HST Focus Status
September 2002
Last mirror move was mid-2000.
Current focus monitoring program (9663) simultaneously
measures focus at multiple times within an orbit in
ACS/HRC and WFPC2/PC every month.
Defocus described here in terms of microns of primary-tosecondary mirror separation.
Focus History
focus in microns @ secondary
(no breathing correction applied)
Focus Measurements Since Last Mirror Move (2000.197)
(no breathing correction)
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-11
-12
WFPC2 8829 Focus
WFPC2 9258 Focus
WFPC2 9574 Focus
WFPC2 8950 Focus
WFPC2 9663 Focus
625
600
575
550
525
500
475
450
425
400
375
350
325
300
275
250
225
200
175
150
125
100
75
50
25
0
-25
-50
-75
-100
-125
-150
-175
-200
DOY 2001
Focus Since Last Move
45
40
30
25
20
15
10
5
Focus (microns)
6
4
2
0
-2
-4
-6
-8
0
-1
0
Frequency
35
Focus (last 6 months)
25
15
10
5
Focus (microns)
6
4
2
0
-2
-4
-6
-8
0
-1
0
Frequency
20
Current Focus Optimization
• Keep WFPC2/PC within ±1.5 m of
optimal focus (mean over orbit).
• Focus varies by ~5 m over the PC field
(more over WFC fields)
• Focus varies by ~10 m among WFPC2
channels (PC best, WF3 worst)
• ACS kept confocal with WFPC2 (WFPC2
has no internal focus adjustment)
Future Focus Optimization
• Keep ACS/HRC within ±1.5 m best focus
• HRC operates at shorter wavelengths than
WFPC2 and is thus more sensitive to focus
• HRC is better sampled and produces less
scatter in focus measurements
• ACS/WFC focus varies by 4 m over field
• Maintain ACS-WFPC2 confocality
HRC F220W Predicted EE’s
0.8
EE
0.6
0.4
0.2
0.0
0.00
0 m
-2 m
-5 m
0.05
0.10
Arcsec
0.15
Recommendations
•
•
•
•
Recommend +3 m mirror move
Optimize focus based on HRC measurements
Maintain WFPC2-ACS confocality
Above are dependent on latest focus monitor
results (data taken yesterday)