was out of the way.

& here it's viewed from below through the hole in M3.

M3 also has a similar ring just above its surface, seen here from underneath - blocking the gap between M3 (left) & the SAC cage (right).

The aperture stop for the inner part of M3 had to be attached directly to the optical surface.

Removing the big CGH

While the lucky ones escaped to San Diego for the SPIE conference, Francois & Ockert took M3 off again & removed the big CGH.

They even photographed the process!  My redundance is now painfully clear...

A great time for low-temperature testing

Now that winter's really setting in up here, it's easier to cool things down to test the SAC's performance at low temperature!  Not much fun for the poor buggers in the clean room, but misery & suffering's supposed to keep us grounded ;)

Encouragingly, the SAC behaved well over the range of operational angles & temperatures tested.  In the meantime, the SALT Ops guys developed a new discipline that will be presented along with Sutherland's bid to host the 2018 Winter Olympics...

M3, & hence The Whole SAC, is aligned!

The May 14 post described how to get the CGH aligned - the next step was to put M4 back in before tipping & tilting the CGH & decentring the interferometer to get the best fringes possible.

The actuator readings indicated where the CGH ended up after optimising the image quality and with the scheme below we could work out where M3 needed to be moved to.  The decentre's set by adjusting M3's tie-rods that attach to the mirror at 1, 5 & 9 o'clock while tilt's taken care of by placing shims (in the form of feeler-gauges) under the mirror's spherical bearings.

After each M3 adjustment, we again had to tweak the CGH & interferometer to get the best image quality & then decide what the next M3 move needed to be.  This seemingly endless iterative process was interspersed with regular re-alignments of the CGH (as described previously) & handfulls of rapidly greying hair were torn out along the way...  But eventually things did converge so - believe it or not, the SAC is now ALIGNED!!

Now we need to tip the whole contraption over & make sure it all still works at angle, then we'll also check it over a range of temperatures...