Second Design Review (May 12, 2010) Optical Design J.J. González, A. Kutyrev, M. Wilson, E. de Leon, F. Cobos, A. Watson (+ others at GFSC, IA-UNAM and elsewhere) RATIR: 4-arm system with (2-arm Visible plus 2-arm NIR channels) Visible channel (u to i bands): dual-arm under direct imaging of the F/13 beam of the OAN 1.3 m telescope (no correcting, field-flattening or reimaging optics) – blue-arm (u-r): • • – i-arm: • • – – – Commercial (FLI) camera (Fairchild 20402 15µm pixels) and focuser Commercial (FLI) 10-slot filter wheel (2” round filters) FLI camera (Fairchild) and focuser Fixed i-band filter Field of view: 5.3x5.3 arcmin2 (Φ ≈ 7.5 arcmin) Plate scale: 10.54 arcsec/mm => Sampling: 0.16 arcsec/pixel (0.32 with 2x2 binning) Future upgrade with demagnification NIR channel (Z to H bands): dual-arm with reimaging optics – – – Demagnification (m=0.66): for larger field of view and better pixel-sampling Pupil: for a Cold-Stop to control thermal background and scattered light ZY-arm • • – JH-arm • • – – Split-filter (half field for each Z and Y band) H2RG detector (cut-off at ~1.7 µm), 20402 18µm pixels Split-filter (half field for each J and H band) H2RG detector (1.7 µm cut-off ?), 20402 18µm pixels Field of view: 9.8x9.8 arcmin2 (Φ ≈ 14 arcmin) Sampling: 0.29 arcsec/pixel For rapid identification of high-z GRB and general-science programs 2 1.5m Mirror Instrument flange Instrument Window 1st (r/i) & 2nd (i/Z) dichroics F/13 beam (10’x10’FoV) Blue (u-r) CCD Camera i-filter &CCD camera JH split-filter & H2RG Cold box ZYsplit-filter & H2RG Cold-stop Cold Y/J dichroic u-r filters (10-slot wheel) Tel. focal surface Field-lens group (3 lenses) Fold mirror Camera (3 lenses) & dewar window 3 Beamsplitters Arms: separated via 3 dichroic mirrors, placed in series along the converging beam Wedged dichroics: being tilted-glass slabs in a non-parallel beam, dichroics are wedged for a partial compensation of their own-induced aberrations Visible channel Near-Infrared cold channel CCD1 CCD2 LWP beam-splitters: all three dichroics are reflect blue light and transmitted the red (WFE an other effects α 1/λ) Inclinations: – – the first two dichroics are inclined ±45º 3rd low-incidence dichroic (20º) has an orthogonal inclination Operation conditions: – – First two dichroics operate at ambient conditions (T=-10º-16º C, P≈0.74 ATM) Last dichroic is within the NIR dewar (T≈-185º C in vacuum) 4 Wedged dichroics in converging beams 5 Visible channel (direct imaging) Performance essentially determined by the image quality and field-curvature of the telescope Field-curvature (Rc=540 mm): images increase by ≤0.1” over the flat CCDs RATIR on an idealized-telescope (curved field) 1” and Airy circles For maximum throughput and lower aberrations at shortest wavelengths, the blue arm only goes through the instrument window before being reflected by the first dichroic (r/i) Conventional 2”-filters located not far from FLI cameras to avoid vignetting u-r band r/i & i/Z dichroic cut-offs limit red camera to the i-band only The wedge of the (i/Z) dichroic was optimized to minimize aberrations within the i-band only The Z/Y-dichroic wedge optimized for the Y to H bands i/Z wedge => residual lateral color within i-band. For longer λs, prism-dispersion ~ balanced by the opposite orientation of 2nd (Z/Y) dichroic i band 6 NIR channel (focal reduction + cold-pupil) Larger FoV (m=0..66 plus 18-µm pixels), better sampling, field-curvature correction, background control “Field-lens” group (3-lenses, all spherical surfaces) Just after the telescope focal surface (~ 20 mm) Controls the pupil location Camera Group Cold-box: Stop, Dichroic, Split-filters & H2RGs Dewar window Focal Surf.. Fold mirror Field-lens Group.. Field-lens Group S-FTM16 2nd (Z/Y) dichroic BaF2 S-TIM6 “Camera” group (3-lenses, all spherical surfaces) Power for image inversion & demagnification Both groups at ambient conditions (-10º to 16º C) Pupil diameter: ≈ 36 mm Pupil walk: < 1 mm (RMS) Cold-stop: ~ 42 mm Telescope Focal Surface (after window & 2 dichroics) JH Split-filter Y/J Dichroic Cold Stop Dewar Window Cold-dichroic (Y/J): 5-mm thick 20º incidence ⅛º Wedge: aberrations for longest bands only (J&H) Split-filters: 4.8x9.7 arcmin2 on each band 5-mm thick & 19.6 mm from detector 0.1% ghosts Φ=110” – 145” (over 380 pixels) Gap Field-loses: 36.5” +16” Δ[mm] (~9%) Camera Group (S-FTM16, BaF2, S-FPL53) Back Focus compensates beyond operation ΔT 7 NIR channel (Design Performance) Image-quality performance not far form diffraction limit Residual Astigmatism/Coma from inclined dichroics reasonably compensated up to 14-arcmin field “Prismatic” residual lateral color present but OK 8 NIR channel (Performance vs. T) EE at 3ºC Z+Y bands (0.83-1.14 µm) EE at 20ºC Z+Y bands (0.83-1.14 µm) Back-focus, moving split-filter and detector together, compensates over the operation T interval (~30µm/ºC) Thermal gradients are very small: ΔT<0.2ºC (lenses have very similar thermal mass & dimensions) 9 Optical Prescription (20º C) 10 After-construction Budget Field and Camera groups (lenses cells and barrels) to be subcontracted as independent units Subcontractor manages his own manufacture tolerances and compensators, but using the same overall-system merit and prescription Integration of the barrel units can be carried out under relaxed tolerances (nominally 100 µm displacements and 3-arcmin tilts). Compensators: fold-mirror, back focus and oversized cold-stop 11 Different glass-combos are possible Allows for interaction with contractors to evaluate: Blank and material properties, availability and delivery timescale Polishing and manufacture difficulty Manufacture sensitivities and compensations AR-coating optimization Cost and delivery of finalized units Glass Combination System ID FT1 FT2 FT3 CT1 CT2 CT3 Demerit BAF2_BAF2 S-TIH6 BAF2 S-FTM16 S-FTM16 BAF2 S-FPL53 δ (%) S-FPL53_BAF2 CAF2_BAF2 CAF2_CAF2 S-FPL53_CAF2 " " " " S-FPL53 S-FPL53 CAF2 S-FPL53 " " " " " " S-NBM51 " " " CAF2 CAF2 " " " " 2.41% 7.31% 8.67% 10.02% S-FPL53_S-FPL53 " S-FPL53 " S-NBM51 S-FPL53 " 12.06% Pupil Blur [mm] Axial 50% EE Diameter ["] 10´x10´ FoV Pupil Radius Min Mean Δ Worst Focus Plate Scale [µm/"] System JH ZY Mn Mx Δ JH ZY JH ZY JH ZY [mm] JH ZY BAF2_BAF2 0.23 1.13 17.64 18.78 1.14 0.24 0.19 0.36 0.27 0.45 0.37 0.00 62.59 62.49 S-FPL53_BAF2 CAF2_BAF2 CAF2_CAF2 S-FPL53_CAF2 0.22 0.30 0.21 0.30 1.30 1.39 1.32 1.28 17.46 17.43 16.75 17.09 18.73 18.76 18.45 18.75 1.26 1.33 1.70 1.67 0.24 0.24 0.24 0.24 0.20 0.21 0.20 0.20 0.36 0.36 0.39 0.37 0.29 0.29 0.30 0.30 0.49 0.51 0.49 0.51 0.40 0.41 0.40 0.42 4.15 2.14 -6.92 2.90 62.59 62.59 62.62 62.62 62.49 62.49 62.47 62.47 S-FPL53_S-FPL53 0.18 1.25 16.76 18.95 2.19 0.24 0.23 0.38 0.32 0.51 0.40 -1.69 62.60 62.48 Nothing is for free, but even a system without crystals can be considered 12 Expected Thermal Background Worst scenario considered: • Warm night (17ºC) • Oversized cold-stop (plus 2 to 8 mm) • Detector cut-off at 2.5 µm • Filters blocked to 10-4 in K • Cryostat K-emission not blocked Instrument background (unmasked structure and RATIR optics): • Out-of-band dominated in Z, Y & J • Mostly in-band emission in H => RATIR NIR-arm is always sky-limited (Sky/Inst >> 1) 13 10-σ limiting magnitudes (AB) GRBs after 20 minutes General Science in ½ hr integration 14