Astrophotography diagnosis of NGC 7380: Tracking drift
MasterO180s23 sept. 2026
The Doc examined this image of NGC 7380 (master, O, 180s). Estimated overall technical quality: 6/10. 1 defect found: Tracking drift (severity 3/5).
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Info
- Cible
- NGC 7380
- Date
- 23 sept. 2026, 22:00
- Lune
- Croissant décroissant 8.6% (33.5° d'alt., 72.4° de la cible)
- Site
- Bortle 2 · rural typique (VIIRS)
- Position
- 22h47m36s · +58°05'24"
Very favourable conditions. Bortle 2 gives a near-ideal sky background, which the background map confirms: no significant oriented gradient (4% amplitude, R squared 0.11), just a mild radial falloff not flagged as a defect. The Moon is a waning crescent at 8.6% illumination, 33.5 degrees high and 72.4 degrees from the target: at that level its contribution is negligible, all the more so behind a narrow OIII filter. In short, nothing in the sky is holding this acquisition back, the quality budget rests entirely on tracking mechanics and cumulative integration time. This is a night worth spending on long OIII layers, which are precisely the most signal-hungry.
Setup
- Type d'image
- Master
- Télescope
- EQMod Mount
- Caméra
- ZWO ASI2600MM Air
- Filtre
- O
- Exposition
- 180s
- Phase de lune
- Gibbeuse croissante (92 %)
- FOV
- 1.49°
Coherent setup for NGC 7380. The 1.49 degree field frames the Wizard Nebula comfortably, filling about a third of the frame, with room for the faint extensions visible in the DSS. Sampling at 0.93"/px with the ASI2600MM at 835mm is slightly oversampled for typical seeing, which is not harmful in itself but makes any tracking drift immediately visible, exactly what the PSF panel shows. On exposure, an e_margin of 2.25 would flag underexposure in broadband, but in narrow OIII a background sitting that close to the black point is normal under Bortle 2: keep the 180s subs, or even extend them to 300s since the sky is not the limiting factor.
The diagnosis in detail
The diagnosis comes down to one point: the stars are not round, and the cause is not optical. The Moffat measurement is unambiguous, elongation reaches 1.31 at the field centre while the instrumental floor measured on this field is 1.22, and the 1.084 corner/centre FWHM ratio is far too low to suggest field curvature, tilt or backfocus error. Horizontal (0.068) and vertical (0.084) asymmetries stay within the noise. A field aberration spares the centre, which is not the case here.
Add to that the directional coherence: 11.2 degrees of PA dispersion across nine zones means a near-unique elongation direction over the whole sensor. That is the classic signature of tracking drift, not of periodic error (which would curve the trails and vary their amplitude) nor of guiding oscillation (zigzag, unstable direction). The slight elongation gradient from top to bottom of the field (1.22 at TR against 1.42 at BL) indicates the drift was not identical across all stacked subs.
Practical consequence: FWHM of 5.6 px, about 5.2 arcsec, is clearly degraded relative to what this Bortle 2 sky allows. By fixing polar alignment and guiding you can reasonably aim for 3.5 to 4 px, which would transform the sharpness of the OIII layer. The rest of the master is healthy, no exploitable flat, dark or gradient residual shows up, and the sky background is remarkably clean.
Priority actions
- Redo a careful polar alignment by drift alignment before the next session, it is the most likely source of the drift
- Recalibrate guiding near the meridian and aim for total RMS under 0.5" given the 0.93"/px sampling
- Check for differential flexure between imaging tube and guider, and for DEC backlash
- Extend sub exposure to 300s once tracking is sorted, the Bortle 2 sky and OIII filter allow it without background saturation risk
- Cull subs on eccentricity before integration to drop the most drifted frames
