Star Correction

Correct star shape -- roundness and elongation, including the FWHM inflation denoise introduces -- without sharpening or deblurring the background or nebulosity.

schema 1 · revision 1

Recognize it

Stars are slightly elongated or irregular after denoise, especially on dense fields (globular clusters, open clusters).
Star FWHM measurably increased relative to the pre-denoise image.

Intended output

Rounder, tighter stars with the background and nebulosity visually unchanged from before this step.

Required input state

Linear, unstretched data, after denoise -- this step exists specifically to correct what denoise did to star shapes.

What NOVA does

NOVA runs bxt_star_correct: PixInsight BlurXTerminator in Correct Only mode, automatic PSF, with sharpen_stars, sharpen_nonstellar, and adjust_halos all forced to zero -- pure star-shape correction, most consequential on dense star fields where denoise inflates FWHM.

Use when

Denoise measurably inflated star FWHM or introduced visible elongation on a dense star field.

Skip when

Stars are already round and FWHM-stable after denoise; there is nothing to correct.
Elongation is from tracking/guiding error, not denoise -- Correct Only will not fix trailing stars.

Limits

This is pure geometric correction, not sharpening or denoising -- 'Star Sharpen' is a misleading name for a step whose sharpening amounts are deliberately zero.
This step happens before the star/starless split (remove_stars_linear); it is not the same operation and does not create or use a separate star layer.
It cannot fix elongation caused by tracking or guiding error -- that is not a denoise side effect and this step will not correct it.

Scientific and aesthetic notes

Jeff's own manual verification renamed this step from 'Star Sharpen' to 'Star Correction -- BlurXTerminator Correct Only' precisely because the old name implied sharpening that does not happen here; the taxonomy's family name reflects that correction.
Do not equate BlurXTerminator Correct Only with Cosmic Clarity stellar-only sharpening -- both improve star shape but are different engines and need their own before/after check.

Evidence and measurements

Measurements	Acceptance criteria
Star FWHM and eccentricity from matched before/after luminance extractions. Star support population (count of stars measured), noted when it differs between before/after. Noise evaluation as a non-regression check, not a success metric.	Eccentricity decreases without material FWHM growth. Star support population is comparable before and after. No new halos, ringing, lost detail, or meaningful noise regression. Background and nebulosity are visibly unchanged from the input.

Cross-tool matrix

Tool	Equivalence	Expected result	Evidence
nova	Exact replay	Lower eccentricity, stable-or-improved FWHM, comparable star support, no change to background/nebulosity.	nova-star-correction-source, nova-m66-run-1.24.7
pixinsight	Exact replay	Lower eccentricity without material FWHM growth, halos, ringing, lost detail, or noise regression.	m66-manual-verification
siril	Same engine, adapted host	Matches the PixInsight/NOVA result closely; same engine, different host.	m66-manual-verification
saspro	Functional alternative	Improved star roundness with the background and nebulosity unaffected.	saspro-source-1.18.0, m66-manual-verification

NOVA Python pipeline workflow 1.24.7

Run immediately after denoise, on the active linear image -- the star/starless split has not happened yet at this point in the pipeline.
Use automatic PSF detection; leave all sharpen/halo amounts at zero.
Compare matched luminance extractions (identical FWHM/eccentricity settings) before and after.

Controls and starting ranges

mode: Correct Only
PSF: automatic detection
sharpen_stars / sharpen_nonstellar / adjust_halos: 0.0 / 0.0 / 0.0 -- forced, not tunable

Mask behavior: Not applicable; BXT Correct Only operates on the full image.

Failure modes

Confusing this step's output with a separate star layer -- none exists yet at this point.

Recovery

Revert to the pre-correction linear image and re-check the denoise step that preceded it.

PixInsight BlurXTerminator, Correct Only Core 1.9.3 Lockhart host; BlurXTerminator version not independently confirmed in this repo

Run BlurXTerminator on the active linear image with Correct Only enabled and Sharpen Stars, Adjust Star Halos, and Sharpen Nonstellar all at 0.
Extract identical before/after luminance copies with the same FWHMEccentricity sensitivity, upper limit, and PSF model.
Use noise evaluation as a non-regression check only; Correct Only is not a denoiser.

Controls and starting ranges

M66 verified: Correct Only on; Sharpen Stars 0; Adjust Star Halos 0; Sharpen Nonstellar 0; automatic PSF

Mask behavior: Not applicable; full-image operation.

Failure modes

Measuring before/after populations with differing star-support counts and treating the delta as precise.
Retaining an older window/file name after later operations and mistaking it for a different input stage.

Recovery

Undo and re-run with matched extraction settings for a valid comparison.

Siril via RC-Astro > BlurXTerminator.py, Correct Only (licensed external integration) RC Astro BlurXTerminator wrapper model v4, verified in Siril 1.4.4

Open Scripts > Python Scripts > RC-Astro > BlurXTerminator.py with Correct Only enabled.
Leave Sharpen Stars, Adjust Star Halos, and Sharpen Nonstellar all at 0.
Automatic PSF is disabled/irrelevant in this mode; do not try to tune it.

Controls and starting ranges

M66 verified: Correct Only on; Sharpen Stars 0.00; Adjust Star Halos 0.00; Sharpen Nonstellar 0.00; Tile Overlap 0.20

Mask behavior: Not demonstrated; full-image operation.

Failure modes

Siril has no native equivalent -- attempting a native star-shape-only correction without this integration.

Recovery

Revert to the pre-correction linear image.

Seti Astro Suite Pro Cosmic Clarity Sharpen, stellar-only 1.18.0 source-inspected; procedure discussed but not independently tested at this exact stage

Apply Cosmic Clarity Sharpen in Stellar Only mode to the active linear image -- not 'the star layer', which does not exist yet at this point in the pipeline.
Keep non-stellar sharpening disabled so this stays a star-shape-only pass.
Do not treat this as the same operation as BlurXTerminator Correct Only; compare before/after star profiles independently, since they are different engines.

Controls and starting ranges

mode: Stellar Only; non-stellar sharpening disabled

Mask behavior: Not demonstrated; full-image operation.

Failure modes

Assuming stellar-only Cosmic Clarity Sharpen and BXT Correct Only are interchangeable without their own comparison.

Recovery

Revert to the pre-correction linear image, or defer star-shape correction to PixInsight BXT Correct Only.

Validation status

Not yet Jeff-validated. Use this as sourced guidance, not a certification.

Sources

M66 manual recipe verification feedback M66_Manual_Recipe_Verification_Feedback.md — Jeff validated
NOVA BlurXTerminator correct-only star-shape implementation and ontology nas_server/seti_astro.py:2623; nas_server/processing_ontology.json:915 — NOVA source-confirmed
M66 workflow 1.24.7 execution record critiques/20260802_045251_seestar_galaxy.json — NOVA execution record
Installed Seti Astro Suite Pro 1.18.0 processing source saspro/pedestal.py; saspro/numba_utils.py; saspro/stacking_suite.py; saspro/abe.py; saspro/sssc.py — Artifact-confirmed

Related evidence

The M66 recipe and Jeff-validation ledger will link here when their static exports are available. Until then, this article remains explicitly unvalidated.