Friday, April 26, 2013

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.

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