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Liverpool Telescope Exposure Time Calculator

Liverpool Telescope
Exposure Time Calculator

Imaging

To use the Exposure Time Calculator (ETC), simply select an Instrument with a corresponding Binning (usually 2x2) and Filter from the pull-down menus. Enter the Apparent Magnitude of your target (in the same band as the filter being used), along with the desired Signal-to-Noise Ratio (SNR) and click on Calculate. The resulting table shows exposure times for different sky brightnesses above the darkest sky and for different seeing (FWHM) for point sources. It is assumed that the diameter of the object and sky apertures used are both twice the FWHM. For extended sources use a target surface brightness (in magnitudes per square arcsecond) and refer to the exposure times in the final column.

IMPORTANT NOTES:

  • 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. However, due to shutter effects a minimum exposure time of at least 10 seconds is strongly recommended. Shorter exposures are unlikely to produce photometric accuracies of less than 1%.
  • 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.

IMAGING Exposure Time Calculator
Instrument: Magnitude:
Binning: SNR:
Filter:
Sky Brightness 1.0 arcsec 1.5 arcsec 2.0 arcsec 3.0 arcsec 4.0 arcsec per sqr arcsec
Dark
Dark + 1.0 mag (V band)
Dark + 2.0 mag (V band)
Dark + 4.0 mag (V band)
Dark + 6.0 mag (V band)
Dark + 10 mag (V band)

Note: the exposure times quoted above are in SECONDS; a value of minus one indicates saturation or an exposure time longer than 3 hours.


In the above table, 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.



Spectroscopy

To use the Exposure Time Calculator (ETC), simply select an Instrument, a corresponding Slit (the IFU must be used for FRODOspec), and a Spectrometer arm - red or blue - depending on the wavelength region you are interested in. Note the difference between the higher-resolution VPH gratings and the lower-resolution gratings in FRODOSpec. Then enter the Apparent Magnitude (B or V band should work best) of your target, along with the desired Signal-to-Noise Ratio (SNR) in the continuum, and click on Calculate. The resulting table shows exposure times for different sky brightnesses above the darkest sky and for different seeing (FWHM) for point sources.

IMPORTANT NOTES:

  • 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.

SPECTROSCOPY Exposure Time Calculator
Instrument: Magnitude:
Slit: SNR:
Spectrometer arm:
Sky Brightness 1.0 arcsec 1.5 arcsec 2.0 arcsec 3.0 arcsec 4.0 arcsec
Dark
Dark + 1.0 mag (V band)
Dark + 2.0 mag (V band)
Dark + 4.0 mag (V band)
Dark + 6.0 mag (V band)
Dark + 10 mag (V band)

Note: the exposure times quoted above are in SECONDS; a value of minus one indicates saturation or an exposure time longer than 3 hours.


This version of the ETC does not take into account slit losses with SPRAT; nor does it correct for the colour of a source. For further information on Sky Brightness and its relation to twilight, moon phase and moon distance, see out Sky Brightness webpage.

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



(version 1.1 - July 2014)