Astrophotography diagnosis of ngc6992: Backfocus error
Processed
The Doc examined this image of ngc6992 (processed). Estimated overall technical quality: 7/10. 1 defect found: Backfocus error (severity 2/5).
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Info
- Cible
- ngc6992
- Position
- 20h54m48s · +31°41'60"
Last-quarter crescent Moon at 4%: negligible lunar illumination, you can ignore it. On an emission target like the Cygnus Veil shot in Hα, the sky background is very well filtered anyway: the low background-to-black-point margin measured (e_margin 2.28) is normal and expected in narrowband, not a sign of underexposure. Conditions were therefore favorable and did not degrade the signal; the quality of the faint filaments captured confirms it.
Setup
- Type d'image
- Traitée
- Phase de lune
- Dernier croissant (4 %)
- FOV
- 1.20°
The 1.20° field frames the NGC 6992 arch well: the structure is fully contained with healthy margins, coherent framing. With sampling yielding center FWHM around 3.4 px and round stars, focus and tracking are good. The only hardware setting to revisit is the backfocus distance: corner FWHM (up to 4.4 px) clearly exceeds the center, indicating a sensor-to-corrector spacing to fine-tune in order to flatten the field at the edges.
The diagnosis in detail
The image is healthy and well controlled. The PSF panel shows round stars across the whole field (elong 1.04 to 1.12, PA dispersion of 48.7° confirming the absence of coherent directional elongation), so no tracking, guiding or tilt issue. The center is sharp (FWHM 3.44 px). The only measured deviation is a symmetric FWHM rise toward the four corners, up to 4.4 px at TL, a corner/center ratio of 1.3: the classic signature of field curvature or a backfocus distance slightly out of spec, not coma (no radial comet tail) nor tilt (no diagonal asymmetry). The impact stays moderate and cosmetic at the edges.
On the sky background, the illumination map reveals only a very slight edge darkening (+5%) with radial symmetry, no clear gradient nor calibration artifact: nothing to correct. Processing is clean, without over-denoising or visible clipping, and the faint Veil filaments are well preserved. In short, a well-made acquisition whose only room for improvement is optimizing the backfocus to even out sharpness all the way to the corners.
Priority actions
- Fine-tune the backfocus distance (0.5-1 mm spacers) to equalize center/corner FWHM
- Verify the result with a new aberration panel after adjustment
