Astrophotography diagnosis of SH2-115: Clipped stars
ProcessedAntlia ALP-T Dualband 3nm Ha & OIII 2"70×300s22 mai 2026
The Doc examined this image of SH2-115 (processed, Antlia ALP-T Dualband 3nm Ha & OIII 2", 70×300s). Estimated overall technical quality: 8/10. 1 defect found: Clipped stars (severity 2/5).
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Info
- Cible
- SH2-115
- Date
- 22 mai 2026, 00:00
Moon at 39% on acquisition night: moderate illumination, so reasonable background management for an emission target shot in 3nm dualband. The Antlia ALP-T 3nm filter effectively rejects lunar and urban light pollution, which explains the well-controlled background despite the Moon. On an extended Ha/OIII nebula like SH2-115, these conditions are perfectly usable: you did not need to skip the session. To push the OIII signal further (more fragile against the Moon), aiming for nights near new Moon would still be preferable, but it is not a constraint here.
Setup
- Type d'image
- Traitée
- Télescope
- Askar SQA106
- Caméra
- Player One Poseidon-C Pro
- Filtre
- Antlia ALP-T Dualband 3nm Ha & OIII 2"
- Monture
- iOptron HEM27 Strain Wave Mount
- Exposition
- 70×300s
- Phase de lune
- 39%
The Askar SQA106 (short focal length, wide field) and Poseidon-C Pro combo is very coherent for SH2-115: the target and its neighbors (SH2-116, Abell 71) fit comfortably in the frame with breathing room, generous and well-centered framing on the region. The sampling yields very tight stars (FWHM ~1.5 px), a sign of a well-matched setup and excellent tracking on the HEM27 strain wave despite the lack of classic autoguiding on this mount type. The ALP-T 3nm dualband is the ideal choice for this emission nebula. 70x300s provide a solid integration of nearly 6h.
The diagnosis in detail
The PSF panel confirms a top-tier optical and mechanical result: FWHM between 1.42 and 1.73 px depending on zone, central elongation of 1.06 right at the field floor (1.05), corner/center FWHM ratio of 0.87 (corners are even tighter than the center) and negligible horizontal/vertical asymmetries (0.059 / 0.034). No sign of tilt, backfocus, coma, astigmatism or tracking error: stars are round everywhere, validating the collimation, backfocus of the SQA106 and the tracking quality of the HEM27.
The only point for improvement concerns the cores of the brightest stars, clipped to white-blue with no central chromatic gradient. This is minor on this kind of nebular field and easily handled via stellar HDR or light highlight control with a mask.
The background is healthy: the measured background variation (+9%) corresponds to the real nebulosity, not a gradient or vignetting, and the histogram shows no dark clipping (correct offset). The low background/black-point margin (e_margin 1.31) is normal and expected in 3nm narrowband, where the background stays naturally low even at 300s. Nothing to fix on that side.
Priority actions
- Recover bright star cores via a few short exposures (stellar HDR) or a star mask in processing
