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 g AB ~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

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