Astrophotography diagnosis of NGC 6992: Astigmatism
RawHOO 1 composite14160s
The Doc examined this image of NGC 6992 (raw, HOO 1 composite, 14160s). Estimated overall technical quality: 6/10. 1 defect found: Astigmatism (severity 3/5).
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Info
- Cible
- NGC 6992
- Site
- Bortle 6 · banlieue dense (VIIRS)
- Position
- 20h55m00s · +31°06'00"
Favorable conditions for this target. In Bortle 6 the Veil, an emission nebula, is a great match for an HOO/dual-band composite that crushes light pollution: the measured background is flat (gradient amplitude 0%) with no color cast, proof the filter is doing its job. The last-quarter Moon at 7% is negligible and introduces no lunar gradient. Nothing to fix on the sky side: it is a usable night for this kind of object, and you can stack many subs without worrying about the sky background.
Setup
- Type d'image
- Brut
- Télescope
- Skywatcher 200/800
- Caméra
- FUJIFILM XT-3
- Filtre
- HOO 1 composite
- Monture
- EQ6R
- Exposition
- 14160s
- Phase de lune
- Dernier croissant (7 %)
- Seeing
- 1,6°
- FOV
- 1.30°
Setup consistent with the target. The 200/800 Newtonian on the EQ6-R yields a 1.30° field that frames the NGC 6992 arc nicely with margin, and the 0.749"/px sampling is comfortable, even slightly fine for average seeing: at this scale any optical imperfection shows, which is why the edge astigmatism reads clearly. The coma corrector is essential on this F/4, and its adjustment (backfocus to the millimeter) drives corner quality, the main lever here. The HOO filter is the right choice for this emission supernova remnant.
The diagnosis in detail
The image is fundamentally sound. The PSF panel confirms a nearly round center (elong 1.11 against a field floor of 1.07) and controlled tracking: PA dispersion stays low and the elongation is not uniform across the field, which rules out tracking drift, PE, or guiding oscillation. The EQ6-R is doing its job on this sub.
The only real defect is optical and purely edge-based: elongation climbs toward the corners (1.5 to 1.64) while FWHM barely rises (corner/center ratio 1.135). This signature, stretching without spreading and major axes that rotate from corner to corner, is that of field astigmatism, typically tied to coma-corrector adjustment on a finely sampled F/4 Newtonian. It is neither tilt (no marked diagonal asymmetry) nor coma (no symmetric radial comet tail).
In practice the core of the field, where the nebula sits, is clean, so the image remains fully usable. Before accumulating the rest of the session, the priority is to fine-tune backfocus and collimation to tighten the corners; failing that, a slight crop will hide the residual.
Priority actions
- Precisely set the coma corrector backfocus (nominal spacing to the millimeter) to tighten edge stars
- Redo a careful collimation of the Newtonian before the next session
- Check for play/tilt in the optical train (corrector, adapter, sensor)
- In the meantime, slightly crop the extreme corners in processing if astigmatism remains visible
