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 mirror and aperture:

Mirror ------------

Aperture ----------

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.

Select the source type and photometric extraction aperture (centered on the target) that will be used for the S/N ratio calculation:

	Point Source
	Use the default region for the camera selected, a circular region containing 80 percent of total light.
	Use a square region of size:
	Use a circular extraction region of radius (arcsec):
	Use a circular extraction region of radius (arcsec):
	[This value must give a circle whose area is larger than one pixel (r ≥ 0.01338 arcsec) but is smaller than the aperture size (r ≤ 1.25 arcsec)]
	Use a circular region containing percent of total light.
	Extended Source that fills the aperture (radius 1.25 arcsec)
	Use a square extraction region of size (pixels):
	Use a circular extraction region of radius (arcsec):
	Use a circular extraction region of radius (arcsec): (see note above)

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, at least one emission line must be specified below; 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
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:

	(arcsec^-2 for extended sources) at
	(arcsec^-2 for extended sources) in filter:

	Filter 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)	Air Glow

Exposure Time Calculator

ETC Version 33.1

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COS Imaging ETC

1. Select mirror and aperture:

2. Specify the exposure parameters:

Point Source

Extended Source that fills the aperture (radius 1.25 arcsec)

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

4. Normalize the target's flux:

5. Specify the expected background levels :