Raise a clear error when Camera.gain is None

[roytsmart]

SensorData.electrons and TapData.electrons failed for any image opened via msfc_ccd.fits.open():

import msfc_ccd
img = msfc_ccd.fits.open(msfc_ccd.samples.path_fe55_esis1)
img.electrons   # TypeError: 'NoneType' object is not subscriptable

Cause

Camera.gain defaults to None and, unlike Camera.sensor (which __post_init__ resolves to TeledyneCCD230()), nothing ever resolves it. AbstractCamera.dn_to_electrons then computed None * a, which yields a TapData whose outputs is None rather than raising. The failure only surfaced later in SensorData.from_taps as TypeError: 'NoneType' object is not subscriptable, pointing at the wrong place entirely.

msfc_ccd.fits.open constructs a default Camera(), so every image loaded the documented way hit this. Passing an explicit gain always worked.

Worth noting the bug was worse than a misplaced traceback: TapData.electrons never raised at all, it returned a TapData whose outputs was None, so anything consuming taps directly got silent corruption.

Fix

dn_to_electrons now raises a ValueError naming the missing parameter before doing any arithmetic:

ValueError: `gain` is `None`, so this camera cannot convert DN into electrons.
Measure the gain of this camera and provide it explicitly, for example
`msfc_ccd.Camera(gain=2.5 * u.electron / u.DN)`.

The alternative was resolving gain in __post_init__ to a measured default for the Teledyne CCD230, mirroring how sensor is handled. That was rejected for now: there is no measured gain anywhere in this repository (the bias and dark-current reports contain none, and the only gain values in the codebase are synthetic test fixtures), and gain is documented as tap-dependent, so a single hardcoded scalar would be wrong for the per-tap case. Baking in a default is a natural follow-up once there is a measurement to bake in.

Docstrings updated to match: Camera.gain now states there is no measured default and that it must be set before dn_to_electrons / .electrons will work, and fits.open's camera parameter notes that the default camera has no gain.

Tests

Nothing previously exercised .electrons on an image from msfc_ccd.fits.open — the existing image tests all build a Camera with an explicit gain fixture.

• test_fits.py gains test_open_electrons (explicit-gain camera through fits.open, asserting both .taps.electrons and .electrons come back in electron and are nonzero) and test_open_electrons_default_camera (default camera raises on both). The path list is now a shared module-level _paths.
• AbstractTestAbstractCamera gains test_gain and test_dn_to_electrons, which branch on whether gain is None. TestCamera is now parametrized over both Camera() and a camera with a per-tap gain array, so both branches are covered.

Full suite passes (123 tests); ruff check and black --check are clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (864910c) to head (9261d62).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Files           19        19           
  Lines          840       875   +35     
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+ Hits           840       875   +35     

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