performance.html

Table 3.1. WFCAM zeropoints (from CASU)

Filter	Mean Zeropoint
Z	22.84
Y	22.77
J	23.02
H	23.24
K	22.57
BrG	20.03
H2S1	19.58

Sensitivity Tables

Point Source Sensitivy

Measurements of point source sensitivity are given in table Table 3.3, “Measured sensitivity”.

These sensitivity figures are based on 5 sec exposures, point sources, 0.6" seeing and a 2" aperture.

For JHK, sensitivity figures are given for different sky brightnesses, corresponding to bright, average and dark parts of the night.Proposers should assume the central value unless there are good reasons to do otherwise.

Table 3.3. Measured sensitivity

Filter	Sky Brightness	5-second, 5σ magnitude
Z	18.1	19.51
Y	17.3	19.13
J	15.6	18.42
J	16.0	18.55
J	16.4	18.65
H	13.7	17.60
H	14.0	17.74
H	14.3	17.87
K	13.2	17.00
K	13.5	17.15
K	13.8	17.26

Extended Source Sensitivity

These extended source sensitivities have been calculted from the point source numbers in table Table 3.4, “Measured sensitivity”.

These sensitivity figures are based on 5 sec exposures.

Numbers are given for aperture areas of 1 square arcsecond, and 1 pixel. For extended sources, signal to noise ratio is proportional to the square root of the aperture area.

Table 3.4. Measured sensitivity

Filter	Sky Brightness	5-second, 5σ magnitude.arcsec^-2 per pixel	5-second, 5σ magnitude.arcsec^-2 per arcsec²
Z	18.1	19.1	20.1
Y	17.3	18.7	19.8
J	15.6	18.0	19.0
J	16.0	18.1	19.2
J	16.4	18.2	19.3
H	13.7	17.2	18.2
H	14.0	17.3	18.4
H	14.3	17.5	18.5
K	13.2	16.6	17.6
K	13.5	16.7	17.8
K	13.8	16.9	17.9

Blank sky - brightness and countrates

Table 3.5, “Blank Sky - Countrates, Brightness, Time to background limit” shows the fluxes of blank sky in each filter, along with the countrate that this generates and the exposure time necessary to become background limited in each filter.

Note: Sky background levels vary on timescales of minutes to hours. The numbers given here are mean background levels for a period of several months in spring 2005. Some statistics of sky level variations are given elsewhere. Also, background limited here is taken to mean that the Poisson noise from the photons from the sky background equals the readnoise of the array. This gives a minimum number, you should generally do longer exposures to diminish the effects of readnoise further.

Table 3.5. Blank Sky - Countrates, Brightness, Time to background limit

Filter	Countrate / ADU.s^-1..pixel^-1.	Mags.arcsec^-2	Flux / W.m^-2.μm^-1	Exp Time to Background Limit / s
Z	11 ± 2	18.1	5.0E-16	18
Y	22 ± 6	17.3	6.7E-16	9
J	122 ± 40	15.8	1.5E-15	1.6
H	760 ± 77	14.0	3.0E-15	0.2
K	712 ± 86	13.4	1.8E-15	0.3
H₂S1-0
Brγ

Twilight flat fields

Twilight flats are the main flat field source for WFCAM. The timing of twilight flat field observations is critical. The following plots shot sky brightness as a function of time for dawn twilight flats.

Filter	Large scale (minutes to sunrise)	Small scale (minutes to sunrise)
Y	-40 to -15	-20 to -13
J	-30 to -10	-20 to -10
H	-26 to -8	-15 to -9
K	-20 to 0	-10 to 0
All	-30 to 0