Optical Property Retention Methods for the T-170M Space Telescope Mirrors Surface in the Project «Spektr-UF» at the Preflight Preparation Stage

Astrophysical observations in the ultraviolet band have many advantages. At present, the «Spektr-UF» project is under implementation to create a large space observatory for operation in the ultraviolet spectrum.Requirements for the ultraviolet telescope optics quality are extremely high. Therefore, both to manufacture such a large space telescope as the T-170M and to transport it to the launch complex are rather difficult challenges in terms of technology.When manufacturing optical elements of the telescope T-170M, a combination of Al+MgF2 coatings has been preferred. At the same time, atmospheric oxygen penetrates through the pores in the magnesium fluoride, thereby forming a Al2O3 oxide layer on the sputtered aluminum, which significantly degrades the UV reflectivity of the mirror surface. It is also necessary to fulfill the requirements for surface cleanliness of optical system elements of the telescope during the finished product transportation and its storage and to provide for the autonomous operation of the system that maintains atmosphere control.To solve the set tasks:1) a dust-proof-and-moisture-proof sheath (DPAMPS) was designed to prevent the optical system mirror surfaces of the telescope from coming in contact with atmosphere;2) to provide a controlled atmosphere inside the DPAMPS the need is justified to blow gaseous nitrogen of special purity (grade 1) in accordance with GOST 9293-74 with a dew point temperature of -50°С, at most, inside the telescope; calculations have proved that charging with the super-atmospheric pressure of 10 kPa provides the optimal conditions for maintaining the optical properties of the space telescope mirrors surface, and also minimizes the loads on the easily damaging elements of the telescope;3) to ensure the required cleanliness of the optical system elements surfaces of the telescope inside the DPAMPS, a class of purity Class 7 ISO, at worst, is established in accordance with GOST ISO 14644-1-2002; contamination tolerance of optical system components being, at most, 0.03% of the total surface area (300 p.p.m.) is provided by the rational choice of the blow-down scheme of the telescope with feeding nitrogen through the equipment bay;4) a rational design of the supercharger using a gas analyzer and boost valves with external feedback provides driving pressure and operation autonomy of the system that maintains atmosphere control.