rretc 0.1.1

LBT/LBC point-source exposure-time calculator

rretc

The Rapid Response Exposure Time Calculator is an exposure-time calculator for point sources observed with LBT/LBC.

Installation

Install (no dependencies) with:

pip install rretc

Install the optional command-line interface with:

pip install "rretc[cli]"

Python API

The package exposes five functions:

from rretc import msat, nexp, snr, texp, tsat

conditions = {
    "magnitude": 22.0,
    "band": "g",
    "moon_zd": 60.0,
    "moon_phase": 0.0,
    "moon_elongation": 90.0,
    "aperture": 2.25,
    "airmass": 1.3,
    "site_quality": "ultra-clean",
    "seeing": 0.75,
}

count = nexp(100.0, 60.0, **conditions)
seconds = texp(10, 100.0, **conditions)
achieved = snr(60.0, 10, **conditions)

saturation_conditions = {
    key: value
    for key, value in conditions.items()
    if key not in {"magnitude", "aperture"}
}
saturation_time = tsat(18.0, **saturation_conditions)
saturation_magnitude = msat(10.0, **saturation_conditions)

nexp returns the minimum exposure count for a target SNR and exposure duration; texp returns the minimum exposure duration for a target SNR and exposure count; and snr computes the SNR for an exposure stack. texp returns math.inf if the requested SNR is above the systematic-noise limit for the specified number of exposures. tsat returns the exposure time at which the peak pixel saturates; msat returns the corresponding saturation magnitude for a proposed exposure time. Both use 90% of the detector full-well capacity. msat returns math.nan when sky and dark current alone saturate the pixel.

Command line

The CLI requires click or the cli installation extra (pip install ".[cli]"). It provides one subcommand for each API operation and prints one numeric value:

rretc nexp --snr 100 --texp 60 \
  --magnitude 22 --band g --moon-zd 60 --moon-phase 0 \
  --moon-elongation 90 --aperture 2.25

rretc texp --nexp 10 --snr 100 \
  --magnitude 22 --band g --moon-zd 60 --moon-phase 0 \
  --moon-elongation 90 --aperture 2.25

rretc snr --texp 60 --nexp 10 \
  --magnitude 22 --band g --moon-zd 60 --moon-phase 0 \
  --moon-elongation 90 --aperture 2.25

rretc tsat --magnitude 18 --band V --moon-zd 70 --moon-phase 0.75 \
  --moon-elongation 90

rretc msat --texp 10 --band V --moon-zd 70 --moon-phase 0.75 \
  --moon-elongation 90

Run rretc COMMAND --help for all options.

Inputs and assumptions

• Supported bands are U, B, V_B, V, R, I, u, g, r_B, r, i, and z.
• Magnitudes are finite values, Vega for Johnson-Cousins bands and AB for Sloan bands.
• Exposure times are seconds. Lunar angles are degrees. Seeing and aperture are arcseconds.
• Saturation calculations use the central pixel, so tsat and msat do not take an aperture.
• moon_phase ranges from 0 for new Moon to 1 for full Moon.
• moon_zd >= 90 places the Moon below the horizon.
• Site profiles are standard, dry, and ultra-clean.
• Default airmass is 1.3, default site quality is ultra-clean, and default seeing is 0.75 arcseconds.

The lunar sky model follows Krisciunas & Schaefer, PASP 103, 1033 (1991). The lunar airmass approximation follows Young, Applied Optics 33, 1108 (1994).

Testing and development

Install with:

pip install . --group dev

Run tests with:

python -m pytest