SALT’s Spectro- graphs: From the inside out! Astro Coffee – Independence Day 2013 The Robert Stobie Spectrograph • Achieving all this requires large, horrendously complicated optics… • Tricky crystals: CaF2 & NaCl • Fluid-coupling, nasty anti- reflection coatings etc • Hugely ambitious UV to NIR instrument, several complicated modes: imaging, longslit, multi-object, polarimetry, Fabry- Perot, high-speed, + N combinations of these! The RSS Beam-Splitter • An optic straight from Hell • Prisms made of incredibly rare & fragile UV calcite crystals • 3x3x2 mosaic: need precision alignment of all 18 elements • Fluid-coupled so complicated plumbing in cells & frame • Suffered from fluid problem so had to be cleaned & 3 prisms cracked along the way • Only re-installed in March 2011 • 8 months later, it sprang a leak! • Repatriated to California in Feb 2012 – still waiting on repair… More is Not Well with RSS… • Imaging-mode data from SALTICAM & RSS indicate a substantial (~40%) throughput loss in RSS • Problem not as pronounced in the blue as it was with the original throughput issue • Suspects included: vignetting, lens fluid problems, degradation of anti-reflection coatings or fogging of the NaCl lenses • Presumed that such a huge effect must produce something observable within the optical system, so climbed up there to take a look… Inspection of the Optics • The camera optics look fine, but the collimator lenses really Don’t! Camera Collimator Collimator Optics • Droplets & streaks in the collimator triplet Droplet Theories • Given the beam-splitter leaks, we suspected that the triplet may too have lost its lens fluid – but no sign of leaks?! • Considered that 2 immiscible fluids could produce such an effect – e.g. old (contaminated) & fresh lens fluids • In fact, camera optics contain fresh “old” fluid but a new type of fluid’s used in the collimator triplet for better NIR performance – are they fully compatible? RSS Throughput Tests • 2 lasers @ 670 nm & 365 nm + filtered photodiodes in a nifty test frame from UW • Flat mirrors with measured reflectivities in filter, slitmask & waveplate holders RSS Throughput Results • Camera: Red = 88% & UV = 78% • Collimator: Red = 59% & UV = 37% • Conclusive proof of a collimator issue! • Total instrument: Red = 52% & UV = 29% Camera Collimator Testing Collimator Sub-Units • Used mirrors to sub-divide the collimator optics & test different lens groups • Collimator main group isolated as worst contributor Doublet + Fold Mirror: 99% & 68% • Measurements & Collimator Main Group: 65% & 69% photos indicate demons in the Waveplate Blank + Field Lens: 92% & 79% collimator triplet Old & New Lens Fluids • Original fluid: Cargille Laser Liquid Code 5610, n = 1.4600, s.g. = 0.88-1.03 • Fresh original fluid used in the camera multiplets – no problem • Different fluid used in collimator triplet & field lens: Certified Refractive Index Liquid Series AAA, n = 1.3950, s.g. ≈ 1.9 • Old & new fluids are supposed to be miscible, but actually Not – they form a cloudy layer at the interface!! Immiscible Lens Fluids • Mixture is cloudy/hazy • The old fluid wets the surface & gets driven around by the new fluid So What Should We Do?! • Can we clean the triplet? No, too risky: last time 2 lenses got hurt/killed & some of the old fluid still stayed behind in the cells! • Instead: keep RSS online while we build a new triplet, swap in the new assembly when ready • Should improve total RSS red throughput from ~50% to ~70% & blue from ~30% to ~40% • Also replace the collimator doublet that has a damaged A-R coating to increase the overall blue throughput of the instrument to ~50% • SAAO/SALT develops valuable new optical capabilities & finally takes full ownership of the last remaining scary RSS sub-system How To Do That? • Source & procure appropriate CaF2, NaCl & Fused Silica blanks (+ spares!) to meet the system’s very tight specifications • Have the blanks figured into lenses • Get anti-reflection coatings applied • Manufacture new lens cells/assemblies at SAAO using our new CNC machines • Align & bond lenses in cells in our lab • Integrate assemblies & test everything • Bring RSS down to install the new optics CaF2 Baby Photos SALT HRS Sneak Preview • Coming Soon to a spectrometer room not far from here… • Due to be shipped in less than a month! SALT HRS Optical Layout • Fibre-fed échelle spectrograph with red & blue channels for λλ = 370–890 nm • 4 observing modes: LR, MR, HR & HS • All sealed in big vacuum tank for stability Really Gorgeous Optics!! Lenses, Mirrors, Light & Magic Summary of HRS Characteristics HRS Timeline • Final pre-ship testing in progress • Pack up & ship by end of July • 4+ weeks to get to Cape Town • Science Verification proposal call open now, closes 15 August • Install, re-integrate & align everything in the spectrometer room under SALT • Commissioning & uncharged Science Verification programmes through end of April 2014 Sodium Testing To Follow….