NEBULAE
Wide Field of Rho Ophiuchi Complex
— IC 4604 & IC 4592 —
Processed by Nicolas Rolland, Paris. 12 hours gathered by Martin Pugh at Heaven's Mirror Observatory, Yass, Australia.
This image is upside down. Turned the other way, the blue shape at the edge of the field does not read as a horse's head.
Twenty-two degrees of sky
Three panels cover 22 by 13 degrees here, which is the width of a hand held at arm's length several times over. That is what it takes to hold the whole complex: IC 4604 and the blue nebulosity around Rho Ophiuchi at one end, Antares and its amber dust in the middle, and IC 4592 — the Blue Horsehead — out at the edge, a reflection nebula lit by Nu Scorpii. The five-degree view of the junction between Antares and Rho Ophiuchi, and the close-up of IC 4604, both fall inside the left third of this field.
Four hundred and sixty light-years, and some depth
Radio interferometry of young stars in the cloud, confirmed by Gaia, places the complex at about 140 parsecs — some 460 light-years. Over a field this wide that number stops being a single value: the measurements separate L1688 from L1689 by ten parsecs, which is to say the near and far parts of what looks like one cloud are thirty light-years apart along the line of sight.
A supernova, a million years ago
The cloud lies well above the galactic plane, at the edge of the Scorpius-Centaurus OB association. A massive star in upper Scorpius exploded around a million and a half years ago; the shock front reached these clouds roughly half a million years later, compressed them, and started the star formation that is still running. At this field width the shape of that push is visible — the streamers all point away from the same place.
Twelve hours, three panels, and a fast lens
The optic here is a 135 mm photographic lens at f/1.8 with a colour sensor behind it, from Yass in April 2021. Twelve hours across three panels means four hours each, which at f/1.8 is a great deal of light — but the price is paid at the edges. Wide open, a lens of this design leaves the corners aberrated and out of focus, so the panels were overlapped by nearly twenty per cent rather than the usual five, and roughly a third of every frame was discarded. There is no luminance channel either: a colour sensor under a UV/IR filter gathers all three channels at once, which is efficient and shallower than a mono sensor would be over the same twelve hours. What comes out is 21,998 by 13,317 pixels, and the whole complex fits inside it. Martin Pugh gathered the data at Heaven's Mirror.
