COS Imaging ETC

This form will calculate the count rates and S/N ratio for a simulated image of ONE source in a COS imaging 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 read the current release notes, updated on 03/24/2008.

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 - a circle of radius 0.4 arcsec.
Use a circular region of radius (arcsec):
Use a circular 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)]

Extended Source that fills the aperture (radius 1.25 arcsec)
Use a circular region of radius (arcsec):
Use a circular 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 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
= (AB 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