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The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) is a special reflecting Schmidt telescope. LAMOST's special design allows both a large aperture (effective aperture of 3.6 m–4.9 m) and a wide field of view (FOV) (5°). It has an innovative active reflecting Schmidt configuration which continuously changes the mirror's surface that adjusts during the observation process and combines thin deformable mirror active optics with segmented active optics. Its primary mirror (6.67 m × 6.05 m) and active Schmidt mirror (5.74m × 4.40m) are both segmented, and composed of 37 and 24 hexagonal sub-mirrors respectively. By using a parallel controllable fiber positioning technique, the focal surface of 1.75 m in diameter can accommodate 4000 optical fibers. Also, LAMOST has 16 spectrographs with 32 CCD cameras. LAMOST will be the telescope with the highest rate of spectral acquisition. As a national large scientific project, the LAMOST project was formally proposed in 1996, and approved by the Chinese government in 1997. The construction started in 2001, was completed in 2008 and passed the official acceptance in June 2009. The LAMOST pilot survey was started in October 2011 and the spectroscopic survey will launch in September 2012. Up to now, LAMOST has released more than 480000 spectra of objects. LAMOST will make an important contribution to the study of the large-scale structure of the Universe, structure and evolution of the Galaxy, and cross-identification of multi-waveband properties in celestial objects.

A telescope with a larger primary mirror can collect much more light and resolve objects much better than one with a smaller mirror, and so the larger version is always pursued by astronomers and astronomical technicians. Instead of using a monolithic primary mirror, more and more large telescopes, which are currently being planned or in construction, have adopted a segmented primary mirror design. Therefore, how to sense and phase such a primary mirror is a key issue for the future of extremely large optical/infrared telescopes. The Dispersed Fringe Sensor (DFS), or Dispersed Hartmann Sensor (DHS), is a non-contact method using broadband point light sources and it can estimate the piston by the two-directional spectrum formed by the transmissive grating’s dispersion and lenslet array. Thus it can implement the combination of co-focusing by Shack-Hartmann technology and phasing by dispersed fringe sensing technologies such as the template-mapping method and the Hartmann method. We introduce the successful design, construction and alignment of our dispersed Hartmann sensor together with its design principles and simulations. We also conduct many successful real phasing tests and phasing corrections in the visible waveband using our existing indoor segmented mirror optics platform. Finally, some conclusions are reached based on the test and correction of experimental results.

Telescopes with large aspherical primary mirrors collect more light and are therefore sought after by astronomers. Instead of using a single large one-piece mirror, smaller segments can be assembled into a useable telescopic primary. Because the segments must fi t together to create the effect of a single mirror, segmented optics present unique challenges to the fabrication and testing that are absent in monolithic optics. A dispersed fringe sensor (DFS) using a broadband point source is an ef fi cient method for cophasing and is also highly automated and robust. Unlike the widely adopted Shack-Hartmann Wavefront sensor and curvature wavefront sensor with edge sensors for calibration of relative pistons, DFS can estimate the piston between segments by only using the spectrum formed by the transmissive grating’s dispersion, and therefore can replace the edge sensors, which are dif fi cult to calibrate. We introduce the theory of the DFS and Dispersed Hartmann Sensor (DHS) for further utilization of the coarse phasing method of DFS. According to the theory, we bring out the preliminary system design of the cophasing experimental system based on DFS and DHS which is now established in our institute. Finally, a summary is reached.

Paul-Baker systems with 4◦ flat field and 5◦ flat field are studied. Their light obstructions under different f/ratios of the primary mirror are analyzed. Due to the strong f/ratio of the system, a focal length extender is designed in order to match the following fiber instrumentation, and two kinds of dispersion prism correctors are designed for correcting the atmospheric dispersion. We compare the designed Paul-Baker system with LAMOST, the national major scientific project now under construction.

A new method to calculate the optical vignetting of LAMOST (Large Sky Area Muti-Object Fiber Spectroscopic Telescope) is presented. With the pilot survey of LAMOST, it is necessary to have thorough and quantitative estimation and analysis on the observing efficiency which is affected by various factors: the optical system of the telescope and the spectrograph that is vignetting, the focal instrument, and the site condition. The wide field and large pupil of LAMOST fed by a Schmidt reflecting mirror, with a fixed optical axis coinciding with the local polar axis, lead to significant telescope vignetting, caused by the effective light-collecting area of the corrector, the light obstruction of the focal-plate, and the size of the primary mirror. A calculation of the vignetting has been presented by Xue et al. (2007), which considered 4 meter circle limitation and based on ray-tracking. In fact, there is no effect of the 4 meter circle limitation, so that we compute the vignetting again by means of obtaining the ratio of effective projected area of the corrector. All the results are derived by AUTOCAD. Moreover, the vignetting functions and vignetting variations with declination at which the telescope is pointed and the position considered in the focal surface are presented and analysed. Finally, compared with the ray-tracing method to obtain the vignetting before, the validity and availability of the proposed method are illustrated.

A telescope with a larger primary mirror can collect much more light and resolve objects much better than one with a smaller mirror, and so the larger version is always pursued by astronomers and astronomical technicians. Instead of using a monolithic primary mirror, more and more large telescopes, which are currently being planned or in construction, have adopted a segmented primary mirror design. Therefore, how to sense and phase such a primary mirror is a key issue for the future of extremely large optical/infrared telescopes. The Dispersed Fringe Sensor is a non-contact method using broadband point light sources and it can estimate the piston by the two-directional spectrum formed by the transmissive grating's dispersion and lenslet array. In this paper we introduce you the current research progress of the successful design, construction and alignment of our dispersed Hartmann sensors together with its design principles and simulations for indoor segmented mirror experiment system and outdoor segmented mirror experiment system. We also conduct many successful real phasing tests and phasing corrections in the visible waveband using our existing indoor and outdoor segmented mirror optics platform. Finally, some conclusions are reached based on the test and correction of experimental results.

The paper reports optical system of a 1 meter diameter telescope dedicated to Stellar Observations Network Group (SONG) for the National Astronomical Observatories, CAS. SONG is an international cooperative project, Optimize the design for Asteroseismology purposes. According to the prototype of SONG, Chinese SONG telescope includes Cassegrain telescope, Nasmyth platform, Coude train, Coude focal plane, Shack Hartmann wavefront sensor and spectrograph. The telescope will provide a field of view of 65 arcsec at the Nasmyth F/36.67 focus, 10 arcsec at the F/6 Coude focus. The rotating tertiary mirror directs the light to the two Nasmyth foci, one for Shack-Hartmann wavefront sensor, the other for Nasmyth platform and spectrograph.

Telescope with much larger primary can collect much more light and it is always pursued by the astronomers. Instead of using a monolithic primary, more and more large telescopes, which are now planed or in construction, invariably adopted segmented primary mirror. Therefore, how to sense and phase the primary mirror is the key technology. Unlike edge sensors, which need careful calibrations, dispersed Hartmann sensor (DHS) is non-contact method using broadband point light sources, and it can estimate piston by the two-direction spectrum formed by the transmissive grating’s dispersion and lenslet array. Thus it can realize the combination of co-focusing and co-phasing. In this paper, we introduce the design of our dispersed Hartmann sensor together with its principle. We also manufacture a DHS sensor and do real tests on our existing segmented mirror optics platform. Finally some conclusions are given based on the test results.

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) project has completed its engineering work, and is going to finish commissioning around the end of 2010. The LAMOST telescope is with both large aperture and wide field of view to achieve the large scale spectroscopic survey observation. It is an innovative large aperture meridian active reflecting Schmidt configuration achieved by an active deformable Schmidt mirror, which could not be realized by the traditional optical system. Its primary mirror and active Schmidt mirror are both segmented, and composed of 37 and 24 hexagonal sub-mirrors respectively. A new active optics method succesfully developed in the active deformable Schmidt mirror of LAMOST. It is a conbination of the thin deformable mirror active optics and segmented active optics. This paper presents the optical performance of the telescope of LAMOST during optical test. It is shown that LAMOST project successfully resolving the big technical challenges, and making the progress in active optics and telescope technology.

The large sky area multi-object fiber spectroscopic telescope (LAMOST) is a special reflecting Schmidt telescope with its main optical axis on the meridian plane tilted by an angle of 25o to the horizontal. The clear aperture is 4m, working in optical band. The light path is 60m long when working in observing mode and it will be doubled if work in autocollimation mode. So the image quality is affected clearly by the ground seeing and the dome seeing. In order to improve the seeing condition of the long light path, we enclosed the spherical primary and the focus unit in a tunnel enclosure and cooled the tunnel. This is an effective but passive method. Corresponding experiments and simulations show the main part of the aberrations caused by the ground seeing and dome seeing is slowly changed low order items such as tip-tilt, defocus, astigmatism, coma and spherical aberration. Thus we plan to develop the low-order AO system based on the low-cost 37channel OKO deformable mirror for the telescope to better the ground seeing and the dome seeing, not aimed to reach diffraction limited image. This work is being carried on now.