Optical Testing for Next Generation Optical Telescopes; Terrestrial and Space Robert E. Parks Optical Perspectives Group, LLC Tucson, AZ September, 2002

Background Difficult to discuss all of optical testing in one hour Brief introduction to optical testing Assume you will be involved in NG telescope design and fab Outline the problems and decisions relative to testing Suggest methods of dealing with testing issues Some ideas never used before, designed to provoke thought No solutions, but places to begin thinking

Introduction to optical testing Test of a spherical wavefront quite easy, many choices Look at the image, “star test”, energy within a circle Knife edge test, Ronchi test to measure slopes Hartmann/Shack-Hartmann test to measure slopes Shearing interferometry measures slopes Fizeau, Twyman-Green interferometry, phase from fringes Phase shifting interferometry, can be very precise Based on self comparison or comparison to a sphere

Difficulty with ‘real’ optical testing Seldom measuring spherical wavefronts Special optics to change aspheric wavefront to spherical –Null lenses/holograms –Alignment, spacing issues/is hologram correct, properly imaged –Problems of partial aspherization/light getting to detector Shearing, knife edge, Ronchi require two measurements Shack-Hartmann must be correct sensitivity Testing of convex surfaces – secondaries Size and stability of reference surface Coherent illumination, speckle, ghost reflections

Testing issues for next generation telescopes Very long optical paths; environment will be big issue Air turbulence, layering and seeing Vibration, seismic and acoustic High precision measurements with large range For space optics, simulation of zero-g support Management often does not see the value in good testing and will not make the necessary capital expenditures Pay up front, or pay later Invest in math modeling that makes testing easier

Environmental issues – how to minimize Try to avoid testing at the center of curvature Use common path tests; measure slopes or curvature Locate test area underground, away from transportation and industrial complexes, isolate support shops Install laminar flow air conditioning with good thermal control Keep visitors out of test area; install visitor’s gallery Keep heat sources like light sources away from test areas Same goes for computers; flat panels and keyboards only

A real testing problem - CELT 30 m aperture, near parabola Vertex radius, 90 m Segments, 1 m diameter, approx segments Segment sag, about 1.39 mm Absolute segment shape good to 20 nm rms, ~100 nm P-V Segment simply supported self weight deflection ~2.8 um Gravity deflection about same as required figure accuracy About 80 different segment figure topographies About 40 um P-V departure from correct radius sphere

Possible approach – Sub-aperture testing Smaller aperture reduces dynamic range of test Common test plate references all segments to same radius Use common mount for all segments Rotating mount allows easy scanning of segment Test set up small and very short optical path Several options for actual test device Drawback – must stitch subaperture data

Segment with test apertures

Sub-aperture tests and stitching Sub-apertures must be stitched to reconstruct surface Generally use overlapping regions and match shapes Curvature is a property of the surface being tested Curvature (and slopes) have two components Measurements of curvature can be averaged Surface found by integrating twice – process noisy Surface could be defined and spec’ed in terms of curvature Look for departure from correct curvature

Alignment issues for off-axis optics Spatial errors become surface figure errors Problem is a two-dimensional one Optical datums must relate to mechanical datums –Centers of curvature and optical axes relate to holes in steel Need tooling to tie optical to mechanical locations Mark segments with mechanical datums, or Add optical datums to the metal structure Use a transfer device such as a point source microscope –Either images or finds centers of curvature

Conclusions Optical testing of NGT optics will not be easy There is great resistance to change proven methodology Methods need rethinking based on computing power Many possible methods never tried or modeled Many ideas in older literature Fertile ground when combined with computational power Don’t be fooled by computer models, either Most models start from ideal, have to find all non-ideal Make hardware models and test new techniques

Star test images of coma and astigmatism

Wire and knife edge test photos of mirror with spherical aberration

Schematic of knife edge test

Comparison of Ronchi and knife edge shadows

Example of Shack-Hartmann samples during a sub-aperture test

Extra focal or out of focus image

Comparison of Shack-Hartmann and Roddier (extra focal) testing

HST null lens and optical layout

Famous interferogram from HST

Segment centered 1.5 m off-axis

Segment centered 10 m off-axis

Segment centered 14.5 m off-axis

Segment on rotary table, test plate on swing arm

Appearance of figure relative to test plate as segment is rotated in 90° steps