Sept 16, 2005Ast 920 Meeting 21 Astronomy 920 Commissioning of the Prime Focus Imaging Spectrograph Meeting 2 Intro; VPH Grating Spectroscopy Ken Nordsieck

Sept 16, 2005Ast 920 Meeting 22 Optics Layout

Sept 16, 2005Ast 920 Meeting 23 VPH Gratings

Sept 16, 2005Ast 920 Meeting 24 Throughput

Sept 16, 2005Ast 920 Meeting 25 NIR Upgrade Path NIR upgrade path preserved by collimator allowing full 320 nm – 1.7 μ wavelength range + support structure

Sept 16, 2005Ast 920 Meeting 26 Assembly and Integration

Sept 16, 2005Ast 920 Meeting 27 Slitmask Cutter Commercial laser MOS: custom slits cut in carbon-fiber Longslit: stainless steel, tilted and polished for viewing by SALTICAM/ slitviewer optics PI planning tool produces slitmask file for inclusion in proposal

Sept 16, 2005Ast 920 Meeting 28 Grating Optics Grating Equation Resolution Why Large Telescope Spectrographs are Big VPH Gratings

Sept 16, 2005Ast 920 Meeting 29 Grating Equation and Resolution m λ / Λ = sin α + sin β = 2 sin α (Littrow) dλ / dβ = (Λ/m) cos β angular size of monochromatic slit: Δα = Δθ D tel / D col Δβ = Δα cos α / cos β Δλ (slit) = (Λ/m) cos α Δθ D tel / D col at Littrow R = λ/Δλ (slit) = 2 (D col / D tel ) tan α / Δθ D tel = dia of primary D col = dia of collimated beam Λ = spacing of grating grooves θ = angle in sky α = angle of incidence β = angle of diffraction m = order of diffraction

Sept 16, 2005Ast 920 Meeting 210 Resolution R = λ/Δλ (slit) = 2 (D col / D tel ) tan α / Δθ Does *not* depend on –diffraction order (!) –grating groove density (!) For fixed seeing, improve only by –going to higher grating angle (eg Echelle) –increasing the size of your collimated beam ($$) Note: for small telescopes, also worry about grating diffraction limit R dif = D grat /Λ For SALT/ PFIS: D col = 150 mm, D tel / D col = 73.3 R = 5640 for 1 arcsec slit, 45 deg grating angle

Sept 16, 2005Ast 920 Meeting 211 VPH Gratings Created by exposing holograph material (“DCG”) to interference pattern from laser Index of refraction of DCG is modulated in space Large; inexpensive; custom design; efficient at high groove density

Sept 16, 2005Ast 920 Meeting 212 PFIS VPH 1300 gr/ mm VPH (Λ = 769 nm)

Sept 16, 2005Ast 920 Meeting 213 Tunable “Blaze” Peak efficiency is at Littrow => “Blaze angle” changes with α If increase β as tilt grating, can use grating at large range of central wavelengths => change camera angle = α + β