Pedestal Removal
Remove a known residual constant electronic offset without confusing legitimate sky signal, noise, interpolation, or an isolated dark pixel with that offset.
schema 1 · revision 1Recognize it
- Calibration records identify a residual constant offset that was not removed.
- Calibrated subframes show left-edge clipping that requires a documented output pedestal during recalibration, rather than post-stack minimum subtraction.
Intended output
A still-linear image with only the confirmed uniform offset removed, no new shadow clipping, and recorded before/after channel statistics.
Required input state
- Linear data with known calibration provenance.
- A documented offset value or calibration diagnosis independent of the stack's single lowest pixel.
- Before-operation channel minima, medians, and clipped-pixel counts recorded.
What NOVA does
NOVA 1.24.7 skips remove_pedestal by default because a global minimum is insufficient evidence. If an operator explicitly forces the current function, it subtracts one global minimum uniformly from every channel and clips only as a numerical safety guard.
Use when
- Capture or calibration provenance establishes a remaining constant offset.
- A controlled calibration diagnosis supplies the amount and the subtraction can be repeated on the original linear data.
Skip when
- The only evidence is a positive minimum, median, or visually raised sky floor.
- The image is an ordinary calibrated stack with no documented residual offset.
- The proposed amount is a sky statistic rather than a calibration offset.
- The data are nonlinear or the original calibration products are unavailable.
Limits
- A positive global minimum does not identify an electronic pedestal.
- Post-stack subtraction cannot restore values clipped during calibration.
- Subtracting separate channel minima can alter color balance.
- An output pedestal added during calibration to prevent clipping is not evidence that an arbitrary post-stack floor should be removed.
Scientific and aesthetic notes
- Sky background is real signal plus noise; forcing its darkest sample to zero is not a calibration measurement.
- Judge this analytically with statistics and provenance, not a before/after beauty slider.
- If calibration clipped negative values, reprocess the subframes with an appropriate calibration-time output pedestal; later addition or subtraction cannot recover clipped structure.
Evidence and measurements
| Measurements | Acceptance criteria |
|---|---|
|
|
Cross-tool matrix
| Tool | Equivalence | Expected result | Evidence |
|---|---|---|---|
| nova | Exact replay | Only the documented uniform offset moves; channel distributions retain their shape. | nova-pedestal-source, nova-m66-run-1.24.7 |
| pixinsight | Algorithmically equivalent | A uniform shift by the confirmed amount without distribution-shape or color damage. | m66-manual-verification |
| siril | Algorithmically equivalent | Means and medians shift by the scalar while sigma/MAD and distribution shape remain stable. | siril-calibration-1.4.4, m66-manual-verification |
| saspro | Conceptual substitute | Each channel minimum moves to zero while spatial structure is translated by that channel's own amount. | saspro-source-1.18.0, saspro-synthetic-2026-08-14 |
NOVA Python pipeline 1.24.7
- Inspect capture and calibration provenance for an explicitly documented remaining constant offset.
- Leave the step skipped when that evidence is absent; this is the default.
- If force_apply is authorized from real provenance, record the input, uniform amount, and before/after channel statistics.
Controls and starting ranges
- default action
- skip
- force_apply
- false; true only with documented calibration evidence
- subtraction amount
- the confirmed offset; never infer from global minimum alone
Mask behavior: No mask: a true constant offset is a full-image calibration property.
Failure modes
- A single cold pixel determines the amount.
- Legitimate sky background is forced to zero.
- Shadow clipping increases or channel balance changes.
Recovery
- Revert to the unchanged linear input.
- Recalibrate from subframes when the failure originated during calibration.
PixInsight PixelMath exact version not recorded; procedure tool-tested
- Confirm a residual uniform offset from calibration provenance; otherwise skip.
- In PixelMath subtract that measured scalar from $T with rescaling disabled.
- Apply consistently to the intended channels and inspect Statistics before and after for minima, medians, and clipping.
Controls and starting ranges
- expression
- $T - confirmed_offset
- rescale result
- disabled
- offset
- measured calibration value; no generic starting number
Mask behavior: No mask for a confirmed full-frame constant offset.
Failure modes
- Using min($T) as proof instead of merely as arithmetic.
- Using a sky median as the offset.
- Per-channel subtraction changes color balance.
Recovery
- Undo and return to the original linear image; recalibrate if clipping occurred earlier.
Siril Pixel Math and calibration 1.4.4 documentation; procedure tool-tested in 1.4.2
- Prefer correct bias/dark/flat calibration and inspect the calibration statistics before considering a later subtraction.
- Skip post-stack subtraction unless provenance confirms a residual offset.
- For the demonstrated manual arithmetic, subtract one confirmed scalar in Pixel Math and compare channel statistics before and after.
Controls and starting ranges
- post-stack scalar
- confirmed offset only; no generic starting number
- display normalization
- disabled while comparing numeric statistics
Mask behavior: No mask for calibration-wide offset handling.
Failure modes
- Treating the lowest channel minimum as an identified pedestal.
- Clamping a large part of the real sky background to zero.
Recovery
- Undo; return to the calibrated linear input and diagnose the calibration chain.
Seti Astro Suite Pro Pedestal Removal 1.18.0 source-inspected and synthetic-array tested
- Treat target class as irrelevant: decide from calibration provenance and linear-image statistics.
- Skip unless each channel's residual offset is independently established; a positive minimum alone is not evidence.
- If applied, record each channel minimum and compare distributions, clipping, and color before and after.
Controls and starting ranges
- operation
- per-channel minimum subtraction; no adjustable amount
- default decision
- skip without independent per-channel calibration evidence
Mask behavior: The active full image or preview is processed; this is not a calibration mask operation.
Failure modes
- A cold pixel or legitimate sky sample defines a channel's subtraction.
- Different channel minima alter the established color balance.
- Clipping or earlier calibration damage is mistaken for a removable pedestal.
Recovery
- Undo immediately and return to the unchanged linear image or recalibrate the source frames.
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 pedestal implementation and conditional gate
nas_server/seti_astro.py:3096; nas_server/auto_process.py:655— NOVA source-confirmed - M66 workflow 1.24.7 execution record
critiques/20260802_045251_seestar_galaxy.json— NOVA execution record - Siril 1.4.4 calibration documentation — Vendor-documented
- 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 - SASpro 1.18.0 synthetic-array operation checks
2026-08-14 local validation: pedestal, cosmetic correction, and ADBE— Tool-tested
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.