Southern Spectroscopy for Dark Energy From Cosmic Visions Report: - - PowerPoint PPT Presentation

Southern Spectroscopy for Dark Energy

• Is Possible next step in

D.E. Science (Stage V)

• D.E. constraints will be

improved from follow-up of LSST imaging

• It is also a natural

progressive step for astronomy

• It is going to happen.
• It is going to be pitched as

“Billion Object Apparatus”, perhaps close to descriptive

• Main Question: On what telescope?

– Prefer a big telescope and a wide area focal plane – Reasonable conversations mention

• The Blanco (weirdly)
• A 6.5M, either one of the Magellans or a

clone

• Gemini S or new S 8m telescope
• A 20,000 fiber focal plane is

conceivable with ~ current

• technology. Fair to call this high

density/high resolution.

• An unbiased optical trade study is

needed to evaluate mirror size/focal plane size/FP densities.

• Needs to be done by a team that

has no conceived bias (us).

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From Cosmic Visions Report: The Stage V DE has large improvements compared to Stage 4. See Josh’s slides.

Dark Energy Constraints / R&D

Counterweight Pivoting ball Carbon fibre tube Trimming weight Tapered tube Stainless steel tube Fibre tip

• R&D is needed to get the fiber positioner

pitch from 8 mm to ~5 mm.

• We successfully convinced “Cosmic Visions

Dark Energy: Small Projects Portfolio” to make this a priority.

• Understand, replicate, improve the FP
• technology. Collaborate w/ AAO, perhaps.
• We have the experience and skill to lead a

instrument project with 20,000 fibers.

Will Saunders, AAO DESpec Mohawk Will Saunders, AAO

Extra Slides

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Support 4 “Skipper CCD” Realtime Control

BTW, a Skipper CCD doesn’t really skip

• At cost of readout time, it enables reduction of RO noise by 1/sqrt(N) reads.
• Applications include

– Ly-a forest spectra with low-signal absorption lines – Low signal emission lines

• If one has a prior for where the lines are then one could do the trick in their

neighborhood only. Or maybe even decide on the fly based on the signal level, decide how many reads are needed, optimizing the RO time.

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Spectrum from LDSS3 w/ DES-style CCD. Has RO noise 7.1 pixels.

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A piece of Fiber Positioner R&D

Cosmic Visions

• Nov. 15, 2017

“DESpec: A good idea ahead of its time” Photo: Will Saunders

Optical Fiber Positioners

• Precisely hold the tip of optical fibers on the desired RA

& DEC of the galaxy (specs typical of Echidna Mark 1)

– 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 – Maximum throughput, highly reliable …

• Tilting Spines and Twirling Posts

– Advantages/Disadvantages for each type (discuss at length) – Relatively high density of positioners on the focal plane made Tilting Spines the best choice for DESPec – The new Tilting Spine design (J. Gilbert & G. Dalton SPIE 2016) alleviates the spine’s disadvantages

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Skipped a step: economics that drives us to fiber positioners

Mohawk Tilting Spine

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Counterweight Pivoting ball Carbon fibre tube Trimming weight Tapered tube Stainless steel tube Fibre tip

• 6 ¾ mm pitch
• Length about 16 cm
• Patrol radius ~ 7 mm
• Positioning accuracy < 10

microns is already achieved

• Improved configuration time is

about 60 seconds.

• Issue of “tilt defocus” drives the

design to long spines.

• New design low-voltage spine

(2016)

Echidna spine picture from Graham Murray (Durham)

MOHAWK fiber positioner

• 4000 spines, 450mm

Focal Plane diameter, 6.75mm pitch.

• Curved modules, each

with two rows of spines, fit together like staves of a barrel to form spherical surface. Will Saunders, AAO

Spines vs Twirling Posts

• Tilting Spines

– Pitch 6 mm – Patrol Radius ~ 1.2 * pitch – Maximum packing ~ 7 – 6 or 7 parts maybe 10 in Model 2 – Better targeting efficiency – More effective for close targets – Light loss with tilt – FRD affects sky subtraction – Both above are mitigated by longer spines

• Twirling Posts

– Pitch 10 mm – Patrol radius ~0.7 * pitch – Maximum packing ~ 2 – ~ 28 parts – Faster to position than initial spine versions

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Fiber Positioner R&D Strategy?

• Understand & replicate

the technology. Collaborate w/ AAO, perhaps.

• Engineering:

– Reduce the pitch, how? – Increase the length

• Incremental

improvements have a big payoff because of r^2

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