Astrophotography diagnosis of IC 1805: Sensor tilt
Rawl-extreme27×300s
The Doc examined this image of IC 1805 (raw, l-extreme, 27×300s). Estimated overall technical quality: 6/10. 1 defect found: Sensor tilt (severity 3/5).
&w=1920&q=75)
Info
- Cible
- IC 1805
- Site
- Bortle 4 · transition rural/banlieue (saisi)
Bortle 4 with a last-quarter moon at 46 percent: that is a workable combination for IC 1805, an emission nebula dominated by Ha, and the L-eXtreme blocks most of the diffuse lunar glow. The measured background confirms this: no detectable gradient, zero plane amplitude, which is rare under moonlight and validates the filter choice. Under a sky of this quality, 300 s subs remain sensible. If you want more signal, save moonless nights for broadband layers and keep this filter plus moon pairing for Ha, where the penalty stays small.
Setup
- Type d'image
- Brut
- Télescope
- askar fra400 with 0,7x reducer
- Caméra
- ZWO ASI2600MC Pro
- Filtre
- l-extreme
- Monture
- am5n
- Exposition
- 27×300s
- Phase de lune
- Last quarter (46 %)
Well-matched combination for the target: the FRA400 reduced to 280 mm of focal length on the ASI2600MC APS-C sensor gives a wide field that comfortably frames IC 1805, a very extended object, and the sampling of roughly 2.8 arcsec per pixel is reasonable under typical seeing, even if it slightly undersamples the PSF. The 300 s subs with the L-eXtreme are consistent: the very dark measured background (significant dark clipping) is normal in narrowband and does not indicate underexposure. The weak point is not optical but mechanical: a train with a reducer demands strict backfocus and flatness, which the tilt measurement confirms.
The diagnosis in detail
The diagnosis is simple and well isolated. The PSF panel shows an exemplary center, FWHM 3.94/3.85 px for an elongation of 1.02, which is the field floor: this immediately rules out any tracking, vibration or global focus component. The AM5N does its job over 300 s, and the seeing that night was decent.
The degradation is strictly peripheral and asymmetric. Both left corners climb to 6.06 and 6.21 px FWHM with elongations of 1.26 to 1.30, while the bottom-right corner stays at 4.73 px and 1.07. The measured horizontal asymmetry (0.101) outweighs the vertical one (0.019) by a factor of five: this is not symmetric field curvature, it is a tilt of the sensor plane about the vertical axis. The 34 degree PA dispersion, incompatible with a coherent tracking elongation, confirms a geometric rather than temporal origin.
The rest is healthy: the measured background is perfectly flat, with no oriented gradient or radial falloff, a good sign for both the sky and the future flat. The priority is therefore single and mechanical: restore the flatness and backfocus of the train before stacking more, because this defect cannot be fixed in processing.
Priority actions
- Tighten and check the flatness of the whole imaging train, starting with the camera to reducer joint, the most likely source of the tilt
- Measure the 0.7x reducer backfocus precisely with calibrated spacers before concluding it is pure tilt
- Fit an adjustable tilt plate and correct the left-right axis first, iterating on short subs analyzed with an aberration inspector
- Aim for a corner/center FWHM ratio below 1.15 before launching another long session on IC 1805
