SOT optical performance
Focus stability in orbit
Y. Katsukawa (NAOJ) and SOT team
SOT17 @ NAOJ
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Apr 17-22, 2006
SOT focus design
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OTA is designed to provide collimated beam into FPP in air, and provides
a weakly converging beam (f200m) in vacuum. Air-to-vacuum difference
of the focus position of OTA and FPP are adjusted by initial setting of
the “re-imaging lens” position.
The 4 optical paths (BFI, NFI, SP, and CT) are designed to be kept in cofocal without any focus adjustment.
Apr 17-22, 2006
SOT17 @ NAOJ
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Focus adjustment by the re-imaging lens
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Focus shift originating in OTA and FPP can be compensated by
movement of the re-imaging lens along the optical axis during the
mission operation. We plan to adjust the focus position occasionally to
compensate seasonal variation.
No need to adjust the position within one orbital cycle because orbital
variation is supposed to be negligible.
Mechanisms for the motion of the re-imaging lens have the
specifications shown below.
Specifications
Stroke range
 25 mm
Resolution
0.17mm /step
Reimaging lens
M2
M1
Apr 17-22, 2006
SOT17 @ NAOJ
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Focus shift in orbit
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Focus shift is caused by displacement of
optical elements along the optical axis.
The change of M1-M2 distance is most
sensitive to the focus shift.
Mechanical environmental (vibration and
acoustic) tests showed no significant
change in focus position.
The thermal environment in orbit is very
different from that on the ground. In
order to predict the focus shift in orbit,
we performed thermal-optical test of the
telescope, and determined the defocus
sensitivity for major components
experimentally.
Focus errors (including margins) are
controlled by a focus error budget table.
The focus position is confirmed to be
well within adjustable range by the reimaging lens for the mission period.
Apr 17-22, 2006
SOT17 @ NAOJ
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Focus position in the first light phase
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After the telescope top-door is opened, the temperatures inside
the telescope increase, and are settled within several hours.
The telescope main structure is made of CFRP (carbon fiber
reinforced plastic). The dehydration of CFRP makes M1-M2
distance smaller in vacuum. The speed of the shrinkage is
temperature dependent.
Focus pos (mm)
Just after door opening
OTA
FPP
Focus pos (mm)
1 month (?) later
CFRP dehydration
0
-4.2 (0.8)
Temperature change
-2.5
-2.5
Initial offset (-2.3)
+12.0
+12.0
(CLU air-vac. diff)
(+4.7)
(+4.7)
0
0
(+1.0)
(+1.0)
+8.6
+4.5
Initial offset
(reimaging lens air-vac. diff)
Total
Apr 17-22, 2006
SOT17 @ NAOJ
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Focus shift by CFRP dehydration
Just after launch
0mm
Apr 17-22, 2006
1 month (?) later
M2
M2
M1
M1
-8.3mm  -4.2mm
SOT17 @ NAOJ
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2nd test on May 2004
OTA temperature was
kept 20C during the
test.
Time constant  400hrs
A20 (lambda, DP)
Temperature dependence of CFRP dehydration
Time (hour)
4th test on Mar 2005
Focus position did not
change during the
cold mode.
Apr 17-22, 2006
Cold mode
SOT17 @ NAOJ
Hot mode
Time (hour)
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Orbital variation by temperature ripple
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The temperature prediction in orbit tells that there is 1-2C temperature ripple
within one orbital cycle especially around M2.
The focus shift within one orbital cycle is expected to be around 0.2mm at the reimaging lens focus. This corresponds to one or two steps of the focus adjustment,
and is within focal depth.
Temperature and focus ripple within one orbital cycle
(Only major components are shown in the table.)
M1-M2 sensitivity
(m/ C)
Temperature ripple
T (C)
M2 support
-0.05
2.3
HDM spider
-0.61
0.9
Ring plate
0.59
0.8
Top ring
0.15
2.0
Upper truss
-0.11
1.1
Lower truss
-0.13
0.2
M1 support
-1.06
0.1
Focus pos at reimaging lens (mm)
Apr 17-22, 2006
SOT17 @ NAOJ
0.22
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Focus change between DC and Limb obs.
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In limb observations, heat inputs to the telescope become smaller
than those in DC obs. This makes the temperatures 1 - 2 C lower.
This temperature change causes small focus shift, but the shift is
expected to be about 0.1mm, and is negligible.
Temperature and focus change between DC and Limb obs.
(Only major components are shown in the table.)
Temperature (C) @ BOL
M1-M2 sensitivity
(m/ C)
Disk center
Limb
M2 support
-0.05
-4.5
-6.2
HDM spider
-0.61
-25.1
-26.9
Ring plate
0.59
-28.0
-29.5
Top ring
0.15
-27.7
-29.0
Upper truss
-0.11
-23.6
-25.3
Lower truss
-0.13
-0.5
-0.9
M1 support
-1.06
10.4
9.6
-2.51
-2.39
Focus pos at reimaging lens (mm)
Apr 17-22, 2006
SOT17 @ NAOJ
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Long-term focus shift
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Because of the contamination on the mirror surface, the temperature
inside the telescope tends to increase gradually through the mission life.
The temperature increase causes gradual focus shift, and will be
compensated by the re-imaging lens.
Temperature and focus change through the mission life
(Only major components are shown in the table.)
Temperature (C) @ DC obs.
M1-M2 sensitivity
(m/ C)
Beginning of life
End of life
M2 support
-0.05
-4.5
11.5
HDM spider
-0.61
-25.1
0.6
Ring plate
0.59
-28.0
-1.1
Top ring
0.15
-27.7
-1.2
Upper truss
-0.11
-23.6
1.4
Lower truss
-0.13
-0.5
18.7
M1 support
-1.06
10.4
27.3
-2.51
-3.78
Focus pos at reimaging lens (mm)
Apr 17-22, 2006
SOT17 @ NAOJ
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Summary
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The orbital and DC-Limb variation are expected to be 12 steps of the focus adjustment, and well within focal
depth.
We should verify orbital and DC-Limb variation in the
first light phase.
The seasonal and long-term focus shift will be
compensated by occasional adjustment of the re-imaging
lens position.
Apr 17-22, 2006
SOT17 @ NAOJ
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