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 Marshall Darren DePoy Steven Villanueva Texas A&M University What is the best spectral resolution (/)?/)? 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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