ViSP Soft De-Scope The ViSP Team R. Casini, A. de Wijn, M. Knölker, R. Summers, L. Sutherland, G. Card, A. Lecinski ATST SWG Meeting, Nov. 7-8, 2011 ViSP Science Mission (from ISRD) • The ATST Visible Spectro-Polarimeter (ViSP) is expected to provide precision measurements of the full state of polarization of the solar radiation, simultaneously at diverse wavelengths in the visible spectrum, and fully resolving (or nearly so) the spectral lines originating in the solar atmosphere. • Such measurements provide quantitative diagnostics of the magnetic field vector as a function of height in the solar atmosphere, along with the associated variation of the thermodynamic properties. ATST SWG Meeting, Nov. 7-8, 2011 2 Science Requirements • Wavelength range: 380‒900 nm [goal: 380‒1600 nm]; up to three lines simultaneously • Fast reconfiguration: 10 min for one line • Spatial resolution: 2⨯ ATST resolution or 0.07 arcsec at 630 nm • Spatial FOV: 2⨯2 arcmin2 • Spectral Resolution: 3.5 pm at 630 nm or R~180,000 -3 • Polarimetric Capability: 10 Icont polarimetric signal in 10 sec • Simultaneous operation with: – Visible Broadband Imager (VBI) – Diffraction-Limited Near-IR Spectro-Polarimeter (DL-NIRSP) – Visible Tunable Filter (VTF) ATST SWG Meeting, Nov. 7-8, 2011 3 Key Design Features Conceptual Design drivers: • “exploration” instrument, quickly reconfigurable to observe any spectral line in the 380‒900 nm range • 3 separate focal planes • large spatial dynamic range (4,000:1) • large resolving power (180,000 at 630 nm) -3 • polarimetric capability of 10 Icont in 10 sec • ability to observe close pairs like Ca II 849.8/854.2 nm Implications: • low-order diffraction gratings (up to m~20) • complete set of order-isolation filters (~2⨯ max # orders) for each camera package • variable β-angles for different focal planes • multiple gratings to achieve spectral diversity ATST SWG Meeting, Nov. 7-8, 2011 4 Soft De-Scope Changes 1) Validated optics quote from alternate vendor against quote presented at PDR (cost reduction of $900K) ✗ Science Impacts: none (unchanged performance of instrument) ATST SWG Meeting, Nov. 7-8, 2011 5 Soft De-Scope Changes 2) Removed Context Imager (cost saving of $150K) ✗ Science Impacts: • need to use other imaging instruments as CIs (must be able to co-register and maintain the respective FOVs within 0.02 arcsec in translation and 2×10-4 rad in rotation, and at different wavelengths) • flux budget reduction due to “gray” beam splitters • atmospheric dispersion characterization capability ✗ Mitigation: • preserve instrument design that allows possible CI upgrade (e.g., tilted focal plane) • maximize photon flux to the ViSP ATST SWG Meeting, Nov. 7-8, 2011 6 Soft De-Scope Changes 3) Provide only 2 stock gratings at first light (cost saving of $210K) Suggested gratings: Newport RGL 316/63 and 632/57 ✗ Risks: reduced wavelength diversity for multi-line observations (e.g., unable to do Ca II 849.8 and 854.2 simultaneously) ✗ Mitigation: future augmentation of the grating library by at least one custom grating NOTE: ViSP Science Verification Plan document needs be revised because of this last de-scope ATST SWG Meeting, Nov. 7-8, 2011 7 A Chromospheric Example He I 587.6 nm + H I 656.3 nm + Ca II 854.2 nm Good efficiency in Hα and Ca II 8542, but significant grating polarization at D3 ATST SWG Meeting, Nov. 7-8, 2011 8 Polarimetric Performance Polarimetric Sensitivity (10 sec integration) t n o c I / ) P ( σ λ [ nm ] • QE data from sCMOS camera vendor • Conservative estimates of light losses (mirrors, lenses, coatings) and grating efficiency (avg. 35%) ATST SWG Meeting, Nov. 7-8, 2011 9 Polarimetric Performance Polarimetric Sensitivity (10 sec integration) 1.0E-01 1.0E-02 t n o c I / ) P ( σ 1.0E-03 1.0E-04 400 500 600 700 800 900 1000 1100 λ [ nm ] • 95/5 gray beam-splitter • 20% grating avg. efficiency ATST SWG Meeting, Nov. 7-8, 2011 10.