Astrophotography diagnosis of sh2-136: no defect detected
RawAntlia Quadri bandes1x180
The Doc examined this image of sh2-136 (raw, Antlia Quadri bandes, 1x180). Estimated overall technical quality: 7/10. No significant defect was found on this acquisition.
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Info
- Cible
- sh2-136
- Site
- Bortle 5 · banlieue résidentielle (VIIRS)
Full Moon under a Bortle 5 sky is, on paper, the worst case for broadband, but the Antlia quad-band changes things: on an emission target like Sh2-136 it only passes the Ha/OIII lines and rejects most of the moonlight and light pollution. The background is well controlled on this sub (e_margin 2.16, low but perfectly normal for narrowband at 180s). So you can shoot tonight without concern. Keep in mind that Sh2-136 is a reflection/dark nebula poor in Ha, where a quad-band is less advantageous than on a true HII region, so useful signal will stay modest, which argues for many subs.
Setup
- Type d'image
- Brut
- Télescope
- C11
- Caméra
- Asi533mc
- Filtre
- Antlia Quadri bandes
- Monture
- Eq6r
- Exposition
- 1x180
- Phase de lune
- Pleine Lune
- Seeing
- 2.5
The C11 at 2800mm with the ASI533 (3.76µm) gives about 0.28"/px: you are heavily oversampled against a 2.5" seeing (a comfortable sampling is ~1"/px). The star spreads over many pixels for nothing, which dilutes per-pixel signal and lengthens the required time. A reducer (f/6.3 or f/7) would bring you to about 0.4-0.45"/px, healthier, with more field and speed, or failing that a 2x software binning at processing. The field (~0.45°) frames the compact Sh2-136 well. The mount handles 180s at this focal length without issue, a good point given how demanding a bare C11 is.
The diagnosis in detail
On star shape, this sub is clean. The usable panel cells (TL, T, TR, R, BL, B, BR) give elongations between 1.13 and 1.34, i.e. at the floor of a monochrome sensor, with Moffat ellipses that hug the stars and scattered PAs: the signature of round stars, no coherent tracking elongation. The two outlier values (L at 4.06, C at 2.63) rest on sub-pixel FWHM (0.42 and 0.74 px) fitted on pure noise tiles, with no real star at the crosshair: they must be discarded and signal neither tilt, backfocus, nor astigmatism.
The EQ6-R tracking on a single 180s exposure at C11 focal length shows no drift or oscillation, which is remarkable and confirms healthy guiding. The background map is radially symmetric (plane/radial dominance 0.51, azimuthal anisotropy 0.01) with only +8% edge darkening: this is the expected optical vignetting of the train, fully correctable with flats, not a calibration defect since no calibration has yet been applied.
The real limit here is not a defect but signal economy: marked oversampling and an emission-poor target call for long integration. The path is simple: stack many 180s subs with dithering, apply flats and darks, and consider a reducer to make the C11 pay off.
Priority actions
- Stack many 180s exposures with dithering to build signal (faint target and moonlit background).
- Shoot flats to correct the edge darkening (+8%) and matched darks.
- Consider a focal reducer (f/6.3-f/7) or 2x binning to escape the 0.28"/px oversampling.