A 2021 study demonstrated a 200 mm diameter optical-grade copolyamide-imide (co-PAI) membrane — coefficient of thermal expansion 0.95 ppm/°C over −150 to +100 °C, coefficient of moisture expansion 13.30 ppm/%RH, sub-nanometre surface roughness, and transmitted wavefront error below λ/30 RMS — establishing a dimensionally stable polymer substrate for membrane diffractive space optics. Its authors explicitly left the scale-up as unfinished work: preparing optical-grade membranes with diameters greater than 400 mm and good space-environment performance. No qualified co-PAI membrane with a diameter greater than 400 mm has been reported to date: same-group membrane work remains at or below 200 mm, the earlier 400 mm spin-cast membranes used a moisture-sensitive polyimide without space qualification, and the largest patterned diffractive elements beyond 300 mm sit on rigid quartz. The open problem is to prepare and qualify an optical-grade co-PAI membrane with free diameter greater than 400 mm, matching the 200 mm baseline's optical quality and thermal/moisture stability, with full-aperture wavefront metrology and good space-environment performance.

Background

Ultra-lightweight transmissive optics for space telescopes replace heavy glass mirrors and lenses with thin polymer films carrying diffractive optical elements. In a membrane diffractive telescope, a large-aperture transparent polymer membrane tens of micrometres thick is patterned with a Fresnel zone lens (FZL) — a concentric-ring diffraction grating that focuses light by constructive interference — so that the aperture mass drops by orders of magnitude relative to glass. The MOIRE (Membrane Optical Imager Real-time Exploitation) ground test-bed program (Domber et al., 2014) demonstrated a large membrane telescope architecture of this transmissive type, and a planned meter-class segmented primary has been proposed as eight 400 mm off-axis membrane FZL segments plus one larger on-axis segment. The practical limit on such films is not diffraction but dimensional stability: orbital temperature swings and moisture absorbed on the ground change the film's dimensions and distort the patterned microstructure. Two material constants quantify this: the coefficient of thermal expansion (CTE, fractional length change per degree Celsius) and the coefficient of moisture (hygroscopic) expansion (CME, fractional length change per percent relative humidity). Traditional aromatic polyimide films have CME around 20–30 ppm/%RH, and their wet-expansion distortion of membrane diffractive lenses is documented (Gao et al., 2019); commercial Kapton film shows a transmitted wavefront error (WFE — the deviation of the transmitted wavefront from an ideal spherical wave, quoted as a root-mean-square fraction of the wavelength λ) of about λ/6, inadequate for precision optics. The same laboratory first established large-film feasibility: Gao et al. (2020) prepared 400 mm diameter, 25 µm optical polyimide membranes by multiple spin-casting and imidization steps and patterned two-level off-axis FZLs on them by vacuum-assisted self-contact photolithography (a contact-printing technique in which evacuating the mask–film gap removes the air pockets that would otherwise distort the exposure over a large area) — but in that moisture-sensitive material and without space-environment qualification. Yin, Mao and Fan (2021) then formulated a copolyamide-imide — a polyimide whose backbone incorporates amide (polyamide) units — and cast optical-grade 200 mm membranes with CTE 0.95 ppm/°C over −150 to +100 °C, CME 13.30 ppm/%RH over 0–90% RH, surface roughness Ra (arithmetic-average surface roughness) below 1 nm, and transmitted RMS WFE below λ/30 at 632.8 nm, together with the thermal and mechanical property characterization underpinning the paper's claim that the membrane simultaneously meets the optical, thermal, and mechanical requirements for space telescope use. Their conclusion states verbatim that the preparation of optical-grade polyimide membranes with good space-environment performance and a diameter of more than 400 mm is under way and will be reported in the near future.

Problem Statement

Prepare an optical-grade copolyamide-imide membrane with free (unobstructed) diameter greater than 400 mm — the scale-up explicitly promised in the conclusion of Yin, Mao and Fan (2021) — and qualify it for space optical applications in the same sense as the 200 mm baseline. Qualification requires demonstrating, on a single membrane: (i) optical-grade film quality, meaning transmitted RMS wavefront error at the λ/30 level at 632.8 nm or an equivalent visible wavelength and surface roughness at the nanometre level (Ra — the arithmetic-average surface roughness — below 1 nm); (ii) dimensional stability matching the baseline, meaning CTE of order 1 ppm/°C over −150 to +100 °C and CME of order 13 ppm/%RH over 0–90% relative humidity, characterized on the membrane material as in the 200 mm baseline; and (iii) good space-environment performance in the same sense as the 200 mm baseline, meaning the baseline's material-level thermal and mechanical properties (glass-transition and thermal-decomposition temperatures, and tensile strength and modulus) that were shown to meet the requirements for space telescope use; this sense does not extend to irradiation or vacuum-exposure testing. The preparation route (monomer composition, solution preparation, casting and imidization conditions, release from the carrier) must be reported in reproducible detail, and the wavefront metrology must cover the full >400 mm aperture rather than cut samples.

References

  1. Yin, Jiajia; Mao, Danbo; Fan, Bin (2021). 'Copolyamide-Imide Membrane with Low CTE and CME for Potential Space Optical Applications'. Polymers 13(7):1001. DOI 10.3390/polym13071001. https://doi.org/10.3390/polym13071001
  2. Gao, Guohan; Wang, Lihua; Shi, Heng; Liu, Dun; Fan, Bin; Guan, Chunlin (2020). 'Facile large-area uniform photolithography of membrane diffractive lens based on vacuum assisted self contact method'. Scientific Reports 10. DOI 10.1038/s41598-020-65990-2. https://doi.org/10.1038/s41598-020-65990-2
  3. Gao, Guohan; Mao, Danbo; Fan, Bin; Guan, Chunlin (2019). 'Effect of Wet Expansion Behavior on Polyimide Membrane Diffractive Lens'. Coatings 9(9):559. DOI 10.3390/coatings9090559. https://doi.org/10.3390/coatings9090559
  4. Yin, Jiajia; Hui, Haohao; Fan, Bin; Bian, Jiang; Du, Junfeng; Yang, Hu (2022). 'Preparation and Properties of Polyimide Composite Membrane with High Transmittance and Surface Hydrophobicity for Lightweight Optical System'. Membranes 12(6):592. DOI 10.3390/membranes12060592. https://doi.org/10.3390/membranes12060592
  5. Wu, Xiang; Fan, Bin; Xin, Qiang; Gao, Guohan; Jiao, Peiqi; Shao, Junming; Luo, Qian; Liang, Zhaoyu (2023). 'The Tailored Material Removal Distribution on Polyimide Membrane Can Be Obtained by Introducing Additional Electrodes'. Polymers 15(10):2394. DOI 10.3390/polym15102394. https://doi.org/10.3390/polym15102394
  6. Yin, Jiajia; Fan, Bin; Du, Junfeng; Yang, Hu; Bian, Jiang (2023). 'Fabrication of a Composite Film Optic with High Transmittance in Vis and IR Regions for an Optical System'. Coatings 13(5):811. DOI 10.3390/coatings13050811. https://doi.org/10.3390/coatings13050811
  7. Gao, Guohan; Shi, Heng; Wang, Lihua; Liu, Dun; Wang, Jiong; Du, Junfeng; Bian, Jiang; Fan, Bin; Yang, Hu (2022). 'Large aperture high diffraction efficiency off-axis Fresnel lens fabrication and analysis'. Optics Express 30(16):28932. DOI 10.1364/OE.464494. https://doi.org/10.1364/OE.464494
  8. Luo, Qian; Gao, Guohan; Liu, Dun; Du, Junfeng; Fan, Bin (2023). 'Wavefront measurement of a large aperture high image quality off-axis Fresnel lens'. Optics Express 31(2):1249. DOI 10.1364/OE.479611. https://doi.org/10.1364/OE.479611
  9. Liu, Xin; Li, Min; Li, Bincheng; Fan, Bin (2022). 'Membrane–Fresnel Diffractive Lenses with High-Optical Quality and High-Thermal Stability'. Polymers 14(15):3056. DOI 10.3390/polym14153056. https://doi.org/10.3390/polym14153056
  10. Domber, Jeanette L.; Atcheson, Paul D.; Kommers, Jeff (2014). 'MOIRE: Ground Test Bed Results for a Large Membrane Telescope'. AIAA SciTech Forum, Spacecraft Structures Conference. DOI 10.2514/6.2014-1510. https://doi.org/10.2514/6.2014-1510
  11. Ageorges, Nancy; Eberle, Sebastian; Kampf, Dirk; Rabien, Sebastian (2024). 'Membrane mirror space telescope: case study of a deployable large aperture'. Proceedings of SPIE 13092, Space Telescopes and Instrumentation 2024: Optical, Infrared, and Millimeter Wave. DOI 10.1117/12.3019764. https://doi.org/10.1117/12.3019764
  12. Fu Jun; Yin Jiajia; Shao Junming; Fan Bin; Bian Jiang; Du Junfeng (2023). '复合薄膜衍射透镜制备及耐空间辐照性能研究' [Preparation and space irradiation resistance of composite thin-film diffractive lens]. Laser & Optoelectronics Progress 60(13):1316018. DOI 10.3788/LOP230453. https://doi.org/10.3788/LOP230453