Recommended conceptual optical system design for China&#x2019;s Large Optical-infrared Telescope (LOT)

Donglin Ma

doi:10.1364/OE.26.000108

1. Introduction

Recently, China is planning to construct a large optical-infrared telescope (LOT) with an equivalent primary mirror diameter of 12m in its 13^th Five-Year program. The primary goal of the project is to construct a general-purpose large optical infrared telescope that can be able to carry out astronomical studies such as the evolution of the universe/galaxies/stars, extreme condition astrophysics, dark energy and dark matter, searching for gravitational wave EM counterpart at a site with reasonably good astro-climatic condition. The telescope should be able to detect faint source, and the sensitivity of the telescope should not be lower than the contemporary 10m class telescope. The telescope should also have multi-object observation capability. The field of view of the telescope should be no less than 15 × 15 arcmin. LOT should also be capable to be upgraded in the future for capabilities and instruments such as Ground Layer Adaptive Optics (GLAO), infrared detection, prime focus instrument etc.

For the preparation of China’s LOT project, Su et al. have proposed an innovative design that consists of four mirrors for its Nasmyth focus [1, 2]. For simplicity, we call it as three-mirror design since it has three aspheric mirrors with non-zero power along its optical path for Nasmyth focus, including a primary mirror, a secondary mirror, and a relay mirror (called SYZ relay mirror). The layout of SYZ design is shown in Fig. 1(a). The design is aimed to achieve diffraction-limited performance for its full field of view (FOV) by adding an additional relay mirror compared to traditional two mirror design such as Ritchey–Chrétien (RC) configuration as shown in Fig. 1(b) and Aplanatic Gregorian (AG) configuration as shown in Fig. 1(c). In fact, modern large optical-infrared telescopes mostly adopt the two-mirror designs [3–8]. Typically, the examples of RC design telescopes can include the 8m Subaru Telescope, the 10m Keck telescope, and the next generation Thirty-Meter-Telescope (TMT) that is currently under construction [3–6]. The most typical examples for Aplanatic Gregorian configuration are the Large Binocular Telescope (LBT) (8.4m × 2) [7], and the next generation Giant Megellan Telescope (GMT) [8]. The Euro50 project also plans to adopt the AG configuration because it is most advantageous to adaptive optics [9]. To demonstrate the efficiency of the so called innovative design to modern large telescopes, we have made a systematic comparison between the three-mirror design and RC design in our previous paper [10]. We have found that the SYZ design cannot compete with traditional RC design for almost all kinds of astronomical observations, including seeing limited observations, extreme adaptive optics (ExAO) corrected observations, and ground-layer adaptive optics (GLAO) corrected observations. Because the SYZ design suffers from lower photon throughput, worse image quality in the center of FOV due to the central obstruction effect, and more complicate structure.

Fig. 1 Optical system configuration for LOT: (a) SYZ design; (b) RC design; (b) AG design.

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To satisfy the scientific requirements proposed by LOT project as well as the necessity of low construction cost and little instrumentation risk, two two-mirror (two powered mirrors) optical configurations are adopted as the optical system configuration for China’s LOT telescope. One is based on the classical Ritchey–Chrétien (RC) system, and the other is based on aplanatic Gregorian (AG) system [11, 12]. Due to the conjugation height of the secondary mirror, AG is more suitable for ground layer adaptive optics, with a compromise of about 30% longer telescope tube which will have a higher overall cost. However, RC design is a proven concept that has been adopted by most of the contemporary largest telescopes in the world. Since the GLAO correction performance is strongly correlated with vertical distribution of site’s turbulence, the final decision must rely on all facets such as feasibility, cost, site’s performance, GLAO’s performance etc. Based on the very limited facts that we have currently, we recommend a RC f/1.6 design for LOT’s Nasmyth focal system in this paper.

Finally, to achieve an engineering feasible design with a reasonable location of tertiary mirror M3, we take a non-identical design of Cassegrain optical system by selecting a different Cassegrain focal ratio compared to Nasmyth focal ratio. We realize the design by shifting the secondary mirror away from the primary mirror a little bit and optimizing the conic constant of the primary mirror for eliminating the residue spherical aberration and coma based on mechanism of warping harness. A further analysis of the engineering feasibility for the special telescope system configuration is provided in the paper.

2. Design and instrumentation of primary mirror

2.1 Design parameters of the primary mirror

The aperture of the telescope will be 12m, which is bigger than the biggest monolithic mirror blank that currently can be produced. Therefore, we recommend using segmented mirror technique combined with active optics technology by active control of the mirror’s surface shape. The layout of the segments is shown in the figure below. As shown in Fig. 2, the primary mirror is composed of 84 segments, the diagonal distance for each projection of the hexagonal segment is 1.44m and the thickness of each segment is set as 45mm. Depending on the distance from the center of the primary, there will be 14 different types of segments. The choice of design parameters for LOT’s primary mirror is mainly based on experience of TMT [6] and E-ELT [13] telescope design.

Fig. 2 Layout scheme for LOT’s primary mirror

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The primary mirror will have a quadratic surface (hyperboloid), which can be described as [4]:

{\begin{cases} z (x, y) = \sum α_{n m} ρ^{n} \cos m θ m, n \geq 0, m \leq n, n - m e v e n \\ α_{20} = \frac{a^{2}}{k} [\frac{2 - K ε^{2}}{4 {(1 - K ε^{2})}^{3 / 2}}] \approx a^{2} (1 + K ε^{2}) / (2 k); (f o c u s) \\ α_{22} = \frac{a^{2}}{k} [\frac{K ε^{2}}{4 {(1 - K ε^{2})}^{3 / 2}}] \approx \frac{a^{2} K ε^{2}}{4 k}; (a s t i g m a t i s m) \\ α_{31} = \frac{a^{3}}{k^{2}} [\frac{K ε {[1 - (K + 1) ε^{2}]}^{1 / 2} (4 - K ε^{2})}{8 {(1 - K ε^{2})}^{3}}] \approx \frac{a^{3} K ε}{2 k^{2}}; (c o m a) \\ ε = \frac{R}{k} . \end{cases}

where a is the projected segment radius of curvature, k is the radius of curvature at the vertex, R is the off-axis distance, K is the conic constant of the primary. The maximum asphericity of the primary is described by α₂₂ + α₃₁. By substituting these parameters into the equation, we can calculate the maximum asphericity is 93.6μm for the f/1.6 design. It is worthy to be mentioned that if we choose primary focal ratio of f/1.2, then the maximum asphericity will become 221.9μm instead.

If Stressed Mirror Polishing (SMP) technique is adopted for polishing the aspherical segments, we must consider the stress of mirror substrate used during fabrication. The stress can be expressed by [14, 15]

{\begin{cases} f (R) = \frac{1}{4 (1 + ν)} [(3 + ν) {(\frac{R + a}{k})}^{2} - (1 + 3 ν) {(\frac{R - a}{k})}^{2}] \\ σ_{\max} = \frac{E h}{2 (1 - ν) k} f (R) R > a \end{cases} .

where ν is the Poisson ratio of Zerodur glass, E is Yong’s modulus of Zerodur glass, h is the thickness of the primary mirror segments, and σ_max is the maximum stress that can be induced in mirror segment. The maximum stress at the edge for Keck telescope is 5MPa, and it has been successfully fabricated [14]. Based on Zerodur’s properties and relevant papers, the material can sustain a stress of 40MPa [16, 17]. Considering that the segment will be stressed while grinding, we set a safe limit for stress at 10MPa.

Under these assumptions, considering that the physical parameters of the segment are: thickness h = 0.045m, a = 0.75m, R = 6m, then the radius of curvature for LOT’s primary mirror must be k>26m according to Eq. (2), which requires the primary’s focal ratio to be slower than 1.1.

2.2 Alignment of primary’s segments

Misalignment of primary’s segments can inevitably increase the size of image spot. The segment misalignment can come from alignment error, gravity effect, wind effect, and so on. With the help of active optics [18, 19], we can realize the co-phasing of primary’s segments. Generally, we use the 80% enclosed energy diameter (EE80) of image spot to evaluate the image quality of large optical-infrared telescopes [20]. As a result, the EE80 diameter caused by radial alignment instrumentation error and azimuthal instrumental error can be expressed separately by [4]

θ (80 %) = \frac{8 K R a δ R}{k^{3}} {[1 + \frac{a^{2}}{16 R^{2}}]}^{1 / 2}

θ (80 %) = \frac{3.58 K R^{2} a δ θ}{k^{3}} {[1 + \frac{0.31 a^{2}}{R^{2}}]}^{1 / 2}

where δR is the radial error of the segment’s location, and δθ is the azimuthal error of the segment’s location. Therefore, based on Eqs. (3) and (4), we can see that large radius of curvature for the primary mirror can greatly release the pressure of segment alignment on both radial and azimuthal directions. This is a very important evaluation criterion for the choice of primary mirror’s focal ratio, which will be discussed with more depth in next section.

3. Design trade-offs for LOT’s optical system

3.1 Choice between AG and RC design

The choice of telescope’s configuration between AG design and RC design is the most critical issue for LOT telescope system. In general, a Ritchey-Chrétien (RC) design has no spherical, or coma aberrations at the designed Nasmyth focus (as well as the identical Cassegrain focus). The optical performance of RC design is limited only by astigmatism that grows quadratically with field angle. The size of the astigmatism scales approximately as 1/F_p, where F_p is the focal ratio of primary mirror. For RC design with a convex secondary mirror, telescope tube is short, leading to a small dome. Since LOT should have the potential to work with GLAO, then the conjugate height of telescope’s adaptive secondary mirror is a very important parameter and can have great effect on the potential maximum FOV of GLAO system. For both RC design and AG design, the conjugate height of secondary mirror can be defined as:

h = \frac{d \cdot f_{p}}{d - f_{p}}

where d is the primary-secondary distance, f_p is the focal length of primary mirror, and h is the distance above the primary mirror. Obviously, the conjugate height h is negative for RC design, and thus the conjugate of secondary mirror for RC system is below the ground. As a result, RC design is a non-optimal choice for ground-layer adaptive optics (GLAO) instruments [21].

The AG design is very similar to the RC design in its optical performance, having no spherical, or coma errors at the designed focus, and being limited by astigmatism that again grows quadratically with field angle. Unlike the RC design, the AG design has a concave secondary mirror, which makes it much easier to test and hence to fabricate. In addition, the conjugate height of secondary mirror for AG design is above the ground based on Eq. (5), which makes it much more attractive for GLAO corrected observations [21–23]. Moreover, the AG design’s focal surface is convex towards the tertiary mirror, whereas the RC design’s focal surface is concave, which makes a difference in mating to different types of instruments, especially spectrograph collimators. The focal plane of a reflective collimator is naturally concave towards the tertiary, and thus matches well to the RC design. The focal plane of a refractive collimator is naturally convex towards the tertiary, and thus matches well to the AG design. However, the most important difference between the two designs is that the AG telescope is longer, which makes its dome larger and thus more expensive. To summarize, the most fundamental trade-off between RC design and AG design is dome cost vs. a larger potential GLAO FOV.

In fact, the performance of the GLAO system is strongly correlated with the turbulence condition at the site. Whether a ground layer turbulence exists, how much fraction of energy this layer contains, and the height of the layer will all contribute to the final performance of the GLAO system [20–22]. However, because by far we still do not have long-term atmospheric turbulence profiles for the preferred site of LOT telescope, it is difficult for us to dictate which design would be better. In addition, the RC design has a slight edge advantage when considering the availability of large collimators for scientific instruments. Because of the large size of the required optical collimator for LOT, the consequent unavailability of key optical glasses needed by the AG design must be taken into consideration. Since LOT’s GLAO performance is highly uncertain, we adopt the RC design as the telescope configuration for China’s LOT project due to its low cost and reliability as well as its easier accommodation of scientific instruments with large apertures.

3.2 Choice among different focal ratios of the primary mirror

The determination of the primary focal ratio must balance among many factors. Compared to a slower focal ratio, a faster focal ratio for the primary mirror has at least three advantages. Firstly, the telescope with faster primary focal ratio will have shorter telescope tube, which will lower the price of the dome, and thus will lower the overall cost for the construction of observatory. Secondly, since the telescope tube is shorter, the distance between primary and secondary will be shorter, and then dome seeing will have a smaller effect for degrading the telescope’s image quality. Thirdly, a smaller focal length of primary mirror will result in a stiffer tube.

However, there exist another three factors supporting a slower primary focal ratio. First, the fabrication difficulty grows rapidly with a faster focal ratio. Because the asphericity of the primary inversely increases with k³ (k is the radius of curvature for the primary), the fabrication difficulty of the primary mirror will be much lowered with a slower focal ratio. For example, an f/1.2 mirror will be more than twice more difficult to be fabricated than an f/1.6 mirror. Second, the tolerances for the alignment of primary mirror’s segments are much stricter for faster ratio. Because the required tolerances for the primary’s segments varies with k³ based on Eqs. (3) and (4), it would be much easier to align those segments with a larger k. Third, with a slower primary focal ratio, aberration (astigmatism) would be smaller based on our simulation in ZEMAX.

Considering these balancing aspects, especially the fabrication feasibility and alignment pressure for China’s LOT, we recommend 1.6 rather than 1.2 as the focal ratio of LOT’s primary mirror.

3.3 Choice among different focal ratios of the Nasmyth focus

Same as the choice for primary focal ratio, it is also necessary to balance many factors for deciding the focal ratio of the Nasmyth focus. The following factors would allow us to prefer a faster focal ratio for LOT’s Nasmyth focus. First, faster focal ratio will necessitate a more hyperbolic primary mirror, which will be helpful for the optical design of the potential primary focus camera. Second, faster Nasmyth focal ratio will simplify the design of the atmospheric dispersion corrector (ADC) with a slightly reduced size, and will have a better image quality within a larger FOV as well. Third, faster Nasmyth focal ratio will shorten the focal length of the collimator designed for spectrographs, which will lower the difficulties for designing scientific instruments at Nasmyth focus. Fourth, faster Nasmyth focal ratio will help reduce the field curvature at Nasmyth focus. As we know, field curvature is one of the most important properties that have large impact on the performance of scientific instruments with large FOV such as multi-object spectrograph. Based on these considerations, it is better to have smaller or faster focal ratio for telescope’s Nasmyth focus.

However, to have a faster focal ratio, it is necessary to have a larger, heavier, more aspheric and thus more expensive secondary mirror, and thus the supporting structure for the secondary mirror will be more complex. In addition, according to scientific instrument construction experience on Keck telescope and Giant Magellan Telescope (GMT) in the design and instrumentation of LRIS, DEIMOS, and IMACS by Dr. Harland Epps [24–26], f/15 is almost reaching the fastest limit of wide field spectrographs. As a result, we will recommend an f/15 as the Nasmyth focal ratio of LOT.

3.4 Location of tertiary mirror

To guarantee the gravitational stability of the telescope during the rotation around the elevation axis, the center of gravity (C.O.G) of the telescope must be located at the elevation axis, where the tertiary mirror locates. Before we can finalize the optical design configuration, we still need to decide the location of the tertiary mirror M3. Generally, we can use the following quantity to specify the relative position of the tertiary mirror for Nasmyth optical system:

o = \frac{| M 1 - M 3 |}{| M 2 - M 3 |}

where |M1-M3| specifies the distance between the primary mirror and the tertiary mirror, and |M2-M3| represents the distance between the secondary mirror and the primary mirror. Generally, the smaller value of the relative position o indicates that the tertiary mirror is closer to the primary mirror, and thus the C.O.G of the telescope is lower. Before determining the relative position of the tertiary mirror of LOT telescope, we firstly check the relative position of the tertiary mirror for several international mainstream telescopes designed at RC configuration, such as Keck [4], Subaru [5], TMT [6], and VLT [27]. The data are shown in Table 1.

Table 1. The relative position of M3 for several RC telescopes.

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As can be seen from the table below, the telescopes designed with monolithic primary mirror will have relatively lower C.O.G compared to those telescopes designed with segmented primary mirror under same aperture size, because monolithic designs can have relatively much thicker primary mirrors. Correspondingly, the larger the primary mirror’s aperture diameter, the center of gravity will be relatively lower. According to the design parameters of several existing mainstream telescopes, and considering LOT’s primary mirror type (segmented) and its aperture size (12m), we choose the relative position of tertiary mirror for China’s LOT to be 0.30.

4. Recommended design of LOT optical system

4.1 LOT’s Nasmyth optical system

By taking selected optical design configuration, primary focal ratio, Nasmyth focal ratio and relative position of tertiary mirror into consideration, the final design parameters of LOT’s Nasmyth optical system can be shown in Table 2. The entrance stop locates at the primary mirror. It is worthy to mention that the maximum unvignetted FOV is set as 15 arcmin. In the final designed RC optical system, the primary focal ratio is set as 1.6, the Nasmyth focal ratio is set as 15, and the relative position of tertiary mirror is set as 0.3. Correspondingly, the 2D layout of the optical system structure can be shown in Fig. 3. The conjugate height of secondary mirror is about 138.3m below the ground based on Eq. (5).

Table 2. Design data of LOT’s Nasmyth system.

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Fig. 3 LOT’s Nasmyth optical system based on RC configuration (f/1.6)

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4.2 LOT’s Cassegrain optical system

If we design the Cassegrain system with an identical focal ratio of the Nasmyth system, then we can derive that the distance between the Cassegrain platform and the primary vertex will be 5.1m based on our previous design parameters for LOT’s Nasmyth system. Obviously, the distance is relatively too long, which will result in the increase of the absolute length of the telescope tube and thus the increase of the construction cost. Besides, too long telescope tube is also detrimental to the gravitational stability of telescope’s mechanical structure.

Based on these considerations, we tend to constrain the distance between the primary mirror vertex and the Cassegrain focus of the telescope within around 4 meters. We can further improve the image quality of Cassegrain focus by optimizing the conic constant of primary mirror along with the distance between primary mirror and secondary mirror. The corresponding optical design data are shown in Table 3, and designed Cassegrain optical system configuration is shown in Fig. 4. In the non-identical Cassegrain optical system, the Cassegrain focal ratio is 14.336, and the conjugate height of secondary mirror is about 139m below the ground.

Table 3. Design data of LOT’s Cassegrain system.

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Fig. 4 Diagram for LOT’s Cassegrain system based on RC (f/1.6)

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4.3 Optical performance of LOT’s optical system: feasibility analysis I

Both optical system design aberrations and telescope site’s atmospheric conditions determine the optical performance of ground-based telescopes. For the Nasmyth optical system which satisfies the aplanatic condition, only astigmatism dominates the final design aberration. While for the non-identical design of Cassegrain optical system that does not satisfy aplanatic condition, both the coma and astigmatism dominate the final design aberration. For the non-identical design of Cassegrain focus, the value of coma is linearly proportional to the field angle and the maximum value of coma across the field is about 12 waves. We generally use EE80 and FWHM (full width at half maximum) of point spread function (PSF) to evaluate the optical performance of telescopes. The two terms follow the following relationship if we take a Gaussian approximation of system’s PSF:

E E 80 = 1.9 * F W H M

The average seeing of LOT’s preferable site is about 0.9 arcsec. If we consider the atmospheric effect and ignore the segment effect as well as instrumentation error, then the final image spot size of ground-based telescope can be expressed by

F W H M_{T o t} = \sqrt{F W H M_{A b e r r}^{2} + F W H M_{S e e i n g}^{2}}

where FWHM_Aberr is the image spot size caused by optical system aberrations. Based on our simulation in ZEMAX software, the optical performance of our designed LOT optical systems can be concluded in Table 4. From the table result, we can see that the atmospheric turbulence still dominates the final optical performance for LOT even at FOV of 20 arcmin. As a result, our optical solutions for LOT provide seeing-limited, as designed performance.

Table 4. Optical performance of LOT’s optical systems (Seeing = 0.9”).

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4.4 Maximum primary segment deformation: feasibility analysis II

Our proposed conceptual optical system design for China’s LOT project ensures the application of Cassegrain focus under the premise of ensuring the best image quality of the Nasmyth focus. In our conceptual design, the two systems have non-identical foci. Therefore, if we switch the working mode of LOT from Nasmyth focus to Cassegrain focus, we need to further optimize the shape of primary mirror (i.e. the conic constant) by using the warping harness of active support system to correct the residue spherical aberration. The maximum deformation of the primary segments caused by the change in conic constant can be derived by Eq. (1) as follows:

{\begin{cases} \frac{d a_{20}}{d K} = \frac{a^{2} R^{2}}{2 k^{3}} \\ \frac{d a_{22}}{d K} = \frac{a^{2} R^{2}}{4 k^{3}} \\ \frac{d a_{31}}{d K} = \frac{a^{3} R}{2 k^{3}} \end{cases} .

Then, when R = 6m, a = 0.72m, k = 38.4m, and dK = 1.0031628-1.00043867 = 0.0027, we have da₂₀ = 0.44µm, da₂₂ = 0.22µm, and da₃₁ = 0.05µm. Therefore, the maximum deformation of primary segment is about 0.71µm, which is smaller than the maximum deformation that can be corrected by warping harness, i.e. about 1.0µm [28]. Of course, there will still exist some residuals left from each of the segments after bending, and we plan to take a further research on the effect of these residuals on telescope’s optical performance in our future work.

In summary, our non-identical design of Cassegrain system and Nasmyth system for LOT project provides state-of-the-art, as-designed performance by the fact by the fact that its key properties are inline with that of existing, operational telescopes – the Keck being a case in point.

4.5 More comments about the Single Conjugate Adaptive Optics (SCAO) corrected observation

Although the SCAO is not planned to be applied to first light instruments of China’s LOT, it is still desirable to discuss the best optical configuration for SCAO corrected observations. If the secondary mirror is planned to be used to SCAO, and thus deformable, then it will offer an opportunity to change the deformation conic constant and obtain an aplanatic Cassegrain configuration in combination with a change of deformation conic constant of the primary mirror. In the case, another aplanatic Cassegrain optical system with a non-identical focal ratio has to be designed for SCAO corrected observations.

Regarding SCAO, the aplanatic Gregorian is a more advantageous choice compared to the RC design, which is supported by at least two arguments. Firstly, it is much easier to manufacture a concave deformable secondary mirror. Secondly, a concave deformable secondary mirror can be tested separately in the telescope by inserting special optics below the secondary, which, however, is not the case for an RC’s convex secondary mirror.

5. Conclusion

In the paper, we have proposed a recommended conceptual optical system design for China’s LOT project. To realize an engineering feasible, economic, and scientific compatible design of the telescope, we fully consider the trade-off with respect to the choice of different configurations, primary focal ratio, Nasmyth focal ratio, and tertiary mirror’s relative location. Unlike the standard RC design of telescope system, we take a non-identical design of the Cassegrain optical system compared to the Nasmyth optical system in order to guarantee a reasonable location of the Cassegrain instrument platform. We have also further proved that this kind of non-identical design can be achieved by warping harness of active optics. In a word, this paper has provided a systematic consideration in designing modern optical-infrared telescopes for scientific applications in astronomy, which will provide an important guidance to researchers in the design of modern telescopes in the future.

Funding

Wuhan Science and Technology Bureau (2017010201010110); Huazhong University of Science and Technology (2017KFYXJJ026).

Acknowledgements

We acknowledge the anonymous referee for a careful reading of the article and insightful comments that led to a significant improvement of this paper. We acknowledge many valuable comments from Prof. Jerry Nelson during the preparation of this paper, who has just passed away shortly. We express our deep memory to his great contribution to optics instruments in astronomy. We acknowledge many valuable discussions from Zheng Cai and Sandra Faber. We also acknowledge the great helps, supports and encouragements from Jiansheng Chen, Suijian Xue, Luis Ho, Lei Hao, and Lu Feng.

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28. “Personal communication” from Jerry Nelson.

Telescopes	Aperture (m)	Primary mirror types	M3′s relative position (o)
Keck	10	Segmented	0.35
Subaru	8.2	Monolithic	0.25
TMT	30	Segmented	0.15
VLT	8.2	Monolithic	0.25
LOT	12	Segmented	-

Element	Radius of curvature (mm)	Thickness (mm)	Aperture diameter (mm)	Conic constant
M1(Stop)	−38400	−1.686E + 4	12000	−1.00316218
M2	−5245.36805	1.2965E + 4	1569	−1.57099786
M3	INFINITY	−9000	1574 × 1113	-
Focus	2430.31	-	784	-

Element	Radius of curvature (mm)		Thickness (mm)	Aperture diameter (mm)	Conic constant
M1	−38400		−1.687E + 4	12000	−1.00043867
M2	−5245.36805		2.087E + 4	1536	−1.57099786
Focus	−2427	-		750	-

FOV (arcmin)	Nasmyth Focus (Diameter: arcsec)			Cassegrain Focus (Diameter: arcsec)
FOV (arcmin)	EE80	FWHM _Aberr	FWHM _Tot	EE80	FWHM _Aberr	FWHM _Tot
0	0.019	0.010	0.9001	0.022	0.012	0.9001
5	0.036	0.019	0.9002	0.083	0.044	0.9011
10	0.10	0.053	0.9016	0.18	0.095	0.9050
15	0.22	0.116	0.9074	0.33	0.174	0.9167
20	0.39	0.205	0.9231	0.52	0.274	0.9408

Telescopes	Aperture (m)	Primary mirror types	M3′s relative position (o)
Keck	10	Segmented	0.35
Subaru	8.2	Monolithic	0.25
TMT	30	Segmented	0.15
VLT	8.2	Monolithic	0.25
LOT	12	Segmented	-

Recommended conceptual optical system design for China’s Large Optical-infrared Telescope (LOT)

Abstract

1. Introduction

2. Design and instrumentation of primary mirror

2.1 Design parameters of the primary mirror

2.2 Alignment of primary’s segments

3. Design trade-offs for LOT’s optical system

3.1 Choice between AG and RC design

3.2 Choice among different focal ratios of the primary mirror

3.3 Choice among different focal ratios of the Nasmyth focus

3.4 Location of tertiary mirror

4. Recommended design of LOT optical system

4.1 LOT’s Nasmyth optical system

4.2 LOT’s Cassegrain optical system

4.3 Optical performance of LOT’s optical system: feasibility analysis I

4.4 Maximum primary segment deformation: feasibility analysis II

4.5 More comments about the Single Conjugate Adaptive Optics (SCAO) corrected observation

5. Conclusion

Funding

Acknowledgements

References and links

Cited By

Figures (4)

Tables (4)

Equations (9)

Optics Express