Optics system wavefront error effects inc' focus

1
Optics system wavefront error effects (inc. focus)

• Martin Caldwell, Rutherford Appleton Laboratory,
  UK
• -with info from current Mission Formulation study
  (D Weise) and from Guido

CONTENTS LISA Optics error effects via phase
jitter Wave Front Error (WFE) of the Tx
beam Requirements, on focus and tilt In-flight
sensing of the errors Actuation of optics to
correct Questions for the study Experiments
2
Optical performance requirements

• Current study 2 effects giving phase jitter
• local optics pointing
• Also called line-of-sight projection effect
• Occurs in Rx beam, whose LOS jitters with the
  local SC
• Gives requirements on pupil locations, and their
  alignment
• Remote (far-field) pointing
• Leads to WFE requirement in Tx beam
• E.g. ?/30 focus for 7 pm/vHz
• Rx beam WFE effect in gives only loss of contrast

3
local optics pointing or LOS projection effect

• Rx beam, LOS jitters with the local SC
• Jitter is around pupil location, produces phase
  jitter if pupils not aligned

e.g. PAA centre mis-aligned from pupil
4
local optics pointing or LOS projection effect

• Unfolded view of LISA Rx path, to show relevant
  pupils.
• Science detector should define the system pupil
• PAA rotation centre at next pupil
• Nature of measurement means that the residual
  effect of SC jitter is that about the PM
• System Ent Pupil set at PM position

5
Remote (far-field) pointing

• WFE in Tx beam

Remote SC

• We use only 0.05km/20km of beam
• This cleans-up the higher-orders of WFE
• Degree of Benefit varies with aberration type,
  Not yet quantified

Local SC
6
Case of power (de-focus) error
Co-linearity of PM, phase centre, and remote SC
700 km ROC ?/30 peak-valley WFE
7
Case of power tilt error

• WFE in tilt, by angle d (e.g. telescope bend)
• Does not produce additional error, providing -
• Optics system Exit Pupil is well-centred on PM
• The pointing accuracy a is with respect to beam
  centre
• Requires alignment to max intensity, not just by
  DWS
• otherwise for in-field case, with d 1-degree
  off-axis, require ?L / - 0.4m !!

8
Converting WFE requirement to telescope
requirement case of focus
Lens equation

• For F/1.2 beam, zr/f5e-6,
• Change from v/f 1 to v/f2 by length variation
  ?u f.(zr/f)2 12.5 pm.
• This is what in-field option would need at a 1
  deg !
• Maximum v 53km, ?u 0.35 micron (cf.
  requirement v lt 700km)
• a more stringent requirement is the degree of
  collimation for received power

9
? How to measure WFE in-flight

• At the ?/30 level, need to measure and correct
  WFE in-flight (at least for focus)
• -use of Active Optics aberration-correction to
  nm levels as in astronomy
• Three options
• 1. Measure just the M1-M2 separation, use model
  values for other parameters
• 2. Measure far-field WFE (over 20km aperture) ,
  by varying the pointing to map it out.
• This is operationally complicated
• 3. Measure the local systems WFE, by
  interferometry of the incoming science beam
• This can be assumed to be near-planar before
  entering the system
• It is already done for the case of one term
  (tilt), by Differential Wavefront Sensing (DWS)

10
Differential Wavefront Sensing of focus and other
aberrations

• Tilt sensing requires just 2 points across
  aperture
• x2 axes, leads to Quadrant photo-diode
• Accuracy 10pm/2mm 5 nrads

• Next higher order terms, focus, astigmatism,
  require 3 points, 2 axes (astigmatism), requires
  3x3 diode array

11
DWS sensitivity to focus, astig coma

• Accuracy required
• e.g. ?/30 at /- 2mm aperture, 30 nm
• A reduced-accuracy phasemeter could be used.
• Dynamic range
• E.g. lt?/4 at /- 2mm aperture, 250nm, ROCgt80km,
  ?u lt 3um
• To increase range should use smaller ratio of
  detector size/beam size
• Problem of measuring coma
• With 3 points, coma is indistinguishable from
  tilt

12
Focussing mechanism

• Vary M1M2 (telescope length)
• Require absolute correction to ?u 1 micron
• Vary L1L2 (beam reducer length)
• Require MF(br)/F(M1)2?u
• (5060mm/500mm)2.1um 36 micron
• These options quite different on mechanism type,
  environment stability

13
Focus actuation -at M2

• The system may have residual astigmatism, coma or
  spherical aberration due to optical quality of
  the many elements in the OB chain.
• Active optics could include the use of M2
  tip-tilt to correct some terms,
• E.g. astigmatism 0.4 deg tilt of M2 gives ?/30
  variation
• Also interesting for in-field option, and
  telescope length interferometry

14
Questions to the current study

• Confirm optimum thermal-mechanical arrangement
• Data on focus stability of the structure ?
• tilt stability ?
• thermal asymmetry affecting
  astigmatism ?
• Decide
• how telescope focus to be sensed
• Where actuation to be placed
• Moving lens on OB
• Movement of M2

15
What to study ?

• Focus stability is main issue
• Astigmatism is the next worse offendor
• Asymmetry in mirrors
• Tilt stability of optics
• Possible use of tilt-actuation to correct
• Other WFE terms to calculate coma, spherical,
  trefoil

• Experiments
• Set up a DWS with telescope structure, 3x3
  detector, focus mechanism
• Characterise the focus measurement sensitivity
• Investigate
• closed-loop focusing,
• stable telescope structures
• in temp - stable chamber
• Extend to tip-tilt mechanism for next-level
  effects

16
Conclusions

• Focus (telescope length to 1um) not driven by
  the phase-centre (WFE) requirement, rather by the
  power-budget.
• Tilt not an issue if aligning to the peak power
  (as well as DWS tilt)
• Best way to measure in-flight focus via DWS,
  with 3x3 elements.
• although for in-field option an L measurement may
  also be needed for scan mechanism piston
• Trade-off for focus actuator location
• On M2 or on OB beam expander lens
• Would currently favor the lens option
• But M2 could provide added functionality (e.g.
  tip-tilt) for higher-order active-optics
  correction,
• Should keep this in mind in case system-design
  requires it, especially likely for in-field
  designs