Astrophotography diagnosis of IC 1805: Backfocus error

Decent conditions for this target. Bortle 4 is comfortable, and a 46 percent last quarter barely penalizes an emission nebula shot through an L-eXtreme: the two narrow bands reject most of the lunar continuum. The background map confirms it, the plane gradient tops out at 4 percent amplitude with an R2 of 0.01, so nothing directional or meaningful as light pollution. The measured background sits well clear of the black point (6.67 margin), so the sky is not the limiting factor here. That said, if you want to push the OIII on the bubble, aim for a moonless night, that channel suffers the most from residual lunar sky.

The setup is well matched to the target. At 6.50 degrees of field, the FRA400 reduced to 0.7x takes in the whole Heart plus Soul complex, and the chosen framing holds both with comfortable margins: a very sensible use of this wide-field combination. The 300s subs are consistent for dual-band, a median of 0.24 with a 6.67 background/black-point margin shows a properly sky-limited regime with no global saturation (0 percent). Dark clipping at 0.76 percent stays marginal but flags a barely sufficient offset, worth watching if you shorten exposures. The one link to adjust remains the backfocus spacing behind the reducer.