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），它在可选的多ghz网格上提供一系列超窄、无漂移、有规则间隔的光学频率。在STFC的支持下，HWU目前由PPRP （ST/S001328/1）和财团（ST/V000403/1）资助，在多个方面领导lfc的开发和部署。该资本设备请求直接支持我们的财团拨款ST/V000403/1，其中一个关键目标是开发基于lfc的概念，以提取天文光谱仪的波长解决方案，即其“像素到波长”映射。在ST/V000403/1中，我们已经开发了一种梳线解析光谱仪来先导绝对校准思想。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膜，在光线被引导到天文光谱仪之前，将其用作高分辨率光谱整形器。我们建议使用高分辨率数字微镜器件（DMD）来实现这一目标，该器件包含400万个微小（7.6微米）的数字可寻址镜像。这个概念为调节发送到天文光谱仪的光提供了一些令人兴奋的可能性。其中最激进的方法是在将其注入天文光谱仪之前，分离出单个梳状模式，并用商用波长计对其波长进行高精度测量。对光谱仪的每一级重复这个过程，可以直接获得每一级的波长解，而不像目前的方法只提供整个光谱的一个基准波长标记。初步计算和实验灵敏度检查表明，这种想法应该是完全可行的。这个概念消除了在摄谱仪的每个阶上观察到的精确梳状模式的不确定性，这个问题通常只能通过使用空心阴极灯进行二次测量或使用锁定在LFC上的复杂单频激光来解决。此外，整形器还可以通过选择性地衰减梳状器中较亮的部分来实现全频谱动态范围控制，并通过选择性地去除每n个梳状线来实现可切换的梳状模式间隔。具体而言，需要两项设备来构建原型光谱成形器（形成单个设备项）：1。DMD和驱动板（Vialux DLP SuperSpeed V-Module） @ 12,202英镑精密波长计（High Finesse WS7-60） @ 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，订单将被置于非竞争性行动之下。这两个项目的交货时间最多为16周，因此两者都可以在资本设备赠款的生命周期内订购，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，可以通过试点新的天文光谱仪数字校准方法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.