Post on 22-Dec-2015
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• Wavelengths 5500 Å 6000 Å 10000 Å
• Emission line redshifts
• [OII] 3727 0.48 0.61 1.68
• [OIII] 5007 0.10 0.20 1.00
• H 6563 N/A N/A 0.52
• For nominal 6000-10000 Å spectral range, the emission line galaxy redshift coverage is z = 0-0.52 with H and z = 0.61-1.68 with [OII] 3727
• Gap from z = 0.52-0.61 can be partly filled using [OIII] 5007 for strong emission line galaxies (as [OIII] 5007 is a weaker line in general)
• Absorption line redshifts
• Ca H+K 3950 0.39 0.52 1.53
• G band 4300 0.28 0.40 1.33
• Mg 5170 0.06 0.16 0.93
• Na 5892 N/A 0.02 0.70
• LRG redshift coverage is about z = 0.52-1.33 using both Ca H+K and G band
• Lower redshifts can be reached by Mg and Na features, but in detail will use simulations to assess redshift success rates
Wavelength range and redshift coverage
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• High-resolution (R 45000) night sky spectral atlas (Hanuschik 2003) derived from echelle spectroscopy using the UVES instrument on VLT
• Wavelength coverage 3140-10430 Å
• Data restricted to dark sky conditions
• Resolution much higher than for DESpec and so good for S/N forecasts
• Before had used Mauna Kea night sky spectrum from Gemini website, but that looks like it’s effectively very low resolution (tens of Å?), even though it’s tabulated with spacing of a few Å
• Not good for forecasts at full DESpec resolution
Night Sky Spectra
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• Consider simple cases of
• Galaxy with constant AB mag = 21.5 continuum
• Unresolved emission line with flux = 1.0 x 10-17 erg/s/cm2
• S/N calculation assumptions
• 2-arcsec diameter fiber, with 70% of light entering fiber (based on galaxy sizes in DECam image simulations)
• Throughput simply taken as 50% of DECam (for now), at airmass = 1.3
• 30 min exposure time
• UVES night sky spectrum
• S/N calculated in fixed resolution element bins of 2.5 Å, corresponding to resolution R = 3600 at 9000 Å
• Working on IDL simulation package (C. Cunha & H. Lin) that simulates 1D extracted spectra, based on input mock galaxy catalogs and template spectra, throughput components, resolution, and night sky data
• Will compute S/N using outputs, as well as run cross-correlations using IRAF rvsao package to compute redshift success rates in detail as functions of galaxy magnitude, color, redshift, and emission line strength
Example S/N calculations