TIPS/JIM 21 Feb 2013
TEL Group
HST’s Recent Wavefront Control
M. Lallo & Colin Cox
(for Telescopes Group)
TIPS/JIM 21 Feb 2013
Hubble’s Optical Telescope Assembly (OTA)
TIPS/JIM 21 Feb 2013
TEL Group
• 2.4 meter f/24 Ritchey-Chrétien (RC) Cassegrain:
- ~20 arcmin FOV suited for imaging
- RC design produces no coma or 3rd order spherical aberrations across the field
- HST’s implementation was flawed, resulting in ~λ/2 spherical aberration
- Current Science Instruments correct the aberration internally
.
FGSs
Axial
Science
Instruments
+ _
Radial
SI
Secondary Mirror
Diameter = 0.31m
Radius of curv. = -1.36m
Multiple rings of
thermal sensors
used by focus
model
OTA METERING TRUSS
4.91m (PM to SM)
1.5m
FOCAL
SURFACE
System
Directions of Secondary
Mirror “piston” or “despace”
movements which alter
focus.
• f/# = 24
• Magnification = 10.43
• 1 micron Sec. Mirror despace = 6.1 nm
RMS wavefront error (focus aberration)
Primary Mirror
Diameter = 2.4m
Radius of curv. = 11.04m
33% central obscuration
Hubble’s Optical Telescope Assembly (OTA)
TIPS/JIM 21 Feb 2013
TEL Group
• Metering Truss
- Thermally passive graphite-epoxy truss/ring structure
- Locates the Primary & Secondary mirrors
- 48 tubular elements matched by measured CTE to minimize overall bending
- Truss constrains mirrors non-axial motions to undetectable levels over the
wide temperature ranges associated with LEO
• Primary & Secondary mirrors
- Actively heated. Controlled to ~ 1ºC
- Exhibit no measurable figure changes
- Primary: 24 actuators for low-level figure
control (not used)
- Secondary: 6 actuators in conventional 3
bipod arrangement
TIPS/JIM 21 Feb 2013
TEL Group
Optical Alignment & Wavefront Changes
• Point Spread Function (PSF) from OTA mainly exhibits focus changes over
time:
- Symmetric dimensional changes in the truss (lengthening/shortening)
- Stable mirror figures
- Any low levels of tip/tilt at the Secondary result in FOV shift and nonradially symmetric aberrations like coma or astigmatism at levels
below our practical abilities to detect.
ACS/HRC Focus Measurements & Lightshield Breathing Model
0.0
• Focus changes occur on a number of different
timescales:
1. HST 96 min orbit (temp.-driven)
2. Days (temp.-driven by attitude changes)
3. Long-term (physical shrinkage of the truss)
Focus (in microns @ Secondary Mirror)
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
17.00
17.25
17.50
17.75
18.00
18.25
18.50
18.75
19.00
Hours (Day 2005.142)
19.25
19.50
19.75
20.00
20.25
20.50
Focus Variations & Evolution
TIPS/JIM 21 Feb 2013
TEL Group
• Temperature-driven focus variations:
- do not trend long term
- have been modeled over the years with reasonable success (Cox &
Niemi 2011, DiNino et al. 2008, Hershey 1998, Bély 1993)
- Peak to peak focus variations typically ~ 6 µm Secondary mirror axial
motion (“despace”) but can be greater.
- are not actively corrected by moving the Secondary mirror
• Long term focus evolution from truss shrinkage:
- ~150 microns over HST life (total 0.003% of truss length)
- Fit by a double exponential
- Temperature-driven variations add +/–2.7 microns rms of “scatter”
- Modeling reduces this to +/–1.5 microns
- is corrected by moving the Secondary mirror in despace
NOTE: 1 micron of Secondary mirror despace induces 6.1 nanometers of RMS wavefront error
(focus) at the HST focal plane, so typical focus variations induce ~37 nm rms wavefront error)
Aberration Measurements - Phase Retrieval
TIPS/JIM 21 Feb 2013
TEL Group
• STScI has used parametric phase retrieval (Krist 1995) to characterize the
nearly in-focus, undersampled HST PSF quite accurately (~1nm rms wfe)
- fourier transform relates image plane (PSF) to pupil plane (wavefront
error)
- iteratively fits wavefront surface with an orthogonal basis function,
specifically the Zernike polynomial series (Mahajan 1991)
where αnZn is the normalized Zernike polynomials & c is the solvedfor coefficients representing rms wavefront error in microns.
For n = 4 to 8, αnZn are given below:
α4Z4=
α5Z5=
α6Z6=
α7Z7=
α8Z8=
3.89(r2-0.55445)
2.31(r2cos(2θ))
2.31(r2sin(2θ))
8.33(r3-0.673796r)cosθ
8.33(r3-0.673796r)sinθ
focus
0 deg astigmatism
45 deg astigmatism
X coma
Y coma
HST’s Wavefront Sensing & Control Scheme
TIPS/JIM 21 Feb 2013
TEL Group
• Sensing (monthly phase retrieval)
- Phase retrieval performed on dedicated regular observations (~1 orbit/
month) with WFC3 & ACS.
- Cameras used have varied over the mission
- SIs have been set to be confocal within the errors during their
commissioning
• Control (periodic Secondary mirror despace adjustments)
- fits of monthly focus determinations distinguish long-term shrinkage
from ± ~3 micron temperature-driven “noise” (± 1.5 micron after
model subtraction)
- Control made when data trends beyond a threshold (tolerance has
varied over the mission, as guided by SI complement and desorption
rate). In recent years, ± ~3 microns (± 18 nm rms defocus). Aiming to
control tighter in the future for WFC3
Mission Lifetime Focus Maintenance
TIPS/JIM 21 Feb 2013
TEL Group
TIPS/JIM 21 Feb 2013
TEL Group
Most recent focus adjustment
&OCUS 4REND 3INCE $EC -IRROR -OVE
in July 2009 (SMOV4) predicted -2.4 +/- 1.4
microns of defocus January 24, 2012 for WFC3
• A +3.6 micron despace of the Secondary
mirror was commanded on January 24th.
Expected to put the Secondary +1.2 microns
from best focus for WFC3 UVIS (1.5 microns
from best focus for ACS WFC).
• Phase retrieval analysis of the standard
monitoring observations on January 26th. After
temperature model corrections, mean focus of
WFC3 over 1 orbit was found to be equivalent
of –0.68 +/-1.5 microns at the secondary mirror.
ACS was found to be +0.14 +/- 1.5 microns.
Not inconsistent with our expected state.
$AYS SINCE (34 DEPLOYMENT
›M
›M
›M
›M
›M
! C C UMULA TE D DE FOC US IN 3 - MIC RONS
• Linear fit to the observed data since refocusing
-IRROR -OVEMENT
%XPONENT &IT
%XPONENT &IT #ONT
"REATHING CORRECTED OTHER 3)S
"REATHING CORRECTED 7&# 56)3
• Subsequent monitoring data will better establish
OTA state.
Wavefront Sensing & Control: JWST vs. HST
TIPS/JIM 21 Feb 2013
TEL Group
• JWST wavefront evolution not expected to be dominated by long-term trend
• We are expecting to actively correct temperature-driven optical alignment
changes during science operations
• Sensing cadence every 2 days
• Control activities cadence unknown, but probably on order weeks to months
• Phase Retrieval much more complex than HST
- Like HST, operational sensing is science-image based, with Fourier
transforms relating image plane to the pupil plane
- Resulting phase map images of wavefront error are combined with a
“target” phase map to solve for the “pose” change required for each of
the 18 primary mirror segments
•
(a)
/
-r - -
4
07 ---/---- -
(b)
Fig. 14. (a) Predicted TBT wavefront error after fine-phasing (84 nm double-pass); (b) calculated PSF in single-pass; (c) calc
Modeled
typical JWST wavefront map & PSF
Measured HST wavefront map & modeled PSF