Astrophotography diagnosis of M33: Residual vignetting, backfocus error and 3 others
ProcessedIR180
The Doc examined this image of M33 (processed, IR, 180). Estimated overall technical quality: 4/10. 5 defects found: Residual vignetting (severity 3/5), Backfocus error (severity 3/5), Noise / underexposure (severity 3/5), Background chroma noise (severity 2/5), Focus miss (severity 2/5).
&w=1920&q=75)
Info
- Cible
- M33
- Position
- 1h33m53s · +30°39'38"
A 66% waning gibbous moon and seeing rated 3: these are unfavourable conditions for M33, a low surface brightness galaxy in broadband. The moon raises the sky background and crushes the spiral arm contrast, which partly explains the measured background/black point margin of 2.92 and the colour mottling. An IR-cut filter alone offers no protection against that scattered light, unlike an emission target that would tolerate a dual-band. Seeing of 3 also inflates the central FWHM measured at 4.8 px. The strategy is simple: keep M33 for nights near new moon and save moonlit nights for narrowband nebulae.
Setup
- Type d'image
- Traitée
- Télescope
- C8 edge F7
- Caméra
- Asi 2600mc
- Filtre
- IR
- Monture
- Rq6 r pro
- Exposition
- 180
- Phase de lune
- Gibbeuse décroissante (66 %)
- Seeing
- 3
- FOV
- 54.5'
The C8 Edge at f/7 with the ASI2600 gives 0.91° of field, which frames M33 somewhat tightly: the galaxy fills a good part of the frame but its outer extensions brush the edges. It is usable, though a shorter reducer or a mosaic would give more room. In sampling terms this combination works well under one arcsecond per pixel, so oversampled for seeing of 3: binning 2 in processing would gain SNR with no real detail loss. The 180s subs are short for a faint galaxy at this focal ratio, 240 to 300s would be more efficient if the RQ6 guiding keeps up. Finally, the Edge corrector spacing needs resetting.
The diagnosis in detail
Two problems dominate and both are instrumental. The first is illumination calibration: the measurement shows a 74% radial falloff from center to edges, perfectly symmetric (R2=0.89, azimuthal anisotropy 0.07). On a galaxy field with no peripheral nebulosity no astrophysical explanation holds, this is uncorrected vignetting. A C8 Edge with reducer vignettes heavily, and proper flats normally cancel it entirely.
The second is optical: FWHM goes from 4.82 px at center to 5.2-6.7 px in the corners, a ratio of 1.31, with oval stars rather than radial comets. That is the signature of field curvature tied to incorrect spacing behind the corrector. The asymmetry between the top left corner (3.06 px, the sharpest) and the bottom right (6.74 px) suggests a slight tilt component on top, to watch once backfocus is reset.
Behind all this, short integration explains the rest: grainy background, colour mottling, barely emerging spiral arms and a background/black point margin of 2.92, far from the sky-limited regime. These three axes are addressed in order: flats, spacing, then total exposure time.
Priority actions
- Reshoot a set of flats on the exact optical train (same rotation, same focus) and verify they are applied at stacking, this is the most immediate gain
- Measure and reset the spacing behind the C8 Edge corrector in 0.5 to 1 mm steps until the corner/center FWHM ratio drops below 1.15
- Accumulate far more integration on M33, aim for 6 to 10 hours, with 240 to 300s subs
- Reschedule the target outside the moon period, a 66% moon kills broadband galaxy contrast
- Rework the processing with SPCC colour calibration and chroma noise reduction targeted on the background
