2.1

Optical Design of Telescope to Re-imager at Focal Plane Instrument

To attain the requirements of high spatial resolution and the large number of photons collection capability, we study a 1-m aperture Gregorian, which can give a theoretical resolution of 0.1 arcsec at Fe I 630 nm (1.22λ/D rad) where accurate polarization measurements can be performed. The distance between the primary and secondary mirror is to be 2 m, after preliminary opto-mechanical tradeoff studies within the allowable size of a launcher’s nosecone. This short Gregorian configuration demands very small static misalignment tolerances for the primary and secondary mirrors, and hence this makes a structural design of the telescope crucial for high optical performance.

We adopt a collimation optical interface between the telescope assembly and accompanied focal plane instruments because of its clear definition of optical interface and also easiness of optical tests of the telescope assembly using a laser interferometer throughout an integration, opto-thermal and system level optical tests. We design a collimator unit to be placed behind the primary mirror and to reduce beam size, making an exit pupil of about 40 mm diameter (beam reduction factor of 1/25) to accommodate clear apertures of critical image stabilization (TTM) and polarization modulation unit (PMU), followed by focal plane instruments. A schematic optical configuration is given in Figure 1.

Figure 1.

Optical configuration of 1-m aperture telescope. Units are in mm. The telescope consists of a Gregorian; the primary mirror (M1) and the secondary mirror (M2) of effective aperture of 1000 mm, a Collimator Mirror Unit (CMU) behind the primary mirror, followed by an image stabilization tip-tilt mirror (TTM) and a polarization mirror unit (PMU). In addition, it has two field stops between the primary and secondary mirrors; one is a heat dump mirror (HDM) at the focus of the primary mirror and the other is a secondary field stop (2FS) at the Gregorian focus.

Figure 2.

Design of secondary field stop (2FS) (left) and confirmation of off-field solar light rejection by HDM and 2FS (right).

To fulfill the achromatism over the wavelength from 280nm to 1100nm, collimator unit was designed with all-reflective system. Among all-reflective collimator designs, we selected on-axis three mirror systems which can accommodate the requirements of wide field performance, compactness in size. A design was found all three mirrors are on-axis aspherical; the surface figure expressed with Zernike first eleven terms, and that is capable to give the diffraction-limited performance, allowing spherically curved focus (field curvature), when combined with the Gregorian within the field of 300 arcsec square. Nominal optical performance of the baseline optics can be designed to give the Strehl ratios in the FOV of 300×300 arcsec² at 632.8 nm greater than 0.999 when a field curvature is allowed (Figure 3). It should be noted that the conic constants of the Gregorian need to be modified and it has a slight aberration even at a field center. By configuring the TTM folding mirror to be an orthogonal reflection to the CMU (Figure 1), an instrumental polarization caused by the CMU can be much reduced.

Figure 3.

Optical layout of tri-mirror collimator unit (CMU, left) and Strehl ratio map of optics combined the Gregorian with the CMU over 300×300 arcsec² FOV (right). Note that a field curvature is compensated in the map.

The field curvature thus designed can be compensated by an off-axis Ritchey Chretien re-imaging optics which has an opposite sign in the field curvature to the Gregorian at an entrance of the focal plane instrument; off-axis design was adopted to avoid any ray vignetting (Figure 4). With this re-imager, an excellent optical performance can be realized in the wide wavelength region from 280 nm to 1100 nm over 180×180 arcsec² FOV (Figure 5). Optical parameters of telescope optics through the Ritchey Chretien re-imager are given in Table 1.

Figure 4.

Optical layout of Ritchy-Chrétien re-imager (left) and Strehl ratio map of whole optics from the Gregorian through the reimager over 300×300 arcsec² FOV (right).

Figure 5.

Strehl ratio vs. wavelength of whole optics from the Gregorian through the re-imager over 300×300 arcsec² FOV (right).

Table 1.

Optical parameters of components for SUVIT telescope assembly

The Heat Dump Mirror (HDM) is located at the primary focus and is a 45deg. flat mirror with a central hole passing through 540 arcsec diameter. The HDM is designed to reflect more than 90 % of incident solar energy out to space through a heat dump window at the side of telescope envelope. The outer diameter of HDM, which is about twice of the diameter of solar image at the primary focus, determines the maximum offset pointing angle from the Sun center allowable for the spacecraft (about 200 arcsec above the limb). The secondary field stop (2FS) is placed at the Gregorian focus for the purpose of further reduction of energy sent to the following optics. The 2FS defines the field of view as 337×337 arcsec², which is slightly oversized the area of the detectors in focal plane instruments with a margin of 15 arcsec (Figure 2). The solar light outside this field is designed to be reflected back through the telescope entrance aperture (Figure 2).

2.2

Structural Design of Telescope Assembly

The framework structure of the telescope assembly should be light weight but sufficiently robust to support and maintain the optical elements with a required positional accuracy against a violent launch environmental conditions and severe onorbit thermal conditions without frequent drives of dedicated alignment mechanisms. The Gregorian optics requires very small static mis-alignment tolerances for the primary and secondary mirrors, on the order of a few tens microns for decenter and de-space or several arcsec for tilt, and a micron-order de-space short-term stability on-orbit during observations. To meet this requirement, the telescope framework will be made of a truss of ultra-low-expansion CFRP (Carbon Fiber Reinforced Plastics) pipes in a Graphite Cyanate matrix as used in Hinode/SOT^[2].

The CTE was proven to be smaller than 0.1ppm K^-1, and the dimensional change due to moisture absorption was measured to be about 30 ppm which is much smaller than conventional epoxy matrix composite pipes: For this Gregorian, aluminum co-cured CFRP pipes and plates will be used to much decrease the dimensional change. Three CFRP sandwich panels are adhesively bonded with upper and lower truss pipes without any metal junctions to save weight and also avoid differential CTE which may cause unexpected telescope thermal distortion.

A conceptual layout of overall framework structure is shown in Figure 3. The center panel ring provides the mechanical interface to the spacecraft; The telescope assembly is supposed to be mounted on the CFRP-made cylindrical optical bench unit (OBU), which also works as a lower tube of the telescope, to the spacecraft with three quasi-kinematic mounting legs with stress-relief spring structures.

Mounting of the primary mirror (M1) is one of the most critical parts of the telescope. The primary mirror, made of light-weighted (about 90% removed) ultra-low CTE glass material, is supported by three stress-free mounting mechanisms (flexible bipod) rooted on a CFRP mirror cell, interfaced with three pads bonded on the side of the mirror. The pad interface of the mounting mechanism is torque-free about three axes and also free in the radial direction, thus providing a kinematic mount for the primary mirror. The pad interface thus avoids stresses to the mirror resulting from dimensional errors in machining or temperature change.

The effect of gravity on the surface figure of such a large primary mirror is a critical issue to verify the optical performance of the telescope on-orbit (zero gravity). We carefully studied the effect of mounting positions on the gravity deformation of surface figure and found that the surface figure deformation is sensitive to the height of the mounting point. Considering that M1 is mounted at three side points and the surface normal is oriented in horizontal direction (horizontal telescope configuration), the minimum surface deformation is found to be about 20 nm rms. Preliminary study of gravity deformation of both framework structure and mirrors in horizontal test configuration shows overall wavefront error due to gravity within a measurable range of laser interferometer.

The secondary mirror (M2) is also made of ultra-low CTE material and light-weighted (about 60 % removed). The secondary mirror unit is mounted on the central disk of a ring plate which is tangentially supported with three spiders from an outer ring. This tangential support works to prevent the M2 from de-spacing causing frequent de-focus error in case of thermal deformation of spider.

The HDM unit is also supported by the ring plate via three tangentially mounting spiders like M2 unit. The mounting points at the ring plate have a mechanism for de-center and tilt adjustment of HDM unit with respect to M2 and HDM. The HDM has through hole through which the HDM can be properly aligned with the M2 vertex. The ring plate is connected to the CFRP top panel ring via three positional and tilt adjustment mechanisms, with which initial optical alignment of the secondary mirror, de-center, de-space (focus), and tilt adjustment with respect to the primary mirror can be performed.

Mirrors of CMU are made of ultra-low CTE material and tangentially supported with leaf springs and configured in triangular structure as shown in Figure 6.

Figure 6.

Structure design of telescope assembly (left) and collimator mirror unit (CMU, right).

The frame structure is covered by a shield tube for the upper half and the OBU for the lower half for the purpose of protecting critical components from molecular and particle contamination, as well as reducing stray light, and ensuring thermal control of the entire telescope assembly. The telescope is estimated to be about 500 kg weigh without the OBU.

2.3

Thermal Design of Telescope Assembly

About 1 kW of solar light is impinged onto the primary mirror at the bottom of the TA during solar observations from its 1-meter diameter entrance aperture. To realize a high-performance solar telescope, a thermal design is critically important which dumps unnecessary heat input to space and maintains critical optical and structural components to within allowable temperature ranges with small temperature fluctuation. In this sense, coating of optical components is critical, which should limit solar light absorption to a minimum, giving high throughput in the observation wavelengths. A silver-based reflective coating is desirable because of low solar light absorption (<6.5%) although it has a problem of very poor reflectivity in the wavelengths below 350 nm[2]. To overcome this drawback, highly enhanced silver coating is under study in which the high reflectivity can extend down to 280 nm (REOSC). In thermal study, we used parameters conventional silver coatings. Provided field stops designed in Figure 2, heat loads into the optical components in the case of BOL and EOL are calculated (Table 2) to design a thermal model of the telescope.

Table 2.

Solar light input and resulted absorbed energy by telescope optical components in the case of begin of life (BOL) and end of life (EOL). In EOL, solar light absorption by mirrors are assumed to be incremented by 5% for each coating.

The significant surface error of the M1 mirror is caused by the difference of CTE between the pads and the mirror substrate; preliminary study of M1 thermal deformation by pads gives sensitive wavefront error (trefoil coma). M1 thermal deformation is very sensitive and its temperature need to be controlled close to a room temperature. Therefore, the thermal design was studied so that M1 temperature is controlled with a cold plate beneath it which has a heater and heat pipes efficiently transferring heats to a radiator placed near the top of telescope. In addition, the temperature of entire telescope structure is maintained close to the room temperature with heater control system. A design was found that the temperature of M1 and entire structure can be maintained close to the room temperature with reasonable radiator area and heater powers.