Chapter 8 - Mirror 1 Lateral Pads


1.- General Description

All mirrors of the 3.6m telescope are of circular shape and made of a solid disk of fused silica. Only the main mirror has a central hole.

they are all held by a system of support pads housed in the cell. The pad system consists always of three fixed points and a number of floating supports placed either only on the back or on the back surface and the front edge of the mirror.

Radially there are only three fixed supports apart from the main mirror where a system of cushions under air pressure reduce the local load peaks around the fixed points.

The cushions get the air from a compressor through a pneumatic control device which delivers a certain pressure depending on the position of the telescope tube.

All mirrors have got a shutter. the design of it varies from a simple flap over a marguarite system to a complicated semi iris shutter of the main mirror.

2.- Technical Details

2.1.- Mirrors

Diameter (mm)
Thickness (mm)
Weight (kg)
Main Mirror
Central hole of 700 mm diameter concave surface
Cassegrain Secondary
Screwed back supports, convexe surface

2.2.- Cells

They are built in the way of a box structure as shown schematically in figure 1MM#3. A circular hollow ring is welded to a bottom structure. Both parts are reinforced and interconnected by ribs.

These ribs take in the bottom structure the fixation for the axial support pads whilst the horizontal support are located in the ring. (figure 2)MM#3

The three axial supports are in contact with the correspondingmirrorthrough a layer of lead fixed to the pad surface. Same configuration is generally valid for the floating lever back supports but in special cases, i.e. overhead suspension of secondary mirror steel fixations with screws or pads with nylon layers can also be found.

All horizontal fixed support are made on nylon blocks apart from the main mirror. Here a thermal dilatation compensating system acts on lead covered pads. The aditional pneumatic pads of the main mirror press arubber cushion against the quartz surface.

  Lead has a big adventage that it can easily follow surface irregularitiesand that it damps mechanical shocks due to plastic deformation. This last property however permits damage during handling which ather standard materials including nylon do not offer from. So a constant care ofthese surfaces is required.  

2.3.- Shutters

There exist three different categories of shutters. The flat mirror are all protected by flap mechanisms. But the one in use at the main mirror consist of 18 specially shaped blades, rotating around a roller bearing in a ring over the outer edge of the mirror. They form in the closed position a tight protection and overlap when opened to a ring configuration similar to an iris system. (figure 5)

2.4.- Compressor and pneumatic control device

Figure 6 shows the diagram of the configuration which supplies the air cushions of the main cell with compressed air. Part A is located in the hydraulic room on the third floor. The air is however taken from the telescope area on Coude floor to reduce humidity and temperature influences through the air conditioned hydraulic room.

The compressor is interconnected with the control device (part C) by a steel pipe system which is altered into flexible lines in the areas of the cable twist in the both telescope axis. (part B)

The control device is operated by Personal Computer that measure the inclination of the telescope tube by an arrenge of load cells. According its measure it gives proper pressure over the air cushions. See the 3p6 Team home page


We are still working....

 [Observing Facilities and Operations]  [La Silla Home Page]  [ESO]  [Index]  [Search]  [Help]  [News]