Astrophotography diagnosis of IC 1396: Astigmatism and sensor tilt
ProcessedAntlia HSORGB
The Doc examined this image of IC 1396 (processed, Antlia HSORGB). Estimated overall technical quality: 8/10. 2 defects found: Astigmatism (severity 2/5), Sensor tilt (severity 2/5).
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Info
- Cible
- IC 1396
- Position
- 21h38m60s · +57°30'00"
Verdict: tough conditions handled very well. A 100% full moon is normally a deal-breaker in broadband, but on an emission target like IC 1396 shot in narrowband (SHO) the background stays dark and well clear of the black point (median 0.16, high e_margin, 0% shadow clipping). The HSORGB choice is exactly the right strategy here: the Ha/OIII/SII signal is barely affected by the moon, only the stellar RGB layers suffer a little. No residual moon gradient visible, the preprocessing handled the background well.
Setup
- Type d'image
- Traitée
- Télescope
- Redcat 91
- Caméra
- ZWO ASI2600MM Duo
- Filtre
- Antlia HSORGB
- Monture
- AM5N
- Phase de lune
- Full moon (100 %)
- FOV
- 1.20°
Verdict: coherent, well-matched setup. The Redcat 91 (1.20 deg field) frames the Elephant's Trunk region comfortably with nebular margin all around, a balanced composition. With the ASI2600MM (3.76 um) sampling lands near 2 arcsec/px, slight undersampling but very reasonable, and the measured FWHM around 1.5 px confirms good optics/seeing agreement. The Antlia HSORGB filter is the right pick to face the full moon on this emission target. AM5N mount flawless on tracking (shape dispersion is optics-driven, not mount-driven).
The diagnosis in detail
On the acquisition side this image is very strong. Moffat metrics give homogeneous FWHM around 1.5 px and round stars in all four corners (elong 1.03 to 1.09), with a field floor at 1.02: the AM5N tracking is excellent (no drift, no oscillation) and the Redcat optics are largely well set. The high elong shown at center (1.29) is a measurement artifact, the test star falling on the bright Trunk nebulosity, where the ellipse fit is biased and does not reflect a real central defect.
The only two attention points are peripheral and mild. Elongation rises at the left and right edges (1.18 and 1.14) without a large FWHM increase, a discreet field-astigmatism signature, and a uni-axial asymmetry (right edge FWHM 1.87 versus 1.42 on the left) betrays a slight focal-plane tilt. Both are marginal and do not affect image readability; they are a mechanical fine-tuning of the optical train, not an acquisition problem.
Processing is well controlled: balanced SHO palette, RGB stars reintegrated with natural hues, clean background with no moon gradient despite the full moon. Nothing to criticize on the post-processing side.
Priority actions
- Check the stress-free tightness and flatness of all threaded couplings in the optical train to reduce edge astigmatism and tilt
- Verify the backfocus distance to within 0.1 mm on the Redcat 91
- Run a dedicated star-field aberration test to confirm and quantify the slight right-side tilt before adjusting a tilt plate
