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Optical Design

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).

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Optical Design

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  1. 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)

  2. 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): • Commercial (FLI) camera (Fairchild 20402 15µm pixels) and focuser • Commercial (FLI) 10-slot filter wheel (2” round filters) • i-arm: • 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 • Split-filter (half field for each Z and Y band) • H2RG detector (cut-off at ~1.7 µm), 20402 18µm pixels • JH-arm • 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

  3. 1.5m Mirror Instrument flange F/13 beam (10’x10’FoV) Instrument Window 1st (r/i) & 2nd (i/Z) dichroics Blue (u-r) CCD Camera i-filter &CCD camera u-r filters (10-slot wheel) Tel. focal surface ZYsplit-filter & H2RG JH split-filter & H2RG Field-lens group (3 lenses) Fold mirror Cold box Camera (3 lenses) & dewar window Cold-stop Cold Y/J dichroic

  4. Visible channel Near-Infrared cold channel CCD1 CCD2 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 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)

  5. Wedged dichroics in converging beams

  6. RATIR on an idealized-telescope (curved field) 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 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 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 1” and Airy circles u-r band i band

  7. Camera Group Focal Surf.. Cold-box: Stop, Dichroic, Split-filters & H2RGs Field-lens Group.. Fold mirror Dewar window Field-lens Group S-FTM16 BaF2 S-TIM6 2nd (Z/Y) dichroic Telescope Focal Surface (after window & 2 dichroics) Dewar Window JH Split-filter Y/J Dichroic Cold Stop Camera Group (S-FTM16, BaF2, S-FPL53) 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 (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 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%) Back Focus compensates beyond operation ΔT

  8. 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

  9. 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)

  10. Optical Prescription (20º C)

  11. 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 over-sized cold-stop

  12. 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 Nothing is for free, but even a system without crystals can be considered

  13. 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)

  14. 10-σ limiting magnitudes (AB) GRBs after 20 minutes General Science in ½ hr integration

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