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NOTCam Zeropoints

New Z and Y filters. Zeropoint magnitudes based on preliminary UKIRT/WFCAM standard star magnitudes.
Date Z
237
Y
236
Gain
(e-/ADU)
Array Comment
25-May-13 - 23.71 ± 0.02 2.5 SWIR3 Only 2 measurements of 1 star.
01-May-12 23.46 ± 0.01 23.75 ± 0.01 2.5 SWIR3
  
10-Jun-11 23.42 ± 0.07 23.72 ± 0.08 2.5 SWIR3
  
12-Feb-11 23.51 ± 0.01 23.82 ± 0.01 2.5 SWIR3
  


Zeropoint (Vega) magnitudes (for 1 e- per second) measured with the WF camera (0.234''/pix). The gain factor is given. The 7th column indicates which array was used: the Engineering Grade (SWIR1), the first Science Grade Array (SWIR2) or the new Science Array (SWIR3), the 7th column gives the Carlsberg Meridian Telescope (CMT) atmspheric extinction coefficient in the SDSS r' band, and the 8th column refers to the latest mirror cleaning, see M1 and M2 reflectivity monitoring.
Date J
201
H
203
Ks
207
Gain
(e-/ADU)
Array CMT r'
extinction
Mirror cleaning Comment
22-Aug-14 23.95 ± 0.01 23.99 ± 0.01 23.40 ± 0.01 2.5 SWIR3 N/A
  
some dust
M1 + M2 realuminisation on 11-Jun-2014
14-Apr-14 24.08 ± 0.02 24.09 ± 0.01 23.46 ± 0.01 2.5 SWIR3 N/A
  
 
25-May-13 24.09 ± 0.01 24.08 ± 0.01 23.48 ± 0.01 2.5 SWIR3 0.101
  
 
01-May-12 24.08 ± 0.01 24.07 ± 0.01 23.46 ± 0.01 2.5 SWIR3 0.104 M1 (CO2) 21/3-12
 
12-Oct-11 23.95 ± 0.01 23.96 ± 0.01 23.35 ± 0.01 2.5 SWIR3 N/A M1 (CO2) 6/9-11 some dust
10-Jun-11 24.09 ± 0.02 24.08 ± 0.01 23.45 ± 0.02 2.5 SWIR3 N/A M1 (washed) 2/3-11
 
12-Feb-11 24.08 ± 0.01 24.10 ± 0.02 23.48 ± 0.01 2.5 SWIR3 N/A
 
 
12-Dec-10 24.06 ± 0.02 24.01 ± 0.02 23.46 ± 0.02 2.5 SWIR3 N/A
 
 
28-Oct-10 24.07 ± 0.02 24.03 ± 0.01 23.47 ± 0.01 2.5 SWIR3 0.095
 
 
01-Aug-10 24.07 ± 0.05 23.89 ± 0.09 23.40 ± 0.05 2.5 SWIR3 N/A
 
some dust
20-Jun-10 24.07 ± 0.01 24.10 ± 0.01 23.46 ± 0.01 2.5 SWIR3 N/A
 
 
19-Jun-10 24.11 ± 0.03 24.03 ± 0.03 23.45 ± 0.03 2.5 SWIR3 N/A
 
 
09-Sep-09 24.15 ± 0.02 24.08 ± 0.01 23.46 ± 0.02 2.5 SWIR3 0.158
 
 
14-Jul-09 24.14 ± 0.01 24.10 ± 0.01 23.48 ± 0.01 2.5 SWIR3 0.073
 
 
M1 + M2 realuminisation on 7-Jul-2009
08-Jun-09 24.14 ± 0.02 24.08 ± 0.01 23.46 ± 0.02 2.5 SWIR3 0.075 M1 (CO2) 2/4-09
 
09-Jun-08 24.01 ± 0.02 23.97 ± 0.02 23.34 ± 0.03 2.5 SWIR3 0.082 M1 (CO2) 28/1-08
 
17-Apr-08 24.06 ± 0.01 24.00 ± 0.02 23.41 ± 0.02 2.5 SWIR3 0.072 M1 (CO2) 28/1-08
 
13-Dec-07 24.11 ± 0.01 24.06 ± 0.02 23.45 ± 0.01 2.5 SWIR3 N/A M1 (CO2) 12/9-07
 
30-Sep-07 23.57 ± 0.03 23.66 ± 0.02 23.13 ± 0.01 3.35 SWIR1 0.089 M1 (CO2) 12/9-07
 
29-Apr-07 23.53 ± 0.03 23.64 ± 0.02 23.14 ± 0.04 2.8 SWIR1 N/A M1 (CO2) 2/3-07
 
24-Feb-07 23.50 ± 0.01 23.58 ± 0.03 23.03 ± 0.04 2.8 SWIR1 0.082 M1 (CO2) 25/7-06
 
11-May-06 23.52 ± 0.01 23.50 ± 0.02 22.98 ± 0.01 2.8 SWIR1 0.085 M1 (CO2) 7/4-06 K (208) = 22.97
14-Jan-06 24.02 ± 0.02 23.97 ± 0.03 23.33 ± 0.03 2.2 SWIR2 0.102 M1 (CO2) 26/9-05
23-Aug-05 23.45 23.43 22.73 2.8 SWIR1 N/A
M1 realuminisation on 21-Jun-2005
15-Jun-05 - - - 2.8 SWIR1 0.084 M1 (CO2) 7/3-05 K(208) = 22.77
16-Sep-02 23.76 23.78 - 2.8 SWIR1 0.069 M1 (CO2) 19/8-02 K(208) = 23.25
8-Jun-02 23.70 ± 0.06 23.64 ± 0.08 - 2.8 SWIR1 N/A
 
K(208) = 23.32 ± 0.09
M1 + M2 realuminisation on 28-May-2002
3-Jan-02 23.62 ± 0.07 23.54 ± 0.04 - 2.8 SWIR1 N/A
  
K(208) = 23.04 ± 0.06


Each standard star observation is made as a 5-point dither. The images are skysubtracted and flatfielded before they are median combined (to minimize bad pixels in the apertures). Each standard is usually measured at several air masses. Exposure times are chosen to keep the peak pixel below the number of ADUs where non-linearity reaches 1%. Both the reset-read-read and ramp-sampling readout modes are used.

Note that the zeropoint magnitudes above are given for 1 e- per second (not 1 ADU/s). The zeropoint magnitudes have been obtained by the best fit to the equations:

J = j_obs + J_zp - J_ext * X 
H = h_obs + H_zp - H_ext * X 
K = k_obs + K_zp - K_ext * X 
where zp is the zeropoint magnitude, ext is the atmospheric extinction coefficient, and X is the air mass. The parameters are obtained from a free fit to several observed standards per night. If the number of standard measurements are sufficient to estimate the atmospheric extinction, then it is given in the table below. Otherwise we use the fixed extinction coefficients: J_ext = 0.086, H_ext = 0.065, and K_ext = 0.080, which are the statistical mode values of a number of measurements at the Teide Observatory in the period 1989 to 1997 (Fuensalida & Alonso, 1998, New Astronomy Reviews 42, 543). See also the document on the Canarian Sky Quality from IAC Sky Quality Group.

Measured extinction coefficients for JHK.
Date J_ext
(mag/airm)
H_ext
(mag/airm)
K_ext
(mag/airm)
3-Jan-02 0.13 ± 0.05 0.03 ± 0.03 0.04 ± 0.04
8-Jun-02 0.15 ± 0.05 0.07 ± 0.05 0.22 ± 0.04
11-May-06 - - 0.02 ± 0.03
29-Apr-07 0.10 ± 0.02 0.09 ± 0.01 0.17 ± 0.03
19-Jun-10 0.18 ± 0.02 0.09 ± 0.02 0.14 ± 0.02
12-Oct-11 0.09 ± 0.02 0.04 ± 0.01 0.07 ± 0.02

Background levels

The IR sky background is strongly variable. For the J and H bands it is dominated by airglow emission (OH lines, which are numerous and highly variable). These lines are also present in the K-band, but here the thermal emission starts to become more important.

Typical sky for NOTCam/SWIR3.
Band Sky
(mag/sq'')
Sky WF-cam
(ADU/min)
Z* ~ 13.2 ~ 15000
Y ~ 17.4 500 - 1200
J 15.5 - 16.2 2000 - 5000
H 13.9 - 14.4 10000 - 27000
Ks 13.1 - 13.4 15000 - 27000

* Thermal leak. Currently under investigation.

Note that the measurements for Z and Y are based on one night only.

Measured background levels in electrons per second per pixel.
Date WF-Y WF-J WF-H WF-K WF-Ks T (° C)
air
Comment Array HR-J HR-H HR-K HR-Ks
3-Jan-2002 - 120 525 750 -
 
 
SWIR1 14 37 123 -
8-Jun-2002 - 90 ± 30 410 ± 60 900 ± 20 -
 
 
SWIR1 - 42 - -
28-May-2003 - 85 480 - 694
 
 
SWIR1 - - - -
03-Jan-2004 - 60 ± 22 306 ± 168 - 531 ± 70 6
 
SWIR1 - - - -
23-Aug-2005 - 90 ± 30 320 ± 30 - 749 ± 56 12
 
SWIR1 - - - -
14-Jan-2006 - 173 ± 17 761 ± 290 753 626 ± 49 2
 
SWIR2 - - - -
11-May-2006 - 84 ± 17 395 ± 118 616 439 ± 32 3.5
 
SWIR1 - - - -
29-Apr-2007 - 87 ± 11 338 ± 32 - 525 ± 16 5
 
SWIR1 - - - -
13-Dec-2007 - 137 ± 28 618 ± 100 - 706 ± 66 1.5 some dust SWIR3 17 69 - 77
17-Apr-2008 - 181 ± 90 804 ± 419 - 923 ± 169 5
 
SWIR3 - - - -
09-Jun-2008 - 121 ± 12 473 ± 51 - 1066 ± 71 10
 
SWIR3 8 33 - 129
08-Sep-2009 - 210 ± 84 927 ± 213 - 1243 ± 90 11 dusty SWIR3 - - - -
09-Sep-2009 - 96 ± 22 594 ± 123 - 949 ± 37 9.5
 
SWIR3 - - - -
19-May-2010 - 193 ± 45 995 ± 325 - 1090 ± 140 7.5 non-photometric? SWIR3 - - - -
19-Jun-2010 - 147 ± 46 658 ± 365 - 1161 ± 153 10.5
 
SWIR3 - - - -
12-Feb-2011 20 ± 3 78 ± 13 425 ± 65 - 605 ± 43 2.5
 
SWIR3 - - - -
1-Apr-2012 47 ± 3 167 ± 13 1142 ± 65 - 777 ± 43 0 non-photometric? SWIR3 - - - -
1-May-2012 37 ± 3 160 ± 5 944 ± 80 - 709 ± 10 -1
 
SWIR3 - - - -
14-Apr-2014 - 135 703 - 768 5 38 deg from fullmoon SWIR3 - - - -

The mean SWIR3 WF-camera background values on 12-Feb-2011 were:
Y = 17.3, J = 16.1, H = 14.3, Ks = 13.3 magnitudes per square arc sec.

The mean WF camera backgrounds with the Engingeering grade array were:
Yn = 15.0, J= 15.5, H = 13.8, Ks = 12.9, and K = 12.8 magnitudes per square arc sec.

The mean WF camera background values on 14-Jan-2006 with the First Science Grade Array were:
J = 15.3, H = 13.6, Ks = 13.2, and K = 13.0 magnitudes per square arc sec.


Limiting magnitude

In the case of background limited observations, the limiting magnitude can be predicted from the derived zeropoints according to the formula (see McLean 1997; relation is modified to be valid for zeropoints for 1 e/s instead of 1 ADU/s):

mlim = zp - 2.5 log (S/N (nB/t)1/2 )

where zp is the zeropoint magnitude (for 1 electron/sec), S/N is the requested signal-to-noise ratio, n is the number of pixels over which a point source is distributed, B is the background level (in electrons per second per pixel), and t is the total on-source integration time. For point sources the seeing is measured as the FWHM (in arcsec) of the point spread function, and the pixel size is 0.234 arcsec for the WF camera and 0.078 arcsec for the HR camera.

  1. Point source: n = 3.1415 * (FWHM/pixel_size)2
  2. Extended source: n = (1/pixel_size)2 and unit is magnitude per square arc seconds..

Photometric scatter across the array

The photometric scatter across the FOV is defined as the standard deviation around the mean of N individual aperture photometry measurements of the same standard star across the array. The number of positions used are currently N=25, although in the past we have used also N=5 and N=16.

The photometric scatter is measured to see if there is a spatial variation of the photometric response. Contributions to the photometric scatter will also be from improper flat fielding and any presence of bad pixels. No bad pixel mask was applied in this analysis. We conclude that the photometric repeatability across the FOV for the broad band filters is relatively good, and that no correction for a photometric surface is needed.

The photometric scatter (in magnitudes) over the FOV.
Date Camera J H K Ks H2 v=1-0 Array N
16-Sep-2002 WF*) 0.05 0.04 0.07 0.03 - SWIR1 16
14-Jan-2006 WF 0.013 0.012 - 0.015 - SWIR2 5
17-Apr-2008 WF 0.018 0.019 - 0.022 - SWIR3 25
09-Jun-2008 HR 0.017 0.018 - 0.015 - SWIR3 25

*) The JHK flat fields were obtained three months earlier, while the Ks flatfield was obtained 19/9-02. This probably explains much of the scatter in JHK.


The 16 positions of the standard star overlaid on the flat field image of the Engingeering Grade Array, SWIR1 (left), corresponding to the first measurement, and the 25 positions of the standard star overlaid on the flat field image of the new science array, SWIR3 (right), corresponding to the 3rd and all subsequent measurements.

Anlaug Amanda Djupvik

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