Astrophotography 101

Wednesday, October 7, 2026

Sh2-132 — The Lion Nebula

Two dying giants, one cloud of gas, and the same light shown two ways — six nights and 33 hours on a lion ten thousand light-years away.

Sh2-132 in the Hubble palette (SHO): sulphur as red, hydrogen as green, oxygen as blue. 33.2 hours of exposure.

How big is it on the sky?

The full Moon, for scale — 30′

This frame (RC8, 1271 mm) — 64′ × 43′

The whole Lion complex — about 90′ × 70′

At roughly 10,400 light-years, this frame spans about 190 light-years of gas.

Somewhere in Cepheus, along the faint band of the Milky Way, two stars are burning themselves out far faster than our Sun ever will. They are Wolf-Rayet stars — massive stars that have already blown away their outer layers and now pour out a hurricane of ultraviolet light and stellar wind. That light hits the hydrogen, sulphur and oxygen around them and makes it glow.

That glow is Sh2-132. Turned the right way up, its bright head and dark mane look like a lion — and what I caught in this frame is mostly the head: the bright bar across the middle, the dark dust lanes cutting down the left side, and a small curled pillar near the centre where the gas is being eaten away by starlight.

None of this is visible to the eye at the telescope. It took six nights, two filters and almost 400 five-minute exposures to pull it out of a suburban sky.

Same light, two ways of showing it

Top: natural colour (HOO). Bottom: Hubble palette (SHO). Same data, different colour mapping.

Both images use exactly the same photons. The nebula glows at a few very specific wavelengths: hydrogen-alpha at 656 nm (deep red), ionised sulphur at 672 nm (also deep red) and doubly ionised oxygen at 501 nm (teal). Narrowband filters let through only those lines and block almost everything else — including most of the light pollution.

In the natural colour (HOO) version each gas keeps roughly its real colour. That is why it is mostly red: hydrogen dominates, and sulphur is red too, so it simply disappears into the hydrogen. The oxygen shows up as teal around the edges.

The Hubble palette (SHO) reassigns the colours so the three gases can be told apart: sulphur becomes red, hydrogen green, oxygen blue. It is false colour, but it is not arbitrary — it is a map of the chemistry. Teal marks hot, oxygen-rich gas close to the young stars; gold is hydrogen and sulphur together; the orange-brown edges trace sulphur-rich fronts where the gas runs into the dark dust.

Sh2-132 in close-to-natural colour (HOO): hydrogen red, oxygen teal.

The long version

Sh2-132 is entry 132 in Stewart Sharpless's 1959 catalogue of hydrogen regions. It lies about 3,200 parsecs away — roughly 10,400 light-years — in the Perseus Arm of the Milky Way, the next spiral arm out from ours. The whole complex covers about 90 by 70 arcminutes, close to three full Moons side by side, which works out to well over 200 light-years across.

The main engines are two Wolf-Rayet stars, HD 211564 and HD 211853 (WR 153), helped by an O-type star and around ten hot B-type stars. Wolf-Rayet stars are the late stage of very massive stars: they have shed their hydrogen envelopes and blow a wind so strong that it carves bubbles into the surrounding gas. Radio surveys show expanding shells around the nebula that are most likely blown by exactly these winds. The open cluster Berkeley 94 sits embedded in the glow.

Star formation here has happened in waves in the past, but there is no sign of anything forming right now. What you see is a region being lit, pushed and slowly dispersed by a handful of stars that will themselves explode as supernovae within a few hundred thousand years.

The data side. This was my first long narrowband project with a one-shot colour camera and two dual-band filters: two nights with the L-eXtreme (hydrogen + oxygen) and four with the L-Synergy (sulphur + oxygen). Sulphur is the weak line — in a single five-minute sub it was barely four counts above the background — so it got most of the time.

The hardest part was not the sky but the calibration. A first practice run came out with pink-lavender edges that no processing could really fix. The cause turned out to be my dark frames: taken a month earlier on a warmer evening, their bias level sat about 33 counts below October's, and the flats amplified that error in the corners. A camera's bias drifts with temperature — something I had read about but never seen this clearly. New darks, a correction measured night by night from the corners of the calibrated frames, and the edges came out flat.

The two filters were stacked separately but aligned to one shared reference frame, so hydrogen, sulphur and oxygen land on exactly the same pixels. The oxygen from both filters was combined, weighted by noise. Bayer drizzle replaced normal debayering, which keeps stars and the faint sulphur noticeably sharper with a colour camera.

Acquisition

Dates: 30 Sep – 6 Oct 2026, six nights
Optolong L-eXtreme (Hα + OIII): 121 × 300 s = 10.1 h (2 nights)
Optolong L-Synergy (SII + OIII): 277 × 300 s = 23.1 h (4 nights)
Total: 398 × 300 s = 33.2 h
Camera settings: −10 °C, gain 0
Calibration: temperature-corrected 300 s dark masters, flats and dark flats for every night
Sub selection: 16 subs rejected (wind, dawn, autofocus failure)

Equipment

Optics: GSO RC8 carbon, 203 mm — 1271 mm, f/6.3
Camera: QHY8L one-shot colour CCD
Filters: Optolong L-eXtreme and L-Synergy
Mount: Sky-Watcher HEQ5 Pro (belt mod) on a permanent pier
Guiding: TS OAG + ZWO ASI220MM Mini, PHD2
Focuser: QFocuser HP with autofocus in N.I.N.A.
Control: N.I.N.A., PHD2
Image scale: 1.27″/px, field 64′ × 43′

Processing

PixInsight WBPP: per-night calibration, Bayer drizzle 1×, distortion-corrected registration, Local Normalization, PSF signal weighting
Channels: Hα and SII from the red pixels, OIII from green + blue of both filters, noise-weighted
Linear: DynamicBackgroundExtraction, BlurXTerminator, BackgroundNeutralization, StarXTerminator, NoiseXTerminator
SHO: SCNR and inverted-SCNR magenta removal, curves, saturation
HOO: Hα and OIII stretched separately, then combined
Stars: colour-calibrated, arcsinh-stretched and screened back in

Shot from a backyard north of Bucharest, Romania.

Sunday, September 20, 2026

The Eastern Veil

NGC 6992 — you are not looking at ribbons. You are looking at the edge of a bubble.

The full Moon, for scale

0.5°

This frame — everything the telescope can see at once

1.0°

The whole Cygnus Loop

3.0°

What you see here is one edge of something six full moons wide.

Somewhere between ten and twenty thousand years ago, a star in Cygnus ran out of fuel. Its core collapsed, the rest of it came apart, and for a few weeks it would have been one of the brightest things in the sky. People saw it. Nobody could write yet, so nobody wrote it down.

What's left is a bubble of shocked gas about 130 light years across, still tearing outward at roughly 1.5 million kilometres per hour.

Here's the part that took me a while to believe. Those ropes and ribbons aren't ropes or ribbons. The shell is thin — thinner, relative to its own size, than a soap bubble. Less than one part in fifty thousand of its radius. You only see it in the places where it curves edge-on into your line of sight. Everywhere else in this frame, the bubble is still there. You're just looking straight through it.

The red is hydrogen. The cyan is oxygen, glowing where the shock front is hottest and moving fastest, which is why it rides the leading edge of the arc rather than sitting inside it.

Five hours of exposure, one 8-inch telescope, a backyard north of Bucharest.

The long version

NGC 6992 is the brightest section of the eastern rim of the Cygnus Loop, catalogued as G74.0-8.5 and known variously as the Veil, the Network Nebula, or Caldwell 33. William Herschel found it on 5 September 1784 — the same year he catalogued NGC 7331, which was the subject of the last post here. His description of this one is unusually good: "branching nebulosity... the following part divides into several streams uniting again towards the south."

The remnant is middle-aged as these things go, with age estimates clustering around one to two times ten thousand years. X-ray spectra of the interior show a metal-rich plasma consistent with a Type II core-collapse event, so the progenitor was a high-mass star, and the remnant's unusually round shape suggests it exploded inside a cavity that the star's own wind had already cleared.

Distance has been argued over for decades. Minkowski put it at 770 parsecs in 1958 from proper motions Hubble had measured in 1937; later estimates ranged from 440 to 1400 pc. The current figure comes from Gaia parallaxes of stars sitting in front of and behind the expanding shell, giving 725 ± 15 pc, or about 2,400 light years.

The filament explanation is the standard one and it's worth stating precisely: the shock front is so thin compared with the radius of the shell that the emitting layer is only optically detectable when viewed exactly tangentially. Undulations in the surface produce several such tangent lines at once, which is why the filaments appear braided. Each one is roughly four billion miles thick — about the distance from here to Pluto.

Framing note: the Cygnus Loop measures roughly 2.8° × 3.5°. This telescope sees 1.00° × 42′. The whole object doesn't fit, and won't, even on the diagonal. The Eastern Veil on its own is a reasonable compromise.

Acquisition

Date	Night of 17/18 September 2026
Integration	40 × 450 s, 5 hours total
Calibration	Matched darks, flats, bias

Equipment

Optics	GSO RC8 carbon, 203 mm, with TS 2″ Photoline 0.8× reducer — 1271.5 mm at f/6.3
Camera	QHY8L one-shot colour CCD at −10 °C, gain 0, offset 140
Filter	Optolong L-eXtreme dual-band
Mount	HEQ5 Pro belt-modded, on PLL C82 pier, EQMOD
Guiding	ASI220MM Mini on TS 50 mm, pulse-guided, 4.13″/px
Control	N.I.N.A. 3.2.0.9001, PHD2 2.6.14
Image scale	1.265″/px, field 1.00° × 42′

Processing

This is HOO from a one-shot-colour Bayer sensor with a dual-band filter, not a mono camera and a filter wheel. The L-eXtreme passes hydrogen and oxygen in narrow windows, and the two lines land in different parts of the Bayer matrix, so they can be pulled apart afterwards into what are effectively two separate monochrome images.

The same frame, without the stars

Almost none of the stars in this picture have anything to do with the nebula. They sit in front of it and behind it, scattered across thousands of light years of the Cygnus arm, and the supernova remnant just happens to lie along the same sightline. Modern processing can separate the two — a neural network trained to recognise stellar profiles pulls them out into their own layer, which is normally done so that stars and nebula can be stretched differently.

Run that separation and throw the star layer away, and this is what's left.

The Eastern Veil with the foreground and background stars removed. The small dark specks are where the brighter stars used to be.

It's a strange thing to look at. The structure is the same, but without the stars to give a sense of depth, the shell reads much more clearly as what it is — a surface, curving away from you, catching the light only along its folds.

Siril — calibration, registration, stacking, then the dual-band data split into separate Hα and OIII channels.
GraXpert — AI background extraction to flatten the light-pollution gradient, then stellar deconvolution and AI denoise on each channel. Stars came down from 4.7 to 2.6 pixels; background noise dropped about 40%.
StarNet — stars separated from nebula, so the two could be treated independently. This is what fixes the cyan-star problem that HOO images usually have: the nebula gets a pure, unmixed Hα-red and OIII-cyan, while the stars get a blend of both channels that restores their natural colour.
Siril again — generalised hyperbolic stretch per channel, HOO composition, and the two halves recombined.
Photoshop — black point, colour balance, curves, a little saturation.

Shot from a backyard north of Bucharest, Romania.

Thursday, September 17, 2026

220P/McNaught
The comet that was supposed to be boring.

Before the outburst, late May 2026

mag 18

Fourteen hours later, 31 May

mag 11

Reported peak, June

mag 8.2

Each step down the scale is roughly two and a half times brighter. It covered ten of them in under a day.

This comet was supposed to be boring.

220P/McNaught, found by Robert McNaught in 2004, loops the Sun every five and a half years. Small, faint, forgettable. Then in June it exploded, brightening by a factor of thousands in under fourteen hours. Then in August it did it again.

The green glow is real. It's carbon gas, C₂, fluorescing in sunlight at 516 nanometres. The fan above the core is dust being pushed off the nucleus.

Here's the problem. Over three hours the comet drifted across the sky against the stars. Stack for sharp stars and the comet smears into a streak. Stack for a sharp comet and every star becomes a trail.

So you do both, then blend them. Ninety frames, three hours, one comet that refuses to die.