DocStellarBeta

Astrophotography diagnosis of IC 405: Tracking drift

RawHa300s06 déc. 2023

The Doc examined this image of IC 405 (raw, Ha, 300s). Estimated overall technical quality: 7/10. 1 defect found: Tracking drift (severity 2/5).

Annotated image
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Info

Cible
IC 405
Date
06 déc. 2023, 08:29
Position
5h16m38s · +34°26'31"

Not enough information for an opinion from the Doc.

Setup

Type d'image
Brut
Télescope
Moana
Caméra
ASI Camera (1) (ASCOM)
Filtre
Ha
Exposition
300s
FOV
45.9'

The 0.77° wide field frames IC 405 well: the nebula sits centered and spills slightly to the right, a coherent composition with no edge cutting the subject. Sampling yields stars at FWHM 3.0-3.7 px, usable but slightly bloated for the available resolution, partly due to the tracking drift. The Hα filter suits this emission nebula and keeps the background well controlled. Gain 90 and a -20°C sensor are healthy settings; the 300s exposure is appropriate in narrowband but seems to exceed what unguided tracking holds, hence the elongation. Shorter subs or guiding would fix this.

- the Doc

The diagnosis in detail

The aberration inspector is clear: elongation is present AT THE CENTER (elong 1.29, above the field floor of 1.19) with a near-identical PA across all nine zones (3.2° spread, around -89°) and a low corner/center FWHM ratio (1.12). This signature, a global constant-direction deformation rather than radial or diagonal edge degradation, rules out tilt, backfocus, coma and collimation, and points to a tracking/guiding drift. The streak is regular and stably oriented, which favors a drift rather than guiding oscillation or an S-shaped periodic error.

On the background side, the background map shows a very slight edge darkening (+7%, low R² 0.47) and a weakly structured 7% micro-gradient: nothing firm, consistent with an uncalibrated sub, so no background defect retained. The histogram confirms a well-exposed Hα sub (no dark clipping or saturation, e_margin 1.69 normal for narrowband at 300s). The filamentary nebulosity is real, confirmed by the DSS.

Overall this is a good sub: the only useful action concerns tracking to tighten the stars before stacking.

Priority actions

  1. Fix the tracking drift: refine polar alignment and/or enable autoguiding
  2. Shorten the sub exposure if tracking cannot hold 300s without drift
  3. Check balance and optical train flexure

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