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Spectroscopic Reference Design Options

This document outlines advanced options for the design of a spectrograph that spans the 550-1000 nm range, leveraging a 2Kx4K CCD with potential resolutions reaching ~4000, ideal for studying galaxy rotation curves and clustering. Key features discussed include the allocation of fibers, such as fixed geometry bundles for closely packed targets, and the potential use of fiber-bundle Integral Field Units (IFUs) to enhance throughput and resolution. Cost implications for incorporating additional spectrographs and optimizing design for high-density targets are also addressed.

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Spectroscopic Reference Design Options

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  1. Spectroscopic Reference Design Options D. L. DePoy Texas A&M University

  2. Notional DESpecunit spectrograph specs • 550-1000nm coverage using DES 2Kx4K CCD • Roughly 4 pixels per resolution element (R~1760 at 775nm; 0.11 nm/pixeldispersion) • ~4000 individually targetable fibres

  3. No metal has been cut (the cuts are not even designed!) • Could use 2 pixels per resolution element • Resolution ~4000 rather than ~2000 • Could “assign” some fibers to fixed geometry bundle • For clusters or other “closely packed” targets • Could use “fiber bundle” IFUs instead of single fibers • Rotation curves of galaxies or higher throughput

  4. Higher resolution • Need longer focal length camera • Or finer grating • Better image quality from camera • “line spread function” <30 microns • However, typical galaxy velocity dispersions are ~150 km/sec • Could split beam into blue and red “arms” • Cost would rise for spectrographs

  5. Fiber bundles Could allocate some fibers to IFU IFU input IFU output Kelz et al., 2006, SPIE 6273-121; Murphy et al., 2008, SPIE 7018-104; Soukup et al., 2010, SPIE 7735-180

  6. Fixed fiber bundles • Closely packed targets may require many exposures • Currently roughly 1 fiber per 2 arcminute diameter patch on the sky • If target density is higher, will require multiple exposures • Galaxy clusters may have 10-100 objects per arcminute

  7. Fixed fiber bundles • Could allocate fixed pattern to central field • ~3 exposures for 100% coverage of area • ~1600 fibers for 1 sq. arcminute • Assumes 1.5 arcsec fiber • ~8 additional spectrographs • Limit would be to carpet entire focal plane in fixed configuration • ~17 million fibers • 70,000 spectrographs • Project cost would be high

  8. Hybrid approach • Use small “IFUs” instead of single input fibers • Each positioner has a bundle of N fibers • N*size = field-of-view of each IFU • e.g. ~15 arcsec field, 1.5 arcsec fibers • ~50% fill factor • Roughly 50 fibers per IFU • Rotation curves for every galaxy larger than ~few arcsec • Need ~50 times as many spectrographs • ~750 spectrographs • Already building 200 for HETDEX • Other combinations possible of course

  9. Summary • Many options still open • Need to allow science goals to drive instrument choices • Should coordinate science ideas with changes in instrument design

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