Direct Comb-Line Measurement: A New Calibration Concept for Astronomical Spectrographs

几十年来，天文学家一直使用空心阴极灯作为光学望远镜光谱仪的校准源，但它们的局限性意味着该社区现在转向激光频率梳（LFC），它在可选择的几GHz网格上提供一系列超窄，无漂移，规则间隔的光学频率。在STFC的支持下，HWU在几个方面领先LFC的开发和部署，目前由巴勒斯坦人民重建方案提供资金（ST/S 001328/1）和联合体（ST/V000403/1）赠款。本资本设备申请直接支持我们的联合体赠款ST/V000403/1，其中一个关键目标是开发基于LFC的概念，以提取天文摄谱仪的波长解，即其“像素到波长”映射。1.我们已经研制了一台梳状谱线分辨摄谱仪，以指导绝对校准的想法。与密封的商业仪器不同，我们可以进入该设备的焦平面，并在那里放置一个掩模，将其用作高分辨率的光谱整形器，然后将光引导到天文光谱仪。我们建议使用一个高分辨率的数字反射镜设备（DMD）来实现这一点，该设备包含400万个微小的（7.6微米）数字可寻址反射镜。其中最激进的是分离出一个单梳状模，并在将其注入天文摄谱仪之前，用商用波长计高精度地测量其波长。对摄谱仪的每一级重复这个过程，就可以直接获得每一级的波长解，而不像目前的方法那样只为整个光谱提供一个基准波长标记。初步计算和实验灵敏度检验表明，这种想法是完全可行的。该概念消除了在光谱仪的每一阶上观察到哪种精确梳状模式的不确定性，该问题通常仅通过采用空心阴极灯的二次测量或使用锁定到LFC的复杂单频激光器来解决。还存在使用整形器通过选择性地衰减梳状的较亮部分来提供全光谱动态范围控制的机会，并且通过选择性地去除每第n个梳状线来实现可切换的梳状模式间隔。具体地，需要两个设备项来构造原型频谱整形器（形成单个设备项）：DMD和驱动板（Vialux DLP SuperSpeed V-Module）@ £ 12，202 2.精密波长计（High Finesse WS 7 -60）@26，130英镑这两个项目的价值都低于50，000英镑的规定采购门槛，因此不需要招标过程。由于每个项目的独特性能，订单将在非竞争性行动下下达。两个项目的交货期最长为16周，因此可以在资本设备赠款的有效期内订购，交付和接收。包括增值税和赫瑞瓦特大学20%的贡献，STFC的最终成本为36，799英镑，这使得这是一个相对低成本的资本设备要求，可以通过对天文摄谱仪的新数字校准方法进行试点，为天文学界提供相当大的价值。

For decades, astronomers have used hollow-cathode lamps as calibration sources for the spectrographs in optical telescopes, but their limitations mean that the community is now turning to laser frequency combs (LFCs), which provide a sequence of ultra-narrow, drift-free, regularly spaced optical frequencies on a selectable multi-GHz grid.Supported by STFC, HWU is leading the development and deployment of LFCs on several fronts, with current funding from PPRP (ST/S001328/1) and consortium (ST/V000403/1) grants.This capital equipment request directly supports our consortium grant, ST/V000403/1, in which a key goal is to develop LFC-based concepts to extract the wavelength solution of an astronomical spectrograph i.e. its "pixel-to-wavelength" mapping.In ST/V000403/1 we have already developed a comb-line-resolving spectrograph to pilot absolute calibration ideas. Unlike a sealed commercial instrument, we can access the focal plane of this device and place a mask there to use it as a high-resolution spectral shaper, before the light is then directed to an astronomical spectrograph. We propose to achieve this using a high-resolution digital micromirror device (DMD) containing 4 million tiny (7.6 micron) digitally-addressable mirrors.This concept offers some exciting possibilities for conditioning the light sent to the astronomical spectrograph. The most radical of these is to isolate a SINGLE COMB MODE and measure its wavelength to high precision with a commercial wavemeter before injecting it into the astronomical spectrograph. Repeating this process for each order of the spectrograph gives a direct means of obtaining the wavelength solution of each order, unlike current methods which provide just one fiducial wavelength marker for the whole spectrum.Preliminary calculations and an experimental sensitivity check show that this idea should be entirely feasible. The concept removes the uncertainty of which exact comb mode is observed on each order of the spectrograph, a problem normally addressed only by resorting to secondary measurements with a hollow-cathode lamp, or using a complex single-frequency laser locked to the LFC.Further opportunities exist to use the shaper to provide full-spectrum dynamic-range control by selectively attenuating brighter parts of the comb, and to implement switchable comb-mode spacings by selectively removing every nth comb line.Specifically, two items of equipment are requested to construct a prototype spectral shaper (forming a single equipment item): 1. DMD and driver board (Vialux DLP SuperSpeed V-Module) @ £12,202 2. Precision wavemeter (High Finesse WS7-60) @ £26,130Both items have a value below the regulated procurement threshold of £50,000, so no tender process is required. Orders will be placed under a non-competitive action, because of the unique performance capabilities of each item.Lead times for both items are a maximum of 16 weeks, so both can be ordered, delivered and receipted within the lifetime of the capital equipment grant.Including VAT and a 20% contribution from Heriot-Watt University, the final cost to STFC is £36,799, making this a relatively low cost capital equipment request that could offer considerable value to the astronomy community by piloting a new digital calibration approach for astronomical spectrographs.