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The Liverpool Telescope Instruments : Exposure Time Calculator

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 2019.
  • Binning: We strongly recommend using 2x2 binning with both IO:O and RISE. Note that we do not routinely obtain un-binned flats for either of these instruments. Please contact us if you plan on taking un-binned data.
  • Minimum Exposure Times: Exposure times as low as 1 second may be quoted by the ETC for IO:O and IO:I. However, due to shutter effects a minimum exposure time of at least 10 seconds is strongly recommended for IO:O; shorter exposures are unlikely to produce photometric accuracies of less than 1%. Please refer to the IO:I webpage for information on exposure times with this instrument.
  • RINGO3: The RINGO3 exposure time corresponds to the duration of the observing group; eight exposures will be obtained each second/per rotation cycle. The values quoted assume a constant gain and are most accurate for "few second" exposure durations. For a more accurate estimate, and for details on how to convert SNR to polarisation accuracy, see the RINGO3 instrument web-page.

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. 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 web-page. See also the ING web-site 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 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 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.
  • 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.

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.

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:
Slit: Seeing (arcsec):
Spectrometer arm: Sky Brightness:

Magnitude Min Magnitude Max Exposure Time Max







(Version 2.0 - Nov 2019, Marco Lam and Doug Arnold)