A.

SEOSAT Optical design analysis

The optical design of the SEOSAT telescope is straight forward since it is defined as a Korsch telescope. The Primary, the Secondary and the Tertiary mirrors are aspherical mirrors and the Folding mirror is a flat mirror.

However, except for the optical design itself, one important design work is to define individual wavefront error specification for each of the four mirrors to finally verify that the wavefront error specification of the full telescope over the field angle of 35nm RMS is fully met.

The tolerancing of the system plays an important role. Sagem work is to tolerance the system to find the best compromise taking into account the manufacturing difficulties of each particular mirror. To do that, Sagem has separated the High frequency and the Low Frequency contributors and has built a budget for each mirror to remain compliant at the scale of the telescope and being coherent with the manufacturing flow chart.

Table 1 summarizes the final specifications for each mirror that have been agreed with customers.

TABLE 1:

HF and LF budget

Sagem capabilities proved that better Surface Errors can be achieved; however the aim for the SEOSAT mirror manufacturing was to perform a trade-off between Sagem best capacities and Sagem best delivery time. Thus, the choices for the mirror specification distribution have to be coherent also with the manufacturing time allowed.

B.

Lightweighting

Individual Zerodur mirror lightweight structure design consists in balancing mirror substrate stiffness under its own weight and 0g sag release, strength under launch conditions, optical surface quilting under polishing pressure, and milling operation complexity.

A 72% lightweighting was achieved for the primary M1 and tertiary M3 mirrors. Figure 5 illustrates the light-weighting of the back side of the primary mirror.

Fig. 5.

Primary mirror back light-weighted surface

The second step of the ‘Light-weighting process’ is applied on the Primary mirror and has been named by Sagem ‘slots opening’. Indeed, for the light to go from the Secondary to the Tertiary mirror, holes must be done on the optical surface of the Primary mirror. The openings consist in four slots as illustrated in Figure 6.

Fig. 6.

Slots opening for the SEOSAT Primary Payload Primary mirror

The ‘slot opening process’ is realised once the Primary mirror is polished up to the specification. Indeed, it is always preferable to polish an optical surface without holes to prevent from any edge effect or fragility of edges. The process used by Sagem to perform these slots after the mirror is polished prevents from edge or quilting effect on the substrate optical surface. Figure 6 illustrates the slots opening realised on the first flight model of the primary mirror M1.

Note that the central hole that is visible in Figure 6 has been manufactured only for alignment reason and not for the Telescope purpose.

C.

Polishing

All mirrors, except for the folding flat mirror MF, are polished to be aspherical with an edge margin of 4mm only between the mechanical and useful optical areas. Thanks to our latest progresses in precision optics polishing technology, no edge effects are occurring.

Traditional polishing, Robot and Ion Beam figuring techniques are used to achieve surface quality in the range of 3-14nm RMS for diameter up to 325mm. Table 2 summarizes the obtained Surface Errors compared to the agreed surface errors specification.

TABLE 2:

Surface error budget compared to results

Figure 7 gives the Surface Error Maps of the first model of the SEOSAT telescope.

Fig. 7.

Surface Error Map of the first model of the SEOSAT mirrors

The level of surface quality is in line with the constructed error budget of the full telescope transmission level.

Indeed, Sagem verified with simulation that the full telescope budget of 35nm RMS is well met all over the field. This has been simulated for the first telescope taking account the four measured surface errors of the Primary, Secondary, Tertiary and Folding mirrors.

Figure 8 illustrates the results all over the field. It confirms Sagem tolerancing skills and assumptions.

Fig. 8.

Simulated transmitted wavefront of the first model of the SEOSAT telescope

D.

MFD bonding

During the mechanical design, the Pad and Mirror Fixation Devices (MFD) were optimized for each mirror taking into account the weight, the mirror geometry, the mechanical specification and the Zerodur material specificity. They were designed to withstand tight mechanical requirements (first eigenfrequency higher than 250Hz, 20°C ±10° operational temperature, up to 75g quasi-static load level, 88g random acceleration).

The high request on random acceleration led Sagem to design and manufacture MFDs in combination with Pads in Titanium and Super Invar with very high precision and stress tolerance. Figure 9 gives an illustration of the SEOSAT MFD choice.

Fig. 9

SEOSAT mirror illustrations of MFDs

During the mechanical design, it is also taken into account the strict request of interface between Sagem and the customer instruments. Indeed, the SEOSAT mirrors have to be implemented in the telescope structure without interface re-adjustment which led to very strict mechanical manufacturing tolerances.

E.

Reflective coating

An enhanced silver coating was deposited ensuring highest reflectivity over the 0.45 – 2.5µm spectral domain. The silver coating specific deposition also enables the bonding of grounding straps on each of the SEOSAT mirrors, so that resistivity and conductivity requirements are fully addressed. Figure 10 gives a picture of the grounding strap on the Primary mirror.

Fig. 10

Grounding straps on the Primary mirror

F.

Environmental tests

Once the mirrors are polished, bonded and coated with grounding straps, the equipped mirrors are thermally and mechanically tested to verify that before and after tests the mirrors are fully stable mechanically at ±3μm and interferometrically at ±2nm RMS.

The mechanical tests are performed on shakers for all axes to verify all their mechanical properties. First eigenfrequencies are typically found around 400Hz, with a discrepancy lower than 10% with respect to the design value and stability lower than 4%. Strength was checked up to a level of 30g specified value in sinus and random vibration. Figure 11 illustrates one of the SEOSAT mirror on a shaker.

Fig. 11

Secondary mirror tested on a shaker

The thermal tests are performed in a thermal chamber. The equipped mirrors go through 10 thermal cycles between -25°C to +60°C.

After each test, the mechanical stability at ±3μm and the stability of the WFE at ±2nm RMS are verified.

G.

Primary & Secondary Sub-assembly testing

Finally, once all equipped mirrors are completed and qualified prior to customer delivery, Sagem performs an optical interferometric test of the sub-assembly composed of the Primary and the Secondary mirrors to evaluate the sub-assembly optical performance specified at 24nm RMS in transmission. The entire set-up is installed in clean room in order to preserve the cleanliness level of the freshly coated mirrors and their MFD’s. The first Primary and secondary mirror optical test proved that the 24nm RMS was achieved.