1. Parameters Challenges in UV Astronomy, October 2013 Paul Bristow Design Considerations at Phase A and Beyond Design team: Beatriz Barbuy Bruno Castilho Hans Dekker Bernard Delabre Clemens Gneiding Jean-Louis Lizon Vanessa B. P. Macanhan Roland Reiss Joël Vernet Phase A team: Florian Kerber, Gero Ruprecht, Harald Kuntschner Cassegrain Ultraviolet Brazilian ESO Spectrograph

2. Challenges in UV Astronomy – October 2013 n Requirements that drive the design n Achieving high efficiency n Opto-mechanical design  Slicer  Detector Array  Optical Bench  Atmospheric Dispersion Compensation  Calibration n Summary Overview

3. Challenges in UV Astronomy – October 2013 n In geometry there’s no 3D object much simpler than a CUBE (fully described by one parameter):  Except maybe a SPHERE (ESO’s already got one, nearly);  or a TETRAHEDRON…. n Simple means:  Quicker  Less risk  Cheaper  Easier to operate  Easier to calibrate Not so many parameters

4. Challenges in UV Astronomy – October 2013 n Top Three Top Level Requirements  Significantly improve upon throughput (or better S/N) of existing ground based UV spectrographs – USP!  Achieve R≥20,000  Cover the wavelength range 310-360nm (302-385nm) n Actually four…:  VLT => 8m Diameter collecting area Paranal seeing and extinction Interface with VLT infrastructure “Campaign mode” “TTTLRS”

5. Challenges in UV Astronomy – October 2013 Achieving high efficiency n Atmosphere n Optical design  Cass focus  Slicer (no AO)  Single dispersive element  Minimum surfaces n Grating n Detector ~20% Airmass=1.0 1.3 1.8 Cassegrain ~77% Nasmyth ~65%

6. Challenges in UV Astronomy – October 2013 Comparison to FORS2, UVES & X-shooter

7. Challenges in UV Astronomy – October 2013 Choices arising from TTTLRs Design AspectImplications CassegrainFlexure away from Zenith Slicer (no AO)Complexity of A&G; DRS; Noise No X-dispersion or pre-dispersionResolution/wavelength range Minimal surfacesDifficulty/cost of optics High efficiency gratingNew technology/cost High QE detectorNew technology/cost Ground based/ParanalAtmosphere

8. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Detector Array n Long detector array:  3 or 4 × 4K × 2K × 15μm × 15μm  ~250mm x 30mm (~200pix gaps)  Large (but feasible) detector vessel n One mode (plus interlace):  No pre-disperser, grating operating in 1 st order => no tuneable wavelength range (without losing efficiency)  Several methods of recovering the wavelengths that fall into the detector gaps are under consideration

9. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Slicer n Phase A slicer design had three very efficient slitlets n Smaller slitlets:  Larger wavelength range for given detector array size and resolving power  More slitlets needed => signal spread over more pixels n Detailed Simulations to investigate optimal number of slitlets and their widths:  Binning, RON, Dark current  Integration times, Targets  Seeing, Sky brightness

10. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Slicer

11. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Slicer 7 x 0.25” 3 x 0.45” 5 x 0.35” 7 x 0.35” V=19 QSO n MUSE style slicer, >=7 slices; <=0.3” slitlet widths

12. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Optical Bench

13. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Camera and DV

14. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Pre-slicer

15. Challenges in UV Astronomy – October 2013 n Observe along parallactic (default)  Flexure easier to handle n Airmass restrictions anyway Evolving Opto-mechanical Design: ADC?

16. Challenges in UV Astronomy – October 2013 Evolving Opto-mechanical Design: Calibration Unit n Talk by Florian Kerber on LDLS for flats n Potential wavecal sources, Hollow Cathode Lamps:  Th-Ar or Th-Ne  Pt/Cr-Ne n Tellurics for absolute wavelength ZP? n Simultaneous wavelength calibration?  Repeatability/stability Automatic flexure compensation  Stray light  To be decided…

17. Challenges in UV Astronomy – October 2013 Summary SlicerNo. slices>=7; slitlet widths<=0.3” Transmission grating~3200mm -1 ; 1 st order; Ruled width~260mm; >80%@320nm Detector Array4 4K × 2K × 15μm×15μm; 250mm x 30mm QE>85% @320nm; Dark current <0.001e - /pix/s; RON <2.5e - Wavelength Range302-390nm (TBC) Resolving Power20,000 n The CUBES design is dedicated to providing significant SNR improvement relative to existing ground based UV spectrographs n CUBES will be easy to build, easy to operate and maintain and easy to calibrate

18. Challenges in UV Astronomy – October 2013 End of Talk