Astrophotography diagnosis of M31: Sensor tilt, clipped stars and 1 other
Processed
The Doc examined this image of M31 (processed). Estimated overall technical quality: 8/10. 3 defects found: Sensor tilt (severity 2/5), Clipped stars (severity 2/5), Over-denoising (severity 2/5).
&w=1920&q=75)
Info
- Cible
- M31
- Position
- 0h42m52s · +41°17'38"
A 3 percent waxing crescent is a non factor: no lunar pollution expected, and the background map confirms it by detecting no instrumental gradient. Conditions were therefore favourable for a broadband target like M31, which is the most demanding on sky darkness since no narrowband filter can rescue it. The measured sky background sits very close to the black point (margin 0.76), which mostly reflects processing that pulled the background low rather than a poor sky. To push further on the outer disk extensions and the surrounding IFN, this kind of moonless night is exactly what you want, simply with more total integration.
Setup
- Type d'image
- Traitée
- Phase de lune
- Premier croissant (3 %)
- FOV
- 2.18°
The 2.18 degree field is well sized for M31: the galaxy fills the frame diagonal with room to spare, and both M32 and M110 are included, the ideal compromise on this target. The monochrome camera and the 5.9 px central FWHM indicate comfortable sampling with no pointless oversampling. The one mechanical item to revisit is the flatness of the imaging train: the left side of the field shows roughly 20 percent more FWHM than the right, which an adjustable tilt plate will fix. On a broadband target like M31 no narrowband filter is relevant, so put the time into integration and into the quality of the luminance layer.
The diagnosis in detail
The diagnosis reads on two independent planes. On acquisition, the PSF is good: elongations between 1.02 and 1.06 across the nine zones, PA scattered over 46 degrees, which means round stars and therefore faultless tracking and guiding. There is no drift, no periodic error, no guiding oscillation. The only instrumental signature is a horizontal FWHM asymmetry of 0.078 against 0.03 vertical, with the centre at 5.92 px and the left side at 7.2 px: that is the signature of slight sensor tilt, not field curvature, which would be symmetric across all four corners.
On processing, two effects echo each other. The stretch was pushed far enough to flatten the M31 nucleus into a neutral white plateau, while the DSS reference keeps a nuanced cream structure at exactly the same place, and a few bright stars share the same fate. In parallel, the denoising smoothed the background to the point of erasing part of the stellar granulation in the outer arms and reducing the count of faint stars compared with the DSS.
Both points are recoverable without reshooting: they come down to how hard the stretch and the denoise were dialled on the same stacked data. The tilt does need a mechanical intervention before the next session, but at its current amplitude it does not seriously degrade the result.
Priority actions
- Redo the stretch with highlight protection to restore the structure of the M31 nucleus and the central gradient of the bright stars
- Lower the denoise strength and mask it off the high signal areas to bring back the granulation of the outer arms
- Correct the flatness of the imaging train with an adjustable tilt plate, aiming for horizontal FWHM asymmetry below 0.05
- Increase total integration so you can denoise less and pull out the faint disk extensions
