The response of COS below 1150 Angstroms (The new cycle 19 settings) The COS team, the IDT (esp, S. Osterman, S. Penton and S. Beland), S. McCandliss (JHU) and P. Chayer (STScI/CSA)

Background: MgF 2 has very low transmission below 1150 Å HST OTA has MgF 2 coatings STIS MAMA window is MgF 2 = thousands of reflections COS FUV gratings have MgF 2 coatings But, COS FUV has only one bounce and windowless detectors Expected to have some FUV response First seen in SMOV -- McCandliss et al. (2010) Two cal programs explored COS FUV: to establish FUV (and more?) G140L response to explore G130M FUV response.

The G140L -- A rock and a hard place

GD50

G130M

Resolution

Problems -- Challenges New settings use FUVB only -- no wavecal Cannot use the BOA -- it has a MgF 2 substrate, and does not transmit below 1150Å. Resolution of the G140L and G130M are comparable in the FUV. G140L has complete coverage, but must contend with the shoulder in the response curve. G130M avoids the shoulder, but need two settings for complete coverage There may be some (~1cm 2 ) EUV response.

Summary ModeRangeA eff R G140L/ Å10-20 cm G130M/ Å15-25 cm G130M/ Å15-25 cm For flat spectra, G130M/1096, segment B only: Bright limit reached f(lam) = 7.5× erg cm -2 s -1 Å -1 S/N = 10/resol in 60 min f(lam) = 2.7× erg cm -2 s -1 Å -1