Astrophotography diagnosis of IC1396_0724: Backfocus error and tracking drift
Raw300s25 juil. 2026
The Doc examined this image of IC1396_0724 (raw, 300s). Estimated overall technical quality: 6/10. 2 defects found: Backfocus error (severity 3/5), Tracking drift (severity 2/5).
&w=1920&q=75)
Info
- Cible
- IC1396_0724
- Date
- 25 juil. 2026, 21:44
- Lune
- Croissant décroissant 12.4% (12° d'alt., 102.3° de la cible)
- Site
- Bortle 2 · rural typique (VIIRS)
- Position
- 21h40m36s · +58°16'59"
Excellent conditions: a Bortle 2 sky is ideal for a faint, extended target like IC 1396, and the Moon (waning crescent at 12.4%, only 12 degrees above the horizon and 102 degrees from the target) has no significant impact. The very dark background of this sub confirms it: no lunar gradient to worry about, and the dark nebulosity already stands out on a single 300 s exposure. You can stack many subs with no light-pollution or Moon limitation, giving a very favorable SNR on the faint structures of this region.
Setup
- Type d'image
- Brut
- Télescope
- EQMod Mount
- Caméra
- ZWO ASI585MM Pro
- Exposition
- 300s
- Phase de lune
- Gibbeuse croissante (86 %)
- FOV
- 1.70°
Setup consistent with the target: at 375 mm focal length the 1.70 degree field frames a portion of IC 1396 well, and the 1.60 arcsec/px sampling is comfortable, neither over- nor under-sampled for typical seeing. Central FWHM of 1.88 px (about 3.0 arcsec) is decent. The item to address is optical: the FWHM rise in the corners reveals a backfocus/spacing to adjust to exploit the full field. On settings, gain 252 is fairly high and the 300 s sub gives a background e_margin of 2.02, low but NORMAL here (mono, possibly narrow band, under a very dark sky). Offset 15 with no dark clipping and sensor at -14C are healthy.
The diagnosis in detail
This sub is well made. Under a Bortle 2 sky with no lunar interference, the dark nebulosity and dusty cirrus of IC 1396 are already readable on a single 300 s exposure, structurally matching the DSS reference: these are real signal, not artifacts. The background is clean, with no visible banding or walking noise, and the histogram (sufficient offset, no clipping) confirms healthy settings at -14C.
The only real job is star shape, measured by the aberration inspector. The center is tight (FWHM 1.88 px, elong 1.15) but FWHM rises symmetrically toward all four corners (ratio 1.87, elong 1.54-1.55 at TR and R with oval, stretched stars): this is the signature of an incorrect backfocus / field curvature, to be fixed at the optical spacing level. Added to this is a subtler component: central elongation slightly exceeds the field floor with a consistent PA everywhere (spread 13.7 degrees), i.e. a small tracking drift present down to the center.
As a priority, set the backfocus: it is what most penalizes the full-field rendering. The tracking drift is minor over 300 s and will partly smooth out during stacking, but it warrants a check of polar alignment and guiding.
Priority actions
- Set the backfocus precisely (thin spacers to the corrector's nominal value) to even out corner FWHM
- Check polar alignment and refine guiding calibration to reduce the directional drift
- Stack a large number of subs: the Bortle 2, Moon-free sky allows an excellent SNR on IC 1396's faint structures
