An experiment of contamination
control for SOLAR-C telescopes
Takamasa BANDO, Hirohisa HARA(NAOJ)

Background{{{{Molecular contaminants are released from organicmaterials.
Due to the accumulation of contaminants on opticalsurfaces, the performance of telescopes is generallydegraded.
The Hinode was one of the satellites that was developed
and operated under very strict contamination controls.
However, it was reported that SOT throughput wasdegraded linearly with time since the start ofobservations.
Observations in an FUV/VUV wavelength band areplanned in SOLAR-C Plan-B. The FUV/VUV optics is mostsensitive to molecular contamination.
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Throughput
degradation of SOT{{{The Hinode SOT throughput
is decreasing with time. Thisis due to molecular
contamination that wasexpected in the pre-launchanalysis.

Observation

The speed of degradation isfaster than expected. The
throughput is largely dropped
at short wavelengths.
(Not critical for observations)
Enhanced absorption may
happen at short wavelengths.
• Prediction from mathematical contamination model
developed for SOLAR-B project before launch.

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Discrepancy from model prediction{ The discrepancy of throughput change fromthe model prediction needs to be understoodfor SOLAR-C Plan-B optical telescope,
because:It requires a high-precision spectro-polarimetry toobserve chromospheric magnetic fields.
z It may have an observing band at UV wavelength
where the telescope optics is easily degraded bymolecular contamination.
z4

Examples of throughput change by molecular contamination

Contaminant FUV/VUV transmittance

T. O’Donell, SPIE{In the FUV/VUV region, it is well known that opticaldegradation becomes greater than that in the NUV/Visregion. See figure above.
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Possible causes of enhanced absorption
in SOT at short wavelength

Focus

1.

A higher outgassing rate than the pre-launch
value that was measured for the flight optics.

2.

A higher deposition rate to optical surfaces thanthat in prediction.

3.

Misunderstanding of absorption coefficient forcontaminants.
zz

A larger absorption coefficient at shorter wavelengths.
An enhancement of absorption coefficient by UV
irradiation, that is, UV darkening.

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Measurement of absorption coefficientand its change by UV irradiation
at JAXA Tsukuba

Chamber to makecontamination samples.

10 solar UV irradiation

Spectrophotometer(measurement of transmittance)
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Degradation by the UV irradiance:
sample contaminant: DEHP

Transmittance （%）

Before UV irradiance
100
99
98
97
96 Masked
95
94
93
Contaminated
92
91
90
89
8887
86
85
250
300
350

400

10 solar irradiation for 2.5days (25ESD)

4505001009998
97
96 Masked
95
94
93
92
91
90
Contaminated
89
88
87
86
85
250
300
350

400

450500

Wavelength (nm)Sample: Diethylhexyl phthalate(DEHP) deposited on MgF2 plate.

{{Lower transmittance was observed inshorter wavelength. (UV darkening?)
Data are being analyzed.
8Measurement of transmittanceat VUV wavelength (110-200 nm) at NAOJSSDUV lamp

Monochromator

HolderUV lamp

{Transmittance at VUV wavelengths is measured
with a VUV monochromator that has a UV lamp asa light source. The UV lamp emits UV photons near
9122nm and 173nm.

A new chamber
to measure outgassing rate at NAOJ

TQCM

LN2
Dewar

ShroudEffusion cell{{{A vacuum chamber to measure the outgassing rate easily
is under preparation by SOLAR-C group at NAOJ.
Geometry of effusion cell and TQCM outgassing sensor isthe same as that defined in ASTM-1559 standard.
Shroud is conductively cooled down by LN2 Dewar.
10Summary{

The throughput of Hinode SOT is decreasing due tomolecular contamination.

{

Investigations on the throughput decrease are going on.

{

An experiment, which focuses on measuring theabsorption coefficient of contaminants on optical surfaces,
has newly started.

{We will answer through this kind of experiment for
SOLAR-C telescopes.
zz{How severe the SOLAR-C contamination control is, comparedwith Hinode.
Whether the “Solar” UV telescope can be made of CFRP.

We think that these are issues of major concern on thefeasibility of the Solar UV telescope.
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