WFC3 IR Spectroscopic ETC

This form will calculate the count rates and S/N ratio for a simulated spectrum of ONE source in a WFC3 IR 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 an appropriate disperser:

Specify additional detector parameters:

# Indep. Exposures (e.g., dither positions)

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 vacuum wavelength of μm.

Filter not displayed pass band

Select the source type.

Select the rectangular photometric region (centered on the target in the spatial direction, at a given wavelength) that will be used for the S/N ratio calculation:

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:
Phoenix M dwarf Models:
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
μm 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:

(arcsec-2 for extended sources) at
(arcsec-2 for extended sources) in filter:
Bandpass selected above in Question 1 (for observations)
Typically select Vega magnitude above
Typically select AB magnitude above

5. Specify the expected background levels :

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