HST Focus Monitoring
&
Phase Retrieval on WFC3
Sami-Matias Niemi (niemi@stsci.edu)
together with
Matt Lallo, George Hartig, Colin Cox
and
John Krist (JPL) & Richard Hook (ESO)
TIPS/JIM meeting, Feb. 18 2010
Outline
How to measure focus?
HST Focus monitoring results
Phase Retrieval
Process of trying to recover the wavefront
error for a given PSF.
Iteratively fitting an observed PSF and
expressing it in terms of a Zernike Polynom.
Nonlinear, similar to deconvolution.
Was used to measure aberrations in HST
mirrors (Burrows 1991; Fienup 1993).
Modeling a PSF
Courtesy: J. Fienup
Mirror Maps
Krist J.E., Burrows C.J., 1995, Applied Optics, 34, 22
Observed vs. Model PSF
Krist J.E., Burrows C.J., 1995, Applied Optics, 34, 22
Zernike Polynomials?
Are a set of orthogonal polynomials that
arise in the expansion of a wavefront
function for optical systems with circular
pupils!
Piston
Tilt (x & y)
Astigmatisms
Focus (0, 2)
Clovers (-/+3,3)
Comas (-/+1, 3)
Higher
order
terms
Implementation of WFC3
Requirements:
Camera pixel size & focal length
Pupil Image with offset mirror maps
Charge Diffusion Kernels (Hartig)
Geometric Distortion Coeffs. (Cox, Platais...)
Modify the existing IDL software
Fix higher order Zernike polynomials
Focus monitoring Data
Proposal 11877:
WFC3 and ACS in parallel
2 WFC3 UVIS and 5 ACS WFC exposures
per orbit (currently, but shall change)
Object NGC 188-73 (open cluster)
Executes once a month
ACS: F502N, WFC3: F410M (currently)
ibcy10bkq_flt
Pupil
Image
HorzCross
Data
VertCross
Model
HorzCross
Residuals
VertCross
Focus Measurements
Focus Stars of ibcy09usq Chip 1
2000
Star 5
Star 3
Star 6
1500
1000
Star 2
500
Star 1
Star 4
0
0
0.0
1000
0.3
0.6
2000
0.9
1.2
1.5
1.8
log10 (Counts)
3000
2.1
4000
2.4
2.7
3.0
Focus Measurements
8
Focus Measurement (No breathing correction)
ACS chip 1
WFC3 chip 1
ACS chip 2
WFC3 chip 2
Mean + standard err
6
Defocus [SM µm]
4
2
0
−2
−4
01:26:23
01:33:35
01:40:48
01:47:59
08 Jan 2010
01:55:11
02:02:24
02:09:35
Focus Measurements
Focus Measurement
10
ACS chip 1
WFC3 chip 1
ACS chip 2
WFC3 chip 2
Mean + standard err
8
Defocus [SM µm]
6
4
2
0
−2
−4
01:26:23
01:33:35
01:40:48
01:47:59
08 Jan 2010
01:55:11
02:02:24
02:09:35
HST Focus Trends
Days since HST Deployment
Accumulated OTA shrinkage [SM µm]
150
0
1000
2000
3000
4000
5000
6000
7000
Double Exponent Fit
No breathing correction
100
50
0
24
Apr
1990
15
Dec
1991
06
Aug
1993
29
Mar
1995
18
Nov
1996
11
Jul
1998
02
Mar
2000
23
Oct
2001
15
Jun
2003
04
Feb
2005
27
Sep
2006
19
May
2008
09
Jan
2010
01
Sep
2011
HST Focus Trends
Focus Trend Since Dec 2002 Mirror Move
Mirror Movement
No Breathing correction
No Breathing correction (WFC3)
Linear Regression
Exponent Fit
Exponent Fit Cont.
5
0
5000
5500
6000
2.97µm
5.34µm
4.16µm
-5
3.6µm
Accumulated Defocus [SM µm]
10
6500
Days since HST deployment
7000
HST Focus Trends
Focus Trend Since Dec 2002 Mirror Move
Mirror Movement
Breathing corrected
Breathing corrected (WFC3)
Linear Regression
Exponent Fit
Exponent Fit Cont.
5
5000
5500
6000
2.97µm
5.34µm
-5
4.16µm
0
3.6µm
Accumulated defocus in SM microns
10
6500
Days since HST deployment
7000
HST Focus Trends
Focus Trend Since Dec 2004 Mirror Move
Mirror Movement
Breathing corrected
Breathing corrected (WFC3)
Linear Regression
Exponent Fit
Exponent Fit Cont.
5500
6000
2.97µm
-5
5.34µm
0
4.16µm
Accumulated defocus in SM microns
5
6500
Days since HST deployment
7000
HST Focus Trends
Focus Trend Since Dec 2002 Mirror Move
Mirror Movement
Breathing corrected
Breathing corrected (WFC3)
Linear Regression
Exponent Fit
10
5
5000
5500
6000
Days since HST deployment
2.97µm
5.34µm
-5
4.16µm
0
3.6µm
Accumulated Defocus [SM µm]
15
6500
7000
Conclusions
WFC3 UVIS phase retrieval works and is
being used for focus monitoring.
Some changes to the current program to
better sample each orbit with WFC3.
Predicted Zero-crossing dates:
April 2010 (data since 2002)
Dec 2010 - Feb 2011 (using all focus data)
WFC3 perhaps -0.5 micron compared to ACS
Thank You!