Astronomy 920 Commissioning of the Prime Focus Imaging Spectrograph Meeting 6 Fabry-Perot Ken Nordsieck Oct 14, 2005 Ast 920 Meeting 6

Bandpass By Mode: Imaging: effective width of filter Grating Spectroscopy: Δλ correspond to slit Fabry-Perot: Δλ of etalon For sky, use arcsec2 corresponding to resolution element Imaging, Fabry-Perot: ΔΩ = area of seeing disk Grating Spectroscopy: ΔΩ = (2 rs) * (2 r0) To calculate readout noise, need number of bins corresponding to resolution element n = ΔΩ/(bx*by / (7.8 pix/arcsec)2) RON = RON/bin * sqrt(n * #readouts) Done! Oct 14, 2005 Ast 920 Meeting 6

Etalon Basics Etalon: adjustable interference filter. destructive interference except at wavelengths: mλ = d cos θ m = order; d = programmable gap Plates must be flat, parallel to λ/m, typically 1/50-1/100 wave The more reflective the coating, the more bounces before exiting: “f = “finesse” distance between orders: δλ = d/m “free spectral range” resolution: Δλ = δλ / f = d/mf m limited by free spectral range, f by practical coatings Oct 14, 2005 Ast 920 Meeting 6

Finesse and Free Spectral Range PFIS etalons: wanted 30, got 20 δλ Oct 14, 2005 Ast 920 Meeting 6

PFIS etalons With constant finesse, higher gap has higher resolution (larger order to get same wavelength) Can’t make one etalon do everything, since cannot practically scan more than about 6 microns (10 waves) PFIS has 3 etalons, used in 4 modes MR and HR always used with LR, for order blocking Mode TF LR MR HR Gap 4 10 26 127 Resolution 250 700 1400 8000 Bullseye >8’ 5.3’ 3.7’ 1.6’ Oct 14, 2005 Ast 920 Meeting 6

Order Blocking Interference Filter HR etalon LR etalon PFIS has 40 interference filters, ~150 Ang wide, 4300 – 9000 Ang HR, MR used in double etalon mode: scanned together with LR Oct 14, 2005 Ast 920 Meeting 6

Fabry-Perot Efficiency FP efficiency at spec in red, slightly below in blue Oct 14, 2005 Ast 920 Meeting 6

Fabry-Perot Resolution Oct 14, 2005 Ast 920 Meeting 6

Scanning PFIS control system allows arbitrary sets of discrete steps (equal exposures) linear scan for line profiles dithering onto continuum for absorption lines Off axis, must take account of wavelength gradient due to cos θ center to edge gradient: λ(r)/ λ0 = 1/ cos θ ~ (1- .00365*(r/4’)2 ) (24 Ang at Hα) Inside “bullseye”, wavelength changed by less than resolution: D(bullseye) = 1.4’ (10,000/R)1/2 - if object larger than bullseye, extend scan to cover same wavelengths - if object not round, pays to move telescope and rescan, instead of extending scan Oct 14, 2005 Ast 920 Meeting 6

Programs FP best for programs requiring good spatial coverage, but few spectral resolution elements Velocity mapping line flux maps Oct 14, 2005 Ast 920 Meeting 6