Sunday, March 3, 2013

SALTICAM Coating Damage

SALTICAM has been undergoing testing at low temperatures to verify the performance of the shutter & other mechanisms.  This has been done in an environmental test chamber at the SAAO in Cape Town, with the instrument variously orientated to simulate different operational rho angles. The instrument has to be warmed up before each angle change & so a safe warm-up & cool-down sequence is employed.  The optics are protected from condensation during this temperature cycling by having the whole instrument enclosed in a plastic bag which is fitted with a dry nitrogen purge.

SALTICAM in the environmental test chamber.

After lunch on Wednesday 27 February, the instrument was removed from the chamber after it had warmed back up to ambient temperature.  The plastic bag was apparently found to have come loose & condensation was seen on the front lens.  The lens cover was left off to allow the surface to dry.  Late Wednesday afternoon, the instrument was still positioned with the lens pointing downwards so one could not easily see into the barrel.  However, a photograph taken of the moisture on the front element revealed a much worse situation further along the optical train.  Click on the individual images to view an enlarged version.

Attempting to photograph the condensation on the front element (green haze in the upper right) revealed even worse condensation further down the barrel.

The next photograph shows the full extent of the condensation on one of the other optical surfaces, while the remaining mositure on the front element is visible in the upper left corner.

Massive condensation on one of the other optical surfaces.

The shutter was closed & the controls were not available, but since the cryostat is not in place it was possible to view the lenses of the rear barrel.  These did not appear to show any condensation.

The SALTICAM rear barrel lenses showed no condensation.  The leaves of the closed shutter can be seen  in the centre of the image.

From this we could infer that the fogged surface pictured above was that of the final element in the front barrel.  A schematic of the optical system is shown below, with the lens surfaces we believe to have been affected coloured red.  The outer surfaces of the rear barrel lenses are coloured orange since we suspect that condensation would have occurred on these as well, but might have dried earlier in the day due to their smaller size & more exposed position.

SALTICAM optical layout.  The surfaces known to have suffered condensation are marked in red while orange indicates those surfaces likely to also have been exposed to moisture earlier in the day.

The next morning it was possible to open the shutter & a preliminary check found all surfaces to be free of moisture, but in need of cleaning due to what looked like dust & water marks.

All lenses were free of moisture on the morning of 28 February, but various surfaces appeared "dirty".

However, closer inspection confirmed fears of substantial coating damage.  The first surface of the front barrel lens group is the only one that is easily accessible so it was studied carefully.  Flash photography focused on this surface produces "globular cluster" like images due to large amounts of scattered light.

Photograph of the front element taken with a flash.  Note the "globular cluster" above & to the right of centre.

The larger drops of water that were present on the front surface transported dust & have left more pronounced scars (bottom, centre).  Note in particular the different colour of the large spot near the bottom of the image.

Most of the condensation on the front surface was in the form of a fine mist, but some larger drops were present too & these have left more obvious residuals.

Close-up shots reveal structure all over the front surface.

Artefacts, not consistent with dust, are seen all over the front surface.

The upper left of the front surface suffered the worst condensation & now has a distinctly spotted appearance.

The part of the front lens that endured the worst moisture exposure.

Closer inspection showed sharply defined edges to these tiny patches that exhibit a range of colours.  This confirms that the Spectrum Thin Films multi-layer coating has been eroded - we are actually seeing the different layers of the coating.

Conclusive proof of coating damage on the front surface.

Photographed through a 4x magnification microscope objective, we see this more clearly.

Viewed through a microscope objective, the full extent of the damage becomes obvious.

While only part of the field is illuminated sufficiently in these images to make the effect visible, panning around with the microscope objective confirms that the whole surface is similarly affected.

Scanning around viewing the surface with a 4x magnification microscope objective shows the whole surface to be severely damaged (note all the detail in the shadowed region).  This is a screen capture from a long video clip taken while exploring the surface.

Attention was then turned to trying to assess the state of the other surfaces.  A piece of paper inserted near the position of the open shutter allowed the front & rear barrel lens groups to be viewed separately, but against a light background.  The next image was taken looking down the front barrel, with the camera focused on some dirt on the back surface of the front barrel lens group.  The reflection from this surface shows the same globular cluster type structure from scattered light that was seen with the front surface, while those from the other (internal) front barrel surfaces (upper right) do not.

The rear surface of the front barrel lens group shows the same scattered light features as were seen on the front surface (which has confirmed coating damage).

Here is a further demonstration of scattered light arising from coating damage on the rear surface of the front barrel lens group.  Note the halo of spots around the top reflection (left of centre).

Reflections from the various surfaces in the front barrel lens group.  The back surface produces a lot of scattered light, as was seen during inspection of the front element of this group of lenses.

Inspection of the rear barrel lenses is more difficult due to limited access, but we expect that those surfaces were exposed to condensation too & will thus be compromised as well.  So far we do not have direct proof of damage to the rear barrel lens group.  However, suspicious-looking dark spots & patches near the bottom of the lenses may be an indication of coating failure as the alternative explanation would be a large amount of dirt, which seems unlikely.

View of the rear barrel lenses showing some dark spots & patches which may be due to dirt, or coating damage.

This report serves to document the situation as it stands; further discussion will follow regarding how to proceed.

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