Wednesday, April 17, 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.

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