Par Genevieve Soucail - 14/04/2008
CFHTLS - D4
RA (2000) = 22h 15m 31s ; DEC (2000) = -17d 44m 06s
lII = 39.2° ; bII = -52.8°
Observations done on D4 : (observations are now definitely closed on D4)
Only observations "within specs" are accounted in the total integration time. The specifications for the Deep survey itself are defined with the following criteria : for all filters except u* and g’ the seeing must be better than 0.9". In u*, the limit is released to 1.1" and in g’ the limit is 1.0". These are measured values in the center "ring" of CCDs (see the MegaPrime web pages at CFHT for more details).
Note that z’ observations are significantly better than 0.8" most of the time, as well as i’ ones.
The official start of the survey is on May 30th, 2003, when the instrumental configuration of MegaPrime became stable. Only data taken after this date are accounted for.
Distribution of usefull observations with the 5 filters :
Average background of the observations :
The average background level of raw images is the following (measured on real data) :
||background = 0.84 e- / pix
sky brigthness = 22.3 AB mag /arcsec2
||background = 4.2 e- / pix
sky brigthness = 21.8 AB mag / arcsec2
||background = 6.1 e- / pix
sky brigthness = 20.9 AB mag / arcsec2
||background = 11.6 e- / pix
sky brigthness = 19.9 AB mag / arcsec2
||background = 11.2 e- / pix
sky brigthness = 19.0 AB mag / arcsec2
Note that these value differ significantly from the canonical values posted on CFHT Web pages. The main differences are due to variations of the sky brigthness with Moon phase for u* and g’, and variations of the sky brightness with airmass for the redder filters (r’, i’ and z’). Finally, these values are subject to changes with the evolution of the baffling of the telescope.
Expected depth of the observations already acquired :
The expected limiting magnitudes were computed by using the MegaCam exposure time Calculator DIET under the following assumptions :
seeing value of 0.8", point source, SNR = 5 computed in an aperture of 1.45*seeing=1.15".
In addition, in the SNR calculation, the sky term is multiplied by a factor n to take into account some additional noise in the sky subtraction or residuals due to crowding or systematics. n can range from 1 for a perfect sky substraction to 2 for a 1-1 pixel type subtraction. in DIET a value of n=1.5 isued.
Limiting magnitudes (AB mags.) are given both for a point source, and a "galaxy" as defined in DIET : a distant and compact source, with a seeing-dominated profile. There is a difference of 0.7 magnitudes between them under the present assumptions.
DSS image of D4. Overlayed are the stars (R<14) from the USNO catalogue
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