Optical design of imaging and spect rograph for 4m ...

Optical design of imaging and spectrograph for 4m telescope in China Hangxin Ji*a,b, Yongtian Zhua,b, Zhongwen Hua,b, Yi Chena,b, Lei Wanga,b, Mingming Xua,b, Songxin

Daia,b, Huatao Zhanga,b

a National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing 210042,China;

b Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing 210042,China.

ABSTRACT

The design and performance of a three-channel image and long-slit spectrograph for the new 4-m telescope in China are described. The direct imaging covers a 3 arcmin by 3 arcmin field of view and a large wavelength range 370-1,600 nm, it has two optical channels and one near infrared channel with different filters. The spectrograph with a long slit is to provide two observing modes including the following spectral resolutions: R1000 and R5000. For dispersing optical elements it use volume-phased holographic grisms (VPHG) at each of the spectroscopic modes to simplify the camera system. The low resolution mode (R1000) is provided by consecutive observations with the spectral ranges: 360-860 nm, however it adopts only one VPHG for the first light. The spectral range of medium resolution mode (R5000) is 460-750nm, it is constrained with the use of a 4k × 4k CCD detector of 15 μm pixel size. Peak efficient in the spectrograph are achieved to be higher than 50% in different resolution mode.

Keywords: Imaging and spectrograph, three-channel imaging, low and medium resolution spectrograph, VPHG

1. INTRODUCTION The IMaging and SPectrograph (IMSP) is a new instrument under development for 4-m telescope in China. Several instrument conceptual designs have been proposed since 2010, three-channel image and long-slit spectrograph was finally selected as the first light instrument in June, 2014. IMSP designed by Nanjing Institute of Astronomical Optics & Technology (NIAOT) has finished the Preliminary Design Phase (PDP) reviewer at the end of 2017 and it is currently in its Detailed Design Phase (DDP) with an expected start of science operations in 2021. The IMSP will be installed on one of the Nasmyth foci of the telescope. General goals for the design of IMSP were to produce a high efficiency with wide range wavelength coverage while minimizing complexity, size and cost. IMSP will be a versatile instrument designed to operate in a number of scientific modes: three-color simultaneous imaging, low and medium resolution long-slit spectrograph, interface for fiber-fed high resolution spectrograph is also preserved. Actually, it is not a traditional imaging spectrograph, the direct imaging and spectrograph has separate subsystems due to different wavelength coverage.

The imaging has two optical channels and one near infrared channel. With the current optical design, IMSP has a large wavelength rage 370-1,600 nm and the blue cutoff wavelength which is constrained by the choice of telescope Ag coating. The imaging is set by the cross over wavelength of the dichroic (950 nm) where CCDs are sensitive in each channel. The optical channels separates at 550 nm and equipped with at least 10 optical filters. In the blue channel it has the capability of high frame rate with EMCCD, while with a conventional CCD in red channel. The near infrared channel cover the whole Y and J bands due to the sensitivity of InGaAs detector and with the capability of extensive application in future. The direct imaging covers a 3 arcmin by 3 arcmin field of view and it is constrained by the telescope fork central hole and the design difficulty of field de-rotation system. The image de-rotator made up by three flat mirrors which is so called “K-mirror”, moves together on a precision rotating stage.

The spectrograph will have two spectral resolution modes for the first light. For dispersing optical elements it use volume-phased holographic gratings (VPHG) at each of the spectroscopic modes to obtain the high efficiency and simplify the camera system. The low resolution spectrograph (LRS, R1000) is provided by consecutive observations with the spectral ranges: 360-860 nm, however it adopts only one VPHG for the first light. The spectral range of medium

Ground-based and Airborne Instrumentation for Astronomy VII, edited by Christopher J. Evans, Luc Simard, Hideki Takami, Proc. of SPIE Vol. 10702, 1070224 · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2309911

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resolution spectrograph (MRS, R5000) is 460-750nm, it needs two exposures to cover the required wavelength range, and it can also be easily extended to other wavelength by using different VPHGs in future.

Light losses in the spectrograph are achieved to be less than 50% in the whole spectral range due to the use of excellent internal transmittance glasses, high efficiency transmission and reflection coating and high quantum efficiency CCD. The loss in the LRS will be a little higher than MRS due to the using of a broadband VPHGs.

This paper will focus on the IMSP optical design and analysis which is under NIAOT responsibility. Section 2 of this paper introduces the IMSP key requirements. Section 3 summarizes the optical design proposed for IMSP in PDP and the key performance of the spectrograph coming from the optical design and associated analysis. Finally, section 4, compiles the summary and future plans.

2. KEY REQUIREMENTS The IMSP key design requirements are derived from main science cases and tried to meet most of the demanding from the astronomy community in China. The main requirements are listed below:

1. The IMSP shall have a separate de-rotator system to accommodate different observation mode.

2. The IMSP shall have the capability of three-color simultaneous imaging, two optical channels and one near infrared channel:

a) It shall provide an unvignetted science FoV of at leastФ3 ′ (Goal: 5′).

b) The wavelength range shall cover the whole optical region down to 370nm (goal: 360nm), and includes the whole Y and J bands.

c) The “blue” channel shall have a high frame rate imaging capabilities: >10 frame per second.

3. The IMSP spectral resolving power shall be:

a) R~1000 for 360 nm≤λ≤860 nm with 1.5″slit, and

b) R~5000 for 460 nm≤λ≤750 nm with 1.5″slit.

c) The IMSP throughput, defined as the percentage of light available at the exit of the slit surface that is delivered to the detector, shall be greater than 45% at the peak efficiency.

d) The slit length shall be at least 3 arcmin long and have at least two slit width options (1.5″ and 1 ″) for different seeing condition.

e) The IMSP spectral sampling shall be ≥ 2.5 pixel (goal 3.0 pixels).

4. All system shall be on the platform with the size of 1.2m x 1.8m and mass < 2 tons.

3. INSTRUMENT DESCRIPTION 3.1 Overview

The IMSP is composed of three sub-systems which including de-rotator unit, imaging and spectrograph. Each sub-system should meet the key requirements listed in section 2. The main sub-units are shown in figure 1. The de-rotator unit will be installed in the fork central hole of the telescope. There are two platforms for supporting the imaging and spectrograph. The imaging, calibration unit and Tip-tilt system will be on the top platform. The spectrograph including collimator, VPHG, cameras and detector will be put on the bottom platform. The slit viewer unit and slit exchanger will be located between the two platforms.

The F/13 light from the telescope went through the de-rotator unit and reimaged into three channels using two dichroic mirrors, each channel had its filed lens, reimaging system, filter wheel and detector. A tip-tilt mirror would be inserted in the optical axis after de-rotator mirrors when spectrograph mode works. After the light imaging on the slit exchanging system, a slit viewer system will monitor the star position. The target through the slit was then collimated by a collimator system, a reflect mirror was used to fold the optical path and make the whole system more compact. Two resolution modes could be exchanged by using different VPHG + prisms and no need to rotate the camera system to accommodate


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different diffraction angle. The camera system adopted the traditional refractive system and the spectrum was finally imaged on the detector.

Figure 1: Block Diagram of the IMSP

3.2 De-rotator Unit

Due to its alt-azimuthal mount, a de-rotator unit has to fulfill demanding requirements to keep the image stability during a scientific exposure. There are many ways to compensate the field rotation, K-mirror design has been selected for its anastigmatic and achromatic characteristics. In view of the telescope’s configuration, it is required to place this unit between M3 mirror and the science instruments. Due to the limited envelope for the de-rotator unit, it will be integrated in the current telescope fork central hole. The providing housing for the de-rotator will be fixed on the Nasmyth flange and the main parts will be in the central hole.

The K-mirror consists of three mirrors, M1, M2 and M3 assembled together in a rigid and stable rotating stage, a simplified K-mirror structure without light-weight shell is shown in figure 2. In order to reduce the difficulty of alignment procedure, the M1 and M3 are manufactured as a monolithic glass. In avoid to add significant aberrations caused by gravity effects, the mirrors must keep their rigidity, planarity and maximization the light-weight ratio. A detailed mechanical structure analysis and optimization will be evaluated during the DDP.

Figure 2: The Opto-Mechanical structure of K-mirror


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The diameter of the allocated space is 610mm and the original distance between flange and Nasmyth focus is approximately 520mm. The telescope system focal ratio is F/13, the closer the distance between K-mirror and the focus, the smaller size of the de-rotator and the larger system FoV can be, however, the smaller distance make the spare envelope for instrument much more challenge, while the larger distance make the whole unit larger and increase the technological difficulty. A careful trade-off has put forward during PDP, the current FoV is 3 arcmin by 3 arcmin and the distance between the M1 center and the flange is 410 mm.

3.3 Imaging

The imaging design are first of all driven by space constraints, as the top platform need to accommodate calibration system, imaging system, tip-tilt system, interface for fiber-fed high resolution spectrograph (HRS) and other exchange systems. Another driver is to simplify the lens design as fewer aspherical surface as possible to reduce the total cost. Based on the basic requirements listed in the section 2, a three-color simultaneous imaging has been considered (figure 3). The optical design of the imaging channels of IMSP are classical focal reducer designs with a filed lens close to the Nasmyth focus, which re-images the telescope pupil on a re-imaging camera. Here, a special plane-convex lens is used as a beam splitter. It has the cross over wavelength about 950 nm where CCDs are sensitive in each VIS and NIR channel. The science requirement of imaging FoV is at least with a diameter of 3 arcmin and goal of 5 arcmin. Design with FoV up to diameter of 5 arcmin has been investigating and found that packaging impossible, it is getting increasingly difficult as the FoV goes beyond 3 arcmin by 3 arcmin. A 3 arcmin by 3 arcmin has therefore been adopted as the baseline. With a 1K x 1K detector, the plate scale is 0.18″/pixel. The design is adapted to a detector with a pixel size of 15μm or 12μm which depended on what kind of detector we can get from different vendor.

Figure 3: Imaging layout of IMSP

3.3.1 VIS channel

The main challenge to design the VIS channel is the packaging constraints and total cost budget. The layout is shown in figure 4. The field lens receiving the reflected visible light from the 20° tilt dichroic which placed at the focal plane. A fold mirror will redirect the light to the reimaging camera system in order to avoid conflict with the NIR channel or calibration system. With the current design, only one dichroic is put after the three lenses group, however we have proposed the use of two dichroics, with crossover wavelengths at 550 nm and 640 nm. This will make it possible to observe the whole V-band or r-band in one of the VIS channel. As there are two dichroics, the crossover does not have to be very steep, which may ease the dichroic design. The penalty will be one more motor and increasing the complexity of optical design a little, the space constraint also need to be double check in the DDP. In order to make the camera system


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simplified, the two channels employ 5 shared lenses with one aspherical surface (* in figure 4) and 2 separate lenses with same parameters to cover the wavelength range of 360 nm – 950 nm, most of the materials used here are CaF2 or i-line glass from OHARA which have a high transmission even in UV. The only difference of two channel is the distance between the last lens and the CCD windows. Filters are placed between the last camera lens and the CCD window in each channel.

Figure 4: VIS channel optical layout of IMSP

The image quality of the VIS should not be degraded by more than 10% in a seeing of 0.8″, which require that the spot quality should be less than 0.36″. In figure 5, a spot diagrams in the V-band is shown, other bands have the similar image quality.

Figure 5: VIS imaging spots in V-band. The box correspond to 0.36", or 2 pixel.


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3.3.2 NIR channel

The NIR channel is a lens based design (in figure 6), in principle quite similar to the VIS channel. According to the design requirements, it should cover the whole Y and J bands due to the sensitivity of InGaAs detector and wavelength shift with a cold condition. The camera system using IR materials also have the possibility to extend to the H or K bands in future. The filter will be placed in the pupil formed by the filed lens. This implies that there will be a color gradient, when using narrow-band filters, however there is no special demanding for this science case in current phase. The design is employing standard IR materials and have not consider the characterized for cryogenic conditions due to the cutoff wavelength of InGaAs detector. A large tilt angle of dichroic will degrade the system spot quality a lot, so here we choose a moderate angle of about 20°. Several iterations were used to come to the conclusion that using a plano-convex lens have a better image quality than dichroic with a toroidal surface. An initial technical consultant with the coating vendor, such kind of plano-convex dichroic is available.

Figure 6: NIR channel optical layout of IMSP

The image quality requirement of the NIR is the same with VIS, it require that the spot quality should be less than 0.36″. In figure 7, a spot diagrams in the J band is shown. The nominal design is almost diffraction limited in all bands, and therefore effectively ‘over-designed’. It is possible to relax the design, while taking into account manufacturing and alignment tolerances.

Figure 7: NIR imaging spots in J-band. The box corresponds to 0.36", or 2 pixels.


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3.4 Spectrograph

3.4.1 Overview

The spectrograph system consist three main parts: slit viewer, calibration unit and spectrograph. The main design drivers of spectrograph are the basic requirements, cost and technique with no big risk. The spectrograph is to provide two observing modes including spectral resolutions 1000 and 5000. For dispersing optical elements it use VPHGs at each of the spectroscopic modes to obtain high efficiency and simplify the opto-mechanical design of cameras. The low resolution mode (R1000) is provided by consecutive observations with the spectral ranges: 360-860 nm. The spectral range of medium resolution mode (R5000) is 460-750nm, however it needs two separate exposures which is constrained by using a 4k × 4k CCD detector (pixel size is 15μm).

The basic optical layout is presented in figure 8. The F/13 light from the telescope is imaged on the long-slit exchanging system after a tip-tilt mirror which guided the light down to the second floor of the IMSP. It is then folded by two flat mirrors and collimated by a Maksutov collimator system, a third reflect mirror is used to fold the optical path and make the whole system more compact. Two resolution modes can be exchanged by using different VPHG and prism combinations and no need to rotate the camera system to accommodate the different diffraction angle. This make the camera system more simple and stable. To optimize the optical performances and minimize the size of the disperser, the VPHGs are positioned close to the pupil image. The camera system adopts the traditional refractive system with focal ratio 1.5, the design consists of 8 lenses with two aspherical surface and one curved windows to achieve a good image quality. The camera system has its own focus adjustment system to compensate the temperature variation during the whole night observation. Light losses in the spectrograph are achieved to be less than 50% in the whole spectral range due to the use of excellent internal transmittance glasses in camera system. The detector will use the commercial CCD with deep depleted chip and the coating will be optimized at the required wavelength. The main spectrograph parameters are listed in table 1.

Figure 8: Spectrograph optical layout

Table 1: Main parameters of the IMSP spectrograph PDP design Parameter Value Comment

Wavelength range LRS: 360-860 nm MRS: 460-750 nm

Current MRS design cover the wavelength range of 459-591nm & 587-756nm

Slit F/13 Angular aperture of input beam 1.5″（or 1.0″for good seeing） Slit width 3′ Slit length

Collimator Focal length: 1885 mm Reversed off-axis Maksutov camera


Field lens

jai"

Camera

Slit Plate

200 mm

Diameter of collimated beam 145 mm

Set by requirements on resolving power, limited by cost/feasibility of VPH dispersers and by constraints on the overall size, volume and camera design difficulty.

Dispersers

LRS: VPH (300 l/mm) + prisms pair (apex angle 22°) MRS 1: VPH(1945 l/mm) + prisms pair (apex angle 21°) MRS 2: VPH(1520 l/mm) + prisms pair (apex angle 21°)

Main consideration is to make the camera system simple and no need to rotate. More VPHGs can be added in future.

Camera focal ratio F/1.5 Set by detector size, wavelength coverage and resolving power

Detectors 4K x 4K ,15µm/pixel E2V depleted chip with optimized coating

Sampling (FWHM) ~ 3 pixels in dispersion direction 3.4.2 Slit viewer

The main purpose of the slit viewer is to allow to move an object onto the spectrograph slit. Actually, the imaging system can be worked as the guiding camera if we make the tip-tilt mirror as a dichroic, however, the top floor for the imaging system is very tight and impossible to accommodate the tip-tilt adjusting structure. So, a separate slit viewer is still needed and will be located after the tip-tilt mirror. The layout of the preliminary optical design is shown in figure 8. The figure includes the tip-tilt mirror, slit plate mirror with a 10° tilt, a fold mirror to redirect the light to the field lens and camera. This subsystem between the imaging and spectrograph optical table will be attached to the bottom of the imaging table. During the exposure of the spectrum, the tip-tilt mirror ahead of the slit exchange system will be used to correct the tracking errors and keep the target on the slit within a certain level with a closed loop method together with the slit viewer detector. The slit viewer camera optics re-images the slit onto the CCD at F/3.7, the image scale allows the entire 3 arcmin x 1 arcmin field to fit onto the 1K CCD with a scale of 0.18″/pixel. This field is constrained by the limited space between tip-tilt mirror and the slit plate with a moderate slit tilt angle. The fields lens consist of two singles and three lenses in the camera, all the materials have the high transmission feature with a wide wavelength range from 0.36 to 1.7μm. To allow for object identification and background suppression, a few filters plan to be available at the interim pupil position.

Figure 9: Layout of the slit viewer.

3.4.3 Calibration Unit

The calibration unit for IMSP is intended to provide flat field illumination and a wavelength calibration reference for the spectrograph. The illumination sources for flat-field and wavelength calibration are plan to feed into an integrating


sphere to provide uniformly distributed illumination and finally optics for projection onto the spectrograph slit. The calibration unit will be located at the top platform between the flange and VIS imaging system. A slide mirror will be inserted in the light path at the position of tip-tilt mirror when a calibration mode is needed. The slit length is about 46mm, so, an area illuminated evenly with a diameter of 50mm is needed. The focal ratio of the illumination must be F/13, identical to that of the telescope. To make room for the tip-tilt and slide mirror exchanging, the distance between the last lens of the projection optics and slit should be about 400mm. Due to the importance of the calibration system, the design will be detailed and some lab test will put forward during the DDP. 3.4.4 Spectrograph design

3.4.4.1 Collimator

The collimator can either user reflective or refractive optics. A trade study was performed and finally a Maksutov type of off-axis collimator was chosen as the baseline design. The primary mirror is spherical and will be coated with an enhanced gold coating which has been applied to the LAMOST project [1]. The corrector is a single lens of fused-silica with spherical surfaces. The overall size and cost of collimator system are not very high and easy to manufacture. The FoV of the collimator system can be large and a long slit will not be the main constrained factor of the design. This kind of design has also been seen in X-shooter [2] and WEAVE [3].

3.4.4.2 VPHG

VPHG has been chosen as the spectral dispersers due to its high efficiency over the relatively broad wavelength range [4]. To optimize the optical performances and minimize the size of the disperser, the gratings are positioned close to the pupil image. In order to make the camera opto-mechanical simple and no need to changing the angle between the camera and the collimated beam, the most convenient approach to change the resolving power is using a combination of prisms and VPH-gratings. The prism are used to change the incidence/diffracted angles on the grating, while keeping the same input angle from the collimator and output angle to the camera. The prism pair parameters are the same and the sum of peak angle is almost the minimum in each VPHG in the current phase to reduce cost and make the intrinsic material absorption less, actually it can also be turned to obtain similar anamorphic magnifications in the LRS and MRS configurations[5], but it will need more prism or make prism peak angle larger, a trade-off study will be put forward in the DDP and try to check whether the camera system can be much simpler and not increase the VPHG manufacture cost and risk much.

3.4.4.3 Cameras

The focal ratio of cameras is about F/1.5, a refractive lens design is a natural choice to have no central obstruction and make the total cost under control. The cameras has an elliptical entrance pupil caused by anamorphic magnification of three VPHGs ranging from 1.05 to 1.2. A telescope field height of about 3 arcmin will be used perpendicular to dispersion, it is constrained by the de-rotator system. The detector will be a flat 4K x 4K by 15µm CCD, as a practical limit, a field of 18 degrees was chosen which produce a 67.2 mm field diameter with the camera’s designed 217.5 mm focal length. With some iterations, the current camera design shown in figure 10 consists of 8 lenses with 2 aspherical surfaces (noted by a symbol of ‘*’), a plane-concaved fused silica was used as the CCD window. In order to reduce the effect of large temperature variations from -20℃ to 40 ℃, there is no glued doublet. This camera shows an image spot diameter within one pixel box over all field angles and over the full 360 to 860 nm spectral range without refocus. During the design process, special attention was given to reducing the number of aspherical surfaces and CaF2. OHARA i-line glasses were used to the extent possible for maximum transmission in the blue end. For the current stage, this camera is still a bit complicated, a more detailed cameras design will be carried out in the DDP.


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Figure 10: Camera lens design.

3.4.4.4 Detector

The choice of particular CCD devices is most important from an instrument efficiency standpoint and cost consideration. A commercial CCD, the e2v CCD231-84, is currently available and is good match for the instrument with a custom curved window. The required spectral range is 360 to 860 nm and may be extended to 1000 nm in the future, so a deep depletion chip with fringe suppression and dual AR coating is the best choice for the instrument. Even better fully depleted devices developed at Lawrence Berkeley National Laboratory [6] or Hamamatsu cooperated with National Astronomical Observatory of Japan [7], which have significantly better response beyond 800 nm than e2v devices.

3.4.4.5 Spectrograph optical performance

In this sections, it will describe the optical performance of the LRS and MRS, including image quality, spectral resolution, throughput and an initial ghost analysis of the camera.

Spot diagrams for the LRS and one of the MRS are shown in figure 11 and figure 12, respectively, the image quality of the two MRS modes is similar due to the almost same pupil shape to the camera. In the figure, the spots are shown with a 15µm x 15µm square box, representing one pixel on the detector. Each figure covers the full respective wavelength range and field points covering the full length of the slit.

Figure 11: Spot diagrams for the LRS. The box are 15µm x 15µm, corresponding to the size of one pixel on the detector. Rows

represent a particular position along the slit, from center to edge, indicated by labels to the left of each row. Each column represents a particular wavelength. From left to right are: 0.36µm, 0.55µm, 0.646µm, 0.8µm and 0.86µm.


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Resolution vs Wavelength @slit =1.5'

- MIS with VPHG 3001/mm-- MRS with VPHG 19451/mm-- MRS with VPHG 15201 /mm

Wavelength: nm

Figure 12: Spot diagrams for one of the MRS.

Spectral resolution defined asλ λΔ , where λΔ is taken to be the spectral FWHM of the slit image on the detector. In reality, the resolution of the instrument will be determined using collapsed spectra, here is just the first order calculation results.

Figure 13: Resolution for the LRS and MRS design.

The expected throughput of the spectrograph with different resolution mode is given in figure 13. The throughput figures include material absorption, coating loss in the mirror and lens, VPH efficiency and detector quantum efficiency (the loss from atmosphere, telescope, K-mirror and slit loss are not included). The coating data for the mirror and lens were provided by the coating team in NIAOT, the average anti-reflection coatings on the camera and corrector will be higher than 99%, the collimator mirror will be with a promising UV enhanced gold coating which have been test at the LAMOST dome for more than a year, the average reflection is higher than 97.5%. The VPH efficiency is taken from initial quotes from the manufactures, the CCD efficiency is taken from valid quotes we received and represent the measured values at the -100�. The initial result shows that it meet the requirement of peak efficiency >45%.


IMSP Spectrograph theoretical efficiency without telescope, .mirror and slit loss

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GH089077.ZMXConfiguration 1 of 1

Figure 14: Theoretical Throughput of the IMSP in the LRS and MRS mode. It includes the fold mirror, collimator system, dispersion

element, cameras and detector (excluding the atmosphere, telescope, K-mirror and slit loss). An initial ghost analysis was performance during the camera optical design. Following the numbering of the lens surfaces shown in figure 10. The expected image and pupil distances to the focal plane are analyzed from ray-tracing. Small distances can be potentially dangerous. The closest image ghost distance from the image plane is -1.4mm which generates by surface 17 (first surface of the CCD window) and bounces back from surface 8 (Lens 4). This distance can be optimized by changing the radius of surface 8 a little, however, the ghost footprint on the image plane is distributed in a large area shown in figure 15. Assuming the AR coating reflection is about 1%, with a double bounce the total reflection will be 1e-4, so it has a little impact on the performance in each pixel. The closest pupil ghost distance is -0.63mm, although it lies near the focal plane, the related image is far away, only a small fraction of light will be sent onto the plane.

Figure 15: Camera ghost analysis result.

4. SUMMARY This paper has presented an overview of the IMSP, listed the main requirements, introduced each subsystem including de-rotator unit, imaging system with VIS and NIR channels, and spectrograph with calibration unit and slit viewer


system. The key performance of the spectrograph coming from the optical design and associated analysis are also described in the paper. IMSP optical design fulfills all the scientific requirements and meet the envelope limitation of Nasmyth platform.

DDP will be finished at the end of October, 2018. During this phase, more attention will be paid to the spectrograph camera design and VPHG evaluation. A complete set of thermal analyses and error budget will be produced and to be used to feedback the opto-mechanical design. Due to the tight project schedule and funds, the de-rotator unit and NIR imaging channel will be manufactured firstly at the end of July, 2018. The other subsystem will begin to construct at the beginning of 2019. IMSP is expected to be on-sky in 2021.

5. ACKNOWLEDGMENTS The authors would like to thank Dominic Speer for providing help with the VPHG technical support and Yoshihiko Kimura for providing equation and some useful information about filters. We also appreciate some coating data provided by Jinfeng Wang and Jie Tian. We acknowledge the financial support of the National Nature Science Foundation of China (No.11503059, 11603054, 11603053, 11473048, 11503061).

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