Brewer Ozone Spectrophotometer Pointing and Levelling C.T. McElroy York University Canada

September, 12-16, 2011Brewer Workshop Beijing, China 2 Underlying Principles Careful, detailed optical design Optical and mechanical mounts are kinematic Loads are carried in tension or compression –Not bending Clear understanding of system integration Temperature compensation when needed “Other instruments are climate controlled and still don’t have good wavelength stability. The Brewer operates at a wide range of ambient temperatures and has good wavelength stability. Why is that?” – Brian Gardiner

September, 12-16, 2011Brewer Workshop Beijing, China 3 Kinematic Design A solid body has 6 degrees of freedom 3 translation degrees 3 rotation Proper design constrains, but does not over constrain these degrees of freedom Over-constraint causes stress and leads to temperature-induced instability and failure

September, 12-16, 2011Brewer Workshop Beijing, China 4 Goals: Pointing and Tracking The Brewer uses an altitude/azimuth mount Axes must be perpendicular to each other Azimuth rotation axis must be vertical The procedure uses levels and gravity The azimuth axis is made vertical Then the elevation axis is made horizontal Therefore they will be perpendicular

September, 12-16, 2011Brewer Workshop Beijing, China 5 Tracker Screws Three instrument levelling screws

Degrees of Freedom 3 translation 0 rotation 2 rotation 1 rotation 3 translation 3 rotation

September, 12-16, 2011Brewer Workshop Beijing, China 7 Brewer Example Diffraction Grating 3 Aluminum Blocks Plane Hole Slot Springs load all three

September, 12-16, 2011Brewer Workshop Beijing, China 8 Tracker Azimuth Rotational Axis Must Be Vertical Elevation Axis Must be Perpendicular To the Azimuth Axis 3 Feet == Kinematic Connection to Earth Internal Screws in Tracker Base are Used to Make the Axes Normal Using Liquid Reflection Software Correction From Mechanical Zero to Earth Coordinates Pointing Accuracy Required ~0.1 o

September, 12-16, 2011Brewer Workshop Beijing, China 9 Levelling You may find it easiest to level the tracker first – then add the Brewer and check the level again.

September, 12-16, 2011Brewer Workshop Beijing, China 10 When is it Right? With the bubble in the level showing ‘level’- Rotate the instrument through +/-180 degrees The instrument should be level at all angles If not: –Level on along a line between a pair of feet first –Adjust level to show level –Rotate 180 degrees –Adjust level half way, adjust leg(s) the rest of the way - iterate back and forth until stable –Do the perpendicular axis

September, 12-16, 2011Brewer Workshop Beijing, China 11 Levelling one Axis Use screw to centre bubble Rotate Brewer 180 degrees: Re-centre with screw – count turns Back off half the number of turns Level with tripod levelling screws. Rotate Brewer back 180 degrees. An iterative procedure…

September, 12-16, 2011Brewer Workshop Beijing, China 12 Orthogonalization - 1 Set zenith prism to look upward Use a circular bubble level Place bubble level on prism surface Rotate prism to centre bubble laterally Adjust centre levelling screw inside tracker to level longitudinal axis

September, 12-16, 2011Brewer Workshop Beijing, China 13 Orthogonalization - 2 If there are persistent sighting problems or in the case of a new instrument –It may be necessary to check that the foreoptics rotation axis is perpendicular to the azimuth tracker axis Assume that the prism surface is parallel to the foreoptics rotation axis (one face is on the bearing) Reflect a laser vertically downward onto the prism (use reflection from a liquid surface to set the laser) Adjust the centre screw in the tracker to make the reflection from the prism travel vertically upward

Brewer Practical Assembly and Disassembly Levelling and Orthogonalization Removing and Replacing the Covers Changing Desiccant Inspection of optics – Mirror, Grating, Lens Servicing the Tracker Resetting the Tracker Limit Switch Various Tests and the Interpretation of the Results RS AP SH FR DT GS SL HG HP Sunscan Results (SC) Cleaning – and NOT Cleaning Hands-on Activities

September, 12-16, 2011Brewer Workshop Beijing, China 15 Now try it yourself….