2. CALIBRATION However, nothing on upper rungs would get done without good instrument calibrations The Cosmic Sexiness Ladder

3. ND Desert Project 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 ND DESERT 0.2x0.2 0.2x0.09 0.2x0.06 0.1x0.03 0.3x0.05ND 0.2x0.05ND

4. lllllllllll*lllllllllll* (10 5 cps)

5. 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

6. Cycle 19 Observing Program Use DA-WD G 191 B 2 B 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

7. 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 G 191 B 2 B with PHOT (upper curve) and NDA,B,C (lower curves) ND 0.6 ND 1.0 ND 1.4 ND 2.0 PHOT NDA NDB NDC

8. (10 5 cps)

9. lllllllllll*lllllllllll*

10. 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 noise Red: ND2 (2990s) Black: NDC ( 850s)

11. 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!

12. 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?