August 2 and 3, 2010 KOSMOS Design Considerations Jay Elias

2 References: –Science Requirements Document –Preliminary Operations Concept Document –SDN 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