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

Brewer Ozone SpectrophotometerPointing and Levelling

C.T. McElroyYork University Canada

September, 12-16, 2011

Brewer 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 goodwavelength stability. The Brewer operates at a wide range of ambienttemperatures and has good wavelength stability. Why is that?” – Brian Gardiner



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



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



5

Tracker Screws

Three instrument levelling screws

Degrees of Freedom

xy

z

>

>

3 translation0 rotation

2 rotation

1 rotation

3 translation

3 rotation



7

BrewerExample

DiffractionGrating

3 Aluminum Blocks

Plane

Hole

SlotSprings load all three



8

TrackerAzimuthRotationalAxis Must BeVertical

Elevation Axis Must be PerpendicularTo the Azimuth Axis

3 Feet == Kinematic Connection to Earth

Internal Screws in Tracker Base areUsed to Make the Axes Normal UsingLiquid Reflection

Software CorrectionFrom MechanicalZero to EarthCoordinates

Pointing AccuracyRequired ~0.1o



9

LevellingYou may find it easiest to levelthe tracker first – then add the Brewer and checkthe level again.



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



11

Levellingone Axis

< >

Top of case m ay not be para lle l to optica l axis

O ptica l 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…



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



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



15

Now try it yourself….

Documents

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