Sh2-261, also sometimes known as the Lower Nebula, is an emission nebula in the constellation Orion. It is located in the northernmost part of the constellation, about 6° west of Alhena (γ Geminorum). It is an H II region located on the outer edge of the Orion Arm, at the boundary of the galactic region between it and the Perseus Arm. It is located in the line of sight of the southernmost part of the Gemini OB1 association. One of the stars responsible for the ionization of gas in the region is believed to be the blue star HD 41997, spectral class O7.5V. In addition, there are other stars, including the blue star BD+15°1079, of class B0. This star has sometimes been indicated as O7 class, thus shifting the distance of the nebula by up to 2,000 pc due to the increase in the gap between the absolute magnitude, obviously brighter in an O-class star, and the apparent magnitude. Thanks to the analysis of the other ionizing stars, a distance of about 1,000 pc (∼3,260 al) was calculated, which seems more reliable, also considering the determination of the star BD+15°1079 as belonging to the B0 class.

Telescope: Askar FRA300pro f5
Mount: Astro Physics Mach-1
Camera: ZWO ASI6200MC pro
Guider: ZWO Off-Axis-Guider / ZWO120MM Mini
Filters: None. Integrated ZWO UV/IR Cut Filter Cover

135×5 mins = 675 mins

Total Imaging Time: 11h 15m

Data Imaged remotely on 4 nights during December, 2025.
Data acquisition & Processing by David Churchill.

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