Astrophotography diagnosis of NGC 6995: Backfocus error
RawHa300s
The Doc examined this image of NGC 6995 (raw, Ha, 300s). Estimated overall technical quality: 7/10. 1 defect found: Backfocus error (severity 3/5).
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Info
- Cible
- NGC 6995
- Site
- Bortle 2 · rural typique (saisi)
- Position
- 20h57m02s · +31°18'21"
Full moon at 100% but a Bortle 2 sky and an Ha filter: this is exactly where that combination works. The background map confirms it, zero radial falloff and a plane gradient limited to 1% amplitude, so the moon barely registers on this sub. On the Veil, an emission target rich in Ha, you can shoot full-moon nights without regret; save moonless nights for OIII, which is far more sensitive to lunar sky background. The reported seeing translates into 2 px FWHM at center, which is excellent and lets the setup deliver its full resolution.
Setup
- Type d'image
- Brut
- Télescope
- Lunette 100mm x 740 mm
- Caméra
- IMX 571 MC
- Filtre
- Ha
- Monture
- Taka EM200
- Exposition
- 300s
- Phase de lune
- Pleine lune (100 %)
- Seeing
- 2"
- FOV
- 1.83°
Broadly coherent. The 740 mm refractor with the IMX571 gives a 1.83 degree field, enough to frame NGC 6995 comfortably, but here the nebula is pushed against the right edge while the whole left half is empty: shift the pointing about one third of a field east to center it. Sampling is healthy, the 2 px central FWHM is neither over nor undersampled. On the histogram, the sky-to-black-point margin is 2.29, low in absolute terms, but that is normal for narrowband Ha even at 300s, with no dark clipping and no saturation: nothing to change on gain or offset.
The diagnosis in detail
The sub holds up well. The PSF measurement gives a center at 1.98/1.92 px for an elongation of 1.03, right at the field floor: no drift, no periodic error, no guiding oscillation over 300s. With a full moon above the horizon, the measured background stays nearly flat, the Ha filter does the heavy lifting and the Bortle 2 sky does the rest.
The single defect is optical and peripheral. FWHM goes from 2 px at center to 4.6 px on the left side and 3.4 px at bottom left, a factor of 1.64, with stars bloating into ovals without forming a clear radial comet tail. That is the signature of uncorrected field curvature, in other words a spacing error behind the flattener. The horizontal asymmetry of 0.63 further indicates the left side suffers more than the right, so a slight tilt is probably layered on top of the spacing issue.
Practical consequence: on a stacked integration those edges will stay soft and you will effectively lose a peripheral band. Fixing backfocus before stacking more nights is the highest-return move here, the rest of the chain is already good.
Priority actions
- Adjust backfocus in 0.5 to 1 mm steps, checking corner FWHM on an aberration panel at each try
- If the left/right difference remains after spacing is set, correct sensor tilt with a tilt adapter
- Shift the pointing about one third of a field east to place NGC 6995 in the middle of the frame
- Use moonless nights for the OIII layer, which is far more sensitive to sky background
