STIS Neutral Density Filters:Bridging STIS’s ND Desert T.R. Ayres (CASA) C.R. Proffitt (CSC)

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

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

lllllllllll*

(105 cps)

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

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

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

(105 cps)

lllllllllll*

(105 cps)

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)

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!

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?