COS Spectroscopy ETC

This form will calculate the count rates and S/N ratio for a simulated spectrum of ONE source in a COS spectroscopic observation.

For general help on how to use this Exposure Time Calculator or for help on various topics, click on the appropriate highlighted words.
Please ensure that you have read the current release notes and recent news before submitting any ETC calculations.

1. Select one Detector+Grating and an aperture:

---------- FUV ---------- ---------- NUV ----------
Grating: Cen. Wave. Grating: Cen. Wave.

Aperture ----------

2. Specify the exposure parameters:

Signal-to-noise ratio is calculated per spectral resolution element.

Exposure time needed to obtain a S/N ratio of
S/N ratio reached in an exposure time of seconds.

S/N or exposure time specified for a wavelength of Å.
Currently selected disperser: not displayed with pass band

Select the source type:
The spectroscopic extraction region is the same for both point and extended sources. All NUV extraction regions use an 8 pixel high and 3 pixel wide resolution element. A 47 pixel high by one resolution element wide region is used for most modes in the FUV. The resolution element is 6 pixels wide, which means the extraction region is 47 by 6 pixels. Two notable exceptions are the G130M grating with either the 1055 or 1096 central wavelength chosen. For these particular modes, the resolution element size and the extraction height change with wavelength and detector segment. Please consult the COS ETC page at for more information.

3. Choose one of the following spectral distributions for the source:

Please note: for User supplied and uploaded spectrum, please use only FITS or text (DAT) format files.
Upload Spectrum File:
Other HST Spectrum:
CDBS Synphot spectrum file
Castelli and Kurucz Models: [Sp Teff log(g) log(z)=0]
Pickles Models: (Sp Teff)
Kurucz Models:
Bruzual Synthetic Stellar Spectra:
HST Standard Star spectra:
Non-Stellar Objects:

Specify the extinction E(B-V) =

Extinction applied normalization.

Specify the redshift z =

Add emission lines to the input spectrum (optional): Note: emission lines are added *after* the spectrum has had extinction, renormalization, and redshift applied.

Line Center FWHM Integrated Flux
Å Å (erg/cm2/s)

Note: integrated flux units are per arcsec2 for extended sources. All three of the parameters (line center, fwhm and integrated flux) must be specified for an emission line to be included.

4. Normalize the target's flux:

= (Vega magnitudes) (arcsec-2 for extended sources)
= (AB magnitudes) (arcsec-2 for extended sources)
Normalize target using bandpass of filter selected above to AB Mag
ergs cm-2 s-1 Å-1 (arcsec-2 for extended sources) at Å.

5. Specify the expected background levels :

a) Specify normalization for zodiacal background
R.A.: (hh:mm:ss or decimal)
Dec.: (dd:mm:ss or decimal)
either degrees
- or -
b) Specify normalization for earth shine background
c) Air Glow