astro-snr-calc

astro-snr-calc is a small Python library and command-line tool for modelling point-source signal-to-noise ratio for astronomical imaging systems.

It supports two operating modes:

• Analytic mode: build a diffraction-limited Airy PSF from first-order telescope parameters in a TOML file.
• Zemax mode on Windows: load a .zmx design through the ZOS-API and compute PSFs from OpticStudio.

The package produces SNR maps over exposure time and source magnitude, reports the SNR-optimal aperture, and marks regions where the brightest pixel saturates.

Features

• Model telescope, sensor, bandpass, and sky background terms.
• Evaluate SNR on a 2D grid of exposure time and apparent magnitude.
• Optimize the photometric aperture from the PSF itself.
• Plot SNR contours and saturation boundaries.
• Use a built-in Sony IMX267 sensor model or load a sensor from TOML.
• Optionally drive PSF generation from Zemax OpticStudio on Windows.

Installation

From the project root:

pip install .

From PyPI:

pip install astro-snr-calc

For editable development installs:

pip install -e .

The package requires Python 3.10 or newer.

Command-Line Usage

After installation, the console entry point is:

SNRCalc <telescope.toml> [detector.toml] [options]

Current CLI options:

usage: cli.py [-h] [-p PUPIL_SAMPLING] [-i IMAGE_SAMPLING]
              [-f [FIELD_INDEX ...]] [--no-plots] [--no-save]
              telescope [detector]

Examples:

SNRCalc telescope.toml
SNRCalc telescope.toml detector.toml
SNRCalc telescope.toml --no-plots

On Windows, you can also pass a Zemax design file instead of a telescope TOML file:

SNRCalc design.zmx detector.toml --field 1 3 --no-save

In analytic mode, the tool prints a model summary and saves an SNR map as a PNG file named after the telescope configuration stem.

Configuration Files

Telescope TOML

The telescope file describes the first-order optical model used in analytic mode.

name = "Example Telescope"
aperture = "80 mm"
effl = "400 mm"
wfno = 5.0
throughput = 0.85

Fields:

• name: descriptive telescope name.
• aperture: entrance aperture diameter.
• effl: effective focal length.
• wfno: working f-number.
• throughput: end-to-end transmission as a fraction from 0 to 1.

Detector TOML

If you do not supply a detector file, the CLI uses a built-in Sony IMX267 uncooled model.

name = "Custom Sensor"
pixel_pitch = "3.45 um"
qe = 0.64
read_noise_e = 2.4
dark_rate = "3 1 / s"
full_well_e = 10700

Fields:

• name: descriptive detector name.
• pixel_pitch: detector pixel pitch.
• qe: quantum efficiency as a fraction from 0 to 1.
• read_noise_e: read noise in electrons RMS.
• dark_rate: dark current in electrons per second per pixel.
• full_well_e: full well capacity in electrons.

Unit-bearing values are parsed with astropy.units.Quantity, so values such as "80 mm", "3.45 um", and "3 1 / s" are accepted.

Python API

You can also use the package directly from Python.

import astropy.units as u

from snr_calc import (
    ExposureGrid,
    PhotometricBand,
    SNRResult,
    Sensor,
    SkyBrightness,
    Telescope,
)

sensor = Sensor(
    name="Sony IMX267 (uncooled)",
    pixel_pitch=3.45 * u.um,
    qe=0.64,
    read_noise_e=2.4,
    dark_rate=3.0 / u.s,
    full_well_e=10700.0,
)

telescope = Telescope(
    name="Example Telescope",
    aperture=80 * u.mm,
    effl=400 * u.mm,
    wfno=5.0,
    throughput=0.85,
)

model = telescope.build(
    sensor=sensor,
    band=PhotometricBand.johnson_v(),
    sky=SkyBrightness.dark_sky(),
)

result = SNRResult.from_model(model, *ExposureGrid.default().arrays())
print(model.summary())

Top-level exports include:

• Sensor
• Telescope
• PhotometricBand
• SkyBrightness
• ExposureGrid
• DetectorPSF
• SNRModel
• SNRResult
• SNRMapPlotter

Zemax Support

Zemax integration is only available on Windows. The module raises an import error on other platforms.

When using a .zmx file, the CLI can request FFT or Huygens PSF calculations through OpticStudio. The Windows-only CLI also supports:

• --huygens: use Huygens PSF analysis instead of FFT.
• --as-extension: connect to a running interactive OpticStudio session.

If Zemax is not available, use analytic mode with a telescope TOML file.

Output

For each evaluated model, the CLI:

• prints a textual summary of telescope, detector, sky, PSF, and aperture parameters,
• computes SNR over a default grid of 1 ms to 100 s and magnitude 0 to 16,
• saves a PNG SNR map unless --no-save is used,
• optionally displays the plot window unless --no-plots is used.

The plot includes logarithmic SNR coloring, contour overlays, and a hatched saturation boundary when the peak pixel exceeds the detector full well.

Development

To run the CLI from the source tree without installing the package:

PYTHONPATH=src python -m snr_calc.cli --help