System Throughputs

This page presents system throughput curves in graphic and tabular form for all of the filters in each of the three ACS channels. These data have been generated with stsynphot and are the most recent transmission curves derived in ACS ISR 2016-03. The system throughputs represent the fractional photon detection efficiency in terms of photons in vs. detected photo-electrons. They are the product of the individual throughputs of the optical telescope assembly (OTA), the window on the instrument housing, the responsive quantum efficiency (RQE) of the detector, and the filter. In other words, they represent the total expected fraction of light detected by the detector.

As the sensitivity changes over time due to degradation of the detectors and filters, the system throughputs change as well. This variability is incorporated in the RQE component file for the ACS instrument. By default, when computing the system throughputs the most recent component files are used, however, time dependent system throughputs can be computed by using stsynphot in the following way:

​import stsynphot as STS # a new and improved replace for pysynphot
import matplotlib.pyplot as plt

obsmode = 'acs,wfc1,f555w,mjd#57000' # desired instrument, chip1, filter, and date

# Instantiate an ObservationSpectralElement object which contains the desired
# information (throughput, wavelength converage, etc..) about the specified obsmode
bp_acs = STS.band(obsmode) 

# Now we get the wavelengths associated with the throughput curve.
wavelengths = bp_acs.binset 

# One of the main features of stsynphot is that it is built using astropy.units
# This means that all values are returned with their correct units, so no more
# wondering what the units are of the values you have!
print(wavelengths.unit)

# Finally, compute the throughput values for the filter over the provided wavelength range
throughput = bp_acs(wavelengths) 

# Check to make sure everything looks good
plt.plot(wavelengths.value, throughput.value, label='F555W')

For more information, please consult the stsynphot documentation.

Determine the value for a specific wavelength 

Hover over a datapoint in the figures below. This will display the coordinates (λ , throughput) and the filter name for the curve the cursor covers.

Select a single filter

Double click the filter name in the legend. This will toggle off all other filters except the one that has been double clicked allowing you to investigate whichever filter you desire.

• To undo this operation, double click anywhere on the legend.
• To view other filters for comparions, single click the desired filter in the legend to toggle it on

Zoom in

Determine the area you wish to examine, then click and hold to draw a rectangle around the desired region.

• To undo this operation, double click anywhere on the plot and it will return to its default state.