STIS Spectroscopic ETC

This form will calculate the count rates and S/N ratio for a simulated spectrum of ONE source in a STIS 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. You may also like to see the current known release notes.

1. Select one Grating/Prism and an associated Slit/Filter:

1st Order CCD
Grating: Cen. Wave.
G750L
G750M
G430L
G430M
G230LB
G230MB
Slit ------------
1st Order FUV MAMA (see special MAMA countrate restrictions)
Grating: Central Wavelength
G140L
G140M
Slit ------------
1st Order NUV MAMA (see special MAMA countrate restrictions)
Grating Cen. Wave.
G230L
G230M
Slit ------------
MAMA Echelle
NUV FUV
Grating Cen. Wave. Slit Grating Cen. Wave. Slit
E230M E140M
E230H E140H

MAMA Objective Prism
PRISM (NUV MAMA)
Filter ----------

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 wavelength of Å.

Select the source type. The indicated extraction region (centered on the target) that will be used for the S/N ratio calculation:

Point Source
Use the default region (height of 7 pixels).
Use a region of height: pixels.

The lambda-pixel extraction region is one pixel wide (one binned pixel for the CCD).

Width of a resolution element is normally two lambda pixels.

When using the CCD with dispersion binning a resolution element is one binned pixel.

Extended Source
Diameter is arcsec

The lambda-pixel extraction region for extended sources is a 1 x 2 box.

Width of a resolution element is normally two lambda pixels.

When using the CCD with dispersion binning a resolution element is one binned pixel.

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
(Å) 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 Å. 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):
c) Air Glow

6. Additional CCD parameters:

Binning Gain CR-SPLIT
Dispersion Axis Spatial Axis