Sh2-261
Lower’s Nebula
1d 26’ x 1d 02’ | 0.7”/px | 7071 × 5159 px
Orion
RA 6h 09m Dec +15° 43’ | 0°
Sharpless 2-261, also known as Lower's Nebula, is a faint emission nebula located in the constellation Orion. Sh2-261 is an ionised hydrogen (HII) region energised by the ultraviolet radiation of nearby hot stars. This nebula lies roughly 3,300 light-years from Earth and spans about 50 light-years in diameter. Observations in radio and infrared wavelengths have revealed dense pockets of gas and dust within the nebula, indicating ongoing star formation. The primary ionising source is believed to be a massive, hot O-type star (or multiple OB stars), whose intense ultraviolet radiation excites the surrounding hydrogen gas, causing it to emit the characteristic red H-alpha glow. The nebula is called "Lower's Nebula" in honour of Harold Lower, an American amateur astronomer and astrophotographer, who first photographed it in 1939 using a homemade 6-inch telescope. His discovery was notable because, at the time, most deep-sky objects were cataloged by professional observatories. Yet Lower managed to capture this faint nebula from his backyard.
source: Deepseek
Data Acquisition
Data was collected over 2 nights in late February 2023 and 2 nights in mid February 2025 using a 106mm refractor telescope with full-frame camera from the backyard in The Netherlands. In 2023, only 4 hours of data were collected, which was not sufficient. The plan was to finish it later. In 2025, I finally had the chance to complete the dataset. Data were gathered in narrowband using Ha, OIII and SII filters with the intention to create a Hubble palette. No additional RGB data was collected for the stars. A total of about 14 hours of data was combined to create the final image.
Location Backyard Observatory in Groningen, The Netherlands (53°N 6°E)
| Sessions | Moon% | Moon° | Hum% | SQM | T°C | Frames | Exposure |
|---|---|---|---|---|---|---|---|
| 20230226 | 45 | 32 | 90 | 17.8 | -1.1 | 18 | 1h 30m |
| 20230228 | 65 | 13 | 94 | 19.3 | -1.6 | 30 | 2h 30m |
| 20250217 | 73 | 115 | 72 | 20.0 | -4 | 66 | 5h 30m |
| 20250218 | 64 | 127 | 75 | 19.5 | -2 | 55 | 4h 35m |
| Total | 169 | 14h 05m |
| Frames | Bin | Gain | Exposure (s) | Frames | Exposure |
|---|---|---|---|---|---|
| Chroma SII - 3nm | 1 | 100 | 300 | 15 | 1h 15m |
| Chroma Ha - 3nm | 1 | 100 | 300 | 17 | 1h 25m |
| Chroma OIII - 3nm | 1 | 100 | 300 | 16 | 1h 20m |
| Antlia SII 3nm Pro | 1 | 100 | 300 | 30 | 2h 30m |
| Antlia Ha 3nm Pro | 1 | 100 | 300 | 43 | 3h 35m |
| Antlia OIII 3nm Pro | 1 | 100 | 300 | 48 | 4h 00m |
| Total | 169 | 14h 05m |
Equipment
Between 2023 and 2025, a few changes were made to the setup. The Sesto Senso focuser was swapped for a Pegasus Astro Motor Focus kit, the Chroma filters were moved to the remote observatory and replaced with Antlia filters that have similar specifications, and the Fitlet2 acquisition computer was upgraded to the Fitlet3.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi FSQ-106 (530mm @ f/5.0), PrimaluceLab Sesto Senso 2/Pegasus Astro Motor Focus kit v2
10Micron GM1000HPS, EuroEMC S130 pier
ZWO ASI6200MM Pro, cooled to -15 ºC
Chroma/Antlia 2” SII,Ha,OIII (all 3nm) unmounted, ZWO EFW 7-position
Unguided
Fitlet2/Fitlet3, Pegasus Ultimate Powerbox v2, DeepSkyDad Flat panel, MBox
Linux Mint, KStars/Ekos, INDI Library, Mountwizzard4, PixInsight 1.9,3
Processing
All processing was done in Pixsinsight unless stated otherwise. Default features were enhanced using scripts and tools from RC-Astro, SetiAstro, GraXpert and others. Images were calibrated using 50 Darks, 50 Flats, and 50 Flat-Darks, registered, integrated and drizzled (2x) using WeightedBatchPreProcessing (WBPP). The processing workflow diagram below outlines the steps taken to create the final image.
The nebula is rather faint, and required careful stretching. None of the automatic stretching methods gave satisfactory results. So a series of small stretches using GHS were applied, each time focusing on a slightly different part of the histogram. This way especially in OIII and SII the nebula could be brightened without exposing the background too much. After combining the channels to the Hubble palette, just some minor tweaks in NarrowbandNormalisation were sufficient to get to the final colour palette. The only thing was that the background was still a bit too green. So with a range mask applied, the greens in the background were reduced using a combination of SCNR, HistogramTransformation and CurvesTransformation.
The Stars-only image had quite a number of red blips/spickles in it. Not sure where that came from. Perhaps some inconsistencies during the SXT process in one of the channels. These were manually removed using the clonestamp tool.
The geometry of the images in 2023 and in 2025 was not exactly the same, even though I had tried to make them similar. Therefore an initial crop had to be made. In the end this crop did not even turn out to be severe enough. The image would benefit from another crop. This time the crop was applied including a rotation of the imaging field so that north would be pointing north. In the slideshow at the top of this page, the results after the first crop are shown as one of the images.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
