Centro de Estudios de Física del Cosmos de Aragón

HELP for JAST/T80/JPLUS Exposure Time Calculator

Important update in August 2018

Since 17th August, from the data collected so far the zero point of the iSDSS filter has been set to 23.30 from the previous 23.50 value.

If you have planned previous observations with the old ETC, we recommend you to recompute your exposure times with this new version.

Important update in August 2016

Since 1st August, 2016, the ETC is using empirical values for the zero points, the extinction coefficients and the sky brightness. Compare with the previous ETC based on theoretical values it has been found that that version overestimated the efficiency of the system, in particular in the bluest and reddest filters.

If you have planned previous observations with the old ETC, we recommend you to recompute your exposure times with this new version.

Quick start

The ETC allows to compute 3 types of figures with different observational conditions.
  • Magnitude vs Signal to Noise. In this plot the exposure time is fixed and the signal to noise ratio (SNR) is computed for the given range of magnitudes. This plot is useful to check the magnitude limits at different SNR, while keeping constant the total exposure time.
  • Exposure Time vs Signal to Noise. In this plot the magnitude is fixed and the signal to noise ratio (SNR) is computed for the given range of total exposure times. This plot is useful to check the needed total exposure times to reach a given magnitude limit at different SNR.
  • Magnitude vs Exposure Time. In this case, the signal to noise ratio is fixed and magnitude (SNR) is computed for the given range of total exposure times.

    Notes on the J-PLUS ETC

    Zero points

    This new version of the ETC makes use of empirical zero points computed from actual observations with the T80/JAST. These zero points (ZP) are defined as: mAB = -2.5 log10(F[ADU/s]) + ZP - kextx; where mAB is the calibrated magnitude in AB system, F[ADU/s] is the flux in ADU/s, ZP is the zero point of the system, kext is the extinction coefficient and x is the airmass of the obsrvation.
    Table 1
    T80Cam@T80/JAST zero points and extinction coefficients.
    Band ZP kext
    uJAVA 21.80 0.55
    J0378 21.00 0.39
    J0395 20.80 0.33
    J0410 21.64 0.28
    J0430 21.61 0.24
    gSDSS 23.80 0.18
    J0515 21.62 0.14
    rSDSS 23.66 0.09
    J0660 21.12 0.11
    iSDSS 23.30 0.05
    J0861 21.53 0.06
    zSDSS 22.66 0.04

    Sky conditions

    The sky conditions are described by three parameters:
    • Sky brightness
    • Seeing
    • Airmass
    Table 2.
    Sky surface brightness.
    J-PLUS Filter System.
      Sky SB (mAB/arcsec2)
    Band Dark Grey Bright
    uJAVA 23.021.219.4
    J0378 23.021.219.4
    J0395 23.021.219.4
    J0410 22.521.019.0
    J0430 22.521.019.0
    gSDSS 22.320.619.0
    J0515 22.020.619.0
    rSDSS 21.119.918.8
    J0660 21.520.519.3
    iSDSS 20.520.019.5
    J0861 19.519.218.8
    zSDSS 19.118.718.4

    The sky surface brightness used in the ETC are empirical values measured from actual observations. The definitions of the sky brightness conditions corresponds to the typical values around specific moon phases:

    • Dark: The week around new moon.
    • Gray: The weeks around first and the third quarter.
    • Bright: The week around full moon.

    Table 2 shows the actual values used by the ETC.

    Be aware that due to variation in the moon illumination and the distance to the moon (and, of course, whether the moon is above the horizon), the actual values of the sky brightness can varied a lot.

    The ETC also takes into account the dependence of the sky surface brightness with the airmass (X) according to the following expression:

    SB(X) = SB(1)*(-0.000278719*X3 - 0.0653841*X2 + 1.11979*X - 0.0552132)

    The input seeing is assumed to be the value at zenith and it is scaled with the airmass(X) following the equation:

    seeing(X) = seeing(X=1) * X3/5





    Object type

    The code allows 2 kinds of objects: point-like sources and extended objects.

    For point-like sources, the photometry is performed within the given aperture and the SNR is computed within that aperture. Either input or output magnitudes are total magnitudes outside the atmosphere. However, the magnitude used to compute the SNR is that within the aperture. The computation of the magnitude within the aperture is donde considering a 2D Gaussian distribution with FWHM equal to the Seeing. In that case, the ratio of the flux within the aperture with respect to the total flux is given by the formula:

    FAper/FTotal = 1 - exp(-0.5 R2/σ2)

    For extended objects, the signal to noise is computed within 1 pixel, although magnitudes are always in mag/arcsec2.

    Magnitudes

    Magnitudes are AB magnitudes outside atmosphere. For point-like sources are total magnitudes while for extended objects are per arcsec2.

    SNR

    The signal to noise is computed within the given aperture for point-like sources and per pixel for extended objects.