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performance.html Expected performance of WFCAM can be derived using the UKIRT Integration Time Calculator

WFCAM zeropoints

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

Typical limit magnitude examples can be found here.

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 Sensitivity

Measurements of point source sensitivity are given in table Table 3.2, “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 3.2. 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 3.3, “Measured sensitivity”.

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, signal to noise ratio is proportional to the square root of the aperture area.

Table 3.3. 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


Exposure time calculator

On-chip exposure times for observing with WFCAM can be calculated using the UKIRT exposure time calculator


Overheads

Table 3.4, “WFCAM Overheads” shows the overhads shows the overhads measured from on-sky observations. These include instrument overheads and telescope dithers/microsteps. An average overhad 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. 

Table 3.4. WFCAM Overheads

Exp time.(s) Coadd Read Mode Overhead (%)
1 1 CDS 156
1 2 CDS 138
2 2 CDS 92
5 1 CDS 58
5 2 CDS 34
5 4 CDS 28
5 1 CDS 58
10 1 CDS 28
10 2 CDS 17.5
20 1 NDR 11.5
60 1 NDR 4


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.

Table 3.6. Twilight flats - Count rates at different times before sunrise

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



Contact: Watson P. Varricatt. Updated: Tue May 19 13:14:47 HST 2015

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