mphot

mphot is a Python package to model photometry for ground or space-based astronomy. Exposure time calculator (ETC) built in.

How it works

Simply put,

• it combines user submitted [telescope * filter * camera qe] efficiencies with generic stellar models and sky transmission/radiance models (for Paranal, 2400m) to generate integrable grids of stellar fluxes and sky radiances.

• Then, mphot uses the grids to interpolate between different

  • atmospheric parameters (PWV, airmass)
  • target star parameters (effective temperature + distance)

• using user submitted

  • telescope/site parameters (primary and secondary diameters, site seeing, sky brightness relative to Paranal with sky_factor)
  • camera parameters per unbinned pixel (plate scale, dark current, read noise, well depth, target well fill, read time), and the binning with pixel_binning and pixel_binning_type ("digital" for CMOS, "on-chip" for CCD)

• to calculate the ideal exposure time and expected precision for a given observation.

Please see the examples for more details on how to use mphot. For further details on the models used, please see https://doi.org/10.1117/12.3018320.

Note, it uses stellar parameters from "A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence".

Galaxies and nebulae

mphot also calculates exposures for images of galaxies and nebulae. Give the Messier, NGC or IC name and the SNR that you need:

import mphot

name, _ = mphot.generate_system_response(
    "resources/systems/speculoos_Andor_iKon-L-936_-60.csv", "resources/filters/r.csv"
)
props = {
    "name": name, "plate_scale": 0.35, "N_dc": 0.2, "N_rn": 6.328,
    "well_depth": 64000, "well_fill": 0.7, "read_time": 10.5, "r0": 0.5, "r1": 0.14,
}
props_sky = {"pwv": 2.5, "airmass": 1.2, "seeing": 1.35}

plan = mphot.get_exposure_extended("M51", props, props_sky, snr=10)
print(plan["t_sub [s]"], plan["subs"])  # 94.5 s, 10 sub-exposures

The sub-exposure is the shortest time that meets two conditions:

• It is background-limited: the sky and dark variance is 10 times the read-noise variance. The read noise then adds less than 5% to the noise.
• The readout uses no more than 10% of the total time.

The sub-exposure is shorter if the brightest part of the target fills the well to well_fill. If one exposure gives the SNR, mphot uses one exposure, because each readout adds read noise. To use a fixed sub-exposure time, set min_exp and max_exp to the same value. The number of sub-exposures gives the SNR per pixel at the mean surface brightness of the target. The result also gives the magnitude of the stars that fill their brightest pixel to well_fill in one sub-exposure, and notes about the assumptions.

Options:

• mu_offset: measure a fainter level, for example 2 mag/arcsec² below the mean.
• area: give the SNR for an area in arcsec², not for one pixel.
• flat_error: the error after flat-fielding and sky subtraction, as a fraction of the sky. This error is the same in all sub-exposures, so it sets a maximum SNR. The default is 0.

Galaxies have a stellar spectrum that agrees with their catalogue colours. Nebulae have emission lines. For stars and star clusters, use get_precision. See the notebook Galaxies and nebulae.

Data sources, built by resources/targets/build_targets.py:

• OpenNGC by Mattia Verga (CC-BY-SA-4.0): positions, sizes, magnitudes and types.
• Acker et al. (1992) and Cahn, Kaler & Stanghellini (1992): Hβ fluxes, line ratios and extinction of planetary nebulae.
• Finkbeiner (2003): Hα fluxes of other emission nebulae.
• SVO Filter Profile Service: Bessell B and V, and 2MASS J, H and Ks filter curves.

Installation

You can install mphot in a Python (>=3.11) environment with

pip install mphot

or from a local clone

git clone https://github.com/ppp-one/mphot
pip install -e mphot

You can test the package has been properly installed with

python -c "import mphot"

Web demo

mphot runs in the browser through Pyodide, see https://etc.withastra.io/. To run it on your machine instead:

python web/build.py
python -m http.server --directory web 8000

Then open http://localhost:8000/. See web/README.md.

Attribution

If you find mphot useful for your research, please cite Pedersen et. al 2024. The BibTeX entry for the paper is:

@inproceedings{pedersen2024infrared,
  title={Infrared photometry with InGaAs detectors: First light with SPECULOOS},
  author={Pedersen, Peter P and Queloz, Didier and Garcia, Lionel and Schacke, Yannick and Delrez, Laetitia and Demory, Brice-Olivier and Ducrot, Elsa and Dransfield, Georgina and Gillon, Michael and Hooton, Matthew J and others},
  booktitle={Ground-based and Airborne Instrumentation for Astronomy X},
  volume={13096},
  pages={1146--1167},
  year={2024},
  organization={SPIE}
}