Astrophotography diagnosis of ngc7023: Sensor tilt, clipped stars and 1 other
Processed
The Doc examined this image of ngc7023 (processed). Estimated overall technical quality: 7/10. 3 defects found: Sensor tilt (severity 3/5), Clipped stars (severity 2/5), Over-denoising (severity 2/5).
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Info
- Cible
- ngc7023
- Position
- 21h03m10s · +68°10'12"
A 3% crescent makes the Moon entirely negligible: no lunar pollution can explain anything here, consistent with the background map finding no instrumental gradient. This is exactly the window you want for NGC 7023, a broadband reflection nebula highly sensitive to any stray light: unlike an emission nebula, you cannot rescue it with a dual-band filter, everything rides on sky darkness. The brownish dust clouds captured around the Iris confirm you had enough transparency and useful integration to reach very faint signal. Nothing to fault on the conditions side.
Setup
- Type d'image
- Traitée
- Phase de lune
- Premier croissant (3 %)
- FOV
- 1.20°
A 1.20 degree wide field is well sized for NGC 7023: the Iris spans about 18 arcminutes, so it fills a comfortable fraction of the frame while you capture the whole dust environment that makes this target interesting. The center FWHM of 2.19 px with 1.02 elongation points to healthy sampling and tracking, with no obvious oversampling. The sensor is monochrome and the image is in color, so you are working in LRGB, the right choice on a reflection nebula. The one thing to revisit is mechanical, not optical: the chain between reducer and sensor is not coplanar, which the top/bottom FWHM gradient confirms.
The diagnosis in detail
The diagnosis comes down to one word: tilt. The Moffat measurements are unambiguous, the center is excellent (FWHM 2.19 px, elongation 1.02, the field floor) but FWHM climbs to 3.45 px at bottom left and 2.86 px along the bottom edge, while the top stays between 2.06 and 2.39 px. Vertical asymmetry of 0.296 against 0.061 horizontal marks a strictly single-axis degradation: this is neither coma (which would be radial and symmetric) nor a backfocus error (which would degrade all four corners together). The sensor plane tips around a horizontal axis, typically a focuser sagging under load or a poorly tightened spacer.
The rest is processing. The cores of the brightest stars are clipped to neutral white, with 0.07% saturated pixels, and the sky background shows fairly heavy smoothing that has probably taken some faint stars with it. Both points are recoverable without going back out, by redoing the stretch with highlight protection and dosing the denoise more carefully.
Good news on the background though: the measured +32% variation corresponds to the region's real dust clouds, not to a gradient or residual vignetting. Your calibration holds up.
Priority actions
- Correct the focal plane tilt: check coplanarity and tightness across the whole chain between reducer and sensor, then adjust with a push-pull tilt plate, iterating on the aberration inspector
- Check whether the focuser sags under the camera's weight, the top/bottom gradient points to mechanical flexure rather than a simple spacer fault
- Redo the stretch with a luminance mask to preserve stellar cores, or add a short-exposure layer blended in HDR
- Lower the denoise strength and apply it in the linear stage to recover faint stars lost to smoothing
