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ACS Exposure Time Calculator

ACS Imaging ETC

This form will calculate the count rates and S/N ratio for a simulated bandpass of ONE source in an ACS 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 list of known problems with the ETC.)

Please repeat any failed or timed-out requests at least twice before contacting support.

Enter exposure description:

Choose gyro mode:

1. Select one Camera and the associated Filter(s):

Detector	Filter Wheel 1	Filter Wheel 2
WFC
HRC
SBC

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 region (centered on the target) that will be used for the S/N ratio calculation:

Point Source

Use the default region for the camera selected, circle of: HRC, WFC 0.2 arcsec; SBC 0.5 arcsec

Use a square region of size:

Use a circular region of radius (arcsec):

Use a circular region of radius (arcsec):

Use a circular region containing percent of total light. The range of supported values for this option is limited. The limits are determined by the PSF and target spectrum.

Extended Source

Diameter is arcsec

The extraction region for extended sources is a 2 x 2 box.

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.

User Supplied Spectrum:
ftp.stsci.edu:outside-access/in.coming/
Upload Spectrum File:
Other HST Spectrum:
CDBS Synphot spectrum file
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)

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 Sun angle: degrees
	- or - Date:
	Normalize zodiacal light to magnitude:
	Scale zodiacal light by factor:
b)	Specify normalization for earth shine background
	Standard earth shine light normalizations:
	Normalize earth shine light to magnitude:
	Scale earth shine light by factor:
c)	Air Glow

6. Additional CCD parameters:

Gain Setting	CR-SPLIT

Please repeat any failed or timed-out requests at least twice before contacting support.

If you have any questions regarding this calculator, please contact the STScI Help Desk (help@stsci.edu). Remember to include your ETC ID number.