1.State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Xi'an 710119, China 2.University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61471357) and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA15020106-03).
Received Date:10 May 2019
Accepted Date:12 June 2019
Available Online:01 August 2019
Published Online:20 August 2019
Abstract:X-ray focusing telescope is one of the most important equipment for X-ray space observation, which is designed based on the grazing incidence principle. The purpose of x-ray observation is to detect the black holes of various sizes in outer space, and the data obtained by X-ray telescope conduces to investigating the basic physical law under the condition of extreme gravity and magnetic field, In this article, multi-layer telescope is designed to satisfy the demand for enhanced X-ray timing and polarimetry mission. in which the telescope is designed based on Wolter-I telescope. The Monte Carlo method and power spectral density are used when the relationship between mirror profile and roughness with angular resolution is investigated. We analyze the relationship between angular resolution and mirror profile, and the result shows that the higher mirror profile possesses higher angular resolution. When the root mean square(RMS) of mirror profile is 0.04 μm, PV is 0.2 μm and roughness is 0.4 nm, the mirror angular resolution is 6.3" and it will change to 30.6" when the RMS of mirror profile is 0.2 μm, PV is 1 μm and roughness is 0.4 nm. The angular resolution out of focus is also investigated in this article, and the more defocusing amount gives rise to the worse angular resolution because defocusing spot will be larger than that of focal plane. So the maximum defocusing amount of 5 mm is required when the focal plane detector is installed. The relationship between effective area with film structure and layers number is also investigated. The film with Au mixed with C has a higher reflectivity than the film with only Au, because the mixed film will generate an interference effect and enhance the intensity of reflecting X-ray. When the telescope layers increase, the effective area and telescope weight are both improved, the requirement for effective area of satellite can be satisfied when the number of nesting layers is 45. However, when the number of nesting layers further increase, the effective area will be improved with a low speed, but the weight of telescope will increase with a high speed. The field of view of this telescope is 16′, which is more than the required value of 12′. Finally, the X-ray focusing telescope with 5.25 m focal length, 45 nesting layers, effective area 842 cm2 at 2 keV, 563 cm2 at 6 keV is obtained. Keywords:X-ray/ focusing observatory/ grazing incidence principle/ effective area/ angular resolution
采用蒙特卡罗方法对聚焦望远镜镜片进行面型仿真, 并计算镜片面型和粗糙度对望远镜焦斑角分辨率的影响, 结果如图2所示, 图2(a)和图2(b)为较高精度的聚焦镜镜片得到的焦斑形状和能量包围函数, 图2(c)和图2(d)为低精度的聚焦镜镜片得到的焦斑形状和能量包围函数. 当聚焦镜镜片表面面型误差PV值为0.2 μm, RMS值为0.04 μm, 粗糙度为0.4 nm时, X射线聚焦镜角分辨率为6.3′′; 当聚焦镜镜片表面面型误差PV值为1 μm, RMS值为0.2 μm, 粗糙度为0.4 nm时, X射线聚焦镜角分辨率为30.6′′. 这是因为当X射线聚焦镜面型精度较高时, X射线光子接近理想反射效果, 所有光子都会聚焦在焦平面上较小的一个范围, 从而焦斑尺寸较小; 但镜片面型精度较差时, X射线光子会由于掠入射角度与设计值不符, 而导致光子被反射到以焦点为中心的一个较大范围内, 从而导致焦斑尺寸和角分辨率较大. 图 2 不同面型精度聚焦镜片的焦斑形状与能量包围函数 (a) RMS 0.04为 μm, PV为0.2 μm, 粗糙度为0.4 nm镜片的焦斑形状尺寸; (b) RMS为0.04 μm, PV为0.2 μm, 粗糙度为0.4 nm镜片的焦斑能量包围函数; (c) RMS为0.2 μm, PV为1 μm, 粗糙度为0.4 nm镜片的焦斑形状尺寸; (d) RMS为0.2 μm, PV为1 μm, 粗糙度为0.4 nm镜片的焦斑能量包围函数 Figure2. Focal points and energy encircle functions obtained by mirrors with different profile: (a) Focal point obtained by mirror with profile of RMS 0.04 μm, PV 0.2 μm, roughness 0.4 nm; (b) energy encircle functions obtained by mirror with profile of RMS 0.04 μm, PV 0.2 μm roughness 0.4 nm; (c) focal point obtained by mirror with profile of RMS 0.2 μm, PV 1 μm roughness 0.4 nm; (d) energy encircle functions obtained by mirror with profile of RMS 0.2 μm, PV 0.1 μm roughness 0.4 nm.
图3是X射线聚焦望远镜角分辨率与离焦量的关系, 从图3中可以看出, 对于X射线聚焦望远镜, 焦距处的角分辨率最小, 当像面处于焦点之前或焦点之后时都会影响聚焦望远镜的角分辨率. 当离焦量为5 mm时, 角分辨率由30.6′′降至32′′; 当离焦量为10 mm时, 角分辨率为38′′. 所以, 要提高X射线聚焦望远镜的角分辨率, 增强其成像效果, 不仅要对其面型精度做精度要求, 而且要对焦平面探测器的离焦量做相应要求, 针对聚焦镜角分辨率要求, 最终提出的离焦量为小于4 mm. 图 3 X射线聚焦望远镜的角分辨率与离焦量的关系 Figure3. Relationship between angular resolution and defocus amount in focusing observatory.
22.3.X射线聚焦望远镜有效面积 -->
2.3.X射线聚焦望远镜有效面积
X射线聚焦望远镜的有效面积与其表面膜层材料有直接关系, 因X射线波长短、光子能量大, X射线聚焦望远镜镜片表面必须溅射高原子序数的膜层才能增大其反射效率. 图4为两种不同的膜层结构对X射线的反射效率, 其中图4(a)是膜层材料为Au时, 不同角度下X射线的反射效率, 图4(b)为在Au膜上面制备10 nm的C时其对X射线的反射效率. 从图4中可以看出, 随着掠入射角的增大, X射线反射效率降低, 而且Au和C的复合膜层反射效率明显大于Au膜的反射效率, 并且复合膜层能有效减弱Au膜在2 keV的吸收作用, 提高X射线反射率, 这是因为多层膜结构会对X射线光子产生相干叠加作用, 从而提高了X射线的反射率. 图 4 (a)膜层材料为Au的X射线聚焦望远镜反射率与不同掠入射角的关系; (b)膜层材料为Au加C复合膜的X射线聚焦望远镜反射率与不同掠入射角的关系 Figure4. (a) Relationship between reflectivity and degree of focusing mirrors with Au film; (b) relationship between reflectivity and degree of focusing mirrors with Au, C multi-layer film.
X射线聚焦望远镜视场的定义是偏轴情况下, 有效面积为在轴有效面积50%时的偏轴角. 图5是X射线聚焦望远镜有效面积与入射光偏轴量的关系, 从图5可以发现, 当X射线聚焦望远镜在偏轴情况下时, 其有效面积随偏轴角增大而减小, 当偏轴角为16′ 时, 其有效面积将降至在轴有效面积的50%, 所以文中设计的聚焦望远镜视场为16′, 而当偏轴角继续增大时, 由于光子已经聚焦在设置的探测器外, 所以其有效面积急速下降. 图 5 X射线聚焦望远镜有效面积与偏轴角的关系 Figure5. Relationship between effective area and off axis in focusing observatory.
图6是X射线聚焦望远镜有效面积随层数、镜片质量的变化关系, 当增大X射线聚焦望远镜有效面积时, 在反射率一定的情况下, 只能通过增多其嵌套层数来解决. 从图6(a)中可以看出, 当嵌套层数增多时, X射线聚焦望远镜有效面积逐渐增大, 但其镜头质量也逐渐增大(图6(b)), 当镜片层数多于45层时, 已完全满足eXTP聚焦望远镜有效面积需求, 并且随着嵌套层数的增加, 内层镜片有效面积贡献能力已非常小, 但卫星总重的增加会提高卫星发射成本, 所以综合考虑有效面积需求与卫星发射成本, 最终确定X射线聚焦望远镜的嵌套层数确定为45层, 其有效面积为842和563 cm2@6 keV. 图 6 (a) X射线聚焦望远镜有效面积与嵌套层数的关系; (b) X射线聚焦望远镜有效面积与镜片重量的关系 Figure6. (a) Relationship between and effective area and mirror layers in focusing observatory; (b) relationship between and effective area and mirror weight in focusing observatory