HIREX

Effect of Solar Illumination on the Primary Mirror

Introduction:

In the early portion of the design study it was hoped that the HIREX primary mirror, unlike those in extreme ultraviolet (XUV) and soft x-ray telescopes in the past, could operate in the full sun. This would avoid having the thin aluminum filters that are used to remove most of the sunlight from previous instruments. There are two key requirements for this system to work:

The primary mirror coating thermal properties were compatible with 20^oC operation in full sun,
A correctly sized field stop placed between the primary and secondary mirror to limit the flux levels at the secondary.

The methods available to meet the first requirement are quite limited since the primary mirror must also reflect in the desired XUV pass band. We have not yet found a suitable mirror coating that will ensure the proper mirror temperature and XUV reflection. Therefore we have baselined the telescope design to include thin aluminum front aperture filters.

The second requirement was met by placing a field stop at the primary focus, which is available in the gregorian-like telescope design.

Coating Performance in the Solar Spectrum and Beyond:

Two zerodur samples were coated using a candidate multilayer and sent to Lon Kauder at Goddard Space Flight Center. There their reflectivity and transmittance, and by extension, their absorptance were measured. The results are shown below:

Reflectivity in the Solar Wavelengths

Reflectivity in Wavelengths Beyond Solar

Combining the transmittance and the reflectivity, and assuming that the remainder of the solar flux is absorbed yields the following graph of absorptance.

Absorptance in the Solar Wavelengths

Sample of Thermal Analysis Results:

Off-Axis Telescope Thermal Analysis:

Analysis indicates that by combining an off-axis telescope design with a uniform mirror and coating, and a backside thermal control system, the primary mirror will operate at a temperature above 100^oC, too hot to be fabricated at the operating temperature and too hot to be mounted.

Using the conditions shown below the mirror temperature distribution and mirror deflection were determined by analysis.

Reflected Energy: 40%
Absorbed Energy: 50%
Transmitted Energy: 10%
Emissivity: 40%

Mirror Deflection due to Solar Illumination, with Focus Error Removed We performed an analysis on a mirror coating whose thermal properties would perform adequately, that is shown here.

On-Axis Telescope Thermal Analysis:

The on-axis performance at this flux level was not run, but past analysis indicates that the temperatures will be the same and the mirror deformation worse.

Questions, comments pcheimets@cfa.harvard.edu <