Simulations of the thermal coating on the BK7 window: where we

stood in 2011Daniele Spiga, INAF/OAB

Thermal coating on the inner side of the BK7 window

The high infrared absorption of the BK7 (> 2.7 µm) has to be limited in order to prevent thermal radiation dispersion in space and to keep the TOU temperature > -80 °C. An IR reflective coating to reflect back the IR radiation toward the TOU can be applied on the inner side of the BK7 window. The BK7 slab will then be ‘cold’.

Reflectivity/transmittance of BK7

Bare BK7G18 in normal incidence exhibits a single-interface reflectivity around 4% in the visible range, and a reflectivity that peaks at 10 µm. The IR radiation beyond 2.7 µm is heavily absorbed. Below 2.7 µm the reflectivity is doubled because of the reflection on the two surfaces.

Thermal emission of BK7 at 190 K

At the target temperature of 190 K, the BB frequency distribution peaks at 75 µm, but there is still enough emission in the near infrared. Because of the high absorption, the BK7 emits inside and outside the TOU almost like a blackbody. Emissivity should be reduced to < 20% (here > 60%).

ITO + MgF2 coating (SELEX galileo)

Already qualified for space, radiation resistant, has been provided by SELEX Galileo. ITO (Indium-Titanium-Oxide) exhibits a high refractive index in visible and << 1 in the infrared range, therefore it is suitable to selectively reject the IR radiation.

Thickness optimization

The optimal thickness can be set to 500 nm thick layer of ITO (to maximize the IR reflectivity) and 130 nm of MgF2 (to minimize the reflection in the 500 – 1000 µm). The ITO coating is also IR absorbing – that rules interference out in the layer – but it reflects IR light TOWARD THE TOU before it gets absorbed. Almost no IR radiation escapes. IR emissivity TOWARD THE TOU is reduced to a few percent. The dependence on the incidence angle is very low up to 17 deg off-axis.

Transparency (single int.) in visible light

Optimal results are obtained by setting a 0.5 µm thickness for ITO and 130 nm for Magnesium Fluoride. The – interferential - reflectivity is smaller than the one of bare BK7. A further reduction would require the insertion of a third layer with intermediate refractive index of ≈1.7 in the visible band, without affecting the reflectivity in the infrared.

Thermal coating on the outer side of the BK7 window

In this case, the IR radiation is almost completely absorbed by the BK7 window, and re-irradiated mainly inside the TOU. The IR emissivity toward space is suppressed, and the BK7 slab is expectedly ‘warm’.

The issue of coating outside the BK7 window

Even though the thermal coating on the outer side of the BK7 window would prevent an excessive freezing of the window. But the interface BK7/ITO then would reflect back to the TOU 5-10% of the stray light (=> ghosts?) unless we insert a layer with an intermediate refractive index!

5-10%

Next developmentsIf the reflectivity in the visible range is still too high, additional layers might be included, or other materials might be considered (SiC?), also considering that only a reduction of the emissivity to 0.2 is requested.

What about reflectivity at wavelengths larger than 20 µm?

Which impact has the insertion of a thermal coating on the temperatures of the telescope?

A dedicated anti-reflex coating on the inner lenses!

The coating recipe provided by SELEX works as a thermal coating when applied on the inner side of the BK7 window. It reflects back to the TOU almost all IR light in the 2 -20 µm range, and, when the MgF2 thickness is optimized to a 130 nm thickness, is transparent to 98% of the light between 1000 and 600 µm.

When placed on the outer side of the BK7 window, it reflects back to the TOU 5 – 10% of the back-scattered visible light.

Conclusions