Astrophotography diagnosis of Trefle: Focus miss, noise / underexposure and 3 others
ProcessedUV ir cut
The Doc examined this image of Trefle (processed, UV ir cut). Estimated overall technical quality: 6/10. 5 defects found: Focus miss (severity 3/5), Noise / underexposure (severity 3/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5), Framing issue (severity 2/5).
&w=1920&q=75)
Info
- Cible
- Trefle
- Position
- 18h02m29s · -23°01'48"
Average conditions: a 74% waning gibbous Moon noticeably raises the sky background, which directly penalizes a mixed target like M20 whose blue reflection component is broadband and unprotected by any narrow filter here (plain UV/IR cut). That matches the noisy background and the missing faint extensions. Two options on this target: aim for a moonless window (late last quarter or new moon), or wait until the Moon is low and far from Sagittarius. Since M20 culminates low from Europe, you also want to shoot at meridian transit to limit the airmass.
Setup
- Type d'image
- Traitée
- Télescope
- Skywatcher Dobson Flextube 305/1500
- Caméra
- Pegasus astro smarteye
- Filtre
- UV ir cut
- Monture
- Dobson
- Phase de lune
- Gibbeuse décroissante (74 %)
- FOV
- 42.0'
The setup is coherent in aperture, less so in mount. A 305/1500 gathers plenty of light and the 0.70 deg field frames M20 tightly but acceptably, even if the blue part spills out. The real constraint is the Dobsonian: with no equatorial platform, subs stay very short and field rotation limits stacking, which explains the modest signal to noise ratio. At 1500 mm focal length, focus is critical and deserves a systematic Bahtinov mask, or even a motorized focuser. The UV/IR cut filter is the right choice here, since M20 has a reflection component that a dual-band would cut out.
The diagnosis in detail
The diagnosis comes down to one central point: stellar resolution. Across the whole frame, stars are soft disks with no preferred direction, which rules out a tracking or field-optics problem and points to imperfect focus, possibly worsened by poor seeing low over Sagittarius. On a 1500 mm Newtonian, focus tolerance is a few tens of microns: without a Bahtinov mask and regular refocusing, this result is expected.
The second axis is signal to noise. The 74% Moon raises the background and the Dobsonian rules out long subs: the result is visible graininess in empty areas and colored mottling in the lower and right parts of the field. This is not a processing artifact but a shortage of photons, confirmed by the DSS comparison where the outer extensions reach much further.
Finally, a few bright stars have cores clipped to white and the framing truncates the blue reflection component. Both points are secondary and easy to fix on the next session.
Priority actions
- Refocus with a Bahtinov mask and check focus every 30 to 45 minutes
- Reschedule the session outside the lunar window and at M20 meridian transit
- Greatly increase total integration time to bring out the faint extensions
- Recenter the field toward the south-west to include the blue reflection nebula
- Add a set of short exposures to rebuild the cores of bright stars
