Parameters

Challenges in UV Astronomy, October 2013

Paul Bristow

Design Considerations at Phase A and Beyond

Design team:Beatriz BarbuyBruno CastilhoHans DekkerBernard DelabreClemens GneidingJean-Louis LizonVanessa B. P. MacanhanRoland ReissJoël Vernet

Phase A team: Florian Kerber, Gero Ruprecht, Harald Kuntschner

CassegrainUltravioletBrazilianESOSpectrograph

Challenges in UV Astronomy – October 2013

Requirements that drive the design Achieving high efficiency Opto-mechanical design

SlicerDetector ArrayOptical BenchAtmospheric Dispersion CompensationCalibration

Summary

Overview

Challenges in UV Astronomy – October 2013

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….

Simple means:QuickerLess riskCheaperEasier to operateEasier to calibrate

Not so many parameters

Challenges in UV Astronomy – October 2013

Top Three Top Level RequirementsSignificantly improve upon throughput (or better S/N) of

existing ground based UV spectrographs – USP!Achieve R≥20,000Cover the wavelength range 310-360nm (302-385nm)

Actually four…:VLT =>

• 8m Diameter collecting area• Paranal seeing and extinction• Interface with VLT infrastructure• “Campaign mode”

“TTTLRS”

Challenges in UV Astronomy – October 2013

Achieving high efficiency

Atmosphere Optical design

Cass focusSlicer (no AO)Single dispersive

elementMinimum surfaces

Grating Detector

16:34:58

Positions: 1-16 19-Jul-12

150.00 MM

~20%

Airmass= 1.01.3

1.8Cassegrain ~77%Nasmyth ~65%

Challenges in UV Astronomy – October 2013

Comparison to FORS2,UVES & X-shooter

Challenges in UV Astronomy – October 2013

Choices arising from TTTLRs

Design Aspect Implications

Cassegrain Flexure away from Zenith

Slicer (no AO) Complexity of A&G; DRS; Noise

No X-dispersion or pre-dispersion Resolution/wavelength range

Minimal surfaces Difficulty/cost of optics

High efficiency grating New technology/cost

High QE detector New technology/cost

Ground based/Paranal Atmosphere

Challenges in UV Astronomy – October 2013

Evolving Opto-mechanical Design: Detector Array

Long detector array:3 or 4 × 4K × 2K × 15μm × 15μm~250mm x 30mm (~200pix gaps)Large (but feasible) detector vessel

One mode (plus interlace):No pre-disperser, grating operating in 1st order =>

no tuneable wavelength range (without losing efficiency)Several methods of recovering the wavelengths that fall

into the detector gaps are under consideration

Challenges in UV Astronomy – October 2013

Evolving Opto-mechanical Design: Slicer

Phase A slicer design had three very efficient slitlets

Smaller slitlets:Larger wavelength range for given detector array size

and resolving powerMore slitlets needed => signal spread over more pixels

Detailed Simulations to investigate optimal number of slitlets and their widths:Binning, RON, Dark current Integration times, TargetsSeeing, Sky brightness

Challenges in UV Astronomy – October 2013

Evolving Opto-mechanical Design: Slicer

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

MUSE style slicer, >=7 slices; <=0.3” slitlet widths

Challenges in UV Astronomy – October 2013

Evolving Opto-mechanical Design: Optical Bench

Challenges in UV Astronomy – October 2013

Evolving Opto-mechanical Design: Camera and DV

Challenges in UV Astronomy – October 2013

Evolving Opto-mechanical Design: Pre-slicer

Challenges in UV Astronomy – October 2013

Observe along parallactic (default)Flexure easier to

handle

Airmass restrictions anyway

Evolving Opto-mechanical Design: ADC?

Challenges in UV Astronomy – October 2013

Evolving Opto-mechanical Design: Calibration Unit

Talk by Florian Kerber on LDLS for flats Potential wavecal sources, Hollow Cathode Lamps:

Th-Ar or Th-NePt/Cr-Ne

Tellurics for absolute wavelength ZP? Simultaneous wavelength calibration?

Repeatability/stability• Automatic flexure compensation

Stray lightTo be decided…

Challenges in UV Astronomy – October 2013

Summary

Slicer No. slices>=7; slitlet widths<=0.3”

Transmission grating ~3200mm-1; 1st order; Ruled width~260mm; >80%@320nm

Detector Array 4 4K × 2K × 15μm×15μm;250mm x 30mmQE>85% @320nm;Dark current <0.001e-/pix/s;RON <2.5e-

Wavelength Range 302-390nm (TBC)

Resolving Power 20,000

The CUBES design is dedicated to providing significant SNR improvement relative to existing ground based UV spectrographs

CUBES will be easy to build, easy to operate and maintain and easy to calibrate

Challenges in UV Astronomy – October 2013

End of Talk