Cleaning the 12" F/15 Brashear Refractor at the University of
Illinois at Urbana-Champaign
by Mike Lockwood
Color pictures by Mike Lockwood
Black and white pictures by Dick Wessling
Since John Dickel had been the advisor to the Astronomical Society at the
University of Illinois, no one could recall the objective of the
12" refractor, housed in an observatory building
on campus that is designated as a national historic landmark, having
been cleaned.
It's not like we hadn't tried to do it, but arranging to clean such a
precious lens is not an easy task. It takes the right people to
arrange and do it, good timing to avoid affecting astronomy classes and
other usage of the telescope, and agreement from the department.
Over the years, the club has become the caretaking organization for the
antique telescope, doing some maintenance, reporting problems, and
teaching its members to use the telescope properly. We noticed
years ago that the lens was VERY dirty, and recent misguided attempts
at cleaning it had been attempted by someone, probably just using their
hand to wipe the front surface clearer of dust. No damage had
been done yet, but it was only a matter of time before someone got out
the bottle of windex and a dirty T-shirt. (Not the proper
cleaning method.)
Below are a few pictures of the dirty lens. The first picture
shows the significant layer of dirt on the outside of the lens.
The second shows the inner surface, which had some sort of deposit on
it. We hoped it could be removed safely, not knowing what it was.
A fortuitous conversation took place between myself and Prof. Dickel at
a Christmas party in late 2004. I mentioned that I had a friend,
Dick Wessling, who had done this sort of work before on even older
telescopes (most notably the Merz refractor, circa 1845, located at the
Cincinnati Observatory Center), and who worked with multi-element
lenses in an industrial
setting every day. John's ears perked up.
John had been around the astronomy department for a long time - long
enough that he was planning to retire in mid-2005. With that date
looming in the future, and the need for his help in getting everything
done, we set about to schedule a date for the
cleaning. John handled the paperwork, red tape, convincing of the
department, and logistics of hiring Dick to come and clean the
lens. I helped everyone communicate, made preparations to remove
the lens, and checked to make sure we could get the lens out of the
cell.
Cleaning day was May 21, 2005. Classes were over, and no events
were planned. John was leaving for New Mexico three weeks later.
Dick and his friend and coworker Marilyn arrived after lunch, and we
set up shop in a lab-type room in the Astronomy Building. The
lens had already been removed by John, Leslie Looney (the new club
advisor), and myself. Here are two photos of the removal.
In the first, note the pile of equipment placed under the secured,
inverted telescope tube. In the second, John Dickel is on the
left, and Leslie Looney on the right, both happy that the lens was
safely and easily removed. They then took the lens to the
astronomy building where we had lab space to do the cleaning.
The first step in the cleaning was to check the alignment of the
elements. The air gap between the objectives should be the same
around the edge of the lens, so that the lenses are aligned with each
other and centered. Monochromatic light was used to view
interference fringes from directly above the lens, and these indicated
good alignment.
The second step in the cleaning process was to remove the retaining
ring from the lens cell. The screws securing it were removed with
only a little difficulty due to the shallow slot that was meant for a
straight screwdriver blade. The ring was then carefully pulled
out. Here are pictures of the lens, in the cell but with the
retaining ring removed (left photo), and the cell alone (lens removed,
right photo).
The lens was removed from the cell by lowering the lens/cell assembly
onto a towel-covered, inverted bucket. The lens is supported by
the bucket, and the cell can be dropped down around the base of the
bucket. With a little careful encouragement, the cell was slipped
down, and the lens was left sitting on top of the bucket. The
tension level in the room plummetted as we admired the ~110 year-old
pieces of glass, free of their cell.
Next, the lens was indexed, providing reference alignment marks for the
two elements and the cell. This was done with a permanent marker
on the edges of the elements, and inside the cell. Here are
photos of the lens and index marks, and John Dickel admiring the lens.
The elements were carefully separated, and the locations of the shims,
which maintain the small air gap between the elements, were
marked. The shims were measured for thickness with a digital
caliper, and turned out to
be about 100 microns thick. They were in bad shape, so new shims
were built up from spacing material.
The separated lens elements were each cleaned front and back.
Dick used a cleaning solution called "Buckeye Star Spray" and
chem-wipes to carefully clean the surface in stages, using each part of
the chem-wipe only once. Here's a
picture of the bottle of cleaner (in case someone wants to get some),
and Dick Wessling working on the upper element.
The front of the front element was covered in dust, which came off
easily. The back of the back element was covered in some sort of
residue that also came off surprisingly easily with the above
cleaner. The two inner surfaces had more dust between them than
we expected, but also cleaned easily. Overall, we were fairly
lucky, with no significant etching on any of the surfaces.
The cleanliness of the elements was checked with a high-intensity
light. Here are a couple of photos showing the cleaning process
being concluded for one of the elements. Some residue is still
present on the glass in the first picture, possibly fingerprints from
handling. Marilyn is helping
hold the lens in the second picture. This sort of work can really
be a two-person job.
Here is the wastebasked full of dirty Chem-wipes after the dirty work
is done, and one of the new shims now stuck to the bottom
element. Note the nice bevel on the glass. Ignore the funny
spot on the towel!
With the elements clean and reassembled on top of the bucket, Dick and
I carefully lifted the cell back up to them, and slid it over
them. We removed the assembly from the bucket, and checked the
element alignment with the monochromatic light source. After
replacing a couple shims, the lenses were once again parallel.
Here's a photo of the interference fringes, shot from off to the side
of the lens. Viewing from directly overhead showed fringes
centered in the lenses, indicating good alignment. Parallax
errors due to the camera position make the fringes appear off center in
the photo. A faint "bullseye" is visible in the center of the
photo, but decentered in the lens.
Finally, we reinstalled the retaining ring in the cell to secure the
lens. Cleaning was concluded in about two and a half hours.
At about 4pm we transported the clean
objective lens to the observatory. Reinstallation was
quick. John backed his car up to the door, we carefully lugged
the box containing the cell and lens up
the stairs, and in about 5 minutes the lens was back in. Pictured
below are the unloading (that's John and I carrying the box up the
stairs), the lens positioned just under the tube, ready
to be bolted back on, and John and I posing with the telescope,
complete with installed objective.
Everyone thanked each other, and Dick, Marilyn and I headed off to an
early dinner. We then drove to Charleston to visit some other
telescope-building friends. It was a day full of optics and
optical talk - lots of fun.
On a Thursday evening a week or two after the cleaning, John and I met
to recollimate the lens. It only took about 10 minutes - John
manned the RA "ship's wheel", slewing the telescope from the viewing
position to a position where I could reach the adjustments for the lens
tilt. I tweaked the adjustment bolts, and then the scope was
moved back to the viewing position and we checked progress. After
three iterations or so, the reflections of my
flashlight in the focuser were centered, and stars were round again.
We spent the next hour or so viewing Jupiter and discussing telescopes
and careers. Jupiter looked excellent, with fine details showing
when the dome seeing settled. (The observatory is a masonry
structure, so heat from the building often limits observations.)
We ventured onto the balcony to view the blindingly
bright lights of the baseball field, which cast a shadow on the inside
of the dome up high. Undeterred by the light pollution, we swung
the scope over to M13, and we
enjoyed a wonderful view. It was a great way to spend the
evening, in the company of someone who has done so much for astronomy
at the University of Illinois in so many ways.
John, we'll miss you. We hope you have a wonderful time in New
Mexico. While we look forward to having such an energetic and
enthusiastic advisor as Prof. Looney, we will miss you every time we
use the 12" scope.
I have already threatened to show up on John's doorstep in New Mexico
with a couple of friends and a 30"
telescope in tow.
On behalf of the Astronomical Society at the University of Illinois,
thanks for everything, John.
Mike Lockwood