The HST Exposure Time Calculators:
Estimating accurate observing times for HST Observations
Rosa I. Diaz, Donald McLean, and Perry Greenfield
Space Telescope Science Institute
Figure 1a

Figure 1b

Exposure Time Calculator

Exposure Time Calculator

What is the ETC and how it works
The ETC is a web-base application (http://etc.stsci.edu) developed to assist Hubble Space Telescopeobservers in preparing their observations with any of the supported modes available at HST. The
main purpose of the ETC is to calculate the Exposure time needed for observations requiring a given
Signal-To-Noise ratio (SNR) and to also calculate the SNR for a simulated observation for a specified
amount of time. These quantities are key for the preparation of proposals and observations during
Phase I and Phase II of the proposing Cycle.

STIS Spectroscopic ETC

Home page and news

ETC Version

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 want to check the main ETC page for relevant new items.

ETC Help
User's Guide (INS link - no return)

ACS ETCs

ETC: 18.0
Pysynphot: 2009-09-02

User's Guide (INS link
- no return)Reset All Parameters

ACS ETCs

1. Select one Grating/Prism and an associated Slit/Filter:
1st Order CCD
Grating:
G750L

COS ETCs

Imaging
Spectroscopy
Ramp Filter
Coronographic Imaging
Coronographic Ramp
FilterCen. Wave.
i 8561

G750M
G430L

Spectroscopy
Spectroscopy Target Acquisition
Imaging
Imaging Target Acquisition
COS Team ETC Help and Release
Notes

G230LBSpectroscopy
Spectroscopy Target
Acquisition
Imaging
Imaging Target
Acquisition
COS Team ETC Helpand Release Notesc 1713

G230MB

Slit ------------ 52 x 0.2
1st Order FUV MAMA (see special MAMA countrate restrictions)
Grating: Central Wavelength
G140L

NICMOS ETCs
Imaging
Spectroscopyc 1222G140M

STIS ETCs

NICMOS ETCs

Slit ------------ 52x0.2
1st Order NUV MAMA (see special MAMA countrate restrictions)
Grating
Cen. Wave.
G230L

Imaging
Spectroscopy
Target Acquisition

WFC3 ETCs

NUV
GratingPrevious results
Previous calculation results

E230H

Imaging
Spectroscopy
Target Acquisition

FUV

Cen. Wave.

E230M

Submit Simulation

Slit

c 27070.2x0.2

c 17630.2x0.09

Grating

Cen. Wave.

Slit

WFC3 ETCs

0.2x0.2

E140M
c 1234

E140H

IR Imaging
UVIS Imaging
IR Spectroscopy
UVIS Spectroscopy

0.2x0.09

Previous results

MAMA Objective Prism
PRISM (NUV MAMA)
Filter ---------- 52x0.05

results
Previous calculation

1. Select one Detector+Grating and an aperture:---------- FUV ---------Grating:
Cen. Wave.
1309

G185M

1850

G160M

1600

G225M

2250

G140L

1250

G285M

2850

G230L

3000

Aperture ---------- Primary Science Aperture

2. Specify the exposure parameters:
Exposure time needed to obtain a S/N ratio of 10.0

Diameter is 8

Please note: for User supplied and uploaded spectrum, please use only FITS ortext (DAT) format files.
Upload Spectrum File:Browse…

seconds.

Castelli and Kurucz Models: O3V 45000 4.0
log(z)=0]

Å.

Pickles Models: O5V 39810.7K

HST Standard Star spectra:
GD50 (DA2, 14.06)

pixels.

Non-Stellar Objects: Elliptical Galaxy [240,34000Å]The lambda-pixel extraction region is one pixel wide (one binned pixel for the CCD).

a Black-body with temperature T = 10000

SNR/Exposure Time
Reporta Power-law: Fλ = λ ** -1

Extended Sourcea Flat continuum in F λWidth of a resolution element is normally two lambda pixels.

Specify the extinction E(B-V) Average Galactic

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

Extinction applied before

Other HST Spectrum:
CDBS Synphot spectrum file
Castelli and Kurucz Models: O3V 45000 4.0
Pickles Models: O5V 39810.7K

Non-Stellar Objects: Elliptical Galaxy [240,34000Å]The design of the COS detectors, on the ther hand, possed significan challenges for the ETC developers. The FUV channel is a photon counter with 2
segments that can operate independently and that have a 9 mm gap in between. The NUV channel creates a three spectrum stripes for the science datawith a gap of 3.70 mm between them. Because of this the ETC needed also toprovide with:- the count rate and global count rate that accounts for the sensitivity of thesegments or stripes and the gap between them- validate the input wavelength for the grating-central wavelength combination- specify the global and maximum local count rates on each channel
Since the FUV channel is regularly used in time tag mode the ETC also estimates the buffer time. Furthermore, due to the vulnerability of photo multiplierdetectors to damage, the ETC needs to calculate various total count rates forsegments and stripes and the corresponding maximum observed count rates
within the segments or stripes, and appropriate buffer time values. Whenthese exceed or are close to exceed global or local limits for the detectors,
these are clearly flagged by the software as shown in Figure 3b.

(Å)Å(erg/cm2/s)

0.

0.

0.

0.

0.

0.

0.

0.

0.

a Black-body with temperature T = 10000
a Power-law: Fλ = λ ** -1

4. Normalize the target's flux:

a Flat continuum in F λSpecify the extinction E(B-V) Average Galactic= 0.0

normalization.

ergs cm-2 s-1 Å-1 (arcsec-2 for extended sources) at

1310

FWHM

Integrated Flux(Å)Å(erg/cm2/s)

5. Specify the expected background levels :a)Average

Compute zodiacal light on sky position:

0.

0.

0.

0.

0.

0.

0.

0.

0.
2

Note: integrated flux units are per arcsec for extended sources. All three of the parameters (line center, fwhm andintegrated flux) must be specified for an emission line to be included.

4. Normalize the target's flux:
Johnson/V

b)

-2

= 15

(Vega magnitudes) (arcsec for extended sources) or(AB magnitudes) (arcsec-2 for extended sources) or

Galex FUV

= 15

1.5e-13ergs cm-2 s-1 Å-1 (arcsec-2 for extended sources) at 5500.0

Å. or

Do not Renormalize Point Source Spectrum. Use only for User Input Spectrum or Calibration Spectra.

5. Specify the expected background levels :c)Specify normalization for zodiacal background

R.A.: 00:00:00(hh:mm:ss or decimal)

Dec.: +00:00:00(dd:mm:ss or decimal)

eitherλ - λSun: 90

degrees- or -

Date: 2009

January

30.0

Scale zodiacal light by factor (1.56 = average):

1.0

Specify normalization for earth shine background
Standard earth shine light normalizations:Average

Normalize earth shine light to magnitude:

30.0

Scale earth shine light by factor (0.5 = average):

1.0

Air Glow
Average

Compute zodiacal light on sky position:

Submit Simulation

R.A.: 00:00:00(hh:mm:ss or decimal)

Dec.: +00:00:00(dd:mm:ss or decimal)

eitherλ - λSun: 90

degrees- or -

Date: 2009

January

1

Normalize zodiacal light to magnitude:

AverageStandard zodiacal light normalizations:

CDBS

Specify normalization for zodiacal background

Standard zodiacal light normalizations:(vacuum)

a)

Å. or

Do not Renormalize Point Source Spectrum. Use only for User Input
Spectrum or Calibration Spectra.

Specify the redshift z = 0.0
Add emission lines to the input spectrum (optional):Line Center(AB magnitudes) (arcsec-2 forGalex FUV
= 15
extended sources) or
1.5e-13Extinction applied before(Vega magnitudes) (arcsec-2

Johnson/V= 15
for extended sources) or

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.

Reset All Parameters

If you have any questions regarding this calculator, please contact the STScI Help Desk (help@stsci.edu). Remember
to include your ETC ID
Copyright Notice1

Normalize zodiacal light to magnitude:

30.0

Scale zodiacal light by factor (1.56 = average):

1.0

Specify normalization for earth shine background
Standard earth shine light normalizations:Average

Normalize earth shine light to magnitude:

30.0

Scale earth shine light by factor (0.5 = average):

1.0

Air Glow
Average

6. Additional CCD parameters:
Binning
Dispersion Axis

Gain

Spatial Axis
1 pixel

1 e-/ADU

CR-SPLIT
2

Reset All Parameters

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

Figure 3a. Output for WFC3 Imaging ETC

Figure 3b. Output for COS Spectroscopic modes

WFC3 Imaging ETC

COS Spectroscopy ETC

ETC Version
ETC: 18.0
Pysynphot: 2009-08-17

ETC Help
User's Guide
Source Model
Release Notes

ACS ETCs
Imaging
Ramp Filter
Spectroscopy
Coronographic Imaging
Coronographic Ramp Filter

COS ETCs
Spectroscopy
Spectroscopy Target
Acquisition
Imaging
Imaging Target Acquisition
COS Team ETC Help andRelease Notes

NICMOS ETCs
Imaging
Spectroscopy

STIS ETCs
Imaging
Spectroscopy
Target Acquisition

IR Imaging
UVIS Imaging
IR Spectroscopy
UVIS Spectroscopy

Previous results
View results from previous
calculations

ETC Version

Exposure ID: WFC3.A173530

ETC: 18.0
Pysynphot: 2009-08-17

Requested Signal/Noise Ratio = 10.00
gives: Time = 0.0306
gives: optimum SNR = 17.73
gives: Time to Saturation (for a single exposure) = 21.59 seconds
Detailed Information

Counts in circle with radius 0.2 arcsec
Source14,144.708
Background
2.493
Sky
2.453
Dark Current
0.040
Read out
Total in selected region
14,147.201
Brightest pixel
3,241.487
Count rate
Source (infinitely large region)
Sky Background (arcsec^-2)(e-)
433.080.08
0.08
1.23E-3
433.1649.62

16,435.67
19.52

Extraction region
circle with radius 0.2 arcsec
Fraction of flux
Area (pixels)
Fraction of flux in brightest pixel
Effective Wavelength

0.86
80.14
0.20
5389.60 Å

Target [point source]

Instrument name: WFC3
Mode:
Detector:
Bandpass:# Frames:

Imaging
UVIS1[F555W] WFPC2 V
2Selected background:

Source spectrum

(e-)
20.81
0.28
0.27
0.04
37.98
43.31

User's Guide
Source Model
Release Notes

ACS ETCs
Imaging
Ramp Filter
Spectroscopy
Coronographic Imaging
Coronographic Ramp Filter

COS ETCs
Spectroscopy
Spectroscopy Target
Acquisition
Imaging
Imaging Target Acquisition
COS Team ETC Help andRelease Notes

NICMOS ETCs
Imaging
Spectroscopy

Model ID = 1+ Flat 1.00 in Fλ. Renormalized to Johnson V = 15 in magnitudes relative to Vega

Sky Background:
Earth Shine:
Zodiacal Light:
Air Glow:Plots

ETC Help

Count rate Total counts Associated noise(e-/s)

WFC3 ETCs

The last HST Servicing Mission added two new instruments: Wide Field
Camera 3 (WFC3) and Cosmic Origins Spectrogaph (COS). Adding support for
WFC3 was relatively strightforward since the similarities between the twochannels covered by WFC3 detectors with ACS (for the UV) and NICMOS (forthe IR) made it possible to implement support for the WFC3 modes with few
changes. The main differences between these ETC are the instrument specific parameters. Other differences have to do with the extraction box size andshape, and the way in with the size of the extraction box and size are indicated.

Integrated Flux

Note: integrated flux units are per arcsec2 for extended sources. All three of theparameters (line center, fwhm and integrated flux) must be specified for an emission
line to be included.

Submit Simulation

Support for new instruments.

(Sp Teff)

HST Standard Star spectra: GD50 (DA2, 14.06)

1 pixel- option for user-supplied input spectra- calculation of optimal S/N as would be obtained by PSF fitting- inclusion of instrument-specific effects. For example, NICMOS spectroscopy
simulations include the degradation in the spectral purity due to the line
spread function.
- verification that user-supplied wavelengths fall within instrumental range- bright-limit checks

FWHM(vacuum)

Bruzual Synthetic Stellar Spectra: O5 V

Pysynphot response(countrate)

Other capabilities of the ETCs are:normalization.

Line Center[Sp Teff log(g) log(z)=0]

Kurucz Models: O5V 44500 5.0c)

The ETC output gives detailed information on the counts rates that are due tothe source and background, the selection of parameters for the source and instrument, as well as other relevant information to the instrument and mode
used. The output information is separated in sections which are common to all
the instruments and modes. However, detailed information and instrument
configuration information will change from instrument to instrument and alsowith detectors.

Browse…

Upload Spectrum File:= 0.0

Specify the redshift z = 0.0
Add emission lines to the input spectrum (optional):

Please note: for User supplied and uploaded spectrum, please use only FITS or text (DAT) format files.

b)

The results of the calculation are displayed to a web page and saved in a filefor future reference. For example, to recall previous calculation results or toplot or show a table of the input spectrum, instrument throughput, or detected counts or signal-to-noise as a function of wavelength. An example ofthe output of a run for an ACS observing configuration as well as that for a
COS spectroscopic observation is given in Figure 3a and 3b, respectively.

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 thelines.

arcsec3. Choose one of the following spectral distributions for the source:

pysynphot request

ETC

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

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

Figure 2. Data flow between web client, ETC, pysyphot, and CDBS datapysynphot(Sp Teff)

Bruzual Synthetic Stellar Spectra: O5 V

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

The ETC relies heavily on the Synthetic Photometry software package, pysynphot. Pysynphot is partof the Space Telescope Science Data Analysis System (STSDAS) and is developed and maintained by
the Science Software Branch (SSB) at STScI. This package performs simulated photometry by computing the detected photons as a function of wavelength (and for spectrographs, detector bin) using a
specified input spectrum and known throughput and detector quantum efficiency curves. The results
are used by the ETC to compute exposure times and signal-to-noise ratios. Its predictions made by
the pysynphot software rely on the throughput information for all the optical components of the instruments that are part of the Calibration Database System (CDBS) at STScI. This database contains
not only the information regarding the sensitivity of the instrument components, but also about thethermal response of the infrared detector and a large number of available source spectra (HST Calibration tables, Catalogs, and Models). Figure 2 shows the flow of data between the Web interface,
ETC and pysynphot. First the user inputs the observational parameters into the ETC web server. The
parameters are then formatted by the ETC in a pysynphot request and submitted to the pysynphot
package. The quantities calculated by pysynphot are the imaging and spectroscopic countrate, thethermal countrate for the IR detectors and the effective wavelength of an observation. The pysynphot
response is retrieved by the ETC application and the count rate used for the calculation of the S/N andexposure time.

web client[Sp Teff log(g)Kurucz Models: O5V 44500 5.0

Width of a resolution element is normally two lambda pixels.

Instrument/Exposure
Level Parametersarcsec3. Choose one of the following spectral distributions for the source:

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

Diameter is 8Å.

Other HST Spectrum:
CDBS Synphot spectrum file

S/N or exposure time specified for a wavelength of 4451

Although the exposure parameters (selection 2.) seem to be identical for all the instruments, the underlying calculations might vary slightly depending on the instrument, observing mode and sometime
detector. The ETC handles these differences by subdividing the calculations in stages, making iteasier to integrate these into the basic equations.

seconds.

Select the source type:
The photometric extraction region is the same for both point and extended
sources. 47 pixels high and a 6 pixel wide resolution element for the FUV.
8 pixels high and a 3 pixel wide resolution element for the NUV.
Point Source
Extended Source

Exposure time needed to obtain a S/N ratio of 10.0

Use a region of height: 7---------- NUV ---------Grating:
Cen. Wave.

G130M

2. Specify the exposure parameters:
S/N ratio reached in an exposure time of 900

Reset All Parameters

S/N or exposure time specified for a wavelength of 1310

STIS ETCs

Slit ------------ 52x0.2
MAMA Echelle

IR Imaging
UVIS Imaging
IR Spectroscopy
UVIS Spectroscopy

Please insure that you have read the current release notes before
submitting any ETC calculations.

S/N ratio reached in an exposure time of 900

Imaging
Spectroscopyc 1687

G230M

This form will calculate the count rates and S/N ratio for a simulated spectrum of
ONE source in a COS spectroscopic observation.

COS ETCs

c 4451G430M

COS Spectroscopy ETC
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 want to check themain ETC page for relevant new items.

ETC Help

Please insure that you have read the current release notes before submitting any ETC
calculations.
Submit Simulation

Imaging
Spectroscopy
Ramp Filter
Coronographic Imaging
Coronographic Ramp Filter

Figure 1a and 1b show specific observing modes for two different instruments. These ETC forms areintentionally separated into 6 well defined sections to allow HST observers and STScI instrument scientist to easily identify the relevant information that is needed in the calculations. As seen in Figure
1a-b, the same structure is maintained for all the ETC modes and instruments, making easier for HST
users to plan observations for different HST instruments. In terms of the management of the software,
it helps to track changes that are instrument dependent to those that affect all the instruments. For
example, options available under 2., 3., 4., and 5. are shared by all the ETC modes, while sections 1.
and 6. depend on the instrument in use and type of observation.

ETC Version

This form will calculate the count rates and S/N ratio for a simulated spectrum of ONE source in a STIS
spectroscopic observation.

ETC: 18.0
Pysynphot: 2009-09-02

Currently the ETC provides support for all the active HST instruments:• Advanced Camera for Surveys (ACS)• Cosmic Origins Spectrograph (COS)• Near Infrared Camera and Multi Object Spectrometer (NICMOS)• Space Telescope Imaging Spectrograph (STIS)• Wide Field Camera 3 (WFC3)

Home page and news

Average
Average
AverageThroughput

STIS ETCs
Imaging
Spectroscopy
Target Acquisition

WFC3 ETCs
IR Imaging
UVIS Imaging
IR Spectroscopy
UVIS Spectroscopy

Previous results
View results from previous
calculations

Exposure ID: COS.A173529

Exposure time (seconds) = 10,000.0000 at wavelength 1310.00 Ågives: SNR = 2,164.81 (per resolution element)
Exposure time calculation HAD WARNINGS.
WARNING MESSAGE: XDL Bright limits: total count rate per pixel exceeds bright limit, value = 20.3,
limit = 0.7
WARNING MESSAGE: Buffer time must be greater than or equal to 80.0 seconds
WARNING MESSAGE: Observation exceeds FUV segment B global count rate limit of 15000.0 counts
per second for non-variable sources.
WARNING MESSAGE: Observation exceeds FUV segment A global count rate limit of 15000.0 counts
per second for non-variable sources.
WARNING MESSAGE: You have requested S/N > 40. Additional calibration steps might be required toachieve S/N > 40 in the FUV" , value = 2164.8, limit = 40.0

Detailed Information

Count rate

Total counts Associated noise(counts/s)
Counts (box 47 pixels high)SourceBackgroundSky
Dark Current
Total in selected region
Brightest Pixel (1225.01 Å)
Count rate entire detector
Count rate segment A
Count rate segment B(counts)(counts)(1 pixel)(6 pix resel)
78.107 4,686,422.07
1.058E-4
6.35
6.806E-12
4.08E-7
1.058E-4
6.34
78.107 4,686,428.42
20.2902,276,168.741
774,565.407
1,501,603.333

Buffer time (sec)
For APT purposes, the recommended buffer timeshould be 2/3 of the buffer time calculated
above, or the exposure time, whichever is shorter.
Please refer to the COS Instrument Handbook
for more details

2,164.81
2.52
6.39E-4
2.52
2,164.82

1

Target [point source]
Model ID = 5504
+ Castelli-Kurucz (O3V): Temp = 45000, LogZ = 0.0, Logg = 4.5. Renormalized to Johnson V = 10 in magnitudes relative to Vega
Instrument name: COS
Mode:
Detector:
Central wavelength:
Grating:
Aperture:

Spectroscopy
FUV
1309[G130M] Grating (R ~ 20,000-24,000)
Primary Science Aperture

Selected background:

PYETC

Sky Background:
Earth Shine:
Zodiacal Light:
Air Glow:

Average
Average
Average

View results in tabular form
Plots

Recently STScI has started an effort to rewrite the ETCs
in Python. The reasons for this re-implementation include
the following goals:

Total Counts

Signal-to-noise

Source spectrum

Throughput- Streamlining the code and data that has been built up organically over many years and instrments.
- Permitting better integration with many of the new STScI science software tools that are being
written in Python, for example, pysynphot, a Python re-implementation of SYNPHOT.
- Making some of the underlying components for the ETC available for other science purposesthat
would be distributed as part of STScI science calibration and analysis software, for example, background models and signal-to-noise calculations. The ETC computational engine will eventually be
distributed with the rest of the science software.
- Anticipating JWST ETC development needs.
The re-implementation will also make possible some other capabilities not yet possible, such as
scripting ETCs from Python for batch use and perhaps drive the web version of the ETC from a
Python script running on a user's computer.
The new ETCs will retain much of the same web user interface as the existing ETCs so users
should not find much difference in how they use the ETCs. The ability to recall past calculations,plot them, and have access to the tabular results will remain.