Transcript DESpec Outline • Concept • Technical Components – – – – Optics Fiber Positioner Fibers & Spectrographs CCD & RO • Some discussion about choices that may be available Tom Diehl, DESpec Meeting in Portsmouth.

DESpec
Outline
• Concept
• Technical Components
–
–
–
–
Optics
Fiber Positioner
Fibers & Spectrographs
CCD & RO
• Some discussion about
choices that may be
available
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Tom Diehl, DESpec Meeting in Portsmouth June 2011
DESpec Concept
• An idea to make a significant impact on the
understanding of dark energy
– Build an instrument to perform spectroscopic
follow-up of targets identified in DES data, taking
advantage of the DECam strengths (redsensitivity).
– It’s necessary that the instrument can be interchanged with DECam in a reasonably short time.
– An instrument that can be built at about the same
cost and schedule as DECam (ready by the end
of DES) is desired.
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Main Technical Challenges
Not particularly ordered by difficulty, risk, WBS, or
amount of R&D …
•
•
•
•
Optics
Focal Plane and Fiber Positioner
Spectrographs and Fibers
CCDs & Readout
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DECam => the Blanco
Telescope @ CTIO
Cage
Filters
Shutter
CCD
Readout
Cartoon from
June 2008
5 Optical
Lenses
Hexapod
For alignment & focus
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DESpec Optics
• Reuse the DECam optics C1-C4
(focal ratio f/2.9)
• The DECam Dewar needs its
window (C5) as the cover. We
designed C5’ and C6 made from
fused silica. C5’ has an asphere on
the concave side.
• Spot size (RMS radius) 0.26” at
center, 0.52” at worst, 0.44” at edge.
• Focal surface has a radius of
curvature -8047 mm.
• Worst chief ray comes in at 0.45 deg
angle of incidence.
Version SK-V3C
See talk by Steve Kent
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FP FoV has Radius = 225.54 mm
ADC or Not
• We will need to decide whether or not to provide
an atmospheric dispersion compensator.
Reasons For (Default)
• Better Spot Size
especially at 50+ deg
from zenith
– Better S/N
– Faster measurements
– Fainter objects
– Provides a better,
more useful Instrument
Reasons Against
• Cost
• Trouble
• Increases time to
change instrument
by 2-4 hours?
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Atmospheric Dispersion Compensator
Example from WYIN
• The WYIN ADC has diameter 635 mm. The prisms are
rotated using a pair of encoded stepper motors.
• Two prisms each made from two wedge-shaped pieces
of different glass materials.
• Issues include optical alignment and position (movement)
tolerance and backlash
• Cementing the
pieces together
must be done
“nicely”.
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Optical Fiber Positioners
• Precisely hold the tip of optical fibers on the desired RA
& DEC of the galaxy
– Premium on small (7 mm) spacing between actuators (pitch)
– ± 0.14” (± 1/2 pixel on DECam) position accuracy corresponds
to ±7.5 um.
– 60” target separation is ~3.2 mm spacing between fiber tips
– Fast reconfiguration time: 90 seconds or less
– Targeting efficiency!
– Maximum throughput, highly reliable …
• See talks by Will Saunders (Tilting Spines) and Richard
Ellis (Twirling Posts)
– The Tilting Spines design is battle-tested.
• These are complicated machines.
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# Fibers & “Pitch”
• Distance between
centers of the
positioners ==
“pitch”
• Here we show 3781
positions on a FP
with R=22.554 mm
using a 7 mm pitch.
• If pitch was 6.3 mm
we get 4675
positioners on the
FP. Probably don’t
need this many.
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Spectrographs/Fibers/CCDs
See talk by Jennifer Marshall
• There’s trade offs and
limitations between the
following design parameters
– wavelength range – want to
take advantage of the red
imaging
– spectral resolution – need R
>3000 at l = 950 nm
– # pixels on CCD – we can get
the as big as 2kx4k
– Fiber size – S/N vs throughput
– f/# of the spectrograph optics –
hard to make them f/1.3, easier
to make them f/1.6
– Cost
Options: 2 arm (above) 1 arm (below)
Parameter
Fiber Diameter
Wavelength Range
CCD
Resolution(Dl nm/pixel)
(use 4000 pixels)
# pixels/fiber
Camera f/#
Spectral Resolution
Blue Side
Red Side
100 mm
500*<l<760
760<l<1050
E2V or DECam DECam 2kx4k
2kx4k
0.065
0.0725
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f/2.2
1923 @ 625 nm
Camera Type
4
f/1.7
3276 @ 950 nm
3621 @ 1050 nm
Reflective or refractive
*why not 450 nm?
Parameter
Fiber Diameter
Wavelength Range
CCD
Resolution(Dl nm/pixel)
(use 4000 pixels)
# pixels/fiber
Camera f/#
Spectral Resolution
Camera Type
Single-Arm
Spectrograph (B)
80 mm
600<l<1000
DECam 2kx4k
0.1
3
f/1.6
3334 @ 1000 nm
VIRUS
Interchangeable w/ DECam
• To install DESPec 1st stow
DECam off-telescope
– We are providing hardware to
install/remove DECam as part
of that project (see right)
• Then pick up DESpec, and
using similar hardware,
install it on the end of the
barrel.
• We bring this into the design
ab initio so that the process
• In reverse, either store
can be done quickly and
DESpec on the telescope or
easily.
produce a convenient way to
connect/disconnect the
• Probably .LE. 2 work days11
fibers.
and can use f/8 in between
Summary
• Science requirements set survey requirements
which set technical requirements.
• We’ve just done a round of science & survey
requirements based on the anticipated range of
technical capabilities.
• We Technical Challenges but No Show Stoppers!
• Next we need to understand what is limiting the
science and whether-or-not that is something to
overcome.
Acknowledgements
• Darren DePoy, Jennifer Marshall, J.-P Rheault,
• Steve Kent, Brenna Flaugher, Rich Kron, Anderson
West, Josh Frieman, Huan Lin
• Ofer Lahav, Filipe Abdulla, Stephanie Joubert
• Matthew Colless, Guy Monnet, Will Saunders, Jon
Lawrence
• Michael Seiffert, Richard Ellis
• David Schlegel
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