M4-M5 aligned & tested!!

Darragh's blasted through the M4-M5 alignment process during the past few days!  X & Y scans (as described in the November 29 post) to sort out the M4 decentre...

& the image-distance vs object-distance scans (described in the December 8 post) to get the axial spacing of the pair right.  Squares indicating the actual measurements are shown along with the theoretical curves in the plot below.  Note the excellent agreement between the large red squares & the central dark blue curve that corresponds to the effective focal length of the pair in the nominal optical prescription of the telescope.

 

After a set of M4 Faro measurements, it was time to remove & replace the steering wheel to check the repeatability of the M4 mount. 

 

While M4 was out of the way, we Faro'd M5 & the pinhole again & confirmed that they're still where they should be.  Then put M4 back, Faro'd it again & found the repeatability to be extremely good - of order 10 microns in decentre & about 35 microns in despace!

 

At this stage we were ready to test the pair at angle.  A wavefront RMS of 0.036 waves was obtained with the SAC vertical.

 

Then it was tilted over to 37 degrees, M4 Faro'd & wavefronts measured. 

 

Only tiny changes in the pinhole position (indicating mirror movements of about 10 microns, in agreement with the Faro readings) were needed to get back to a RMS of 0.038 waves :)  We obtained equally good results for the SAC at the other extremes of its operational envelope (tipped over to + & - 6 degrees in X & from 31 to 43 degrees in Y).

 

Lastly, it was time to cycle the temperature in the clean room - down to about 11 C & back up to 23 C, making wavefront measurements with the SAC vertical & tilted to 43 degrees at various intervals during the temperature change.  This all looks great too!

So the pair's happy at angle & over a range in temperature!  With that - we're out of here, for nearly a month :)  The team members will attempt to unwind & possibly even re-establish some semblance of their pre-clean room lives...  Enjoy the Holidays everyone!

Keeping it tidy...

After getting the pinhole into the right position with the Faro, it was time to put M4 back in.  Obviously we're extra keen to keep M5 clean now so some new measures have been introduced.  First of all: a ring of soft plastic out beyond M4 - the melted cheese that covers the pizza.

Then a hard plastic lid that fits over M4.  We should point out that, despite the new SAC colour scheme, the IQ Team remains strictly apolitical.

 

We decided a major clean room tidy-up was long overdue so we charged around like Tazzie Devils for a while, getting everything cleaned up & squared away before putting the super-structure back up.  By lunch time no one would've guessed that anything much had happened in here over the past few days!

 

Came across this guy outside the hostel earlier in the week...  It was about 10 cm long & moved surprisingly fast - a really impressive specimen!

Fortunately it wasn't in the clean room so it didn't have to put on a little blue hat & 4 pairs of shoes :)

 

M5 back in the SAC

A spring clean of the SAC & surrounds last night, a few more photos this morning & then M5 had to go back where it belongs...

A fluorescent light directed up through the central hole reveals the worst of the pinholes (original coating defects) & makes the Zerodur (the low thermal expansion glass-ceramic that all the SAC mirrors are made of) glow spectacularly!  The image on the right was illuminated with the camera flash.

Then the covers were put on & the removal (replacing?) jig slid into place underneath the mirror.

The lifting frame was attached & the tie-rods secured.

Stand-in photographer Nic took over the D200 while the whole lot was lifted & carried across to the scaffolding.

 

Then handed to the guys up there who shuffled it back over to the SAC.

Safely hurdling over the Faro!

 

 

& then lowered back into place.

 

The lifting gear could then be removed & the tie-rods re-attached to the cage.

 

Lastly, the pizza pan was reinstalled & the mirror cover retracted to reveal the zippy new-look M5 - WOW!! 

 

So, did you do anything exciting this weekend? :)

M5 gets a wash!

Most of the comparative shots showing the results of various actions are grouped towards the end of this post to make it easier to assess the effects of each stage in the cleaning process.  In the meantime, here's the story as it unfolded.

The dirty beast was carefully wheeled to the wash bay...

At some stage there was a glycol spill in the payload & a few drops made it onto M5.  These spots were really bad so we didn't particularly rate our chances of removing them.  The images below show that part of the mirror in the normal fluorescent light of the clean room (left) & illuminated with a LED flash-light (right).

Clean, dry compressed air was blown over a quadrant of the mirror to see what effect that would have on all the loose particles on the surface.  The effect was noticeable, but not nearly enough.

 

Next, that quadrant was rinsed with de-ionised water for a few minutes.

 

& that was followed by a distilled water rinse for another few minutes.

Drops that remained on the surface were then herded either down the central hole, or over the outer edge with the clean, dry compressed air.

The result of the rinsing was encouraging - the images below compare the dirty side with the rinsed quadrant (top & bottom frames respectively).

 

However, closer inspection with the brutal LED flash-light showed that although the rinsing had removed a lot of the dirt, the process had also produced awful streaks...

 

Further rinsing efforts made no significant improvement so it was time to haul out the horse soap (see yesterday's post for more about sodium lauryl sulphate).  Since the outermost 20 mm of the mirror isn't used, we opted to try the soapy cotton wool on a section of the edge first.  The soap was left to run down the rest of the mirror in that sector & rinsed, as before, down the central hole.

 

The effect was dramatic & so the rest of the edge was given the cotton wool treatment.  The whole mirror was kept wet with the soapy solution while the edge was being cleaned, then the usual rinsing & drying processes followed.

It was a team effort with Jonathan keeping the mirror wet, Francois providing the soaked wads of cotton wool & Hitesh dragging the cotton wool.

 

Not to mention the Irish barman in the back, shaking up his legendary sodium lauryl sulphate cocktails...

The edge had clearly benefited greatly from the cotton wool cleaning so the next step was to do the same for the rest of the mirror.  Larger pieces of cotton wool were used for this, dragged uphill, 2 passes per strip.

 

Yet more rinsing (& drying) after all that...

 

 

& finally - a beautifully clean M5 :)  Note that the white spot in the image below's Not glycol related, it's actually light from the LED flash-light bouncing off M5 & onto the plastic sheeting that covers the roof & walls of the wash bay!

 

One of the major highlights of the day was emerging from the clean room for a much needed break & finding that a batch of Hannah's outrageously wicked chocolate brownies had been delivered to the SALT kitchen!  Thank You Thank You Thank You :) :) :)

The next image shows the results of the main steps in the cleaning process, from left to right & top to bottom:

- The filthy start.

- Dust blown off the top right quadrant with clean compressed air.

- Right half rinsed with de-ionised & distilled water, dried with clean compressed air.

- Cotton wool soaked in sodium lauryl sulphate dragged along a sector of the outer edge & the soap allowed to run down in that sector, then rinsed & dried as before.

- Whole outer edge treated as in the previous step.

- As above, but the entire mirror cleaned with cotton wool soaked in sodium lauryl sulphate.

 

Before & after (left & right), illuminated with a LED flash-light & the camera flash (top & bottom).  Spots on the dirty mirror are from Glycol while those on the clean mirror are reflections off the plastic sheets that line the wash bay.

 

More than 500 photos were taken in here today...  A thoroughly amazing day & a superb job by all involved!

Extracting M5!

It's the scary weekend...  M5's coming out of the SAC for a much needed clean!

James designed a nifty removal jig & a dummy M5 - the same 18 kg mass & overall dimensions - was made up out of aluminium to allow us to practise the removal & replacement procedure.

The mirror was covered, the removal jig + lifting frame installed in the SAC & the M5 tie rods undone.

 

With scaffolding in place on either side of the SAC, the lifters could pick up M5, clear the Faro & shuffle across to the hand-over position where the carriers were waiting.

 

We then ferried the precious cargo over to the wash trolley waiting nearby.

 

A gentle touch-down & breathing could resume...  Thanks to substitute-photographer Hannah!

 

After dismantling the removal jig & taking the covers off, the full horror of the dirt was revealed!

 

So we're now down to a 2 mirror corrector!  Surely this must make things simpler?

 

With M5 safely down on the wash trolley, we could turn our attention to trying out the cleaning process on one of the witness samples.  This small specimen has the same coating as the SAC mirrors & has been living underneath the primary mirror truss since the SAC was installed in 2004.  As a result, it's been exposed to much the same conditions as the upward-looking mirrors in the corrector (M5 & M3).

 

The first step was simply to rinse the sample with de-ionised water & then with distilled water for a few minutes.  The distilled water has a distinctly different character to either ordinary water, or even the de-ionised water - just magic stuff!

The rinsing process removed most of the dirt, but the surface remained somewhat dull.

The next stage was to soak unbleached, lint-free cotton wool in a 10 grams/litre solution of sodium lauryl sulphate & distilled water.  This mild soap's used on horses, & increasingly in dealing with dirty telescope mirrors!  The cotton wool's simply dragged over the surface, no extra pressure's applied other than the weight of the cotton wool.

This really did the trick - the surface looks as good as new!