Astrophotography diagnosis of M 16: Backfocus error, clipped stars and 1 other
ProcessedSII / Ha / OIII3 x 20 x 600"
The Doc examined this image of M 16 (processed, SII / Ha / OIII, 3 x 20 x 600"). Estimated overall technical quality: 8/10. 3 defects found: Backfocus error (severity 2/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- M 16
- Site
- Bortle 4 · transition rural/banlieue (VIIRS)
- Position
- 18h18m50s · -13°52'17"
Bortle 4 and a 30% moon are comfortable conditions for an emission target like M16 in SHO: narrowband filters cut most of the residual light pollution and moon scatter, which explains a clean background with no measured instrumental gradient. The background map confirms it, the +87% variation matches the nebula itself, not an artifact. Under this sky the limiting axis is not transparency but time per channel: OIII and SII stay the weak links in SHO, and that is where the background chroma noise comes from. On nights like these you can safely push OIII acquisition even with the moon up.
Setup
- Type d'image
- Traitée
- Télescope
- Planewave CDK 12,5"
- Caméra
- Moravian C4 monochrome
- Filtre
- SII / Ha / OIII
- Monture
- Paramount MX+
- Exposition
- 3 x 20 x 600"
- Phase de lune
- 30%
- FOV
- 48.9'
The 12.5 inch CDK with the mono C4 gives a 0.81 degree field, which frames M16 comfortably: the nebula and its halo fit easily and the Pillars are resolved with real finesse. The SHO trio is the right pick on this HII region, a fully legitimate palette here. The 6.4 px central FWHM indicates generous sampling relative to seeing, which leaves headroom but visually amplifies any backfocus drift: hence the value of fine tuning the imaging train, the gain at the edges will be immediate. The background to black point margin of 1.11 is low, normal for narrowband at 600 s, nothing to fix on exposure.
The diagnosis in detail
The core of the diagnosis is optical, not mechanical. Stars are round everywhere, elongation 1.02 at center against a field floor of 1.01, PA dispersion of 26 degrees: in other words no tracking, guiding or field rotation signature. The Paramount is doing its job. FWHM, however, goes from 6.4 px at center to 7.6-8.1 px at the edges and 12 px on the bottom edge, a corner/center ratio of 1.27 with no associated elongation. That is the typical signature of field curvature or a slightly off spec backfocus, not tilt (no diagonal asymmetry on the panel) and not coma (no radial comet shapes).
The rest is processing. The bright NGC 6611 star cores are flat and white, a consequence of 600 s subs with no short exposure set for the highlights. And in the dark areas, notably the lower left quadrant, the background shows colored mottling absent from the DSS: this is chroma noise, consistent with a background to black point margin of 1.11 and OIII and SII channels less fed than Ha.
None of these three points undermines the quality of the image, which stays well above average. They are three fine adjustments that together would push it up a level.
Priority actions
- Adjust backfocus in 0.5 mm steps and re-validate with the aberration inspector until center and edge FWHM match
- Acquire a short exposure set (30-60 s) per filter to rebuild the bright star cores in HDR
- Add integration time on OIII and SII to reduce background chroma noise
- Apply masked chroma noise reduction on the background during the next processing pass
