Flat Calibration

Detailed documentation of the ALS RemoveFlat process
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Overview

The RemoveFlat process divides each sub by a user-provided master flat to remove optical vignetting, dust motes, and pixel-to-pixel response variations.

Its configuration is managed via the ALS preferences page.

Configuration

Source Data type Required Default value
ON/OFF Preferences: Processing Tab ON/OFF ∅ OFF
Master flat path Preferences: Processing Tab File path Yes ∅

Control

This process is triggered by the Preprocess pipeline.

Input

Data Type
image received from the Preprocess pipeline Image
master flat read from configured path Image

Behavior

graph LR

START([START])

TEST_ENABLED{{Processing enabled?}}
READ_FLAT[Read master flat]
TEST_SHAPE{{Identical dimensions?}}
NORMALIZE[Normalize by maximum value]
SAFEGUARD[Replace zeroes with ones]
DIVIDE[Divide image by normalized flat]
CLIP[Clip to 16-bit range]
RETURN[Return calibrated image]
UNCHANGED[Return unchanged image]

END([END])

START --> TEST_ENABLED

TEST_ENABLED ----->|No| UNCHANGED
TEST_ENABLED -->|Yes| READ_FLAT

READ_FLAT ----->|Failed| UNCHANGED
READ_FLAT -->|Ok| TEST_SHAPE

TEST_SHAPE ----->|No| UNCHANGED
TEST_SHAPE -->|Yes| NORMALIZE

NORMALIZE --> SAFEGUARD
SAFEGUARD --> DIVIDE
DIVIDE --> CLIP
CLIP --> RETURN

RETURN --> END
UNCHANGED --> END

classDef bounds fill: #333, stroke: #666, stroke-width: 2px, color: #BBB, font-family: 'Poppins', sans-serif
classDef step fill: #444, stroke: #622, stroke-width:2px, color: #c6c6c6, font-family: 'Poppins',sans-serif
classDef wait  fill: #444, stroke: #262,stroke-width: 2px, color: #c6c6c6, font-family:'Poppins', sans-serif
classDef test fill: #444, stroke: #226, stroke-width: 2px, color: #c6c6c6, font-family: 'Poppins', sans-serif

class TEST_ENABLED,TEST_SHAPE test
class START,END bounds
class RETURN,UNCHANGED,READ_FLAT,NORMALIZE,SAFEGUARD,DIVIDE,CLIP step

The master flat is loaded from disk, normalized by its maximum value, and any zero entries are replaced with ones before dividing the sub.

  • If the master flat cannot be read or its dimensions differ from the sub, the division is skipped and the unmodified sub is returned to the Preprocess pipeline.
  • After division the pixels are clipped to the 16-bit range and converted to unsigned 16-bit integers to maintain downstream compatibility.

Output

The calibrated sub is sent back to the Preprocess pipeline.

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