STAR CLUSTER
The Wishing Well Cluster
— NGC 3532 —
Processed by Nicolas Rolland, Paris. 6 hours gathered by Martin Pugh at El Sauce Observatory, Rio Hurtado, Chile.
On 20 May 1990 the Hubble Space Telescope opened its aperture door for the first time and pointed at this cluster.
First light
That first image was released the same day. It was sharper than anything taken from the ground, and it was also the first evidence that something was wrong: within weeks, engineers had traced the softness in Hubble's optics to a spherical aberration ground into the main mirror, and it would be three years before a servicing mission corrected it. NGC 3532 is therefore the object on which the world first saw both what the telescope could do and what it could not.
Coins in a well, or a rugby ball
The cluster lies about 1,300 light-years away in Carina and holds around 400 stars, spread across twice the apparent diameter of the full Moon. It has two unofficial names and neither is catalogued: some see the scatter of a handful of coins thrown into a well, others the oval of a rugby ball. At roughly 300 million years it is middle-aged for an open cluster — old enough that its heaviest members have already exploded, young enough that the next tier down are only now swelling into red giants while the rest still burn white and blue.
The red veil behind it
Most images of an open cluster show stars and black sky. This one does not, and the difference is the direction it faces: NGC 3532 sits against the Carina arm of the Milky Way, one of the densest concentrations of ionised hydrogen in the southern sky. The faint red wash across the field is that gas, recorded through an H-alpha filter and laid behind the cluster. It has nothing to do with the cluster itself — it is simply what happens to lie beyond it.
Six hours, and a frame too small
A Planewave CDK17 at f/6.8 with adaptive optics ahead of the sensor, from Rio Hurtado in July 2020 — six hours in all, four and a half across red, green and blue in ten-minute frames, and an hour and a half of H-alpha in three frames of thirty minutes. That is enough for the stars, which are bright, and only just enough for the veil: three frames establish that the hydrogen is there and stop short of giving it any structure. The other limit is the framing. This field runs 28 by 29 arcminutes and the cluster covers close to fifty, which means the frame holds its centre and leaves the outer members outside. Martin Pugh gathered the data at El Sauce.
