Liverpool Telescope
Exposure Time Calculator

Imaging

To use the Exposure Time Calculator (ETC), select an Instrument with a corresponding Binning (usually 2x2) and Filter from the pull-down menus. The Plot Mode option is used to select whether the plot shows multiple Signal to Noise Ratios (SNR), Seeing Values (FWHM) or Sky Brightnesses. The comma separated list can be modified to the desired values.

IMPORTANT NOTES:

Mirror Reflectivity: The zero points are derived with clean mirror coatings. Mirror reflectivity will naturally degrade over time. The IO:O ZP (two reflections) is measured to decline at a rate of 0.15mag per year. Other instruments (three reflections) are likely to decline slightly faster than that. The most recent mirror coating was Sept 2024.
Binning: We strongly recommend using 2x2 binning with both IO:O and RISE. Note that we do not routinely obtain unbinned flats for either of these instruments. Please contact us if you plan on taking un-binned data.
Sky brightness: "Dark+10 mag" corresponds to civil twilight (sun altitude < -4 degs), and "Dark+6 mag" to nautical twilight (sun altitude between -8 and -12 degs). Roughly speaking, "Dark+4 mag" corresponds to observing >30 deg from a full Moon, and "Dark+2 mag"to observing >30 deg from a 65% illuminated Moon. We have assumed that the Moon has no affect on J and H-band imaging. For further information on Sky Brightness and its relation to twilight, moon phase and moon distance, see our Sky Brightness page. See also the ING web page on La Palma Night-Sky Brightness.

If you have any problems with or questions about this ETC, please don't hesitate to contact us.

Log x-Axis:	Off On	Plot Mode:
Instrument:		SNR:
Binning:		Seeing (arcsec):
Filter:		Sky Brightness:

Magnitude Min

Magnitude Max

Exposure Time Max

Spectroscopy

To use the Exposure Time Calculator (ETC), select an Instrument with a corresponding grating option from the drop-down menus. The Plot Mode option is used to select whether the plot shows multiple Signal to Noise Ratios (SNR), Seeing Values (FWHM) or Sky Brightnesses. The comma separated list can be modified to the desired values.

IMPORTANT NOTES:

Mirror Reflectivity: The zero points are derived with clean mirror coatings. Mirror reflectivity will naturally degrade over time. The IO:O ZP (two reflections) is measured to decline at a rate of 0.15mag per year. Other instruments (three reflections) are likely to decline slightly faster than that. The most recent mirror coating was Sept 2019.
Red and Blue wavelengths: the exposure times listed below are calculated close to the peak transmission of each arm/wavelength region, corresponding to 450 nm and 700 nm in the blue and red, respectively.
Definition of SNR: This calculator displays signal-to-noise ratio per resolution element. I.e., the SNR you would see if you rebinned the spectrum to the resolution of the instrument. On SPRAT that is approx 18Å.
SPRAT: this version of the ETC does not take into account slit losses with SPRAT; nor does it correct for the colour of a source. The minimum exposure time is 0.03 s.

Dark+10 mag corresponds to civil twilight (sun altitude < -4 degs), Dark+6 mag to nautical twilight (sun altitude between -8 and -12 degs). Roughly speaking, Dark+4 corresponds to observing >30 deg from a full moon; Dark+2 to observing >30 deg from a 65% illuminated moon. For further information on Sky Brightness and its relation to twilight, moon phase and moon distance, see our Sky Brightness webpage.