The Long wave (11-16 µm) spectrograph for the EChO M3 Mission Candidate study.

Transcription

1 The Long wave (11-16 µm) spectrograph for the EChO M3 Mission Candidate study. N. E. Bowles M. Tecza J. K. Barstow J. M. Temple P. G. J. Irwin L. N. Fletcher S. Calcutt Department of Physics Clarendon Laboratory Parks Road Oxford OX1 3PU UK Fax: J. Hurley M. Ferlet Science and Technology Facilities Council Rutherford Appleton Laboratory Harwell Campus OX11 0QX Didcot UK D. Freeman Kidger Optics Associates Great Malvern UK Keywords: EChO Instrument Spectrometer Exoplanet Transit Spectroscopy Abstract The results for the design study of the Long Wave Infrared Module (LWIR), a goal spectroscopic channel for the EChO ESA medium class candidate mission, are presented. The requirements for the LWIR module were to provide coverage of the µm spectral range at a moderate resolving power of at least R=30, whilst minimising noise contributions above photon due to the thermal background of the EChO instrument and telescope, and astrophysical sources such as the zodiacal light. The study output module design is a KRS-6 prism spectrograph with aluminium mirror beam expander and coated germanium lenses for the final focusing elements. Thermal background considerations led to enclosing the beam in a baffle cooled to approximately K. To minimise diffuse astrophysical background contributions due to the zodiacal light, anamorphic designs were considered in addition to the elliptical input beam provided by the EChO telescope. Given the requirement that measurements in this waveband place on the performance of the infrared detector array, an additional study on the likely scientific return with lower resolving power (R<30) is included. If specific high priority molecules on moderately warm giant planets (e.g. CO 2, H 2 O) are targeted, the LWIR channel can still provide improvements in determining the atmospheric temperature structure and molecular abundances. Thus, the inclusion of even a coarse-resolution (R 10) LWIR module would still make an important contribution to measurements of exoplanet atmospheres made by EChO. 1

2 1 Introduction This paper describes the optical and mechanical design of the Long Wave Infrared (LWIR) channel for the proposed EChO mission payload instrument (Eccleston et al. 2014, Tinetti et al. 2012). The EChO payload is a multi-module spectrometer that allows simultaneous wavelength coverage from 0.5 to 11.5 µm with a goal to extend the spectral range to 16 µm. Each module is designed to cover a separate spectral range with divisions at approximately , , and µm. Measurements at wavelengths longer than 11 µm present challenges due to the comparatively lower flux in this spectral region from the target star compared with shorter wavelengths, hence the LWIR channel was defined as a goal within the original EChO design study. At these wavelengths, thermal emission from both the telescope and the instrument module, combined with the astrophysical background, can form a large fraction of the signal at the detector (e.g. Puig et al. 2014). The design described in this paper includes features to minimise these contributions, and also describes options for optimising the resolving power λ/ λ (nominally R=30 defined in the study (Puig et al. 2014)) of the instrument. Given the limited availability of high performance detectors with high technology-readiness and operating temperatures of > 7 K, a study that targets specific molecules (e.g. CO 2, H 2 O) at lower resolving power (R 10) is also described. The LWIR module would measure the µm spectra of a wide variety of planetary types, from hot and temperate Jupiter-class worlds to smaller Super Earths and Neptunes. Although a goal for the mission study, this spectral range contains several key diagnostic features for exoplanetary dynamics and chemistry. The spectral range includes the rotation-vibration bands of common species such as H 2 O and CO 2, plus a host of more exotic hydrocarbons (e.g., ethane, C 2 H 6 and acetylene, C 2 H 2 ); nitriles (e.g., HCN); ammonia (NH 3 ) and phosphine (PH 3 ). Specifically the inclusion of the LWIR module would extend EChO s spectral coverage to capture the key 15 µm CO 2 band and better constrain the overall shape of planetary spectra, permitting more unambiguous extraction of atmospheric temperature profiles (via fitting of the CO 2 band and broad collision-induced spectra of H 2, He, N 2 and CO 2, among others) and aerosol signatures, particularly long-wave silicate features. Furthermore, the peak blackbody emission from a more Earth-like world occurs at longer wavelengths, making this spectral range crucial for characterising the climate of these terrestrial planets. 2 The Long Wave Infrared (LWIR) Channel: Minimum Requirements and Derivation of Performance The LWIR channel provides spectral coverage from µm, with a minimum resolving power (λ/ λ) of R=30. To provide some initial signal-to-noise estimates, typical stellar photon fluxes were defined based on bright and faint cases as well as an empirical model of the zodiacal light (Puig et al. 2014, Ecclestone et al. 2014). During this study several designs were considered, including classical dispersive spectrometers based on prisms and gratings, as well as a static, spatially modulated Fourier transform interferometer (Reininger.2001) One of the main concerns for measurements at wavelengths greater than 11 µm was managing the signal contributions at the detector due to broadband thermal emission from the telescope, instrument enclosure and final focusing optics of the spectrograph. Based on the radiometric model described in (Puig et al 2014), it was also clear that contributions due to the diffuse zodiacal light would become a dominant astrophysical noise source, which the design would also need to accommodate. In summary, in addition to the spectral range (11-16 µm) and resolving power (R 30), the following constraints were also included in the study to support the overall photometric requirements of the complete EChO spectrometer instrument: Static designs: no moving parts in the optical system. Good optical throughput (> 0.25) across the LWIR spectral band. Photon noise-limited operation, if permitted by available detector arrays, for the bright and faint limits defined in (Puig et al. 2014). Flexible design allowing for o o Contributions from the diffuse zodiacal astronomical and payload module thermal backgrounds to be managed; Adjustment of (fixed) resolving power to accommodate near-term future developments in LWIR detectors. Analysis of a static Fourier transform (FTS) design based on beam-shearing prisms showed that whilst having advantages of excellent (> 60%) optical throughput and programmable resolution, sources of uncertainty inherent for this type of interferometer could reduce the instrument s ability to work in the photon noise limit. In particular, the 2

3 noise model described in (Reininger.2001) showed that the phase error introduced by uncertainties in the position of the pixels in the detector array could produce a source of noise comparable to the photon noise ( 11 e - /s). The data rate for a FTS design would be higher by approximately a factor of 2, as it is necessary to include at least some (~10-20) pixels on the other side of the central burst to allow a phase correction to be applied. As a result of these considerations, the static FTS design was not pursued further. Of the purely dispersive options, a straightforward prism spectrograph offered the simplest option for the LWIR module. The only complication was the availability of a suitably dispersive, yet transparent material in the µm wavelength range. Materials such as ZnSe were considered, but were rejected due to the presence of strong absorption bands with associated temperature dependence at wavelengths > 14 µm. Multi-prism designs using cadmium telluride were studied, but rejected due to the increased number of optical surfaces. Further analysis of available prism materials (using data from e.g. Palik 1998) showed that a system based on the thallium bromide/chloride glass (KRS-6) allowed a comparatively straightforward design of beam expander, single prism and germanium lenses that met all of the EChO mission requirements for the channel. With the basic configuration of the LWIR module, a prism spectrograph, now fixed the detailed design of the module and its connection to the rest of the EChO instrument is discussed. 3 The LWIR module as part of the EChO instrument common optics layout In the EChO instrument all the opto/mechanical systems are attached to a single, monolithic optical bench that is passively cooled to ~45 K (Ecclestone et al. 2014). To maximise the efficiency of the thermal interface to the 45 K optical bench (Figure 1, Figure 2, Eccleston et al. 2014), the LWIR module is attached via four M6 screws (Figure 2, Figure 3). However, given the sensitivity of the module s performance to the thermal background in the instrument, (Ecclestone et al. 2014) additional thermal straps from the active cooler stages used by the LWIR and mid-wave detectors would also be required to cool regions of the module close to the optical path to approximately K. This reduces the thermal load on the detector from the instrument, especially in regions close to the final focal plane assembly (Figure 3). The thermal load is further reduced by controlling the focal ratio of the final lens (Section 4) to decrease the solid angle subtended by the final transmissive components to the detector array. However, the contribution to the background noise due to the zodiacal light can also be significant, so a trade-off between spatial and spectral sampling was also included in the study (Section 4.1). Light from the EChO telescope is passed to the LWIR module via a chain of three channel division dichroics (Figure 2) mounted on the instrument s common optical bench. 3

4 SWIR From telescope D1b VNIR D1 BS D2 D3b MWIR 2 D3 MWIR 1 LWIR Figure 1. The LWIR as part of the EChO front and common optics concept (module labels as in Figure 2) (Ecclestone et al. 2014). Upper right shows the relative size, nominal position and orientation of the LWIR module s field of view at the telescope s intermediate focus in relation to the other optical modules. The lower right shows a schematic of the channel divisions by dichroics. Figure 4-9:Left: chosen layout of the optical modules. Right: MICD extract showing dimensions and Figure 2. Position of the LWIR channel in relation to the other EChO optical modules on the Instrument Optical Bench (IOB) and Telescope Optical Bench (TOB),(Ecclestone et al. 2014). The other channels are labelled Shortwave Infrared (SWIR), Visible-Near Infrared (VNIR), Fine Guidance Sensor (FGS) and Mid-wave Infrared (MWIR). 4 LWIR Module Design overview 4.1 Optical Design Combining the performance and accommodation requirement for the LWIR module as part of the EChO instrument study gives a baseline design for the instrument that is based on a prism spectrograph (Figure 3). The LWIR module takes the elliptical 25mm 17 mm diameter collimated input beam from the common optics chain described in Section 3, and passes it through a 1.5 two-mirror beam expander. The beam expander includes a slit mask compatible with an 8.3" x 20" rectangular field baffle at the intermediate focus. The beam is then dispersed by a KRS-6 4

5 prism with a 30 apex angle. As described in Section 2, the choice of prism material having both sufficient dispersion and low absorption (<0.7) is somewhat limited in the µm wavelength range. The material selected for the baseline design is KRS-6, a thallium bromide/chloride crystal, which was considered a useful compromise. The dispersed beam is then focused onto a two-dimensional detector array by a single, rotationally-symmetric germanium lens. The spectrum is recorded on ~50 pixels, assuming a 25 µm pitch array. The focal ratio of the final lens was selected so as to minimise the thermal load on the detector from non-astrophysical background sources, such as selfemission from the payload module itself, resulting in a working focal ratio of the system of approximately 2. To prevent longwave (>16 µm) thermal radiation from reaching the detector, a bandpass filter window with cut-offs at 11 and 16 µm is fitted to the detector array. Although contributions from astronomical scene at wavelengths <11µm are removed by the dichroic chain feeding the module from the telescope (Figure 2), some contribution remains due to thermal emission from the spectrometer module. Using the model of the zodiacal light described in Puig et al. 2014, this diffuse source of infrared radiation is expected to be a significant contribution to the signal in the LWIR channel, especially for the fainter star targets (Puig et al. 2014, Ecclestone et al. 2014). One approach considered to reduce the relative spatial contribution of this diffuse zodiacal light is to use an anamorphic system to maintain the same spectral sampling whilst increasing the spatial sampling. This can be achieved in the presented optical design by combining the elliptical pupil, provided by the common optics chain, with the option of an anamorphic beam expander. Figure 3. Baseline LWIR optical layout. The approximate dimensions are 139 x 347 x 182 mm, including a JWST MIRI derived detector housing. 4.2 LWIR Optical Performance. From the design described in Section 4.1, and assuming that three 25-µm pixels are used per spectral sampling element, the spectral resolving power varies between approximately 27 and 56, with an average value of 42 across the LWIR wavelength range of µm, meeting the requirements described in (Puig et al. 2014). The resolving power estimate for the module is based on the assumed input telescope beam and diffraction but no other aberrations have been included. The mechanical envelope of the module on the instrument optical bench has sufficient margin to accommodate a prism with increased dispersion (e.g. increasing the resolving power to across the LWIR band) allowing higher spectral resolution. However, any increase in spectral resolution will necessarily result in a decrease in signal to noise so is restricted by the choice of detector. KRS-6 has good transmission properties across the LWIR bandpass, with an estimated transmission of 75% at an incidence angle of 30 (Figure 5). The transmission was calculated using the data from Palik 1998 and the FilmStar software (Goldstein 2014). The Germanium lens is coated with a multilayer broadband anti-reflection coating based on the flight design used by the mid-infrared spectrometer on the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft, currently in orbit around Saturn (Cole and Bowen 1994). The anti-reflection coating gives transmission across the LWIR spectral range varying between approximately 0.65 and 0.8 (Figure 5). 5

6 Figure 4. Approximate resolving power for the LWIR baseline design using a single KRS-6 based prism. 4.3 LWIR Mechanical Design Overview Figure 5. Approximate transmission for the coated Ge lens and KRS-6 prism. The mechanical accommodation of the LWIR channel spectrometer (Figure 6) uses aluminium optical mounts and assumed diamond-turned mirrors. The mirrors are supported using three axis kinematic mounts. The cryogenic germanium lens and KRS-6 prism are mounted using arrangements derived from the Cassini/CIRS instrument (Kunde et al. 1996). The estimated mass with 20% margin applied was 5.47 kg, with dimensions of a maximum of 139 x 347 x 6

7 182 mm. To control the thermal background at the detector, a housing based on the MIRI instrument for the James Web Space Telescope (JWST) has been used (Ressler et al. 2008). Figure 6. Cut-away showing mechanical accommodation for the LWIR channel. Approximate dimensions are 139 x 347 x 182 mm, with a mass of 5.47 kg with 20% margin and thermal multi-layer insulation blankets (not shown). 4.4 LWIR module Thermal Design Considerations As described in Section 2, the module enclosure is kept in good thermal contact with Payload Module Optical bench via four M6 bolts, at a nominal 45 K (Figure 6). The exterior of the module is covered in multi-layer insulation blankets to reduce the radiative load from the rest of the payload module. To reduce the background signal at the detector due to thermal emission from the instrument enclosure, additional baffles are included around the optical path and are cooled to approximately K using the active cooler (Figure 6). Accurate temperature control (±100 mk) of the enclosure is also essential for maintaining the goal photometric stability of ~10-5 described in Puig et al The baffles create a low-temperature inner-sanctum within the instrument enclosure (Ecclestone et al. 2014), and the transmissive optical component mounts are thermally-isolated from the optical bench. For example, the germanium lens mount is based on the Cassini/CIRS instrument cryogenic design, and a similar mounting system is used for the prism that includes the necessary thermal isolation (Kunde et al. 1998). 5 Performance vs. Requirements The LWIR module design has sufficient spectral resolving power (between 27 and 56) across its waveband (Puig et al 2014) and sufficient optical throughput of to meet the requirements for the goal LWIR channel. The LWIR slit is sized so that the given (full) widths are determined by the size of the Airy discs at 16 µm, with the necessary added margin for static random pointing error (fine Absolute Pointing Error (APE) and quasi-static Relative Pointing Error (RPE) residual) and co-alignment. The optical design of the channel has been optimised for a 25-µm pitch detector array and assumes 3 pixels per spectral sampling element. With the optimisations described above, the final limiting factor in the noise performance of the module is the choice of detector array. For a detector based on the Si:As detector arrays used in the the MIRI instrument that is part of the JWST the performance of <1 e - /s (Ressler et al. 2008) allows higher resolving powers than R=30. However, inclusion of MIRI-like detectors places additional complications on the cooling system as they require an additional stage to reach their operating temperature of 7 K. Signal to noise calculations using the design parameters described in this paper are detailed in Eccleston et al using the faint (GJ1214, (G0V, Ks=9.0)) and bright (55Cnc (K0V, Ks=4.0)) targets defined in Puig et al and the simulation tool of Waldmann and Pascale The analysis of Eccleston et al concluded that for the bright case that photon noise from the star would dominate the total noise measured by the the EChO instrument, including the LWIR channel. For the faintest target the detector performance becomes the major contribution. The performance described above assumes JWST MIRI-like detectors, with the associated complication of cooling to 7 K. At the time of the study, alternative detector technologies were under consideration (e.g. Hogue et al. 2010) that had higher operating temperatures (>20 K) but lower dark current performance, typically of order a few 100s of e - /s. This reduces the requirement for additional cooling, but has lower performance in terms of dark current compared to the MIRI Si:As detector. If detector dark current noise is a significant noise source then a set of science-derived trade-offs for the LWIR module at lower resolving power is necessary and this is discussed in the next section. 7

8 5.1 Options for resolving powers R<30 a spectrophotometer Although the goal requirements for the LWIR channel are R>=30, if the spacecraft resources available restrict the cooling solutions for detectors and thermal background in the instrument even this resolving power may be difficult to achieve. However, given the lack of measurement data in this spectral range, there is still much useful information to be gained from including a revised µm channel with even a comparatively low (R<10) spectral resolving power or a few (~5) photometric points. Although a higher spectral resolving power is always preferable for a survey-type mission such as EChO, this section briefly summarises the science case for including an instrument capable of measuring radiances with as few as roughly five spectral points within the µm range. The analysis is based on the approach described in Barstow et al using the NEMESIS retrieval model described in Irwin et al and assumes that specific chemical species such as H 2 O, CO 2, CO, CH 4 and NH 3 are targeted. 5.2 Example spectrophotometer science case for microns at resolving powers of R< Example: Warm Jupiter For Hot Jupiters, there is sufficient signal in the thermal emission spectra at shorter wavelengths such that their atmospheric properties can be inferred without measurements at wavelengths longer than 11 µm (although the µm region remains useful for constraining the overall shape of the spectrum and providing redundancy in the detection of molecules such as water and carbon dioxide, as described in the introduction). However, cooler planets emit at longer wavelengths. For a warm Jupiter (equilibrium temp ~650 K) orbiting a sun-like star 35 pc away, EChO would need to observe at least 30 eclipses to retrieve the temperature structure and abundances of trace species H 2 O, CO 2, CO, CH 4 and NH 3 from the planet/star flux ratio spectrum (Barstow et al. 2013a). We repeat the analysis of Barstow et al. 2013a for this case (Figure 7), including only wavelengths up to 11 µm and then testing the effect of also including four broadband points up to 16 µm. We find that observations up to 11 µm can provide good constraints on the temperature profile between and 1 bar, and can also result in an accurate retrieval of H 2 O, CO and NH 3 abundances; however, CO 2 and CH 4 are not correctly retrieved to within 1σ error. If instead four broadband points (width 1.4 µm) between µm are also included, we find that the temperature below 1 bar, CO 2 abundance and CH 4 can now also be correctly retrieved. This is good evidence that, even at low resolving power, the µm wavelength range provides important information about the atmospheres of cooler exoplanets, and its inclusion as part of EChO is strongly justified. 8

9 Figure 7. Temperature retrievals for 30 transits of a warm Jupiter orbiting a sun-like star if different spectral ranges are observed. It can be seen that including µm improves the quality of the temperature retrieval in the deep atmosphere below 1 bar, and the region just above the tropopause. This improvement is seen in the deep atmosphere even for the 4-channel case. The shaded regions indicate the 1σ error on the temperature retrievals. Input (ppmv) Retrieved (11 µm) Retrieved (4 points µm) Retrieved (11 16 µm R=30) H 2 O (2.88) (2.94) (2.70) CO (34.3) (25) (23.3) CO (242) (304) (363) CH (15.9) (14.8) (16.5) NH (104) (103) (111) Table 1. Retrieved trace gas abundances (volume mixing ratios in 100 ppmv) compared with the input. The +/-1σ range is given for the retrievals, with the best-fit value in parentheses. CO 2 and CH 4 VMRs are not retrieved correctly to within 1σ error if the long wavelength cut off is at 11 microns, but including four points up to 16 microns results in a correct retrieval for all gases. However, including microns at R=30 results in an incorrect retrieval for CH 4 and NH 3, although the retrievals for H 2 O and CO2 are further improved. This serves to illustrate the complexity of the retrieval problem and the degeneracy of solutions. 6 Conclusions The goal channel for the µm spectral range (LWIR) was included in the EChO instrument study and an optical and mechanical design developed that is capable of achieving the spectral and thermal requirements demanded by measurements at these longer wavelengths. The eventual study design was a compact prism-based spectrograph with options to minimise the effect of instrument thermal loads and diffuse background due to the zodiacal light. The main spacecraft system complexity drivers for the LWIR channel come from the cooling requirements for the instrument module. The detectors with the highest level of technical maturity are Si:As BIB detectors; however these require an 9

10 additional cooling stage to reach 7 K. In addition, to reduce the background due to thermal emission from the module itself, the transmissive components and optical path require cooling to K. However, even at reduced spectral resolving power, the LWIR channel can make a valuable contribution to the science goals of the EChO or similar mission by constraining the overall shape of an exoplanetary spectrum, helping to break the significant degeneracies inherent in inversions of exoplanet transit spectroscopy. 7 Acknowledgements Support for this work was provided by the United Kingdom Space Agency as part of UK EChO study team. We would also like to acknowledge the significant help and assistance provided throughout the study by the whole EChO team, especially B. Swinyard, P. Ecclestone, G. Tinetti and E. Pascale for many useful discussions. 8 References: Barstow, J et al : On the potential of the EChO mission to characterize gas giant atmospheres MNRAS 430 (2013) Cole, C. and Bowen, J. W.: Synthesis of broadband anti-reflection coatings for spaceflight infrared optics, Proc. SPIE 2210, doi: / Eccleston, P., Swinyard, B., Tessenyi, M., Tinetti, G., et al.: The EChO payload instrument an overview. Experimental Astronomy (2014). doi: /s Goldstein, F.: Filmstar Thin Film Optical Design Software. (2014). Accessed December Hogue, H. et al : Update on Blocked Impurity Band detector technology from DRS. Proc. of SPIE Vol , Irwin, P. G. J., et al.: The NEMESIS planetary atmosphere radiative transfer and retrieval tool. Journal of Quantitative Spectroscopy and Radiative Transfer 109, (2008) Kunde, V. et al., in Cassini/Huygens: A Mission to the Saturnian Systems, L. Horn, Ed., SPIE Proc. 2803, 162 (1996). Palik, E. D.: Handbook of Optical Constants of Solids volume 3. Academic Press (1998). Puig, L., Isaak, K., Linder, M., Escudero, I., Crouzet, P. et al.: The Phase 0/A study of the ESA M3 mission candidate EChO. Experimental Astronomy (2014). doi: /s Reininger F. M. : Infrared Physics and Technology 42 (2001) Ressler et al.: Performance of the JWST/MIRI Si:As detectors Proc. Of SPIE Vol 7021, (2008) / Tinetti, G., et al.: EChO Exoplanet Characterisation observatory. Experimental Astronomy 34, 311 (2012) Waldmann, I., Pascale, E.: Data analysis Pipeline for EChO end-to-end simulations arxiv, 1402,4408 (2014) doi: 10