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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. - PowerPoint PPT Presentation
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DESpec spectrographs
Jennifer MarshallDarren 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?
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
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
Optimal Spectral Resolution
Jennifer MarshallDarren DePoy
Steven VillanuevaTexas A&M University
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?
Low resolution red spectra compromised by night sky emission lines
Fewer compromised pixels at higher resolution
Much less of a problem at bluer wavelengths
Lower resolution in “blue” not substantially compromised
Fraction of “uncontaminated” pixels (SNR > 0.9 relative to no night sky emission lines)
SNR per pixel versus resolution
SNR per pixel versus resolution
Conclusions
• Red spectra require relatively high resolution– R > 2500– Optimization is soft
• Blue spectra can be lower resolution– R > 500
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