CFH12K Direct Imaging Exposure Time calculator

Table of contents:

What is DIET?
DIET is a versatile calculator allowing the observer to compute various quantities related to the CFH12K observing performance.

The magnitude system in DIET is Johnson (Vega star has V=0.03 and all colors equal to zero). The following link (courtesy of D. Patton) provides information on the various magnitude systems and ways to go from one to another:

A calculator is available on-line (see "graphical interface") but a complete set of tables covering the typical observing conditions is also provided on this page. Only the main photometric bands B, V, R and I are supported now.

The observing conditions (sky brightness, camera zero points, ...) are derived directly from the tables given in the "Optics and optical performance" page of the CFH12K documentation.

DIET was throughoutly tested and calibrated using a large number of CFH12K frames taken in various conditions. CCD09 is the reference for these calculations, hence as mentioned in the "CCD focal plane" section, better performance can be expected from the high resistivity CCDs.

The interactive graphical interface allows the user to experiment with some custom parameters. This iterative process can be time consuming: this is when the tables come handy: they summarize on one single sheet many conditions. It offers the user a better feeling of the global results evolution.


Some basic information on how DIET computes magnitudes and SNRs
The following figures 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:

      • Point sources: stars & QSOs
      • Galaxies: distant compact sources - seeing dominated profile
      • Extended source: uniform illumination over square arcsecond scales

For nearby galaxies, the extended source scheme is more appropriate.

Figure 1: How DIET works - Part 1

Figure 2: How DIET works - Part 2


The tables
The following tables (also available in high-resolution PostScript format on these pages) will give the total exposure time to accumulate to achieve a given signal-to-noise ratio at a given magnitude or vice-versa.

This total exposure time is not the telescope time needed to realize the observations: the telescope and camera overheads have to be taken into considerations. See the page "Observing Overheads" in the CFH12K documenation page.


The graphical interface
Prior using the graphical interface, it is a recommended to consult the tables and graphs listed above to get a good feeling on the camera performance in various conditions.

Click here to launch the graphical interface.

Click here to access the graphical interface help page.


How to optimally fragment the total exposure time
To efficiently remove the cosmic ray hits and cosmetic defaults (gap between the CCDs, bad columns), a minimum of 4 dithered exposures per field is required.

Each exposure should however be in sky photon noise regime so that when exposures are later added, the expected signal to noise ratio will be obtained. The readout noise is low on CFH12K and quickly dominated in 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 to use to be dominated by a factor of 10 by the sky background photon noise is:

  • B band: 3 mn
  • V band: 1 mn
  • R band: 30 sec
  • I band: 15 sec

The saturation level should also be a consideration: exposing too long will indeed save a couple of minutes by skipping some readouts but will result on high sky background and several objects reaching saturation (tables available above). Typically, the following exposure times are a good compromise to achieve low overhead while keeping the signal in a reasonable range:

  • B band: 15 mn
  • V band: 15 mn
  • R band: 10 mn
  • I band: 10 mn

   Comments on the CFH12K pages to Jean-Charles Cuillandre:
   DIET was developed by Simon Kras (CFHT/UVic Coop) and Jean-Charles Cuillandre (CFHT).
   CFH12K is the CFHT CCD wide-field imaging mosaic.