Astrophotography diagnosis of NGC 281W: Tracking drift
Raw300s31 juil. 2026
The Doc examined this image of NGC 281W (raw, 300s). Estimated overall technical quality: 6/10. 1 defect found: Tracking drift (severity 2/5).
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Info
- Cible
- NGC 281W
- Date
- 31 juil. 2026, 22:01
- Lune
- Croissant croissant 5.3% (14.3° d'alt., 73.2° de la cible)
- Site
- Bortle 2 · rural typique (VIIRS)
- Position
- 0h54m09s · +56°36'40"
Excellent conditions for this target: Bortle 2 and Moon at only 5.3%, 14° above the horizon and 73° from the target, so lunar pollution is negligible. For an emission nebula like NGC 281 shot in Hα, these conditions are ideal: maximum contrast, minimal background gradient. The sky-to-black-point margin (e_margin 2.15) may look low but that is normal in narrowband even at 300s, the sky being naturally dark. Nothing to fault about the choice of night.
Setup
- Type d'image
- Brut
- Télescope
- EQMod Mount
- Caméra
- ZWO ASI585MM Pro
- Exposition
- 300s
- Phase de lune
- Gibbeuse décroissante (93 %)
- FOV
- 1.70°
Setup consistent with the target. At 375mm focal length and 1.70° field, NGC 281 (about 35') fits comfortably with margin, well-centered framing. The 1.595"/px sampling is comfortable and well matched to typical seeing, neither over- nor under-sampled, confirmed by the tight 2.3 px FWHM. The cooled mono ASI585MM at -13.5°C is healthy (dark current under control). Gain 252 is high but the 300s exposure is consistent in narrowband; offset 15 is sufficient, no dark clipping (0%). Settings are overall well chosen.
The diagnosis in detail
The image is a well-made Hα sub. The PSF panel shows fine, regular stars everywhere: FWHM around 2.3 px, corner/center ratio of just 1.1 and low horizontal/vertical asymmetry (0.04/0.02). No sign of tilt, backfocus, coma or astigmatism: the optics and focus are clean, and the Moffat ellipse fits each star well.
The only real defect is slight tracking drift. The central elongation (1.18) clearly exceeds the field floor (1.08) and appears identically at center and edges, with a very consistent PA (11.3° spread, orientation ~30-35°). This signature, present at the center where no optical aberration is at play, points to a directional mount motion during the exposure rather than guiding oscillation or periodic error. The amplitude stays moderate and the image is perfectly usable in a stack.
On the background side, the map notes a slight plane gradient (14%, R² 0.73) but under a Bortle 2 sky with a negligible Moon it will be trivially removed in processing (GraXpert/DBE) and is not a defect at this stage. In short: a very good sub, only the tracking needs a tweak.
Priority actions
- Fix the tracking drift: check polar alignment then tune or enable autoguiding for the 300s exposures
- If unguided, shorten the sub length to keep stars round
- Remove the slight background gradient in processing (GraXpert/DBE) after stacking
