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KOSMOS Design Considerations

KOSMOS Design Considerations. Jay Elias. 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. KOSMOS Design. Basic principles:

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KOSMOS Design Considerations

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  1. KOSMOS Design Considerations Jay Elias

  2. 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

  3. KOSMOS Design • Basic principles: • Modify as little as possible consistent with requirements • Above all, avoid “scope creep”

  4. 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

  5. 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.

  6. KOSMOS Design – Facility Issues • Larger telescope requires faster camera to preserve pixel scale • Field of view size a related issue, see later

  7. 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)

  8. 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

  9. 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

  10. 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)

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

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