The OSIRIS mechanical design is driven by the stringent top level requirements of alternative use in both Cassegrain and Nasmyth focus, large optical components, image stability during one hour better than ±7.2micro-meter in imaging mode and ±3.6 micro-meter in spectroscopic mode at the detector focal plane (with a goal of ±3.6 micro-meter and ±1.8 micro-meter respectively) and a flux variation through the long slit less than 3.7% during all the night.
The concept of the OSIRIS structure has been developed to fulfil the constraints imposed by the optical layout and the accessibility criteria, to minimize flexures (increasing its stiffness) and to meet the interface requirements.
Structural Concept
The OSIRIS structural concept is based on a 3D optical bench that supports all optical components except the collimator. Three legs leave the optical bench in order to save the Wavelength Selection Unit and hold the collimator support structure.
The whole structure is attached to the interface flange by means of three hyperstatic supports. Calculations demonstrate that the rotator deformation do not affect the image stability. The optical bench and the legs are hollow structures manufactured with welded flat plates and reinforced with internal ribs. We prefer this alternative regarding a structure made of beams or bars due to the reliability of the numerical model. Additionally, with a correct optimisation process, these structures are more rigid and lighter than beams structures since it take advantage of the membrane behaviour of the shell. The collimator support structure is made of hollow beams.
Hardware electronics has been isolated from critical optical elements and attached directly to the telescope rotator flange to minimise gravitational loads over the structure
Flexure Compensator
A flexure compensator is needed according the predicted image movements due to gravitational loads in the most unfavourable position. A compensation is performed tilting the collimator about two axis perpendicular to the incoming optical beam. The compensator has been selected according the sensitivity analysis, the increase of high order optical aberrations due to flexures and compensator movements and the practical problems to implement the mechanism. The compensator mechanism actuates on the triangular frame that supports the collimator cell.
A flex pivot is located near one extremity, while two linear actuators provide the tip-tilt degress of freedom The pivoting point is materialized by a flexure hinge with an in-plane membrane, thereby effectively blocking 4 degrees of freedom. An open-loop flexure control system will be implemented where the collimator tilt is controlled by using a look-up table. It is proposed by simplicity, at the cost of having non-predictable terms (hysteresis, image movements due to temperature variations) This approach forces to control the hysteresis (structure design and fabrication, compensator control) to avoid values higher than 2% of the total non-corrected flexures. The temperature variations must be maintained below 0.2 degrees Centigrated per hour to fulfil the values assigned in the error budget.
The image movement compensation is made always in the central field. Displacements on the field edges are due to high order optical residuals. The flexure model will be obtained from 400 calibrated positions on the collimator angular travel. It is equivalent to a mean value of 9 calibrations in elevation and 45 calibrations in the rotator angular position (for the Cassegrain), or 100 calibrations in the rotator angular position (for the Nasmyth)
Compensator of Image Movement due to Plate Scale Thermal Variations
The camera barrel will be passively athermalized to avoid place scale variations. Athermalization is obtained mounting a large CTE spacer between the third doublet and the second singlet to increase this distance at a rate of 13+/-1 micro-meter/oC.
Last update July 19, 2005, by Héctor Castañeda