Optimising DESpec for Dark Energy science Will Saunders, DESpec workshop, College Station

1. Optimising DESpec for Dark Energy science Will Saunders, DESpec workshop, College Station 9th December 2012

2. Working on DESpec is wonderful • Because • the design is science driven • It’s the same science, for all of us! • we even have a metric (FOM or maybe FOM/$) • So design can be properly optimised • But there will be other users!

3. Working on DESpec design is wonderful • Because: • the design is science driven • it’s the same science, for all of us! • we even have a metric (FOM or maybe FOM/$) • So design can be optimised • But there will be other users! • Fixed things: • Telescope, site, seeing • Field-of-view • Positioner technology • Required sample size • Sample sky distribution • Partially fixed things: • Number of fibers • Telescope image quality • Total budget • Target types and magnitudes • Data quality • Time available • WFC/ADC design • Free things: • Spectrograph design – • Number of arms in red • Blue channel? • Fiber size

4. WFC design Image quality matters!20% slower at 25m than at 15m Blue ZD=0 Aperture efficiency vs rms for faint ELG Red ZD=0 Red ZD=40

5. Aperture losses Aplosses2.nb is a Mathematica code for modelling aperture losses.Includes seeing, telescope optics, galaxy size (assumed exponential), differential refraction.Also includes sky brightness and extinction as function of Air Mass, to give relative survey speeds for a wide variety of observation variables (e.g. telescope image quality, source type, observing conditions)Can integrate over expected Air Mass distribution to find relative overall survey times for different strategies.Can easily be used to test new variants, or extended to include other issues.Working with Brian Nord for incorporation into simulation pipeline.

6. ADC or no ADC? ADC improves polychromatic throughput at high airmass. But, all ELG and LRG targets have one main expected spectral feature with known photo-z.QSOs would benefit from an ADC, except that the ADC absorption costs as much throughput as is gained from reduced aperture losses.Even a perfect ADC with no degradation of image quality would not be worthwhile. Why does BigBOSS have one?DECam WFC design cannot accommodate an ADC in any case, without ruining the imaging in both red and blue. QSOs ELGs

7. Optimal fiber diameter Aplosses2.nb used to simulate overall survey speed as a function of fiber size for various targets and strategies. In red (ELGs, LRGs), I assumed monochromatic observations.For QSOs, I assumed polychromatic 350-525nm.I included effect on S/N for partial resolution of spectral features.Both QSOs and ELG’s have optimal fiber size 1.7-1.8". LRGs prefer much larger fibers, 2.25" .1.76" is very convenient size – largest diameter allowing standard cladding diameter, with safe cladding thickness. ELGs, median and diffuse QSOs

8. A Blue arm for QSOs? • Would be nice to include QSOs. WFC goes down to 340nm! • But only a few hundred targets per field, daft to equip all spines with blue capability • Also absorption in fibers in UV (2%/m@350nm) • Tilting spine design means full area coverage possible with only 1/3 of fibers • Could add 2nd fiber to 1/3 of spines, feeding 2 dedicated 1-armed blue spectrographs mounted on telescope. • Don’t care about flexure in blue (TBC!) • Throughput would be pretty good, 20% peak

9. 1 armed red spectrographs? • Minimum required red coverage and resolution is just compatible with single-armed VPH-based design, with 4K spectral pixels. • Compromises in wavelength coverage, resolution, VPH efficiency, fiber diameter, collimator speed. • Strawman design is 600-1000nm, with 1.57" fibers, Rc~2900, F/2.75 collimator and F/1.5 camera. • Efficiency good at 800nm, but very peaky.

10. 2-armed red spectrographs • 2-armed design removes the compromises. Can use larger fibers, peakier VPH’s (and in principal tuned CCDs and coatings). • Strawman design is 540-770nm + 750-1050nm (dichroic in A-band), with 1.76" fibers, Rc~3900, F/2.75 collimator* and F/1.5 cameras. • Efficiency now much flatter, and good right up to 950nm. * Faster would give a gain in throughput, at the cost of resolution. The smart speed would be that which balances defocus and non-telecentricity losses, around F/2.65-F/2.7.

11. 1 vs 2-armed throughput • 2-armed design removes all the compromises. • Strawman design is 540-770nm + 750-1050nm (dichroic in A-band), with 1.76" fibers, Rc~3900, F/2.75 collimator* and F/1.5 cameras. • Efficiency now much flatter, and good right up to 950nm. • * Faster would give a gain in throughput, at the cost of resolution. The smart speed would be that which balances defocus and non-telecentricity losses, around F/2.65-F/2.7.

12. 2-armed red spectrographs • Not obvious that 2-armed is better overall! 1-armed design is more efficient for ELG’s at 700-840nm. 2-armed design is better for ELG’s everywhere else. 2-armed design is better for LRG’s (because it allows larger fibers). • But, what wavelength is the ‘critical path’? i.e., where does a change in efficiency have greatest impact on FOM? Effective relative throughput