Astrophotography diagnosis of Nébuleuse de la bulle: Focus miss, residual vignetting and 2 others
ProcessedUV ir cut
The Doc examined this image of Nébuleuse de la bulle (processed, UV ir cut). Estimated overall technical quality: 4/10. 4 defects found: Focus miss (severity 3/5), Residual vignetting (severity 3/5), Noise / underexposure (severity 3/5), Clipped stars (severity 2/5).
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Info
- Cible
- Nébuleuse de la bulle
The first quarter Moon (60% illumination) is the limiting factor of this session. On the Bubble Nebula, a faint Ha emission target, a Moon-raised sky background directly crushes the contrast you are after: that is exactly what the image shows, faint noisy red patches instead of a structured shell. With a broadband UV/IR cut you have no protection against that scattered light. Two concrete options: shift the session to late night once the Moon has set, or move to a dual-band Ha/OIII filter that will give back much of the contrast despite the Moon. Meanwhile, greatly extend total exposure to compensate for the lost SNR.
Setup
- Type d'image
- Traitée
- Télescope
- Skywatcher Dobson Flextube 305/1500
- Caméra
- Pegasus astro smarteye
- Filtre
- UV ir cut
- Monture
- Dobson
- Phase de lune
- Premier quartier (60 %)
A 305/1500 Dobson on an altazimuth mount remains primarily a visual instrument: without an equatorial table, subexposures must stay short and field rotation caps integration, which partly explains the weak signal. The aperture collects a lot, but sampling at 1500 mm focal length is demanding and requires flawless focus, which is missing here. The most damaging point is the fully illuminated field, visibly smaller than your sensor: check the focuser, adapters and filter, a stop somewhere in the optical train is likely. On an emission target like the Bubble, a dual-band filter would be a clear gain over the current UV/IR cut.
The diagnosis in detail
The dominant defect is sharpness. Stars are soft disks, uniform in size from center to edges, with no directional elongation and no asymmetry between opposite corners: this is neither tracking, nor tilt, nor coma, it is missed focus, likely worsened by a tube not at ambient temperature. At 1500 mm, the slightest focus drift shows immediately, and that is what keeps the Bubble shell from standing out.
Next comes an optical train and calibration issue. The fully illuminated circle is clearly smaller than the sensor, all four corners are black in a perfectly circular way, and nothing was corrected by flats. This is mechanical, not cosmetic: either something stops the beam down, or the Newtonian is used without a corrector with a sensor too large for its usable field.
Finally the signal. The Ha nebulosity exists only as traces, consistent with short integration under a 60% Moon and a plain UV/IR cut. The chain of causes is cumulative: soft focus, clipped illumination, low SNR. Fixing focus and flats will pay off immediately, the rest needs more time and a suitable filter.
Priority actions
- Refocus with a Bahtinov mask after letting the tube reach ambient temperature, and recheck it during the session
- Find and remove the stop in the optical train that shrinks the fully illuminated field (adapters, filter, focuser)
- Shoot and apply a flat series in the exact configuration of the session
- Accumulate several hours of integration on the Bubble, with no Moon present
- Consider a dual-band Ha/OIII filter matched to the 305/1500 f-ratio for this emission target
