Exposure Time Calculator

PEPSI Exposure Time Calculator (ETC)
V mag: Spectral type:
Airmass: Seeing (arcsec):
Exposure time (s): Cross disperser:
Setup/Fiber: S/N @ center of CD:

PEPSI Exposure Time Calculator v2.10 2021-12-20

The Exposure time calculator (ETC) estimates the flux from the star by using flux templates from Pickles (1998) and from Francis et al. (1991), scaling them using the V-band vega VEGA flux (Bohlin & Gilliland 2004). The flux is then adjusted by using the V-magnitude supplied, and by applying extinction correction using the airmass and the extinction values from the Multi-Object Double Spectrograph (MODS) instrument manual. According to the selected instrument setup, the correct entrance aperture for each fibre diameter is used (7.5, 0.74, 1.5, 2.3 for VATT, 100µm, 200µm/Polarimeter, 300µm respectively) according to Strassmeier et al. (2007)), and the losses are calculated for that aperture according to the specified seeing value.

The flux is computed for the size of the central pixel in each Cross Disperser (CD), multiplied for the binning if applicable (the 200µm fiber is 2x binned, the 300µm fiber is 5x binned). The read-noise of the Charge-Coupled Device (CCD) (on average 3.5 for the blue CCD and 4 for the red CCD; see PEPSI STA1600 10K science CCD camera implementation details) is added to the photon noise, and the size of each slice (20, 9, 17, 35 for VATT, 100µm, 200µm/Polarimeter, 300µm respectively) times the number of slices (9, 7, 5, or 3 for VATT, 100µm, 200µm/Polarimeter, 300µm fibers respectively) is used to determine the number of pixels (see PEPSI image slicer implementation details). For the overall instrument efficiency values are taken from the PEPSI image slicer implementation details. We determined the peak of the efficiency using the 95% quantile for each CD, fiber and telescope. The average value for both telescopes is used in the ETC.

The output is an estimate for the peak (i.e. in the center of the blaze function) Signal to noise ratio (SNR) of the specified cross-disperser for an individual spectrum. As of Oct 2023, the efficiencies for SX and DX are calculated separately since they differ by up to 30\% (DX having lower throughput). When using PEPSI in binocular mode, the SNR is the geometric sum for the two sides, in monocular mode SX values are used. For the polarimeter, the Foster prism splits the light into two, we give the value of the sum of these two spectra. As there is only one fiber from the Vatican Advanced Technology Telescope (VATT), the monocular mode is always used in that setting.

  • Bohlin, R. C. & Gilliland, R. L. 2004, AJ, 127, 3508
  • Francis, P. J., Hewett, P. C., Foltz, C. B., et al. 1991, ApJ, 373, 465
  • Pickles, A. J. 1998, PASP, 110, 863
  • Strassmeier, K. G., Woche, M., Andersen, M., & Ilyin, I. 2007, Astronomische Nachrichten, 328, 627