Astrophotography diagnosis of M16: Sensor tilt and clipped stars
ProcessedSII / Ha / OIII3 x 20 x 600"
The Doc examined this image of M16 (processed, SII / Ha / OIII, 3 x 20 x 600"). Estimated overall technical quality: 8/10. 2 defects found: Sensor tilt (severity 3/5), Clipped stars (severity 2/5).
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Info
- Cible
- M16
- Site
- Bortle 4 · transition rural/banlieue (VIIRS)
- Position
- 18h18m49s · -13°52'41"
Bortle 4 and a 30% moon make for very favorable conditions here. In SHO on an emission nebula like M16, the narrowband makes lunar and residual light pollution nearly negligible: this is exactly the kind of target you can shoot without waiting for new moon. It shows in the result, the sky background is neutral and the measurement finds no instrumental gradient, the background variation matching the nebulosity itself. With a sky of that quality you can afford to push the OIII and SII exposures, the signal-poor channels, rather than extending Ha which is already well served.
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
- 48.7'
The 12.5 inch CDK with the mono C4 gives 0.81 degree of field, a tight but correct framing for M16: the nebula fits entirely with reasonable margins, and the resolution resolves the Pillars of Creation, which a shorter instrument would not. The SII/Ha/OIII trio is the right call on this emission region. However, the 6 px central FWHM points to marked oversampling relative to the seeing: 2x2 binning on the SII and OIII channels would gain signal without losing real detail. That same oversampling is what makes the tilt so visible in the measurement.
The diagnosis in detail
The only structural defect in this image is a tilted focal plane, and it is clear cut: the field center is the sharpest after the bottom-right corner, while the bottom edge and the top-right corner reach twice the FWHM. Elongation stays flat everywhere (1.02 to 1.06), which rules out tracking, astigmatism and coma: the Paramount is doing its job perfectly, and the CDK optical design is not at fault. This is mechanical coplanarity between the sensor and the focal plane, to be looked for in the adapters and the tilt plate.
On the processing side the result is well controlled. The Hubble palette is coherent, the SII/Ha/OIII separation readable, and comparison with the DSS reference confirms that every visible structure is real, including the reddish extensions at the bottom of the frame that follow the field nebulosity. The background shows no notable chroma noise, no posterization, no over-denoising signature. Only the cores of the NGC 6611 stars are clipped, which is common on 600 second subs and easily fixed with a short HDR layer.
As a priority, fix the tilt before the next session: it is the only thing genuinely limiting the resolution the CDK can deliver.
Priority actions
- Map and correct the imaging train tilt (tilt plate, adapter coplanarity) before the next session
- Check for focuser flexure under the weight of the C4 by pointing at different declinations
- Add a series of short exposures per filter to rebuild the clipped star cores of NGC 6611 in HDR
- Consider 2x2 binning on SII and OIII to gain signal, the 6 px central FWHM indicating oversampling
