Object Data: The very bright star Alnitak (Zeta Orionis, the eastern-most star in Orion's belt) is at the centre of one of the most intriguing and picturesque regions in the night sky. The radiation from this very hot star excites the gases in the surrounding nebulae causing ionized emissions. NGC 2024 (Flame Nebula) to the north (left in this photo), one of the most remarkable reflection nebulae in the night sky, has an underlay of glowing Ha (red) emission but is strongly overlaid with reflected light from Alnitak resulting in almost unique and beautiful colours, ranging from shell-pink through yellow to deep orange. Running south from Alnitak (right in this photo) is a strong band of glowing emission nebula known as IC 434. High resolution photographs show this gas throwing off ionized partcles in response to the energised gases below - the effect can just be discerned in this photo as an upwards 'spray' of red light. A large mass of obscuring dark matter underlays the eastern part of this region (lower part of photo) and its protrusion into IC434 in the shape of a horse head gives rise to the famous name the Horsehead Nebula (Barnard 33). It is speculated (and seems likely) that the dark core of the Flame Nebula is caused by a similar protrusion from this same mass of dark matter. Please also see this later high resolution version of the Horsehead.
Location: Southern France
Conditions: Calm, moderate dew, transparency=8, seeing=6
Optics: Astro-Physics 155 EDF f/7 with 4" focuser and integral field flattener working at f/7
Mount: AP 900 GTO on Portable Pier
Camera: SBIG STL-11K, SBIG LRGBC filter set, -30°C
Guiding: Integral STL-11K autoguider
Exposure: Normal LRGB Seq: 20x 10 minutes; 3x 10 : 6.5 : 10 minutes (binned 2x2). Short LRGB Seq: 12x 1 minute; 3x 1 : .65 : 1 minutes (binned 2x2).
Processing: Image acquisition and initial processing using Maxim DL, subsequent processing in RegiStar and Photoshop.
Notes: After looking carefully at the STL-11K QE curve and the RGB filter responses I decided that the SBIG recommended RGB ratio of 1 : 0.65 : 1 should be most accurate. The results of this and other image sets endorsed this - even so the images were biased a little towards the green and I compensated for this in Maxim Combine Color with RGB weights of 1.07 : 0.93 : 1. I did careful comparisons of the different combine algorithms and decided to use SD-Mask for all image calibration and combine operations as it seemed to give best results overall, though I saw little difference between this and Sigma Clip.