Orion’s Sword & the Horsehead

Featured as the 8 August, 2020 AAPOD2

Image Details:

This is a generally bright area of the Orion molecular cloud complex best seen during mid-winter in the Northern Hemisphere. Shot as a 3-panel mosaic, this photo shows a large area of sky covering more than 13 times the apparent width of the Full Moon in our sky and includes famed objects like the Horsehead & Flame to the top or left, the Great Orion Nebula (Messier 42) to the bottom or right, and oceans of interstellar dust between it all. In most regions of sky one set exposure time is enough to capture a nebula, but bright regions like the core of M42 add an additional challenge since most cameras lack the dynamic range to capture both bright and dim regions at the same time - in other words, the brighter regions quickly overexpose and turn white, losing all detail in the process. To compensate for this shortcoming I took varying sets of exposure times and blended the shorter exposures (which capture brighter areas with useable detail) over the longer ones (wherein the brighter details have become whited out) to retain both background dust and detail in brighter areas.

M42 is visible to the naked eye even in moderately light polluted conditions and has been found be in the process of forming hundreds of new stars and solar systems. The bright core of the nebula features a color rarely observed in blackbody-dominant populated space, which is the teal green of ionized Oxygen. The pinkish areas are caused by a combination of emission spectrum of ionized Hydrogen - firstly red which is seen throughout the area (though attenuated by interstellar dust into dark orange / brown) and higher-energy blue & violet light in brighter (higher energy) areas. This is caused by the bright stars in the area, particularly Sigma Orionis, which is located near the center of the Horse’s “Mane” region.

2022 Reprocess:

In the newer edit I focused more on the “accurate” color of the background. Most astrophotographers will invariably combine narrowband Hydrogen into the Red channel, and while this is aesthetically pleasing I believe it can lack some nuance. In addition to Red channel contribution, I have also begun applying this same narrowband imagery to Green in order to maintain the orange / brown hue of interstellar dust as well as the Blue channel to simulate emission lines of Hydrogen -beta and -gamma (which are blue and violet, respectively).

Equipment:

• William Optics Star71-II Petzval APO (345mm Focal Length, F/4.9)

• ZWO ASI1600MM-P, ZWO Filters

• Celestron CGEM-II

• Autoguiding: Orion 50mm Guidescope + ZWO ASI224MC

Exposures:

• Red, Green, Blue: 418, 349, 280 x 90” (Total: 26h 10m 30s), 100, 100, 100 x 10” (Total: 50m)

• Hydrogen-Alpha 7nm: 379 x 300” (Total: 31h 35m), 150 x 30” (Total: 1h 15m)

• Oxygen-III 7nm: 98 x 300” (Total: 8h 10m), 50 x 30” (Total: 25m)

Misc Details:

• Capture Software: AstrophotographyTool, PHD2 (guiding), Celestron CPWI (mount control), Pegasus Powerbox (dew heater control, power management)

• Processing Software: PixInsight

• Taken from: Wichita, KS, Bortle 5

• Capture Dates: 18, 20 30-31 Dec, 2019; 1, 4-6, 18-20 January, 18, 20-21, 28 February, 2020

Annotation