Harry continues the urban sky challenge – seeing deep sky objects from the suburbs – by finding and sketching “The Grus Quartet”

grus cluster1

A sketch of the Grus Quartet galaxies from a suburban location. Image and copyright Harry Roberts ©, all rights reserved

After dusk overhead in spring and towards the south-west in the middle of summer, the Grus star group hosts some remarkable deep sky objects, objects bright enough to be seen with ‘scopes big and small from urban sites. And yet the patch of sky dominated by Grus, Sculptor etc looks a bit empty compared to Milky Way areas like Orion or Canis Major say, that rise later in the night.

Grus contains about 30 galaxies brighter than magnitude 13, many grouped in clusters; all are seen best from a “real” dark site, but some are visible in suburbia. After recent views of bigger, closer, galaxies, I tried to track down some of the more distant ones.

A pair of mag 4 stars, Iota and Theta, on the east side of Grus were the starting point, as a cluster of galaxies is charted nearby, 2º NE of Theta. These galaxies are NGC numbers 7552, 7582, 7590 and 7599 (Fig, shortened to N7552 etc), and the red-dot finder soon had the 10in ‘scope on target, with a fairly bright compact spiral in the field with some mag 10 stars nearby. This, the chart’s said, was N7552, an 11.6 mag barred spiral.

Continuing NE from Theta half a degree led to a trio of galaxies, heralded by N7582, a shapely mag 11.8 edge-on spiral, a nice object seen well with direct vision, and appearing much larger with averted vision. N7528 has an elongated bright core surrounded by fainter outer regions.

The other two galaxies, N7590 and N7599 were harder objects with direct vision at my suburban site; N7590 was brightest of the pair with a central core and irregular outline. N7599 was a challenge; distinct with averted vision, but almost invisible with direct, and oval shaped. N7590 and 7599 are magnitudes 11.9 and 12.0 respectively, and tougher “gets” than their brighter companions.

The three brighter members of the ”Grus Quartet” of galaxies were discoveries of James Dunlop, working at Parramatta in 1826 (perhaps with the three inch refractor) [More likely with his home-built 9-inch (23-cm) reflecting telescope that he used to produce the catalogue of 629 clusters and nebulae – Nick] – but fainter N7299 was found by John Herschel at Cape Town ten years later, suggesting a better scope than Dunlop’s [Herschel’s reflecting telescope was twice the size of Dunlop’s and, according to Herschel, seven times the light-gathering power – Nick].

The “Quartet” members are spirals – all about 70 million light years distant and around 80,000 L.y. in diameter – their recession velocity is 1600 km/s – so they are truly a quartet. The sketch suggests they are all “edge-on” – but this is misleading – their elongated cores are much brighter than their outer regions – and mostly the cores were seen – their spiral arms recorded in photographs were not seen either.

While some prefer UHC filters for galaxy viewing none were used for the sketch – but full dark adaption and a head covering were employed – as well as a higher power of 150, which darkens the background and improves contrast.

There is something satisfying in gazing at a cluster of galaxies with your own eyes (rather than Hubble’s) – countless stars and “worlds” in the field of view with, perhaps, myriad unimaginable life forms. Astronomy – it’s so amazing!

Harry Roberts is a frequent contributor to this blog and a member of the Sydney City Skywatchers.

Share

2 responses to “Harry continues the urban sky challenge – seeing deep sky objects from the suburbs – by finding and sketching “The Grus Quartet”

  • Just noticed a typo in my post that reads:

    ” There are three HII regions along the plane of NGC 55 that can be seen using this technique with 10-12″ ‘scopes. In an 18″ ‘scope, almost a dozen HII regions will brighten to the application of a UHC filter including NGC 604 (a Tarantula Nebula — NGC 2070 analogue) and many northern hemisphere observers report that M101 displays several in an 8″.”

    Should read:

    “Viewing M33 in an 18″ ‘scope, almost a dozen HII regions will brighten to the application of a UHC filter …”

  • Hi Harry,

    Great post mate and sketch much enjoyed. Just one or two comments:

    UHC filters should almost never be used to observe galaxies whatever the environment. These filters are designed to pass three wavelengths relatively unhindered — the dual doubly ionised Oxygen lines at 496 & 501 Nm and the Hydrogen Beta line at 486Nm. They pass relatively little visible light apart from these. They will dim starlight by about 90% (ie approx 3.5 magnitudes) because the filter is designed to reject it (and light-pollution too). They are designed specifically to boost visual contrast on planetary and emission nebulae because these are usually strong line-emitters of these three wavelengths.

    On the other hand, the light we see from galaxies at visual wavelengths is almost exclusively continuous spectrum starlight, so a UHC filter will only serve to make the galaxy dimmer in the eyepiece no matter the observing conditions.

    The only time you would use it is on a handful of relatively nearby galaxies that have large, (relatively) bright HII regions (nebulae) embedded within them. A few indicative examples include M33, M101 & NGC 55 where using a UHC will dim the starlight made by the galaxy, but will boost the HII regions. There are three HII regions along the plane of NGC 55 that can be seen using this technique with 10-12″ ‘scopes. In an 18″ ‘scope, almost a dozen HII regions will brighten to the application of a UHC filter including NGC 604 (a Tarantula Nebula — NGC 2070 analogue) and many northern hemisphere observers report that M101 displays several in an 8″.

    But to see the galaxy itself, unfiltered is the go. Some observers are of the opinion that a wide band-pass filter such as a “Deep Sky” filter or a “Light Pollution Reduction” filter is of limited use in urban areas on galaxies. I respectfully disagree. I believe, as you demonstrated in using x150, that maximum visual contrast on small low surface-brightness objects is best achieved with an eyepiece that provides an approximately 2mm exit-pupil. Exit pupil is approximated by dividing the aperture in mm by the magnification used. x150 on a 10″ ‘scope is about x1.7mm exit pupil.

    Completely agree with your comments about gazing at a cluster of galaxies with your own eyes !

    Best,

    Les Dalrymple
    Guide-Lecturer Sydney Observatory
    Contributing Editor, Australian Sky & Telescope Magazine.

Leave a Reply

Your email address will not be published. Required fields are marked *