August 2 and 3, 2010

KOSMOS Design Considerations

Jay Elias

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• References: – Science Requirements Document– Preliminary Operations Concept Document– SDN 1.01-1.04 on science requirements– SDN 2.02 & 2.03 on SW requirements– Functional Performance Requirements

Document

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KOSMOS Design

• Basic principles:– Modify as little as possible consistent with

requirements– Above all, avoid “scope creep”

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KOSMOS Design

• Focus on two areas:– Differences between MDM 2.4-m and KPNO

4-m– Differences between science needs of

NOAO user base• Derive input from ReSTAR, KPNO staff, NOAO

Users’ Committee

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KOSMOS Design – Facility Issues

• Larger telescope requires faster camera to preserve pixel scale– 0.3 arcsec/pixel (or slightly coarser) is a

good match to seeing at both telescopes.– Finer scale plus binning is not a good

solution because 4k pixels then provide fewer resolution elements; in this case a larger CCD could be used but require more $$, new dewar, etc.

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KOSMOS Design – Facility Issues

• Larger telescope requires faster camera to preserve pixel scale– Field of view size a related issue, see later

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KOSMOS Design – Facility Issues

• Want to use NOAO standard CCD system (dewar + Torrent controller)– Easier to support– Existing dewars save money– Interchangeable with other

instruments/telescopes– Considerations reinforced if we implement

2 CCDs (as we did)

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KOSMOS Design – Facility Issues

• Software interfaces different– Telescope, CCD system, data archive– Only instrument controls common to

OSMOS– Choice of adapting existing top-level OSU

software or NOAO software• Adopt NOAO software (NOCS) after evaluation;

see later presentation for more on the NOCS• We spent time trying to make this decision

rationally

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KOSMOS Design – Science Issues

• User community differences– Not much (not surprising)– Less emphasis on the low-resolution prism

mode– More interest in higher spectral resolution– Initial disperser complement 2 moderate

resolution grisms; prism remains an option for the future

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KOSMOS Design – Science Issues

• Field of view– Physical field of view of OSMOS only 10

arcmin on 4-m; with faster camera could (probably) provide a larger field on CCD

– This requires (at least) a larger collimator and makes the slit wheel, probably the whole instrument much larger• A lot of re-design• Doesn’t fit in the cass cage any more without

fold(s)

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KOSMOS Design – Science Issues

• Field of view (cont’d)– ReSTAR did not identify maximum field as

a strong science driver• A lot of the science programs involved single

objects• KOSMOS AΩ already as good as GMOS• Science value added not considered enough to

offset added cost, delivery delays, and performance risk

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KOSMOS Design – Science Issues

• Higher resolution– Resolution R>2300 (4000 goal)– RC Spec will go higher (about 10,000) but

demand is limited– A requirement for higher max resolution

requires larger beam size, hence a larger instrument; similar issues as larger FOV

– Fixed-angle layout limits coverage at higher resolution

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KOSMOS Design – Science Issues

• Wavelength coverage– OSMOS does well in the UV down to ~365

nm– Desirable to keep this level of performance

for KOSMOS• Performance likely to be limited not by design

(which is good) but by differences between design and actual materials; mitigate by index measurement (see later discussion) but don’t put in the maximum possible effort (blank selection via testing) because of time and cost

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KOSMOS Design – Science Issues

• Wavelength coverage (cont’d)– OSMOS performance in the red limited by

CCD– Option to acquire a thick LBNL chip

appeared, took advantage of this– LBNL CCD is not the commissioning CCD

and probably will not be the most-used CCD on KOSMOS; purchasing e2v CCD for that purpose• Need to define scheduling policy for these

CCDs

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KOSMOS Design – Science Issues

• Flexure– OSMOS worst-case performance about

1/pixel hour• Flexure is along direction of changing gravity so

it’s simple to understand• Flexure leads to need for more night-time

calibration (fringing could be a serious problem but not with CCDs selected)

• OSMOS performance acceptable but not desirable

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KOSMOS Design – Science Issues

• Flexure (cont’d)– Greater stiffness possible in 2 areas:

• Higher-grade focus stages – modest cost increase, otherwise no impact

• Stiffer enclosure – reduce aggressive light-weighting needed for MDM 2.4-m; don’t pursue extensive re-design & analysis effort

• Requirement is to meet OSMOS performance; goal is factor of 2 improvement