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. You may also like to see the release notes.

1. Select an appropriate disperser

G102 (Blue grism, high resolution)
G141 (Red grism, low resolution)

2. Specify the exposure parameters:

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.

Select a 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:

Use the default region (a rectangular region of 3x1 pixels)
Use a rectangular region of size (pixels):

Select the source type.

Point Source
Extended Source
Diameter is arcsec

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 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:
a Black-body with temperature T =
a Power-law: Fλ = λ **
a Flat continuum in
No continuum. If selected, at least one emission line must be specified below, and the reference wavelength from part 2 above must correspond to one of the lines.

Specify the extinction E(B-V) =

Extinction applied normalization.

Specify the redshift z =

Add emission lines to the input spectrum (optional):

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

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) or
ergs cm-2 s-1 Å-1 (arcsec-2 for extended sources) at μm. or
10 -23 ergs cm-2 s-1 Hz-1 (Jy) (arcsec-2 for extended sources) at μm. or
Do not Renormalize Point Source Spectrum. Use only for User Input Spectrum or Calibration Spectra.

5. Specify the expected background levels :

a) Specify normalization for zodiacal background
Standard zodiacal light normalizations:
Compute zodiacal light on sky position:
R.A.: (hh:mm:ss or decimal)
Dec.: (dd:mm:ss or decimal)
either λ - λSun: degrees
- or - Date:
Normalize zodiacal light to magnitude:
Scale zodiacal light by factor (1.56 = average):
b) Specify normalization for earth shine background
Standard earth shine light normalizations:
Normalize earth shine light to magnitude:
Scale earth shine light by factor (0.5 = average):

6. Additional detector parameters:

# Frames