Science Operations

Sensitivities & Sky Brightnesses

The LBC exposure time calculator (ETC) should be used whenever possible for LBC exposure time estimates. However, the most recently acquired TiO and CN filters, do not yet have their zeropoints incorporated into the ETC. Also, the predictions based on the ETC should always be considered as the upper envelope of what is possible, since aside from moon phase, sky brightnesses can vary with telescope pointing and time, and furthermore, they are based only photon statistics and do not account for uncertainties introduced in the data processing (e.g. flat-fielding errors). This page provides background information on the zeropoints, sensitivities measured from data taken during the commissioning phase and plots of measured sky brightnesses which can provide some context and guidance for predictions based on the ETC.

The LBC Exposure Time Calculator

The web-based LBC Exposure Time Calculator can be found here.

Zero Points

Broad-band LBC filters:

Zero points through all filters in LBC Blue and all filters, except for the narrow-band PI filters: F972N20; CN_814 and TiO_784; in LBC Red, were determined from observations of the Landolt and the Galadi-Enriquez standard star fields made during the LBC Blue and Red commissioning periods. Color terms and extinction were also determined. The adopted calibrations relations, from the commissioning reports, were:

MAG(i) = mag(i) + zero_point(i) + color_term(i,j)*[mag(i)-mag(j)] + K(i)*X

where:

mag(i) indicates the instrumental magnitude at filter i
MAG(i) is the standard magnitude
K(i) is the extinction at i
X is the airmass

Calibration Coefficients for LBC Blue Filters
Filter Zero Point Color Term Extinction
SDT_Uspec 27.33 (0.02) *
0.220 (0.004)(u-g) -0.47 (0.02)
U-BESSEL 26.23 (0.03) 0.036 (0.009)(u-b) -0.48 (0.02)
B-BESSEL 27.93 (0.02) -0.123 (0.003)(b-v) -0.22 (0.01)
V-BESSEL 28.13 (0.01) 0.021 (0.005)(b-v) -0.15 (0.02)
g-SLOAN 28.31 (0.01) -0.086 (0.002)(g-r) -0.17 (0.02)
r-SLOAN 27.75 (0.01) -0.016 (0.002)(g-r) -0.11 (0.02)

 

Calibration Coefficients for LBC Red Filters
Filter Zero Point Color Term Extinction
V-BESSEL 27.94 (0.04) -0.041 (0.019)(v-i) -0.16 (0.03)
R-BESSEL 27.86 (0.02) 0.067 (0.009)(v-r) -0.13 (0.01)
I-BESSEL 27.59 (0.02) 0.019 (0.005)(v-i) -0.04 (0.01)
r-SLOAN 28.03 (0.02) -0.014 (0.008)(r-i) -0.09 (0.02)
i-SLOAN 27.57 (0.02) -0.072 (0.005)(r-i) -0.03 (0.01)
z-SLOAN 27.20 (0.04) -0.020 (0.004)(i-z) -0.04 (0.02)
Y-FAN 24.58 (0.08) -0.05 (0.02)

* NB: The ZP at SDT_Uspec appears to be too high. In the ETC, the difference between the ZPs at Uspec and U-Bessel is not that large: ZP = 26.52 for Uspec and ZP = 26.2 for U-Bessel.

Narrow-band TiO and CN filters*

For the TiO_784 and CN_817 filters, the following zero points, which express the I-Bessell magnitude (Vega) for which 1 count per sec would be expected through the narrow-band filter, are listed below.

 Filter ZP(Vega)
TiO_784 ~25.45
CN_817 ~25.33
The count rates in these two filters, relative to the count rates measured in the LBCR I-Bessell filter for 3 different types of stars and for the sky background are given in the table below:
 Filter stars with V-I <= 1
Carbon stars
M-type stars
sky background
 TiO_784  0.13      0.16  0.11  0.19
 CN_817  0.13      0.10  0.16      0.11

* Please note that the zero points and relative count rates through the narrow-band TiO and CN filters were kindly provided by Andreas Koch and Matthias Frank (LSW).

F972N20 filter:

The ETC adopts a zero point of 24.43 mag (AB) for the F972N20 filter, and we are in the process of measuring the zeropoint from observations of two spectrophotometric standard stars made during the observatory restart in September 2017.

10-sig 1-hr sensitivities

The sensitivities as measured during the Commissioning phases of LBCB and LBCR are tabulated below. Conditions may not have been optimal when these data were taken — skies not moonless and perhaps not perfectly photometric.

Sensitivity Limits for LBC Blue Filters
Filter Mag(AB) 1hr 10 sig Mag(AB) 1hr 1 sig Seeing (arcsec)
SDT_Uspec 24.8 27.0 1.05
U-BESSEL 24.5 26.9 1.05
B-BESSEL 24.2 26.8 1.00
V-BESSEL 25.1 27.6 1.00
g-SLOAN 26.6 28.9 0.95
r-SLOAN 25.2 27.7 1.00

 

Sensitivity Limits for LBC Red Filters
Filter Mag(AB) 1hr 10 sig Mag(AB) 1hr 1 sig Seeing (arcsec)
V-BESSEL 25.9 28.5 1.05
R-BESSEL 25.2 27.8 0.90
I-BESSEL 25.1 27.6 1.00
r-SLOAN 25.5 27.9 1.00
i-SLOAN 25.2 27.7 0.85
z-SLOAN 22.9 25.3 0.90
Y-FAN 22.7 25.2 0.85

Sky Brightnesses

UBVRI Sky Brightnesses for Mt. Graham

The Mount Graham sky brightnesses [mag/sq.arcsec] at U, B, V, R and I were presented by Pedani (2009), PASP, 121, 778.

Count rates per pixel

The histograms below plot the sky backgrounds (counts/s/pix) in all of the LBC filters as measured from the set of LBC images taken between 2011 and 2016. Images taken during twilight or bright moon were not excluded and may skew the results, especially when there were few images taken at that filter. In comparison with the sky count levels used by the ETC, the median backgrounds measured in I-BESSEL, i-SLOAN and Y are ~2 and 1.4 times are higher.  Note also the wide spread, especially at these longer wavelengths where the variable OH Meinel bands make a contribution. To keep background levels below 20,000-30,000 counts, we recommend keeping exposure times to <=180 sec at i-SLOAN and I-Bessell and <= 120 sec at z-SLOAN.

Figure 1: Background count rates (cnts/s/pix) measured from LBC images taken between 2011-2016 for all of the LBC Blue filters (top panel) and all of the LBC red filters (bottom two panels). The upper left corner gives the filter (note that all of the LBCR filters are prefaced by “R”, Bessell filters are indicated by capital letters and SLOAN filters by lower-case letters, and narrow-band filters are denoted as RF, F972N20; RC, CN_814; and RT, TiO_784); the mean and median of the distribution; and the number of images (N) used for the measurement. The data come from the output of skyseeing (skyseeing is based on the LBTtools.LBC.allseeing task and outputs background sky count rates, which are computed as the clipped mean of the background levels measured by source extractor around each detected source minus the median value of the overscan, all divided by the exposure time.