SOT optical performance Focus stability in orbit

1
SOT optical performanceFocus stability in orbit

• Y. Katsukawa (NAOJ) and SOT team

2
SOT focus design

• 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 co-focal without any focus
  adjustment.

3
Focus adjustment by the re-imaging lens

• 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.

Reimaging lens
M2
M1
4
Focus shift in orbit

• 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 re-imaging lens for the mission period.

5
Focus position in the first light phase

• 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.

6
Focus shift by CFRP dehydration
1 month (?) later
Just after launch
M2
M2
M1
M1
-8.3mm ? -4.2mm
?0mm
7
Temperature dependence of CFRP dehydration
2nd test on May 2004 OTA temperature was kept
?20C during the test.
A20 (lambda, DP)
Time constant ? 400hrs
Time (hour)
4th test on Mar 2005 Focus position did not
change during the cold mode.
Cold mode
Hot mode
Time (hour)
8
Orbital variation by temperature ripple

• 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 re-imaging
  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.)
9
Focus change between DC and Limb obs.

• 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.)
10
Long-term focus shift

• 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.)
11
Summary

• The orbital and DC-Limb variation are expected to
  be 1-2 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.