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STIS Neutral Density Filters:Bridging STIS’s ND Desert T.R. Ayres (CASA) C.R. Proffitt (CSC)
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Warning: talk deals with
bottom rungs of Drake’s Ladder, where, sadly,
sexiness is low
CALIBRATION
However, nothing on upper rungs would get done without good instrument calibrations
The Cosmic Sexiness Ladder
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ND Desert ProjectSTIS MAMA Global Count Rate ceiling
(200,000 cps) imposes severe caps on S/N of bright objects: <10
cps resol-1 vs. 700 for Local Count Rate limit
Worse, if echelle setting just exceeds GCR limit w/ smallest clear aperture,
have to use 0.2x0.05 ND2: 50x exposure time penalty
ND DESERT
0.2x0.20.2x0.09
0.2x0.060.1x0.03
0.3x0.05ND
0.2x0.05ND
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lllllllllll*
(105 cps)
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STIS “Hidden” Options to Rescue
Usefully, there are three “available, but unsupported” aper’s in STIS slit wheel that cover the intermediate Neutral Density range: NDA (0.6), NDB (1.0), and NDC (1.3)
These slits are tall: 31”x0.05”; normal echelle aper’s are only 0.1”-0.3” high
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Cycle 19 Observing Program
• Use DA-WD G191B2B to validate slit throughputs for 31x0.05 NDA,B,C relative to PHOT aperture (0.2x0.2)
• Take high-res (E140H) spectra of bright UV source Vega (α Lyrae: A0V) to evaluate any side effects from tall slits compared w/ short 0.2x0.05 ND2
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Lower: Transmissions from flux ratios of spliced spectra (error bars) or total cnts order-by-order (open symbols) [connected dots: earlier estimates]
Upper: E140M+E230M spectra of G191B2B with PHOT (upper curve) and NDA,B,C (lower curves)
ND 0.6
ND 1.0
ND 1.4
ND 2.0
PHOTNDANDBNDC
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(105 cps)
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lllllllllll*
(105 cps)
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E140H spectra of Vega taken w/ NDC and ND2 both are identical: tall slits OK for
echelles (thanks to excellent image quality of STIS)
10σ noiseRed: ND2 (2990s)Black: NDC ( 850s)
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OUTCOMEValidation of ND tall
slits for STIS echelles enabled Cycle 21 Large
Treasury Project Advanced Spectral Library: Hot Stars (ASTRAL-II: 230
orbits; half completed 8/2014)and hopefully many more STIS echelle programs in future
Thanks Bruce!
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ISSUES• ND throughputs sometimes
below predictions even following peak-ups (“Breathing” effects compounded by unevenness of slit width along length?)
• How to accommodate in STIS ETC, given lack of reproducibility?
• Refined throughput curves based on higher S/N sources?