1.

INTRODUCTION

The on-ground calibration play a crucial role on the development process of space instrumentation. The overall instrumental performances are in most cases dependents of the quality and accuracy of these calibrations. The italian National Institute for Astrophysics (INAF) have developed the Optical Payload System (OPSys) facility for the AIV (assembly, integration and verification) activities and the calibration of optical instrumentation. The facility is hosted by the Aerospace Logistics Technology Engineering Company (ALTEC S.p.A) in Turin, Italy. The facility environment is controlled in terms of temperature/humidity and cleanliness (see par. 2). In the cleanest part of the lab, is located the optical bench. This is part of the so called Space Optics Calibration Chamber (SPOCC), described in the paragraph 3. The OPSys has been used for the integration and calibration of the Metis instrument, the coronagraph on board the ESA/Solar Orbiter mission. The calibrations performed on this coronagraph will be reported in the par. 4 in order to show how the facility can be used.

2.

The OPSys facility

The OPSys facility consists of a controlled area of about 100 m². As depicted in Figure 1, the laboratory is composed by three different rooms with different cleanliness level:

Figure 1.

The OPSys laboratory and the layout of the SPOCC chamber

→ The “Grey room” classified as ISO-8 following the ISO 14644-1 standard (see Figure 2a). This room of about 50 m² hosts:

Figure 2:

OPSys facility rooms: (a) ISO-8 grey room; (b) ISO-7 clean room; (c) ISO-5 clean-room

The dedicated instrument controllers are positioned here during the activities. Moreover in this area is performed the cleanliness of the items to be introduced in the clean rooms.

→ The “ISO-7 Clean room”. (see Figure 2b) This room of about 35m² is dedicated to support the activities performed in the ISO-5 Clean room. The equipments of this area are:

The SPOCC cover is placed in this area when open.

→ The “ISO-5 Clean room” (see Figure 2c), of about 11m² host the SPOCC optical bench and is the place where the AIV and the calibration activities are performed. The cleanliness level is guaranteed by 12 HEPA filters.

All the environments are controlled and continuously monitored in terms of temperature, relative humidity and particle contamination. In the last years several improvements has been introduced with respect to the original facility configuration described in [1] especially in terms of cleanliness control and monitoring.

3.

The Space Optics Calibration Chamber (SPOCC)

The SPOCC is the optical test and calibration vacuum chamber housed in the OPSys facility. The vacuum calibration chamber was designed for performing integration and verification activities on optical payload. However, thanks to its features, the chamber is extremely adaptable for performing the following activities:

The SPOCC is composed of the following four main sections:

The main optical features of the calibration chamber consist of a Sun simulator, a light trap and a variety of baffles (see Figure 3 and Figure 4).

Figure 3:

Space Optics Calibration Chamber optical features: (a) light trap, (b) off-axis parabolic mirror, (c) baffles

Figure 4:

Optical Payload Calibration Chamber ray tracing

The SPOCC solar simulator is an off-axis parabolic mirror focusing the light incoming from a source. The mirror within the chamber is coated with a SI/Mo multilayer having an Ir-Mo capping layer, designed for operation in the visible and UV, and at an EUV wavelength of λ = 30.4 nm at 4.93° incidence. Its diameter is of 165 mm with a curvature of about 400 mm. A dedicated mechanism allows movements along the directions perpendicular to the mirror optical axis. Due to the flexibility of this system, it is possible to simulate a collimated beam with the Sun divergence at the Earth-Sun distance (i.e. 1 AU), up to 0.5 AU, or a focused beam with Sun divergences corresponding to closer distances to the Sun. The spectral reflectivity of the collimator is plotted in Figure 5.

Figure 5:

The SPOCC collimator reflectivity

Due to the off-axis mirror optical features, the SPOCC allows performing tests in the visible and in the ultraviolet light spectrum. Consequently, a variety of light sources is present in the OPSys facility: a monochromator working in the wavelength range from 30 nm to 1200 nm can be coupled with an hollow cathode source for performing test in UV and EUV light, a VUV Lyman Alpha source and, the Illumination System in Visible Light (ISVL). Moreover, a suitable vacuum equipment permits to change the light source without compromise the vacuum level previously established in the test chamber. For interfacing the visible light source, the chamber has been provided with a 101.6 mm fused silica optical window with two anti-reflection coated surfaces optimized in the 500 ÷ 700 nm interval and able to sustain a delta pressure of 1 atm. The off-axis parabolic mirror inside the calibration chamber is surrounded by a light trap consisting of two concentric cones coated with VEL-BLACK®, that guarantees 99.95% absorption in the visible range.

As a result, the computed level of brightness of the light trap is less than 10^-11 time the source brightness. This minimizes the probability of entering stray-light rays to exit back into the chamber toward the payload under test. To the aim at further improving stray-light suppression, a series of baffles has been inserted along the pipeline. The purpose of these baffles is to suppress rays coming from the edge of the mirror. Moreover, the SPOCC inside wall has been painted with Aeroglaze^© Z306, characterized by a low level of volatile emissions.

The SPOCC optical features guarantee the minimum possible level of stray light inside the chamber. In order to allow the stray light rejection measurements down to 1x10^-9 a suitable visible light source has been developed. The Illumination System in Visible Light ISVL [2] has the purpose of simulate the Sun disk. Two re-imagers are used to properly operate ISVL, namely the IC re-imager (IC, infinite conjugated) and the FC re-imager (FC, finite conjugated). Figure 6 shows the ISVL layout. The optical performances of the source are reported in [2].

Figure 6:

ISVL Layout

The optical bench inside the SPOCC is made of aluminum profile and its dimensions are 900 mm x 3500 mm, that is the maximum useful planar surface (see Figure 7). Due to the dimensions of the SPOCC test section, the optical bench can host instruments high, at least, 550 mm.

Figure 7:

Space Optics Calibration Chamber and its optical bench

The main feature of the optical bench is the possibility to tilt it in pitch and yaw and to translate it perpendicularly to the chamber longitudinal axis. As the matter of the fact, the SPOCC bench is provided with two groups of actuators located near its edges. The actuators are servo-controlled by a remote workstation hosted in the ISO 8 clean room. Table 1 reports the optical bench performances in terms of maximum allowed translations and rotations.

Table 1:

Optical bench performances

The SPOCC vacuum subsystem is a two-stages pumping system; it is composed of two tri-scroll pumps of 500 liters/minute each and two turbo-molecular pumps of 2000 liters/second each.

The tri-scroll pumps bring the pressure from atmosphere to 1*10^-2 mbar in about half hour. As a consequence, the air contained in the chamber passes from the viscous or laminar to the molecular regime. At that point, the turbo molecular pumps can be safely turned on. The SPOCC pumping system reaches a pressure of 1*10-6 mbar after 24 hours in dynamic vacuum condition hampered by the outgassing from the internal coating of the chamber, when the cold trap is used. It is also possible to isolate the chamber from the pumps by closing two servo-controlled gates-valves.

Six vacuum gauges (two thermocouples for the low vacuum, two cold-cathodes for the high vacuum and two full range gauges) monitor the internal SPOCC vacuum level. These sensors are distributed along the SPOCC in order to keep under control the vacuum level in each section of the chamber. An example of the pressure plot is shown in Figure 8.

Figure 8:

SPOCC vacuum plot when the cold trap trap is not used

The SPOCC is equipped with a thermal subsystem consisting mainly of a cold plate cooled by a LN2 pipelines system and used as cold trap for contamination. A copper bar connects the cold plate to six removable thermal straps that can reach the cold spots (if any) of the optical payload under test.

The temperature at the interfaces, on the cold plate and along the copper bar are real-time monitored and controlled through a dedicated control panel.

4.

The Metis instrument assembly and calibrations

Metis is the coronagraph of the ESA Solar Orbiter mission and it will acquire, for the first time, simultaneous imaging in the visible light (580-640 nm) and in the Ly-α line (121.6 nm) of the solar corona [3]. The integration and the calibration activities on the Metis coronagraph have been performed in the OPSys facility.

Here are listed the main calibrations performed on the Metis coronagraph are [4], [5], [6], [7] and [8]:

→ Visible Light Channel:

→ UV Channel:

5.

Conclusions

The OPSys facility located at ALTEC SpA has been designed to be a solar instruments test facility. Nevertheless, due to its features, it is extremely adaptable to test a variety of contamination sensitive optical payloads.

The SPOCC can recreate environmental conditions representative of space and, with its motorized optical bench, permits to perform tests considering different line-of-sight of the instrument, preserving the vacuum conditions. Its thermal subsystem offers the possibility to control the temperature of sensitive parts of the instrument under test.

In the frame of future space missions, the OPSys facility could represent a suitable laboratory in which perform the integration and the calibration activities on a variety of space flight instruments and optical payloads.