The goal of the "Global Statistics" page is to provide an up-to-date (on a per run basis) status of the CFHT Legacy Survey in terms of 1) integration time per survey and per field, 2) image quality, and 3) sky conditions (brightness, airmass and transparency).

The statistics are based only on the validated exposures, this means that after analysis of the characteristics of the image (image quality, sky background) and the observing conditions (sky transparency) at CFHT by the QSO coordinator, the exposure was qualified as fitting within the constraints set by the CFHTLS survey coordinator, or if slightly out of range for a given parameter, it was still judged adequate by the CFHT QSO coordinator to match the primary scientific goal of that survey component (which explains why some exposures with poor image quality got validated: they still have a value in terms of the time analysis which is a severe constraint for the Deep and Very Wide surveys).

A validated exposure does not however necessarily mean that the time is accounted for in the Legacy Survey time budget. Indeed, if some exposures from an Observing Group (OG) are not all validated, then the OG is not validated and the time not accounted for (QSO will in consequence try to capture the entire OG again until all its individual exposures get validated). QSO has the precise information for inter-connecting the validation of exposures and in absolute only its statistics should be considered to account for the survey progress (they can be consulted on this page). The global statistics based on the validated exposures are however a good way to follow the collection of useful data gathered for the survey.

These statistics are based exclusively on the exposure catalogs presented and available on this site. The photometric bootstrapping exposures (Q98) are not included in these statistics, nor are other calibrations fields (photometry, PSF).

The 3 surveys relative fraction

At the time the CFHT Legacy Survey was designed (in 2001-2002), the relative use of the global time made available (approximately 450 nights over 5 years) by the three components was precisely defined in order to reach specific scientific goals: 44% for the Deep survey, 34% for the Wide survey and 22% for the Very Wide survey.

The updated current ratio for the three surveys is simply based on the relative fraction of total integrated exposure time of validated exposures per survey. Note that the numbers encompass the survey since its "official start" on May the 30th of 2003 when the instrumental configuration became stable: improved image quality, guiding, full mosaic.

The Pre-Survey is a shallow mapping in the r' filter of the three Wide survey patches with large overlaps between frames (180 seconds integration each) and a total sky coverage half a degree beyond the limits of the Wide patches. This mapping will allow a very precise anchoring of the Wide survey astrometry. Since the Pre-Survey serves primarily the Wide survey, its integration time is included in the Wide survey budget and the total number of hours spent on the Pre-Survey on the "Global Statistics" page is only informative, i.e. not used independently to build the statistics.

Individual statistics per survey and field

- Field name & Filter

Field Name	Ratio (%)	Itime (sec) = itime x Nexp	Med Avg Min Max Dsp	A.	Bkg.	Ph.
r	21.9	8.9 = 360 x 89	0.87 0.88 0.60 1.28 0.16	1.21	3.47	67

The left column entries define the field name and the relevant set of filters for the given survey. Since there is no field per say on the Very Wide survey, all the exposures for a given filter are found together in a same set.

- Integration ratio for the given filter

Field Name	Ratio (%)	Itime (sec) = itime x Nexp	Med Avg Min Max Dsp	A.	Bkg.	Ph.
r	21.9	8.9 = 360 x 89	0.87 0.88 0.60 1.28 0.16	1.21	3.47	67

This entry simply provides the percentage of the total integration time on the given field in the given filters versus the other filters on that same field. Check the page "Survey Depth's Goals" for an estimation of the expected end ratios.

- Integration time and number of exposures

Field Name	Ratio (%)	Itime (sec) = itime x Nexp	Med Avg Min Max Dsp	A.	Bkg.	Ph.
r	21.9	8.9 = 360 x 89	0.87 0.88 0.60 1.28 0.16	1.21	3.47	67

This field provides the total validated integration time (Itime) for the given filter on the given field. The median integration time per exposure (itime) as obtained by MegaPrime for that given set is also provided as well as the re-computed number of validated exposures obtained (Nexp) based on the median integration time (it can differ slightly from the actual real number of exposures dues to changes in the observing strategy in the course of the survey).

- Image Quality

Field Name	Ratio (%)	Itime (sec) = itime x Nexp	Med Avg Min Max Dsp	A.	Bkg.	Ph.
r	21.9	8.9 = 360 x 89	0.87 0.88 0.60 1.28 0.16	1.21	3.47	67

As explained in the pages "Guide to browsing the catalogs" on this site, the image quality is estimated on the central part of the mosaic (12 CCDs) and on the outer ring (24 CCDs). The image quality statistics presented here are based on the average of these two measurements, hence it is an accurate representation of the image quality across the entire image. The following estimators are proposed: Med = Median, Avg = Average (mean), Min = Minimum, Max = Maximum, Dsp = Dispersion.

- Airmass

Field Name	Ratio (%)	Itime (sec) = itime x Nexp	Med Avg Min Max Dsp	A.	Bkg.	Ph.
r	21.9	8.9 = 360 x 89	0.87 0.88 0.60 1.28 0.16	1.21	3.47	67

The average airmass for the set of exposures.

- Sky Background

Field Name	Ratio (%)	Itime (sec) = itime x Nexp	Med Avg Min Max Dsp	A.	Bkg.	Ph.
r	21.9	8.9 = 360 x 89	0.87 0.88 0.60 1.28 0.16	1.21	3.47	67

The median sky background in ADU per second for the given filter.

- Sky Transparency

Field Name	Ratio (%)	Itime (sec) = itime x Nexp	Med Avg Min Max Dsp	A.	Bkg.	Ph.
r	21.9	8.9 = 360 x 89	0.87 0.88 0.60 1.28 0.16	1.21	3.47	67

The fraction (in percent) of the exposures taken in photometric conditions. For the exposures not acquired under photometric conditions, a special short exposure (1/10th of the original exposure time) for photometry bootstrapping is acquired when the sky is clear (Q98 program).