DESpec spectrographs

Jennifer Marshall

Darren DePoy

Texas A&M University

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

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

VIRUS spectrographs

• Simple design– Single reflection spherical collimator– Schmidt camera

• Two lenses + one spherical mirror– VPH grating

• High throughput

Unit spectrographs packaged in pairs

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

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

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

DESpec as VIRUS clone

• Relatively straightforward redesign of VIRUS can produce DESpec– Change grating– Reoptimize coatings– Refractive camera?

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

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

How to decide

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

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

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

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

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

Sky lines

Resolution Percent Free Resolution Percent Free

6833 75 10000 72

5198 70 8846 70

3089 60 5610 60

1984 50 3538 50

1347 40 2383 40

905 30 1565 30

799 25 1288 25

312 10 603 10