Development of Ultimate Transmitting Dispersive Grating (Grism)

为了在8.2m Subaru望远镜上安装FOCAS和CIAO日冕成像仪，研制了高效率的棱镜（光栅）。首先，建立了一个测量系统，通过对衍射光强度分布的FFT来评估光栅的刻线形状。该系统随后被扩展以实现衍射光束中效率的绝对测量。采用超精密三维加工系统，在氟树脂红外透射材料上直接刻划沟槽，制备了用于近红外波段的CIAO光栅。以这种方式实际制造的棱镜已经显示出在近红外中给出65 - 70%的衍射效率。用标准的复制方法在玻璃基片上淀积树脂材料，制成FOCAS和Trispec的低色散和中色散光栅，衍射效率可达80%，FOCAS的高色散光栅和CIAO分光模需要用高折射率材料制成的光栅。在折射率大于1.9的光学材料上制作深槽的方法有很多种，但目前还没有一种可行的方法。锗棱镜可以使红外高色散光谱仪的尺寸足够小，从而节省成本并提高仪器的稳定性。该研究表明，实际制造的这种棱镜在10微米波长应用中显示出可接受的性能，但表面粗糙度对于在2微米波长下使用是值得关注的。

Research and development of high efficiency prisms (transmitting gratings ruled on prisms) were carried out for implementation at Faint Object Camera And Spectrograph FOCAS and for Coronagraphic Imager with Adaptive Optics CIAO both to be mounted on the 8.2m Subaru telescope.First of all, a measuring system was set up to evaluate the ruling shape of grisms through the FFT of the intensity distribution of diffracted light. This system was then expanded to enable absolute measurement of efficiency in diffracted beams. Fabrication of grisms for CIAO, to be used in the near infrared wavelength, was tested by ruling grooves directly on infrared transmitting material made of fluorine resine using a hyper fine 3D machining system. The grism actually fabricated in this way has shown to give 65-70% diffraction efficiency in the near infrared. Low dispersion and medium dispersion grisms for FOCAS and for Trispec were made by using the standard replicating method on resin material deposited on glass base plates and was shown to give 80% diffraction efficiency.Grisms made of high refractive index material are required for high dispersion grisms of FOCAS and for CIAO spectroscopic modes. Various methods were tested to produce deep grooves on optical material with refractive index larger than 1.9, but no promising method has yet been identified.Germanium grisms are useful in making the size of the high dispersion spectrograph for infrared small enough to save the cost and to enhance the stability of such an instrument. It was shown in this study that such a grism actually manufactured shows acceptable performance in 10 micron wavelength application but the surface roughness is of concern for use at 2 micron wavelength.