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DIET is a versatile exposure time calculator allowing the computation of
various quantities related to the MegaCam observing performance.
The magnitudes in DIET are expressed in the Vega system and given for the SLOAN filters u*, g', r', i', z'. The following link (courtesy of D. Patton) provides information on the various magnitude systems and ways to go from one to another: http://www.astro.utoronto.ca/~patton/astro/mags.html. The observing conditions (sky brightness, camera zero points, ...) are derived from data obtained during the first two engineering runs and will be refined (specially sky brigtness vs. Moon phase) as more data get accumulated. DIET has yet to be throughoutly tested and properly calibrated using a large number of MegaCam frames that have been and will be acquired during the entire commissioning phase to be completed this winter 2003. The interactive graphical interface allows the user to experiment with some custom parameters (see the interface help page for the description of each parameter).
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The following sheets propose a rapid tutorial on the magnitude
and signal to noise calculation schemes used in DIET.
There are three classes of objects considered in DIET:
For nearby galaxies, the extended source scheme is more appropriate. |
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Figure 1:
How DIET works - Part 1
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Figure 2:
How DIET works - Part 2
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Click here to launch the graphical interface. Click here to access the graphical interface help page. |
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To efficiently remove the cosmic rays hits and cosmetic defaults (gaps
between the CCDs, bad columns), a minimum of 4 dithered exposures
per field is required - 5 is optimal (those 5 positions are the most
external of alls position proposed in the CFHT default dithering patterns).
Each exposure should however be in sky photon noise regime such that when exposures are later stacked together, the expected signal to noise ratio will be achieved. The readout noise is low on MegaCam and quickly dominated by the sky photon noise when observing with the broad band filters. Taking a typical readout noise of 5 electrons, and using the darker sky brightness (dark time, 1.0 airmass), the minimum exposure time required to be dominated by a factor of 10 by the sky background photon noise is approximately:
The saturation level should also be a consideration: exposing too long will indeed save a couple of minutes by skipping some readouts (MegaCam readout time is 30 seconds long - exposure to exposure duty cycle is less than 40 seconds) but will result on high sky background and several objects reaching saturation. Typically, the following exposure times are a good compromise to achieve low overhead while keeping the signal in a reasonable range:
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Comments on the DIET pages to Jean-Charles Cuillandre:
jcc@cfht.hawaii.edu
DIET was developed by Simon Kras (CFHT/UVic Coop) and Jean-Charles Cuillandre (CFHT).
MegaCam
is the new CFHT CCD wide-field imaging mosaic.