despec spectrographs
DESCRIPTION
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 PresentationTRANSCRIPT
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DESpec spectrographs
Jennifer MarshallDarren DePoy
Texas A&M University
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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
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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
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VIRUS spectrographs
• Simple design– Single reflection spherical collimator– Schmidt camera
• Two lenses + one spherical mirror– VPH grating
• High throughput
Unit spectrographs packaged in pairs
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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
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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
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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
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DESpec as VIRUS clone
• Relatively straightforward redesign of VIRUS can produce DESpec– Change grating– Reoptimize coatings– Refractive camera?
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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
![Page 10: DESpec spectrographs](https://reader035.vdocuments.us/reader035/viewer/2022062302/56816387550346895dd4722f/html5/thumbnails/10.jpg)
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
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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
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How to decide
• Need science input to provide instrument requirements:– Wavelength range– Resolution– Density of targets/number of fibers– Fiber size on sky
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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
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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
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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
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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
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Optimal Spectral Resolution
Jennifer MarshallDarren DePoy
Steven VillanuevaTexas A&M University
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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?
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Low resolution red spectra compromised by night sky emission lines
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Fewer compromised pixels at higher resolution
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Much less of a problem at bluer wavelengths
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Lower resolution in “blue” not substantially compromised
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Fraction of “uncontaminated” pixels (SNR > 0.9 relative to no night sky emission lines)
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SNR per pixel versus resolution
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SNR per pixel versus resolution
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Conclusions
• Red spectra require relatively high resolution– R > 2500– Optimization is soft
• Blue spectra can be lower resolution– R > 500