Overview

Some ZWO ASI676MC cameras occasionally produce an image with a strong purple or magenta cast between otherwise normal images. indi-allsky can detect and exclude these frames, or repair them automatically after calibration while continuing to capture in RAW16.

This guide covers the built-in Fix ASI676MC purple frames tool. For the correction algorithm, all settings, and detailed calibration requirements, see ASI676MC Purple Frame Repair Details.

RGB24 workaround: Jump directly to the original instructions. The built-in repair described below supports ASI676MC only; it does not support ASI224MC.

Purple Images

The failure affects individual frames. A persistent colour cast on every image is not enough to identify this problem. Leave purple-frame handling off unless your ASI676MC actually produces the intermittent failure.

The same original RAW16 exposure before and after correction with the current repair implementation and its default repair values. Both panels use the same display scale and gamma, without separate white balancing. This illustrates the correction; calibrate using your own camera's frames before enabling it.

The repair corrects an observed one-row displacement and colour-channel imbalance in the raw image, and estimates green values lost in clipped bright areas. It does not fix the underlying camera or driver cause. Reconstructed highlights are estimates, so a repaired frame should not be treated as an untouched measurement. The original investigation is in issue #2007.

Before you begin

• Use a local ASI676MC, capturing RAW16, with RGGB Bayer pattern, 1x1 binning, and zero Bayer offsets. Other formats and layouts are not supported by this repair.
• Use an account that can save configuration settings. With normal login enabled, this means an administrator.
• If your installation does not have these controls, follow Updating indi-allsky.
• This tool is ASI676MC only. Similar-looking problems on an ASI224MC or another camera are outside its supported scope.

Collect calibration frames

1. Open Config > Image and find ASI676MC purple-frame handling.
2. Turn on Enable ASI676MC purple-frame handling.
3. Leave Detect and exclude only on. This keeps detected purple frames unchanged and excludes them from standard timelapses while you collect calibration evidence.
4. Turn on Save purple and following normal FITS for calibration. This saves the original purple FITS and its next compatible frame without requiring every image to be saved as FITS.
5. Optionally turn on Also save the preceding normal FITS. A normal frame on both sides of the purple frame gives better evidence. This option uses about one extra FITS frame of memory.
6. Save the configuration and let the camera collect more frames.

Use daylight captures with visible bright areas, at at least two different exposure settings. The tool needs at least seven purple frames and seven distinct compatible normal references. For each purple frame, include the normal frame immediately before or after it, preferably both, with matching capture settings. Bright areas are needed to calibrate highlight reconstruction; seven purple events alone do not guarantee a successful result.

Standard FITS saving can stay off if you do not need it for another purpose. Diagnostic files follow normal FITS retention, so allow enough time and disk space to retain the evidence until calibration finishes.

If purple frames are not being detected and diagnostic saving misses them, temporarily keep Detect and exclude only on and set standard FITS saving to Every Image. This lets the tool inspect failures outside the current detection thresholds. Occasional FITS saving can miss a random event.

Run calibration

Open Tools > Fix ASI676MC purple frames. Select the ASI676MC if the page offers a camera selector.

Use saved FITS

Choose Use saved FITS to search the camera's retained FITS files. Leave the target at 20 purple-frame groups for a first attempt. The tool finds suitable purple frames and nearby normal references automatically and reports missing or unsuitable evidence. Seven usable groups is the minimum.

Wait for analysis to finish. The progress view shows the current stage, and you can cancel if needed. Searching and calibration leave the original saved FITS files untouched.

Upload a FITS collection

Use Upload a FITS collection when you have collected the files elsewhere. Select the purple and adjacent normal frames together: 14 to 80 uncompressed .fit, .fits, or .fts files, up to 256 MiB per file and 2 GiB in total. The same evidence requirements apply. Fourteen arbitrary files are not enough; the collection needs matching purple and normal captures.

Upload copies are private and deleted after analysis. Your original files remain in their original location.

Understand the result

Analysis does not change your settings automatically. Use Download details to keep a report of the result and any warnings.

Result	What to do
Full calibration	Review the matched frames, exposure coverage, and warnings. If the evidence is suitable, choose Save calibration values, then enable repair as described below.
Detection settings need adjustment	This is a preliminary result, not a repair calibration. Check that the likely-purple files show the actual camera failure, using the previews when available and the filenames and capture times. Confirm this, choose Save detection settings, then Start over and run calibration again. Keep Detect and exclude only on.
Analysis failed	Read the retained explanation, correct the problem or collect more suitable frames, then choose Try again. No calibration settings were applied.

If the tool recommends more complete normal/purple/normal groups or more varied evidence, collect those before relying on the result. Do not change thresholds just to make an unsuccessful analysis pass.

Enable repair and check the results

After saving a suitable full calibration:

1. Return to Config > Image > ASI676MC purple-frame handling.
2. Turn Detect and exclude only off and save the configuration. Saving calibration values alone does not enable repair.
3. Optionally enable Show purple-frame status in gallery to review repaired, excluded, and failed frames and use the corresponding filters.
4. Check subsequent purple events in the gallery and logs. Return FITS-saving options to your preferred long-term settings once you are satisfied.

Repair happens automatically on newly captured frames before normal image processing. The calibration tool does not repair an existing archive of images or regenerate old timelapses.

Frame status	Meaning
Normal	The frame did not match the purple signature and was not changed by this feature.
Excluded	A purple frame was preserved unchanged in exclusion-only mode and omitted from standard timelapses.
Repaired	Correction passed validation and the repaired image continued through normal processing.
Validation failed	Correction could not be validated. The original image was retained and excluded from standard timelapses. Collect its diagnostic FITS group and recalibrate.
Skipped	The input layout or configuration was unsuitable. Check the explanation in the log or Camera notification.

Excluded frames and failed repairs are also omitted from stacking history and automatic exposure control. The feature reports skipped checks and failed repairs through Camera notifications; successful repairs do not create one.

Common questions

Why is the tool missing from Tools?

Enable purple-frame handling and save Config first. A visible local ASI676MC must be available, and your account must be able to save settings.

Why does the search find too few frames?

The capture settings affect future images. Older originals may never have been saved or may already have expired. Collect more diagnostic FITS. If detection misses the purple events, use the every-image FITS fallback described above.

Can I use FITS saved while repair was already enabled?

Untouched diagnostic FITS are suitable. Standard FITS from successfully repaired frames contain the corrected image and cannot serve as purple originals. Save FITS Pre-Calibration refers to dark-frame calibration; purple-frame handling still runs before that save point.

Does each camera get its own calibration?

No. Repair settings are shared across the installation. If you change physical ASI676MC units, review or recalibrate the shared profile. Selecting a camera in the tool does not create a separate profile for it.

For upload limits, rejected evidence, save errors, and further explanations, see the detailed reference.

Older RGB24 workaround

Before the built-in repair was available, this page recommended switching from RAW16 to RGB24, which was reported to reduce the frequency of purple frames. That remains historical workaround information for installations that cannot use the repair. RGB24 is outside the repair tool's supported input formats.

The original INDI configuration example was:

{
    "SWITCHES": {
        "CCD_VIDEO_FORMAT": {
            "on": [
                "ASI_IMG_RGB24"
            ],
            "off": [
                "ASI_IMG_RAW16"
            ]
        }
    },
    "PROPERTIES": {},
    "TEXT": {}
}