Astrophotography diagnosis of Altair: Sensor tilt and clipped stars

Excellent and clearly under-exploited conditions. Bortle 2 puts the sky background at the darkest level available, and the 96.7% waning gibbous Moon was below the horizon during the exposure, so no lunar contribution is possible: the measured plane gradient (4% amplitude, R² 0.18) is not meaningful and signals nothing. With such a sky, the limit is not light pollution but exposure: 30s in L at gain 125 leaves a background-to-black-point margin of only 1.94, far from sky-limited. Under Bortle 2 you can safely stretch to 120 or 180s in luminance, the background will stay clean and you gain depth directly on faint stars.

A coherent wide-field combination. The 530mm FSQ at F/5 with 3.76µm pixels yields 1.463"/px and a 3.89° field, a comfortable sampling that matches typical seeing without oversampling. On Altair, a point-like and very bright target, the 3.89° field is obviously oversized, but it suits an interest in the surrounding star field. On settings: gain 125 sits slightly above this sensor's unity gain, no noise concern but a bit less highlight headroom on bright stars. Offset 50 leaves 0.21% dark clipping, a low and acceptable value. The weak point is the 0°C sensor temperature: going down to -10°C will clearly reduce dark current and stabilize dark frame matching.