Post on 05-Apr-2018
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October 1, 2009 Copyright R. E. Parks 2009 1
Testing flats or plane surfacesand metallic, reflective coatings
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October 1, 2009 Copyright R. E. Parks 2009 2
Objectives
Learn methods of testing flats for figure and absolute flatness
Where is absolute flatness required and where not
Concept of absolute testing
Learn about inspecting for surface finish quality
Grey, pits and full polish
Scratches, digs and sleeksSurface texture and mid-spatial frequency roughness and
orange peel
Learn about cleaning
When is it necessary and when it should be avoided
Introduction to coatingMetal reflective coatings and overcoats
Fixture marks and coating defects
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October 1, 2009 Copyright R. E. Parks 2009 3
Where is absolute flatnessimportant?
A flat is a spherical surface of infinite radius
A test for flatness is measuring the difference between two flats
Does a zero difference means the two are flat?
And does it matter?
If a plane mirror is used at normal incidence it usually doesnt matter;
System can be compensated by re-focusingIf a plane mirror is used at non-normal incidence, it will matter:
This introduces astigmatism since footprint of beam is elliptical
Longer one way than other, more sag one way
Also suggests how truly flat mirrors should be tested;
Test at non-normal incidence, Ritchey-Common testAnother method is the three flat test
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October 1, 2009 Copyright R. E. Parks 2009 4
Ritchey-Common test for flatness
Plane mirror under test is used to rotate center of curvature of concave
spherical mirror
Footprint of beam on plane mirror is roughly elliptical so sag in plane of
page is greater than perpendicular to page if flat has power
Orientation of astigmatism will indicate cc or cx power
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October 1, 2009 Copyright R. E. Parks 2009 5
Three flat test - example
If all 3 have same powerthen the difference
(match) is the same.
Therefore if all 3 are
truly flat the matches
will be zero in all cases
If there is a difference in
power it becomes
obvious the matches will
be different
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Idea of absolute testing
Idea used to be unique to optics but now in allprecision work
Machinists rule; test device 10x better thantolerance
In optics, test optics no better than optic undertest
How are test optics errors removed from opticunder test
Basically, two tests are done, one with opticmoved Then test maps compared to see what errors moved
with optic
For complete separation, three tests needed One movement along each of two axes
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Schematic of test optics errorremoval
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Testing system fold flat for figure
Need collimated beam large enough to cover clearaperture Or test flat the size of clear aperture
Flat held in interferometer test fixture with threefingers Use lightest force possible to just hold flat stable
Align reference flat to interferometer Two dots superimposed
Align fold flat to reference flat
Two dots superimposed behind reference flat dots Switch to measure mode
Check that fold flat is in focus, look at edges
Mask test aperture to clear aperture on fold flat
Measure
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1st step setting up interferometer
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Install transmission flat (TF), reference surface, in bayonet mount
In Align mode, align TF return dot behind cross hair
Monitor screen with mis-aligned, then aligned return spot
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2nd step, aligning optic under test
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Pair of dots are reflection from both sides of plane window with wedge
Align by placing one or the other dot behind the crosshairsDimmer dot tends to be from rear surface
Misaligned, left, and rear surface dot aligned, right
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3rd step, check focus
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Once aligned, switch to view mode to see fringes
Generally, view will need focusing to avoid diffraction at edge
Barely visible fine, vertical fringes are wedge between the two surfaces
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4th step, measure
September 17, 2009 Copyright R. E. Parks 2009 12
View on monitor screenData will depend on settings
entered on control screen
Image has 75764 good
data points
Vertical wedge fringes more
easily seen hereBlank areas around edge
due to hot glue blocking
view
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Control screen with profile
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Control screen with Zernikepolynomials
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Problems with testing flats
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Noise due to coherent source partially removed with Ring of Fire, lower
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Interference between surfaces
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Very parallel (1-2 fringes) but non-flat window
Interference between surfaces affects phase unwrappingProblem obvious in fringe pictures, maybe a coating problem also
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Interference between surfaces only
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Wedge fringes only gottenby removing transmission
flat and using
interferometer as an
illumination and imaging
system only
Cannot phase shift sinceno reference surface
Can shift by tuning laser
source
Time delay source is way
of phase shifting to
measure wedge,effectively also measuring
transmitted wavefront
Fringes do not move when
window tilted
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Details of figure testing
Test on vibration isolated table; two fringe printthrough Couple part under test to interferometer
Keep fold flat close to reference flat; air turbulence
Use reference flat with same reflectivity as testpart; contrast
For phase shifting interferometer, break out onefringe
Reduce tilt to near zero; tilt will introduce false coma For test plate need 5 or so fringes to judge power and
irregularity
In analysis over elliptical aperture, do not useZernikes Just refer to p-v and rms results
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Inspection of surface finish
In general, test against dark background With fold mirror, more difficult because of ground surface
background
Mirror will be coated for high reflectivity; defects reallyvisible then
Use bright light like microscope illuminator, LEDflashlight Reflect light back until just outside eye pupil when looking
through lupe
This type examination best for checking complete polish,orange peel
If in complete polish, specular surface will have a greylook due to scattered light
Scratches are long defects where glass surface has
fractured
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Inspection for texture and orangepeel
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A straight line separation of bright and dark is best type of source
Scratches readily apparent if aligned parallel to light/dark line with
scratch over dark area. Scratch seen by scattered light against dark
Similarly, orange peel seen at grazing incidence against dark/light line
Boundary will show a waviness like shadows of orange peel
A bare fluorescent line bulb with black paper behind is excellent source
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Cleaning
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In general, dont
For inspection of surface quality it will be necessaryIf obviously dirty, blow off, raise with soap and water, blow dry
If still dirty, use edge of folded tissue wet with IPA, or acetone
Wear gloves or finger cots, otherwise grease from fingers will contaminate
Wipe from above center to edge, discard tissue and use another for
next wipe
Idea is to avoid dragging contamination from edge onto surface and toavoid contamination from previous wipe
Keep tissue handy because sometimes an apparent defect is dirt
Dont want to reject surface by mistaking dirt for a defect