Wednesday, March 23, 2011

Pins, pellicles & puzzles

Another busy day over here...  Ant continuing to work on his RSS software - a process known as ADT: Anthony's Destructive Testing.

Janus installing the fibre-fed guidence & focus cameras.

Charl still engaged in the endless task of wiring up the contents of the rotating structure.

Darragh & Francois aligning SALTICAM while Peter & Vic (partly visible underneath RSS) contemplate a future with their treasured spectrograph back up on the telescope.

Up on the tracker, Eben, Johan & Jonathan set about replacing the fork pins that connect the pairs of hexapod legs.  A stress analysis indicated that the pins would be overloaded once everything's back up on the tracker.

Struts were put in place to support the load while each pair of pins was removed in turn & replaced.

The old pins (left) were made of 340 steel & the new ones (right) are made of EN 19 steel, which has a much greater yield strength.

Here they are, in position (before & after) - connecting the hexapod forks.

The SALTICAM alignment effort was facilitated by the introduction of a neutral density filter for the laser.  In the characteristic spirit of ingenuity, this took the form of a piece of the shiny material that teabags are packaged in.

Nice head-gear Darragh - it's sure to be all the rage in Europe next season!

Later it was time for another scary optic to make its appearance - the dreaded pellicle!  This is a 5 micron thick(!) film of nitrocellulose which acts as a beamsplitter.  To put that in perspective, typical human hair's about 100 microns in diameter...

Here's 95% of Janus - the other 5% is being sent off in another direction...

Being so fragile, it's a nightmare to handle.  But the advantage of a pellicle is that because it's so thin, it doesn't produce ghost images - reflections off the second surface coincide with the original beam.

Janus successfully installed the pellicle, which will allow SALTICAM to be used for guiding, or even limited imaging, while RSS is in use.  It will generally provide additional flexibility within the payload, by enabling some light to be fed into a second instrument while one is in use.

Next, it was time to set things up to enable SALTICAM to view the RSS slit plane, such that the image will end up in a convenient position on the CCD & in focus.  

Fortunately this didn't require any adjustment of the 4 mirrors that make up the Offner relay - the system that bounces light from the slit plane, down through the payload & into SALTICAM.

One of the enduring puzzles in this telescope is why when one photographs the primary mirror with a flash (bottom), you get a double image of the segments.  Look closely to see how the reflections of the structure seem to lie between these two "layers".  Very odd!

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