Functions of the telescope and star simulators

One forcing requirement from ESO is the handover of the instruments in Europe in a well and fully tested manner before they will be shipped to the remote site of Cerro Paranal in Chile.

The experience that is gained with the instrument at the simulator within a period of a year

0

Distance from instrument front flange [mm]

’Front_1.2kN’

Figure 7.1: Displacements of the top section housing obtained in a bending simulation. The measured curves show the displacements for the nominal 12 000 N load at the front and the back side of the symmetry plane as well as for half this load for checking the linear scaling. The floor effect is indicated by the linear increase of 1/14.3 for the front and 1/18.4 for the back side.

fully supports that demand. Lots of necessary modifications of the instrument and the software interaction could otherwise not be performed.

Several simulators, specially dedicated to the Forsinstruments, were built by the VIC consor-tium. The largest one was the telescope simulator that was motor driven in two axes, in the elevation and the flange rotation axis. These two-axes motion did simulate any motion of instru-ment within the gravitational field with all possible aspect angles relative to the gravitational vector.

This telescope simulator did allow the checking of

• all electro-mechanical functions and its performances regarding fine-tuning adjustments, reproduction accuracy and reliability,

• all opto-mechanical adjustments, sensitivities and stabilities,

• the optical qualities in imaging mode, spectroscopy and polarimetry under real i.e. bending conditions etc.,

• the interaction between software and electronic hardware in a realistic environment,

• the contamination of the detector and its controller system by homemade electro-magnetic noise from the instrument itself,

• the behaviour of the cable wrap with all its power and cooling lines and the fibre link for data transmission

under realistic and fully operational conditions.

Additionally to this telescope simulator, two star simulators were build to provide adequate light sources for all these tests. One formed star images via projection optics while simulating the telescope optics as close as possible except for its astigmatism – which is inherent to the RC-system – and the small residual of mismatching coma. It provided stellar images with the right size and F-number that could be positioned at every location within the instrument’s field of view allowing the analysis of image qualities and ghost images and to determine the amount of straylight. The second star simulator was specially build for testing purposes of the instrumental flexure. This was designed to illuminate pinholes in the instrument’s focal plane without introducing errors in image shifts by its own bending through a stiff mounting.

These three simulators could cover in their combination the telescope functions almost com-pletely except for the environmental effects. The consortium was not able to simulate thermal loads or behaviour of the instrument or of the internal modules by changing temperatures or gradients of the range to be anticipated at the Paranal Observatory. These tests were considered to be dispensable because detrimental effects were not expected according to the proper design and the results of corresponding thermal calculations.

Another effect that could not be simulated realistically is the electro-magnetic environment of the observatory to test the electromagnetic compatibility (EMC). Due to this lack we are unable to test the irradiation of electro-magnetic noise. That would – if any – most probably affect the instrument but not the telescope or the dome functions due to the power consumption of the instrument, the telescope and the dome driving motors, respectively. A contamination of the near-field environment by electro-magnetic noise, produced by the most powerful drives of the instrument for filter and collimator exchange, could be detected. It did not impair any electro-mechanical function but it affected the detector and its controller system. An improved shielding could not counteract the induction of noise into the data lines but the right change of the motor frequency shifted the noise into a domain that was uncritical for the detector system.

Even though the EMC problem could be solved for the instrument alone, no simulation strategy could be found so far for the observatory’s environment. Thus, the compensation for a possible electro-magnetic incompatibility had to be figured out for the telescope site.

The last lack in simulating the observatory’s environment were the loads that will be induced by possible earthquakes, emergency brakes or wind buffeting. The probability for an earthquake is in this seismic very active region of the pacific plate extremely high, compared to Europe. I was not surprising that since the beginning of constructing the observatory, several weak and one strong earthquake took place. Up to now, the strongest quake happened in 1995 on July 30.

with an intensity of grade eight on the Richter scale which produced some minor damages to the equipment like the domes etc. that were already installed on the mountain at that time. Acceleration curves exist for the telescope site which are transformed by finite-element calculations to specific locations like basement, control room or the different foci. The ground acceleration for the Unit Telescope was given with 0.34 g or 8.5 magnitudes to the maximum which is magnified by the telescope structure (yoke, tube and mirror cell) to amounts of about 3 g with a maximum just below 5 g. The instrument structure withstands these loads according to our finite-element analysis. The FE-models clearly indicate that additional loads are allowed to amount to a factor of 20 at least. This will be by far enough reserve in strength for the loads expected for earthquakes, deceleration of any emergency brake or the wind buffeting. They all are only a small fraction of the load that would be induced by an Operational Based Earthquake

‘OBE’ or the even stronger Maximum Likely Earthquake ‘MLE’. Due to the experience that the consortium unfortunately gained with the instruments in the course of testing, and which

strongly supports the analysis, no doubt exists that the instrument will withstand all these loads.