When using a robotic telescope, it is sometimes difficult to estimate what exposure to use - when observing live or remotely, one can simply take a short exposure and then scale up to the correct exposure.   Rather than first ordering an image based upon a guess and then ordering again when one has a better idea, it's better to use the known brightness of an object, the filter desired, and the properties of the telescope and camera to calculate what the exposure should be.  This is the purpose of an EXPOSURE TIME CALCULATOR and such are used at all major observatories around the world to help in the preparation of future observations (e.g. Hubble Space Telescope, ESO, ...).

The MONET Exposure Time Calculator (METC) is just such a tool.  Given an astronomical object, the weather conditions (seeing), and the properties of the MONET telescope, filters, and camera, it can calculate any one of three quantities (input fields on the METC page) given two of them:

  • the brightness of your object (in magnitudes);
  • the exposure time (in seconds);
  • the final signal-to-noise (a measure of the final quality of a measurement).

For example, given the brightness and the desired signal-to-noise (should be at least 5 or so for a good detection, much higher for a high-quality measurement), METC can tell you how long your exposure should be.  Alternatively, given a brightness and an exposure time (e.g. limited by the constraints of an eclipse), METC will tell you what signal-to-noise to expect.

The results of a measurement are affected by things like


  • the extinction (how much light is absorbed when it passes through the Earth's atmosphere);
  • the sky brightness (the brighter the sky, the more noise in the measurement due to background light);
  • the seeing (the better the seeing, the fewer pixels are needed and so there should be less sky background and less read-out noise);
  • the binning of the CCD (by binning, you have fewer pixels in the final image and hence less read-out noise);
  • the choice of aperture size for photometry (the larger the aperture, the more light from a star but also the more background and read-out noise);
  • the wavelength corresponding to the filter chosen (e.g. CCD cameras tend to be more red-sensitive and less blue-sensitive).


These secondary aspects can also be changed in the METC.


When you have made a calculation by pressing any of the METC buttons ("EXPOSURE TIME", "SIGNAL-TO-NOISE", or "TARGET BRIGHTNESS"), the quantity calculated is highlighted in green while the other two are highlighted in blue.  If you change anything important for the calculation, the fields are highlighted in red.

With each calculation, you are also presented with other useful information, like

  • the total number of ADUs in the target and sky;
  • the total number of ADUs due to dark-current (due to the warmth of the camera and not produced by light);
  • the total number of pixels used in the estimate.

While METC was produced for MONET, it can also be used for other telescopes, filters, and cameras.  Thus, useful things like the pixelscale (how many arcseconds per pixel) and the total field-of-view (given by the pixel sizes and numbers) are also shown.