Seestar S30 & S50 Rejection Rate Analyzer
Enter your session data – the tool prioritizes Top Causes and provides you with the next 1–2 best steps instead of an endless list of tips.
Inputs
Tip: If you are unsure, leave "Auto" active – frames will be estimated from Integration & Exposure (incl. 1s overhead per frame).
Advanced (Setup & Conditions) ⌄
Result
Status: Green < 10%, Yellow 10–25%, Red > 25%. "Confidence" is distributed across the Top-3 causes.
Rejection Rate
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Frames (Accepted / Total)
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Rejected (Count)
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Priority
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Next Best Steps
What do "Rejected Frames" mean in stacking – and how to debug systematically?
In live stacking, the software decides for each individual frame whether it is "good enough" to go into the stack. A rejected frame is therefore not a "broken photo", but a frame that does not pass the quality check – typically because stars cannot be registered cleanly or image sharpness/structure fluctuates too much. The Rejection Rate is thus a practical health indicator for your session: It tells you if the setup, sky, or settings are currently "stable".
Typical reasons with smart astro-scopes are surprisingly mundane: Wind and Vibration (tripod wobbles, ground vibrates), Seeing and low target altitude (air turbulence + more atmosphere near the horizon), Veil clouds (brightness and star profiles change), Exposure too long (every disturbance becomes more visible), Dew/Humidity (contrast drops, stars bloat) and Moon/Light Pollution (background becomes bright, details get drowned out). Important: Often it is not "one" factor, but a multiplier – for example, 20s exposure in light wind is often worse than 10s exposure in the same wind because the movement per frame has a stronger effect.
Debugging works most reliably when you change only 1–2 variables and observe the rejection rate. Start with the biggest leverage: Stability → Sky → Settings → Restart/New-Alignment. Specifically: First check if the tripod is really standing still (no soft grass, no loose clamps, avoid cable drag). If wind is noticeable, lower the exposure significantly (e.g., from 20s to 10–12s) and compensate with more total time. Then comes the sky: With veil clouds, the rate can jump sharply, even if "it doesn't look that bad". Short pauses, a restart of the stack, or simply: aborting the session if it becomes obvious, help here.
S30 vs. S50 in practice: In practice, a narrower field of view/more "zoom" often acts more sensitively to small movements and bad seeing. This does not mean that one model is "worse" – but that you benefit faster from shorter exposures and very stable setup in difficult conditions. A somewhat wider image angle often forgives slight drift/wobbles. Therefore, it makes sense with the S50 (and generally with more detail/magnification) to expose more conservatively when wind or low target altitude are involved.
A simple example: Exposing 20s in wind often leads to oblong stars or frames that do not register cleanly – the rejection rate rises, the stack becomes sluggish. If you switch to 10s exposure stably (or stabilize the setup), registration remains clean; you get more frames, but above all more usable ones. The target is not to expose "as long as possible per frame", but to achieve the highest usable yield per minute. This is exactly what the rejection rate is an excellent signal for.
FAQ
1) What rejection rate is "normal"?
2) Are rejected frames "lost"?
3) Why do rejects rise so sharply at low target altitude?
4) Alt-Az or EQ: Which is more "robust"?
5) What is the fastest fix for wind?
6) How do I recognize veil clouds as the cause?
7) Dew/Humidity: What to do if stars get soft?
8) High Moon – is it worth it at all?
9) Why can "Strict" harm the quality check?
Seestar Rejection Rate Analyzer: Find the Root Cause Fast
A high frame rejection rate in the Seestar's live stacking wastes precious imaging time and degrades the final image quality. This analyzer takes your session inputs — exposure time, sky conditions, temperature, mount setup, and more — and outputs the top probable causes ranked by confidence score, plus one or two prioritized action steps to fix each.
- Enter Session ParametersModel (S30/S50), exposure time (s), observed rejection rate (%), moon phase or sky brightness (Bortle estimate), temperature (°C), humidity (%), wind speed (Beaufort), and whether you are in alt-az or EQ mode.
- Analyzer Scores Each CauseA confidence score (0–100%) is assigned to each of 8 possible root causes based on the combination of your inputs. For example, a 70% rejection rate + high humidity = dew as the top-scored cause; 70% rejection rate + strong wind + short exposure = vibration as top cause.
- Top Causes Displayed with ConfidenceThe top 2–3 causes are shown with their confidence percentage and a brief explanation of why that cause is likely given your specific inputs.
- Prioritized 1–2 Action StepsEach identified cause comes with exactly 1–2 concrete actions — not a generic checklist, but the specific fix for your combination of symptoms. Example: "Activate dew heater to Medium — do not wipe the lens."
The 8 Root Causes of High Rejection Rate
| Cause | Typical rejection rate | Key indicator inputs | Primary fix |
|---|---|---|---|
| Dew / Condensation on lens | Progressive rise to 80–100% | High humidity (>80% RH), temperature near dew point | Activate dew heater; pause session to let lens clear indoors |
| Polar alignment error (EQ mode) | 40–70%, worsens with longer exposure | EQ mode + long exposure + consistent N-S or E-W trailing | Redo compass calibration; check wedge tilt angle |
| Wind / Vibration | Sporadic 30–60% | Beaufort 3+, lightweight tripod, unstable ground | Add weight to tripod; use windbreak; wait for calmer conditions |
| Field rotation (alt-az) | Increases with session length | Alt-az mode + exposure >20s + object near zenith | Shorten exposure to 10–15s; switch to EQ mode for long sessions |
| Satellite / Aircraft trails | Sporadic 5–20% | Clear sky + low rejection on most frames + sudden spikes | Increase sigma-clipping threshold in app; normal and unavoidable |
| Cloud transparency variation | Sporadic 20–50% | Partly cloudy forecast; brightness variability between frames | Wait for clear night; use the rejected frames to identify cloud windows |
| Focus drift (thermal) | Gradual increase over 1–2 h | Large temperature drop (>5°C during session); blurred frames | Re-run autofocus; enable periodic refocus if available in app version |
| App stacking threshold too strict | Consistently >50% even in good conditions | Good sky + stable setup + still high rejection | Lower the rejection sensitivity threshold in Seestar app Settings → Stacking |
What Is a "Normal" Rejection Rate for the Seestar?
<20% — Excellent
Very good conditions, stable setup, optimal calibration. Typical for dry clear nights, good polar alignment, and no wind.
20–35% — Normal
Expected in typical conditions. Occasional satellite trails, minor atmospheric seeing fluctuations, and light wind contribute to this range even on good nights.
35–55% — Investigate
Elevated — likely one identifiable issue (wind, humidity, alignment). Worth analyzing inputs and correcting before continuing the session.
>55% — Stop & Fix
More than half your data is being discarded. The session is effectively wasted — identify and fix the root cause before resuming. Running longer will not help.
Frequently Asked Questions
Where do I find the rejection rate in the Seestar app?
During a live stacking session, the Seestar app shows a running frame counter in the format "Stacked: X / Total: Y" or as a percentage indicator in the session overlay. The rejection rate = (Y − X) / Y × 100%. In some app versions this is shown directly as a percentage next to the stack count. If your app version does not show it explicitly, calculate it manually from the stacked vs. attempted frame counts shown at the end of the session in the session log.
Can a high rejection rate actually improve my final image?
Paradoxically, a moderate rejection rate (20–35%) can indicate that the Seestar's stacking algorithm is working correctly — it is successfully identifying and discarding poor-quality frames (blurred by seeing, trailed by wind, dimmed by thin cloud). The concern is when rejection is so high that the stacked image has too few frames to achieve good signal-to-noise ratio. As a rough rule: 200+ accepted frames at 10s each (33+ minutes effective exposure) produces a good result for bright targets; faint nebulae benefit from 500+ accepted frames.
Does rejection rate differ between S30 and S50?
At identical conditions and the same exposure time, the S50 typically shows slightly higher rejection rates than the S30. This is because the S50's longer focal length (250mm vs. 120mm) magnifies tracking errors, vibration, and seeing effects that the S30 partially averages out with its wider field. In practice, the difference is 5–10 percentage points on a typical night. This is why experienced S50 users pay more attention to polar alignment quality and wind conditions than S30 users — the narrower field is less forgiving.
My rejection rate is fine but the final stacked image still looks poor — why?
Low rejection rate confirms that individual frames are being accepted by the stacking algorithm, but image quality also depends on: (1) Sky quality (Bortle class) — light pollution limits the faintest detail visible regardless of frame count. (2) Total integration time — even 5% rejection with only 30 minutes of imaging is insufficient for faint targets. (3) Seeing conditions — atmospheric turbulence blurs individual stars even in accepted frames. (4) The target itself — some objects (large galaxies, faint nebulae) require multiple hours of integration even under dark skies. Use the exposure time recommendations in the Seestar app or the FoV tool on this site for target-specific guidance.
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