Introduction

This page follows the structure of the various sections found in the "General Summary" table and provides further details related to the impact they may have on an observing strategy.

1 Geometry

1.1 Mosaic organization

Figure 1. MegaCam readout layout. North is at the top, East to the left.

1.2 Wide-Field Corrector optical distortion

Nevertheless, depending on the scientific goals, recovering the medium (13 arcsec wide, or 70 pixels) and large (80 arcsec wide, or 425 pixels) gaps in the mosaic is not always a necessity. For that reason, three types of dithering pattern are offered in the QSO interface:

• Large Dithering Pattern (ellipse 30'' x 180''):
  LDP1 -> LDP21 = "Dither Large Gaps"
  This in the pattern of choice to cover the largest gaps in the mosaic with 4 (and up) exposures. The pattern, as shown on Figure 2, actually becomes a very elongated ellipse as the largest vertical gaps are 6 times wider than the horizontal gaps. After proper resampling and stacking, single monolithic 1 by 1 square degree images can be created.
• Medium Dithering Pattern (disk diameter 30''):
  DP1 -> DP21 with scaling factor 1.5 = "Dither Small Gaps"
  This is the pattern to pick if only three large stripes of sky with a one arcminute gap between them is fine to conduct the scientific program. Resampling is still a must-do here.
• Small Dithering Pattern (disk diameter 20''):
  DP1 -> DP21 with scaling factor 1.0 = "Dither CCD"
  This is the pattern to pick if all 36 CCDs are to be processed separately. Global resampling to stack all the individual chips is not necessary as the optical distortion remains minimal at the scale of a single CCD with the applied offsets.

Figure 2. Distribution of optimally dithered exposures. (the disk becomes an ellipse for the large pattern).

1.3 Evaluation of the image quality across the field of view

We have defined image quality estimators to quantify this effect on the observed image. They rely on ratio of image quality measurements between various regions (shown on Figure 3) of the mosaic.

Figure 3. Regions of image quality analysis.

 IMAGE:      712976
 ------------------
 IQ MAP & CLASS TO MEDIAN 1-9 | *0 |+/-1|+/-2|+/-3|+/-4|+/-5|+/-6|+/-7|+/-8|+/-9|
 IQ difference in arcsec ---> <0.02<0.06<0.11<0.17<0.22<0.28<0.34<0.39<0.41< more
 |0.81|0.77|0.74|0.74|0.72|0.73|0.76|0.82|0.88|    |+1|*0|*0|-1|-1|-1|*0|+2|+3|
 |0.80|0.75|0.69|0.67|0.67|0.68|0.70|0.74|0.80|    |+1|*0|-2|-2|-2|-2|-2|-1|+1|
 |0.83|0.77|0.71|0.68|0.68|0.70|0.72|0.76|0.82|    |+2|*0|-1|-2|-2|-2|-1|*0|+2|
 |0.90|0.84|0.80|0.77|0.77|0.77|0.80|0.85|0.93|    |+3|+2|+1|*0|*0|*0|+1|+2|+4|
 
 CENTER     = 0.69"
 OUTER RING = 0.79"
 
 RADIUS R = 1.144 [ center / (top+left+bottom+right) ]
 PISTON X = 1.018 [ right / left ]
 PISTON Y = 0.937 [ top / bottom ]
 

2 Photometry

2.1 Filter set

The following table gives basic filter charateristics. The measured mean bandwidth and mean transmission over that bandwidth are derived from scans performed at CFHT for each filter, as discussed below.

Figure 4. MegaCam filter set transmission and average CCD quantum efficiency.

2.2 Telescope and MegaPrime/MegaCam optics

The reflectivity model for the primary mirror is of a freshly coated, bare aluminum. The curve is shown on figure 5 and the data used to generate that plot are available here.

The transmission model of the Wide-Field Corrector and the camera window is derived from combining all the transmission and diffusion effects of the various glass and coatings. The curve is shown on figure 5 and the data used to generate that plot are available here.

Figure 5. Primary mirror and MegaPrime+MegaCam optics (minus filters) transmission.

2.3 CCD quantum efficiency

The measurements were performed individually on each chip at CEA.

The data from this table are available in ASCII format here.

A more precise table with a point every 10 nm is available here. Note that these are results from a simulation for a given set of CCD characteristics that match best the table of real measurements provided above.

2.4 Narrow band filters: CFH12K filters forcing a reduced field of view

These narrow band filters cover 14 chips at the center of the mosaic (10 to 16 and 19 to 25) though the external ones will have their external sides strongly vignetted. The resulting fielf of view is 42 by 28 square arcminutes. More information on this filter set can be found here.

2.5 AB Magnitude equation

The zero points advertised on the MegaPrime pages come from photometric frames processed by Elixir. The flat-fielding step takes care of making the zero point uniform over the entire 36 CCDs of the mosaic by applying a multiplicative factor scaled on CCD00, e.g. the response is normalized to CCD00. The result is a flat looking image with a unifom sky level but with a detection limit changing slightly from CCD to CCD due to their intrinsic differences in quantum efficiency and read noise.

When applying a zero point to some Elixir processed data, one should use the gain of 1.62 by default, hence apply the equation "In ADUs" provided in the table above.

2.6 Transition from AB to Vega magnitude system

2.7 Sky brightness

The measurements were derived from data obtained exclusively in photometric conditions. The combination of cirrus and the Moon in the sky is basically impossible to predict, but the effect will be less in i' and z' than in u*, g' and r'.

It is remarkable that the sky remains the same brightness at zenith in the i' and z' band no matter what the phase of the Moon is (we tested between 0% and 70%). This demonstrate that MegaPrime is a prime grey time instrument. However the sky brightness in these two bands depend greatly on the airmass.

2.8 Fringes in the i' and z' bands

MegaCam is not a great instrument to observe in the z' band as the CCDs sensitivity is low at those frequencies and the sky background is very high resulting in very strong fringing.

3 Observing

3.1 Immunity to bright stars scattered light

• Nearby off-field bright stars:
  As shown on the "Off-field bright stars immunity" page, this kind of contamination is not an issue anymore with MegaCam.
• Reflection off the wire bonds on the focal plane:
  There was a concern that the exposed golden pads in the middle of the mosaic (the part where the CCD signal are connected from the substrate to the connectors) would cause some problem. No such thing as they sit below the sensitive surface and the light can not in consequence cause a contamination. A very bright star was moved around in that area and no contamination could be noticed.
• Reflective halos:
  They are caused by the low level reflections from the CCD surface back to the optics and then back to the CCD. If the star is very bright, then it ends up becoming an obvious feature. They are shown on Figure 6. They can not be avoided and pretty much ruin any signal within the area of the halo, typically 7 arcminutes in diameter. While this effect was most prominent on the CFH12K in the B filter, here it is present in all filters, a result of the different anti-reflection coating. The example on Figure 6 shows a u*g'r' "true-color" image of the NGC 2244 cluster, the bright star on the upper right is V=6.0 mag (USNO) and the one on the lower left is V=6.8 mag (USNO).

Figure 6. Reflection halos in MegaPrime due to the optics (NGC 2244 cluster).