1. DESpec spectrographs Jennifer Marshall Darren DePoy Texas A&M University

2. Prototype design: VIRUS clone • 10 fiber-fed unit spectrographs, 400 fibers each • Wavelength range 550-950 nm in one arm • Resolution at 950 nm = 3167 • Uses 2 DECam CCDs in each arm • Based on VIRUS design

3. VIRUS • The first highly-replicated instrument in optical astronomy • 150+ channel fiber-fed Integral Field Spectrograph placing >33,000 1.5” dia fibers on sky • 350-550 nm coverage and R~700

4. VIRUS spectrographs • Simple design • Single reflection spherical collimator • Schmidt camera • Two lenses + one spherical mirror • VPH grating • High throughput Unit spectrographs packaged in pairs

5. Texas A&M’s role in HETDEX • Participate in optical and mechanical design of VIRUS • Fabrication and procurement of VIRUS components • Assemble VIRUS unit spectrographs • Optically align instruments in lab • Ship to McDonald

6. HETDEX+VIRUS specs • Wavelength: 350 – 550 nm • Resolution: R~700 • Integration time: t=20 minute • Fiber diameter: 1.5” on sky • Sensitivity • Line flux limit 3.5e-17 • Continuum detection gAB~22 mag

7. Flexibility of VIRUS design • VIRUS design is readily adaptable to other fiber-fed spectrograph systems • Easy to change resolution, wavelength range, etc. with simple redesigns • Has already been used as basis of new spectrograph design • LRS2, a moderate resolution red-optimized spectrograph for HET

8. DESpec as VIRUS clone • Relatively straightforward redesign of VIRUS can produce DESpec • Change grating • Reoptimize coatings • Refractive camera?

9. Prototype design: VIRUS clone • 10 fiber-fed unit spectrographs, 400 fibers each • Wavelength range 550-950 nm in one arm • Resolution at 950 nm = 3167 • Uses 2 DECam CCDs in each arm • Based on VIRUS design

10. Alternate design: two arms • 10 fiber-fed unit spectrographs, 400 fibers each • Increased wavelength range • Two arms, blue (500-760) and red (760-1050) • Different resolution in each arm • 625 nm, R~1923 • 950 nm, R~3276 • Uses 2 DECam CCDs in each arm • Significant design modification from VIRUS • Similar optical layout to GMACS

11. GMACS • Wide-field, multi-object optical spectrograph for GMT • Four quadrants with two arms (red and blue) each • One quadrant could be modified to become DESpec unit spectrographs

12. How to decide • Need science input to provide instrument requirements: • Wavelength range • Resolution • Density of targets/number of fibers • Fiber size on sky

13. Work required to design DESpec as VIRUS clone • Science input for instrument requirements • New optical design for camera • Mechanical redesign of camera • Mechanical design of instrument mounting scheme on telescope • Cooling system redesign

14. Work required to design DESpec as VIRUS clone • We would need about 2 years of engineering effort for redesign • A&M could assemble and test spectrographs in ~2 years • Lots of experience from VIRUS! • These are estimates; will require more careful schedule/planning

15. Work required to design DESpec two-arm design • More optical and mechanical design work required • Increases cost • May need non-DECam CCDs for blue channel • Increases cost

16. Summary • VIRUS design could be easily and relatively cheaply adapted to DESpec spectrographs • Two-arm re-design is more involved but possible • Would need ~10 spectrographs • 3-4 years of effort in redesign and assembly

17. Optimal Spectral Resolution Jennifer Marshall Darren DePoy Steven Villanueva Texas A&M University

18. What is the “best” spectral resolution (λ/Δλ)? • Science objectives set broad constraints • Various considerations suggest low resolution • Easier optics • Smaller CCD format • Cheaper spectrographs • Low means R=1000-1500 • 200-300 km/sec • Night sky emission lines are bright in the red • Suggest resolution should be higher • Isolates lines and allows for more “clean” pixels • What does “higher” mean?

19. Low resolution red spectra compromised by night sky emission lines

20. Fewer compromised pixels at higher resolution

21. Much less of a problem at bluer wavelengths

22. Lower resolution in “blue” not substantially compromised

23. Fraction of “uncontaminated” pixels (SNR > 0.9 relative to no night sky emission lines)

24. SNR per pixel versus resolution

25. SNR per pixel versus resolution

26. Conclusions • Red spectra require relatively high resolution • R > 2500 • Optimization is soft • Blue spectra can be lower resolution • R > 500