Astrophotography diagnosis of NGC 4535: Backfocus error and clipped stars
ProcessedOptolong L-Para 2''241x300s
The Doc examined this image of NGC 4535 (processed, Optolong L-Para 2'', 241x300s). Estimated overall technical quality: 8/10. 2 defects found: Backfocus error (severity 2/5), Clipped stars (severity 2/5).
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Info
- Cible
- NGC 4535
The 38% moon stays modest and does not seriously affect a broadband galaxy target like this one: the measured background shows a weak plane gradient (4%, R²=0.54) and clean radial symmetry, meaning residual light pollution is well controlled. The background/black-point margin (e_margin 2.12) is low in absolute terms, but over 241 subs of 300s under dark skies this is consistent with a deep galaxy integration where the background is intentionally kept contained. The positive Bowley skew (0.69) reflects a background well lifted off the black point with no dark clipping (0% clipping). Overall favorable conditions properly used.
Setup
- Type d'image
- Traitée
- Télescope
- TS-Optics 200mm/8" ONTC f/5
- Caméra
- ZWO ASI2600MC Pro
- Filtre
- Optolong L-Para 2''
- Monture
- ZWO AM5
- Exposition
- 241x300s
- Phase de lune
- 38%
The ONTC 200mm f/5 combo (1000mm focal length) with the ASI2600MC Pro gives a sampling around 0.78 arcsec/px, rather fine for current seeing, which explains the 2.3-3.2 px FWHM (about 2 arcsec, very good). The field nicely frames NGC 4535 while capturing a rich Virgo galaxy group, a relevant and composed framing. The coma corrector works well (no clear radial coma), just a residual field curvature to fine-tune via backfocus. The AM5 delivers exemplary tracking over 300s. A very coherent setup for this galaxy target.
The diagnosis in detail
The PSF panel confirms a sound optical train and tracking. The center is sharp (FWHM 2.69 px, elong 1.04) and the 20.8 degree PA dispersion is low, which rules out any mount issue: no drift, no visible periodic error, the AM5 does its job over 300s. The only optical signature is a symmetric FWHM rise toward the 4 corners (ratio 1.21), with no diagonal asymmetry or radial comet shape: this is field curvature / a slight backfocus offset, mild and usable, that would benefit from a millimeter adjustment of the spacing to even out the edges.
On the processing side, the background is clean: the background map shows only a 4% plane gradient (perfectly manageable), healthy radial symmetry and no residual vignetting. I inspected the dark zones for chroma noise: the background stays neutral enough, with no notable color mottling for such a deep integration. The point to watch is the clipping of the bright-star cores (yellow-orange ones on the right), which lose their central chromatic gradient.
Overall this is a polished image: the depth reveals many small background galaxies, the galactic bulge colors are nuanced, and the remaining defects are minor and cosmetic.
Priority actions
- Fine-tune the backfocus spacing in 0.5-1 mm steps to even out corner FWHM
- Add short subs (10-30s) to rebuild the clipped bright-star cores
- Preserve stellar color by limiting the highlight stretch during processing
