Exposure Modes & Auto-Gain

The Exposure Mode subsystem controls how indi-allsky regulates sensor exposure duration and analog/digital gain across varying daylight and nocturnal sky conditions.

Configuration parameters are located in the web interface under Configuration → Exposure.

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1. Operating Modes

A. Basic Mode

In Basic mode, camera gain remains fixed at static preset values for each discrete astronomical solar/lunar state, with image brightness regulated strictly by adjusting exposure duration:

• Day Mode: Sun altitude $> -6.0^\circ$ relative to the horizon.
• Night Mode: Sun altitude $\le -6.0^\circ$ relative to the horizon.
• Moon Mode: Triggered when in Night Mode, the Moon is $> 0.0^\circ$ above the horizon, and lunar disk illumination exceeds the configured threshold (default: $> 33%$).

Basic mode is appropriate for sensors with narrow dynamic ranges or setups where constant gain is required for standardized photometric comparison.

B. Auto-Gain with Exposure Priority (autogain_exposurepriority_dB.py)

This algorithm optimizes signal-to-noise ratio (SNR) by prioritizing exposure duration over gain. It increases exposure time until reaching the configured maximum night exposure limit (e.g., 30–60 seconds). Only when maximum exposure is reached does the algorithm increase gain to maintain target frame brightness (mean ADU). Conversely, as the sky brightens, gain is reduced to baseline before exposure time is curtailed.

Because different camera manufacturers configure gain on distinct mathematical scales, select the matching sub-mode:

Sub-Mode	Unit / Increment	Camera Platforms
1/10 dB	0.1 dB increments	ZWO ASI, Player One Astronomy
1/100 ISO	1/100 nominal ISO	Raspberry Pi libcamera (typical range: 1.0 to 16.0 / 22.26)
Native ISO	Direct ISO values	ToupTek, Altair Astro, Omegon, Ogma, RisingCam
Native dB	Direct dB values	Select QHY models

Consult the Camera Gain Reference for vendor-specific gain ranges and unit mappings.

C. Legacy Auto-Gain (legacy_autogain.py)

Divides the total camera gain range into linear increments once exposure thresholds are crossed. Because it assumes a linear gain curve, it performs sub-optimally with non-linear ISO-based sensors. It is retained solely for backward compatibility and is not recommended for new deployments.

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2. Key Parameter Reference

Parameter	Type	Default	Description
TARGET_MEAN_DAY	Integer	128	Target mean image brightness (0–255 scale in 8-bit, scaled in 16-bit) during daylight.
TARGET_MEAN_NIGHT	Integer	35	Target mean image brightness during night capture.
EXPOSURE_MIN_DAY	Float	0.0001	Minimum allowed daytime exposure duration in seconds.
EXPOSURE_MAX_DAY	Float	1.0	Maximum allowed daytime exposure duration in seconds.
EXPOSURE_MIN_NIGHT	Float	1.0	Minimum allowed nighttime exposure duration in seconds.
EXPOSURE_MAX_NIGHT	Float	60.0	Maximum allowed nighttime exposure duration in seconds.
EXPOSURE_STEP_DAY	Float	0.001	Exposure adjustment step size for daylight transitions.
EXPOSURE_STEP_NIGHT	Float	0.5	Exposure adjustment step size during nighttime transitions.