One of the more infrequent but necessary jobs undertaken at a large observatory. A cross between
heavy engineering (moving heavy weights around) and precision instrument work making sure that the
mirrors go back to exactly the right position and that everything works correctly afterwards.
Usually this is done at the onset of winter. It generally takes 3 days to remove a mirror, strip
off the old coating, re-aluminise it and re-fit the mirror and its cell back on the telescope. Although
the re-coating process only takes a few hours (with most of that time being taken to produce the high
vacuum in the aluminising tank) the mirror is left overnight in the tank for the new coating to harden.
It is only a few microns thick and very delicate when fresh.
It was once common practice to re-aluminise the mirrors annually, but to keep the telescope downtime
to a minimum, in recent years other methods have been adopted such as snow cleaning (with CO
2) or washing at regular intervals. The mirrors are only re-aluminised if the coating has
deteriorated to the point where cleaning using the methods stated above are ineffective.
The 1 metre JKT mirror
Photos taken in 1987
This shows the 1 metre mirror after aluminisation and being craned out of its protective crate. A
special plug is fitted into the central aperture which must lock into a keyway in the mirror before
it is raised. This system is used on all the ING telescope primary mirrors and is crucial for a safe
lift. The mirrors are to all intents irreplaceable so extreme care is always necessary
when handling them.
The mirror is now being lowered into its cell. This is a very critical part of the operation as it
must seat perfectly. To ensure that its does, dial gauges mounted on the cell measure both the radial
and axial alignment of the mirror and the readings must be the same as those recorded before the
mirror was removed. On all the ING telescopes, the mirror rests on concentric rings of pneumatic
pressure pads which are controlled through load cells and a servo system. This ensures that the
optical axis of the mirror remains constant no matter in what position the telescope is working at.
Seen in the photo from L to R: Amado Guillen, Ken Maris, Andy Inchley, Vincente Reyes
and Servando Rodriguez.
Ken Maris and Nick Ferneyhough making final checks before winching the mirror cell
back into place
One of the three load cells which measure the compressive/tensive forces of the mirror can be seen
in this photo. To the right of the load cell is one of the three radial position defining rods which
prevents the mirror from twisting in the cell.
The 2.5 metre INT mirror
Photos taken in 1986
These series of photos are quite historic as the aluminising tank shown (the original
from the RGO at Herstmonceux) was decommissioned in the late 1980's and all aluminising operations
since have been carried out using the aluminising plant in the WHT. From what I remember this tank
took many hours to evacuate and the WHT equipment it is much quicker.
Another reason why large telescopes have a central hole in the primary mirror!
One of my favourite photos showing Roy Wallis drying off the mirror after its been washed with
distilled water. The old aluminium coating was first removed using caustic soda. Its
imperative that the mirror is perfectly clean and dry before it goes into the aluminising tank.
The mirror now suspended on the dome crane is being lowered into the tank.
With the mirror now secured in the tank, Roy brings it down to the vertical position to allow the
other half of the tank (which ran on rail tracks) to be connected. The many hours of waiting for
the required vacuum pressure to be reached then started.
The next day - With the halves of the tank separated, the freshly aluminised mirror now
awaits to be hoisted back up to the observing floor. Spot the difference!
A view up through the building with the top floor hatches open as the dome hoist is lowered.
The mirror now on its way up. Photo taken (very carefully!) from the balcony on the second floor.
The mirror is now above the observing floor hatches. Once these are closed, the mirror cell handling
trolley moves out on rail tracks from below the telescope and is positioned under the mirror. Using
the dome crane and small movements to the handling trolley, the mirror is aligned back into the
The brackets around the circumference of the mirror must line up perfectly with the
radial defining arms mounted on the mirror cell.
The 4.2 metre WHT mirror
Photos taken around 1989
As the WHT is the largest of the ING telescopes, re-aluminising the mirror is
a very impressive (and at times a stressful) operation. Before the mirror cell can be removed
from the telescope, a large handling trolley is craned up to Cassegrain access platform, but first
a section of the platform is removed to make enough clearance for the trolley (and the mirror) to be
raised or lowered to the ground floor. The working space is very tight. The mirror weighs 16 and
its cell 21 metric tonnes.
The aluminising process
When the mirror has been released from its cell, it is lowered to the ground floor and sits
on three pedestals. The old aluminium coating is removed using caustic soda and the mirror hosed
down. It is then given a final wash with distilled water and dried to ensure that no contaminents
The WHT aluminising tank consists of two sections. The lower section holds the mirror and can
move out on rail tracks into the drum of the building. The upper section is fixed but can be driven
upwards to allow for tank separation. One of the first jobs is to prepare the upper section of the
tank by placing pure aluminium wire loops on to a circular busbar. When the mirror is clean, it is
craned into the lower section of the tank which is driven back into the aluminising area and the tank
sealed. Powerful roughing pumps begin the evacuation process and when a certain level of vacuum
pressure has been reached, turbo molecular pumps kick in to take the vacuum down to the required
level. Other processes are also involved to remove any gas molecules still left in the tank. At the
point when aluminisation will take place, a heavy current passes through the busbar and vaporises the
aluminium onto the mirror. It is possible to see this process taking place through the 'porthole'
shown in the photo below. A purple mist being seen. The whole process is automated and takes just a
Showing the lower section holding the mirror being brought back under the tank. When perfectly
aligned, the upper section moves down and the tank is sealed.
As a point of interest, the WHT aluminising tank was at the time the widest load to be carried up
the winding mountain road from the harbour in Santa Cruz to the observatory. Not the sort of thing
that could be sent up in sections like dome or telescope parts that were assembled on site.
Dave and Ken carefully rotating the mirror so that it lines up correctly in the
The counterbalanced radial defining arms can be clearly seen in this image. As the WHT is an
alt-azimuth mounted telescope, these arms are only required at the axis of where the telescope
moves in elevation. The INT (and JKT which uses a different arrangement) being equatorially
mounted require radial definers equally spaced around the circumference of the mirror.
"Bring it down a bit more!"
Eddie Penny signalling to the crane operator up on the dome balcony. Also seen in this photo is
Dave Benham to the left and Ken Maris with another guy (who I can't make out) at the rear.