Transcript 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.

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 highlyreplicated 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
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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
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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 (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