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: • MgF2 has very low transmission below 1150 Å

• HST OTA has MgF2 coatings

• STIS MAMA window is MgF2 = thousands of reflections

• COS FUV gratings have MgF2 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:• 12081 to establish FUV (and more?) G140L response• 12082 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 MgF2 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 (~1cm2) EUV response.

SummaryMode Range Aeff R

G140L/1280 920-1160Å 10-20 cm2 2100

G130M/1096 1040-1081Å 15-25 cm2 2300

G130M/1055 900-1041Å 15-25 cm2 1800

For flat spectra, G130M/1096, segment B only:•Bright limit reached f(lam) = 7.5×10-11 erg cm-2s-1Å-1

•S/N = 10/resol in 60 min f(lam) = 2.7×10-13 erg cm-2s-1Å-1