WFCAM Performance

WFCAM Performance

The expected performance of WFCAM can be derived using the UKIRT Integration Time Calculator (currently being updated).

WFCAM Zeropoints

Table 3.1, shows measured zeropoint magnitudes for each filter in WFCAM under photometric conditions. 

Typical limiting magnitude examples can be found here.

WFCAM mean zeropoints from CASU

FilterOld21 Nov 2019
Z22.8422.88
Y22.7722.85
J23.0223.02
H23.2423.26
K22.5722.58
BrG20.03
H2S119.58
Table 3.1. Variation in the mean UKIRT+WFCAM zero points over the years (from CASU). 

The zero points plots below are from the data reduction at the Cambridge Astronomy Survey Unit (CASU). The large jump at the end of 2019 is the result of re-coating the UKIRT primary and secondary mirrors. 

Sensitivity Tables

Point Source Sensitivity

Measurements of point source sensitivity are given in table Table 3.2.

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.

FilterSky Brightness5-second, 5σ Magnitude
Z18.119.51
Y17.319.13
J15.618.42
J16.018.55
J16.418.65
H13.717.60
H14.017.74
H14.317.87
K13.217.0
K13.517.15
K13.817.26
Table 3.2. – Measured sensitivity of point sources

Extended Source Sensitivity

These extended source sensitivities have been calculated from the point source numbers in table Table 3.3.

These sensitivity figures are based on 5-sec exposures.

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.

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

FilterSky Brightness5-second, 5σ magnitude per arcsecond^-2 per pixel5-second, 5σ magnitude per arcsecond^-2 per arcsecond^2
Z18.117.320.1
Y17.318.719.8
J15.618.019.0
J16.018.119.2
J16.418.219.3
H13.717.218.2
H14.017.318.4
H14.317.518.5
K13.216.617.6
K13.516.717.8
K13.816.917.9
Table 3.3 – Measured sensitivity of extended sources

Exposure time calculator

On-chip exposure times for observing with WFCAM can be calculated using the UKIRT exposure time calculator (currently being updated).

Overheads

Table 3.4, shows the overheads measured from on-sky observations. These include instrument overheads and telescope dithers/microsteps. An average overhead of 1.5 minutes for slew and acquisition of each target and a maximum overhad of one minute per filter change should be added to your total time estimate. 

Exp time. (s)CoaddRead ModeOverhead (%)
11CDS156
12CDS138
22CDS92
51CDS58
52CDS34
54CDS28
101CDS28
102CDS17.5
201NDR11.5
601NDR4
Table 3.4 – WFCAM overheads

Blank sky – Brightness and Countrates

Table 3.5 shows the fluxes of the 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.

FilterCountrate / ADU per s^-1 per pixel^-1Mags arcsec^-2Flux / Wm^-2 micron^-2Exp Time to Background
Z11 +/- 218.15.0E-1618
Y22 +/- 617.36.7E-169
J122+/- 4015.81.5E-151.6
H760 +/- 7714.03.0E-150.2
K712 +/- 8613.41.8E-150.3
H_2S1-0
Br-gamma
Table 3.5 – Blank Sky – Countrates, Brightness, Time to Background Limit

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 show sky brightness as a function of time for dawn twilight flats. 

FilterLarge 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
Table 3.6 – Twilight Flats – Count Rates at Different Times Before Sunrise