STIS
Neutral
Density
Filters:
Bridging
ND
STIS’s
Desert
The Cosmic Sexiness Ladder
talk deals with
bottom rungs of
Drake’s Ladder,
where, sadly,
sexiness is low
However, nothing on
upper rungs would get
done without good
instrument calibrations
CALIBRATION
ND Desert Project
0.2x0.2
0.2x0.09
0.2x0.06
0.1x0.03
0.2x0.05ND
0.3x0.05ND
STIS 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
l
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(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
PHOT
NDA
NDB
NDC
ND 0.6
ND 1.0
ND 1.4
ND 2.0
Upper: E140M+E230M spectra of G191B2B with
PHOT (upper curve) and NDA,B,C (lower curves)
Lower: Transmissions from flux ratios of spliced
spectra (error bars) or total cnts order-by-order
(open symbols) [connected dots: earlier estimates]
(105 cps)
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(105 cps)
Red: ND2 (2990s)
Black: NDC ( 850s)
10σ noise
E140H spectra of Vega taken w/ NDC and ND2
both are identical: tall slits OK for echelles
(thanks to excellent image quality of STIS)
OUTCOME
Validation 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?