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:

G102 (Blue grism, high resolution: ~ 0.9 - 1.15 μm)

G141 (Red grism, low resolution: ~ 1.1 - 1.65 μm)

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.

	Point Source
	Extended Source
	Diameter is arcsec

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:

	Use the default region (a rectangular region of 3 pixels by 1 resolution element).
	Use a rectangular region of size (N pixels by 1 resolution element):

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:
	a Black-body with temperature T =
	a Power-law: Fλ = λ **
	a Flat continuum in
	No continuum. If selected, the reference wavelength from part 2 above must correspond to one of the lines; and no extinction, redshift, or renormalization may be applied.

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)
(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:

	(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

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 (not a supported observation if target is within 50 deg of sun):
	R.A.: (hh:mm:ss or decimal degrees)
	Dec.: (dd:mm:ss or decimal degrees)
	either λ - λ_Sun: degrees
	- or - Date:
	Normalize zodiacal light to Johnson V 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)	Specify normalization for Helium I background