Monday, March 11, 2013

Further Inspection of the SALTICAM Lenses

A Taylor Hobson alignment telescope was used to further investigate the damage to the anti-reflection coatings on the SALTICAM lenses.  By adjusting the alignment telescope focus, one can sequentially isolate individual lens surfaces for inspection.  Illumination was usually provided by a single white LED.  The straight black lines in the images below are from the cross-hair of the alignment telescope.  Clicking on the individual images will display enlarged versions & reveal more detail.

Here we see a small central section of the first surface of the front barrel lens group.  The sharp, irregularly shaped bright flecks mark where the coating has been eroded.  The diffuse features are due to the white light source used to illuminate the system glancing off the various other optical surfaces.

Coating damage on the front surface of the front barrel lens group.

This is the back surface of the front barrel lens group (which endured the worst condensation), seen through the alignment telescope.  The blue region is the reflection of the light source off this surface - the array of spots in the foreground indicates the damage to the coating.

Extensive coating erosion on the rear surface of the front barrel lens group.

The features seen all over this surface are identical to the worst damage seen on the first surface of the front barrel group (see previous blog post).  The flakes show the same irregular shapes & colours due to partial removal of the multi-layer coating.

Close-up of the extensive damage to the back surface of the front barrel lens group.

The back panels of the instrument were removed to permit access to the rear barrel lens group & a piece of paper was again inserted between the front & rear lens barrels.  The alignment telescope was moved to the other side of the lenses & focused on the flat final surface of the rear barrel lens group.  Coating damage was again evident on this surface - note the various small spots & the large dark patch to the upper right of the green disk.

Confirmation of coating damage on the back surface of the rear barrel lens group.

The alignment telescope was then focused on the front surface of the rear barrel lens group (the surface closest to the shutter) & this too showed the distinctive signs of coating failure.  Here we illuminated the lens surface with 2 white LEDs, hence the 2 bright regions.  The circular structures in these patches are related to the front element of the alignment telescope.  Of relevance are the fine spots that reveal pitting of the coating on this surface.

The coating on the front surface of the rear barrel lens group has also been degraded by exposure to moisture.

Here is a close-up view of the front surface of the rear barrel lens group, displaying the now-characteristic moisture-induced coating damage.  The green background is the reflection of the white LED from this surface.

Detail of coating degradation on the front surface of the rear barrel lens group.
 
The piece of paper between the front & rear barrels was removed & this photograph was taken looking through the rear barrel lenses first.  The alignment telescope, aimed at the front barrel lens group, can be seen to the left of centre.  As noted previously, the rear surface of the front barrel lens group generates a great deal of scattered light. 

Reverse view through the SALTICAM optics (from the rear barrel side).

Although the camera flash & the light source used to illuminate the lenses are both white, we noted an alarming range of colours in the reflections arising from the different optical surfaces.

 Reflections from the various surfaces do not match the colour of the incident (white) light.

This was also noticeable when viewing the various surfaces through the alignment telescope.  The effect is easier to see in out of focus images. 

Note the range of colours in the spots produced by reflections off the lens surfaces.

Here we view a flashlight consisting of 4 white LEDs through all the lenses.  The back surface of the front barrel lens group produces an array of small spots over the entire field, but the green & yellow patches reveal that the coatings have also de-natured in a more global way.

The optics now have a greenish cast & reflections off the individual surfaces produce a range of colours.

Shining a bright white light through all the lenses demonstrates the impact of the increased scattering, as well as the colour-dependence of the reflections off the various surfaces. 

The combined effects of the coating damage on white light passing through the SALTICAM optics.

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