Further Cleaning of the SALTICAM Optics

The successful cleaning of SALTICAM's front lens prompted us to apply the same techniques to the other lens surfaces that suffered coating damage.  Once the cryostat is removed, the back surface of the rear barrel lens group is relatively accessible.  Other than having to avoid the pick-off prisms for the auto-guider, the cleaning of this surface was a fairly simple matter & the results were encouraging.

The rear surface of the rear barrel lens group was relatively easy to clean.

The remaining 2 surfaces were extremely difficult to access as the gap between them, where the shutter & filter gripper mechanisms fit in, is only ~35 mm.  The image below shows the space between these 2 lenses.

The 35 mm gap between the front (lower) & rear (upper) barrel lens groups, viewed from above with the shutter & filter gripper mechanisms removed.

The concave rear lens surface of the front barrel lens group suffered the worst coating damage & thus had plenty of residue deposited on the surface.

The concave rear surface of the front barrel lens group that suffered the worst condensation & hence coating damage.

The convex front lens of the rear barrel lens group is made of thermally sensitive calcium fluoride.  This surface was heavily contaminated with what appeared to be a dark, dusty material.  An additional concern about cleaning this surface stemmed from the fact that uncoated calcium fluoride may be eroded by water when subject to prolonged exposure.  The surface does have a Spectrum Thin Films coating, but this was compromised by the condensation event & so we were weary of using de-ionised water on this lens..

The convex calcium fluoride front surface of the rear barrel lens group which was heavily contaminated with "dust" from mechanical wear of the shutter mechanism.

Given the limited space between the 2 lenses, we planned to use a shaped wad of cotton wool to reach down to first clean the concave surface at the back of the front barrel.

Heroes of the day: Darragh & John (Booth) preparing to take on the impossible pair of lens surfaces.

This was soon found to be impractical, but fortunately John Booth's hands proved to be ideally suited to the task.  With a protective sheet of thin card taped over the convex lens, he was able to reach into the narrow gap, with a damp wad of lint-free cloth, to clean the concave surface.

The essential combination of long, thin fingers, an extremely flat hand that could somehow reach down within the narrow gap between the 2 lens groups & a contortionist's dexterity to be able to sweep around the whole lens surface.

Several passes were required to cover the whole lens to remove all of the dirt - including a number of dark spots visible in the image below.

Numerous dark spots, due to the accumulation of coating residue, were visible on the back surface of the front barrel lens group & were successfully removed by the cleaning process.

As with the front lens surface, an acetone/isopropanol solution was applied first, followed by de-ionised water which removed any residues from the coating damage. 

The de-ionised water that so effectively removed coating residue from the surfaces also left substantial drops of water which needed to be dried.

Finally, the lens was dried with more of the acetone/isopropanol solution.

The acetone/isopropanol solution readily removed the drops of moisture left by wiping the surface with de-ionised water.

This process significantly improved the condition of the back surface of the front barrel lens group.  However, looking through the lenses at this stage revealed just how badly contaminated the front lens of the rear barrel lens group was.

With 3 of the 4 damaged surfaces cleaned, it was apparent that the fourth badly needed cleaning as well, even though we were reluctant to tackle the calcium fluoride element.

A protective card was taped over the clean concave lens &, as before, the surrounding lens cell was cleaned with the acetone/isopropanol solution prior to tackling the cleaning of the lens itself.  Huge amounts of fine black dust were removed from the cell & surrounds.

The area surrounding the front lens of the rear barrel lens group was heavily coated with fine, black dust liberated by the shutter mechanism.

This material is believed to be from mechanical wear on the shutter mechanism which is situated closest to this lens surface. 

Cleaning the front surface of the rear barrel lens group was an extremely delicate process.

The cleaning process for this surface was slow & particularly careful so as to remove all of this fine material without producing scratches.  Given the concerns about the damaged coating on the calcium fluoride surface, we decided not to apply de-ionised water, but only used the acetone/isopropanol solution.  Most of the contamination on this surface was in the form of "shutter dust" & so the strategy proved highly successful, as evidenced by the images below.

Looking backwards through all of the lenses after the damaged surfaces had been cleaned (front lens cap on).

An encouraging reverse view through all of the lenses after they had been cleaned (ambient light).

Another post-cleaning reverse view, lens cap removed & illuminated with a LED flashlight + ambient light.

Yet another reverse-view, illuminated with a LED flashlight but no ambient light.

Front view with the optics indirectly illuminated from behind with a LED flashlight.

Illuminated with both a LED flashlight & a camera's (red) autofocus-assist lamp, we see the various surfaces of the SALTICAM lenses.  The coating damage on the rear surface of the front barrel lens group is clearly visible around the red spot to the upper left of the image.

Post-cleaning front-view, illuminated with a LED flashlight & a digital camera's (red) auto-focus assist lamp.

Lastly, we also cleaned the dewar window in front of the CCDs using the combination of the acetone/isopropanol solution & de-ionised water.

Dirty cryostat window prior to cleaning.

All residues on this surface - including fine condensation marks near the centre of the window, were removed by the cleaning process.

Cryostat window after cleaning with the combination of acetone/isopropanol & de-ionised water.

All of the above was done on Tuesday 23 April 2013.

Cleaning SALTICAM's Front Lens

On Monday 15 April, we attempted to clean SALTICAM's front lens.  In order to access the lens edge, we had to remove the retaining ring that secures the front lens barrel in place.  Although the instrument remained level throughout the process, we used screws with spacers & washers to ensure that the lenses could not move - thanks Craig!

Washers used to clamp the front barrel in position after the retaining ring was removed.

A section of 1.5 mm thick o-ring cord was used to seal the gap exposed by the removal of the retaining ring as we did not want any moisture from the cleaning process to get into the lens barrel.

Removing the outer retaining ring left a narrow space that could have accumulated cleaning fluids so a piece of o-ring cord was used to seal the gap.

We inspected the lens & took photos of the worst of the spots left by the large drops of water that dried on the surface.  The colour & texture of the spots is believed to be due to residue from the damaged coating: material that was stripped from elsewhere on the surface & then re-deposited on the lens after the mositure evaporated.  Please click on the photos to view enlarged versions.

Two of the worst spots on the front lens surface - sites of large-scale deposition of the eroded coating material.

A group of 4 large spots (the biggest being ~3 mm long & distinctly green, high-lighted in red in the image below) was noted above & to the left of the centre of the lens.  Further to the upper left one can see the section of the lens with the most severe coating damage (high-lighted in blue).

A group of spots below the worst of the coating damage on the front lens.  Note the greenish colour of the largest drop.

We planned to follow Chris Clemens' recommendation of using a mixture of isopropyl alcohol & acetone to clean the lens, before applying the highest grade of de-ionised water (with a resistivity of at least 18 megaohm.cm) to try to remove residues from the damaged coating.  Before tackling the lens, we mixed a 50/50 isopropanol/acetone solution & tested it on one half of a clean test mirror, to check if it left any streaks or marks - it did not.  We also tested a 50/50 mixture of ethanol & ether on the other half of the mirror, but found that it did not work quite as well.  Thus we proceeded with the isopropanol/acetone solution.  

At first, we only used a small amount of extremely pure cotton wool, wound around the end of a long cotton swab & saturated with the isopropanol/acetone solution, to try to remove one of the water marks near the bottom edge of the lens.  Each swab was rotated while being lightly dragged over the surface (before being discarded & replaced with another).  Although this did successfully remove dust from the lens, the spot remained unaffected.

The region at the edge of the lens that we initially cleaned can be seen just to the right of the lower set of bright reflections. The faint spot in this region was immune to the isopropanol/acetone solution.

Disappointing as this was, we agreed to try the de-ionised water on the same area to see what effect it might have.  The same treatment led to some reduction in the spot & so we applied more pressure & found that the spot was further reduced & eventually removed completely.  We repeated this process on 2 more spots in the same region (radially inward, between the 2 bright reflections in the image above) & convinced ourselves that the technique was indeed working & that it should be applied to the entire lens.

Deciding how to go about cleaning the whole lens after the successful removal of a few spots near the edge by lightly rubbing with cotton wool soaked in de-ionised water. Thanks to Ockert for grabbing the camera while the usual photographer's hands were full!

Before doing so, the edge of the lens barrel, the orange elastomer that bonds the lens in place & all the surrounding areas were carefully cleaned with more isopropanol/acetone swabs, to avoid dragging contamination from there onto the lens.  Large wads of cotton wool were then rolled around the full length of the swabs & soaked with the cleaning solution before the stick was cracked to produce bow-shaped lens wipers.

Francois rolling & soaking the wads of cotton wool for each wipe.

These could not be rotated, but were gently dragged down over the entire lens before being discarded.

The process of dragging a wad of cotton wool (soaked with an isopropanol/acetone solution) over the entire lens surface.  This was repeated several times before replacing the solution with de-ionised water & then going back to the original solution to dry any remaining moisture on the lens.

This was done a number of times with the cleaning mixture as it initally produced a lot of marks at the bottom of the lens.  These were removed on subsequent passes, but as previously - the original spots remained.

Early passes with the isopropanol/acetone solution left a lot of residue at the bottom of the lens.

The procedure was then repeated a few times with wads of cotton wool soaked with de-ionised water.  More pressure was applied when dragging these over the lens surface & then a couple more isopropanol/acetone swabs were used to dry the water that had remained.  The image below shows the substantially cleaner lens with all of the large spots removed, but of course the speckling due to the coating damage remains.

The same area as shown above, after wiping with de-ionised water & then again with the isopropanol/acetone solution.  The fine speckles that remain are from the pitted AR coating producing a lot of scattered light.

The photos taken before, during & after the cleaning process are repeated below to facilitate comparison.

Progress during the cleaning process is shown from left to right in this panel.

Below are multiple views of the cleaned lens.  Many of the fine specks are on the inside of the lens, but most are due to the condensation-induced coating damage.

Damage to the coating is clearly visible in this image, but note that the large spots have been removed.

A close-up view for comparison with the pre-cleaning images.

Another close-up showing the scattering due to the coating damage.

A side-on view of the lens illuminated from the opposite edge.

This image shows a green laser shone into the front barrel lens group, striking a piece of lens tissue positioned behind the back element of the group.

Although access to the other damaged lens surfaces is considerably more limited, it might be worth trying to clean them in a similar way before the instrument goes back up to SALT.

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.