NGC 6438

A Distorted Galaxy Pair, or Maybe a Triplet System, can be Found in the Modest Constellation Octans, Home of the South Pole Star.

The faint constellation Octans seen from Pampa El Leoncito (San Juan, Argentina). 

Photo by the author.

Octans, a constellation with 3 primary stars charted by Abbe Nicholas Louis de Lacaille in the 1750s encompasses the south pole star, Sigma Octantis, a faint 5.5 magnitude star you can glimpse with the naked eye from a dark sky site. Another object contained in this constellation is NGC 6438, discovered in 1835 by John Herschel. First, let`s try to answer the question formulated on the tittle above. Is this galactic pair the closest one to the south celestial pole? Certainly no, another interacting system (i.e. NGC 2573A & B) is situated even closer to the pole star and not too far from the southernmost NGC object, the galaxy NGC 2573 (polarissima australis). However, that pair, with a magnitude around 14, is a target out of reach for owners of an 8-inch telescope, even in 6.5 limiting magnitude skies. Thus, we could say that NGC 6438 is the southernmost interacting system to observe with a common amateur telescope (e.g. 6, 8 or 10-inch). More than that, we can consider NGC 6438 the southernmost interacting system with its components very close to each other in projection on the sky (the components of NGC 2573 are not so close, as you can see in Figure 2). Of course, you need to observe somewhere south of the Equator. Another requirement is that you will need at least a latitude that makes possible to see this object at a reasonable altitude above the horizon. Being a circumpolar object for most of the Southern Hemisphere, another advantage is that NGC 6438 is a target you can study at any time throughout the year, specially for those living south of latitude -20° where this galaxies never go too low in the sky.

Figure 1. The NGC 6438 system and its surrounding field. North is up.
Photo by STSCI DSS.

I observed this system around local midnight from a dark sky site in Uspallata Valley (central west Argentina), not far from the Aconcagua Provincial Park in Los Andes mountains). The seeing conditions that night were really good, I should say excellent, to try to see as many details as possible using an 8-inch telescope. The system under study, NGC 6438 / 6438A* (also ESO 10-1 / ESO 10-2), lies about 3° 50` from the south pole star Sigma Octantis. 


Morphology of the system

NGC 6438, described in Dreyer`s catalog as "pretty bright, very gradually brighter in the middle" was also described by Shapley & Paraskevopoulos (1939) as a strange pair, probably a physical double, one spheroidal and the other belonging to the Magellanic type. A second description is given well afterward by Sérsic (1966) in which he notes the weak extension toward the north-preceding of the SO (NGC 6438) galaxy and the remarkable appearance of the irregular object (NGC 6438A) showing a nucleus and a disk of slightly smaller dimensions than the main body of the SO galaxy” (read more in the paper NGC 6438: A Triple System? C. J. Donzelli1, and M. Espíndola 1996). These researchers state that their analysis suggests that NGC 6438 is an interacting triplet, one S0 galaxy and two disk galaxies undergoing a merger, rather than a double system (E+S). In its now old paper "Southern Peculiar Galaxies II - NGC 6438" Sèrsic (1966) states that there are no radio sources coinciding with these galaxies. The Revised Data for NGC 6438 says that one of the galaxies is S0 type while the other one is a type ring B. This system appears also in the "Catalogue of Southern Peculiar Galaxies and Associations" (H. Arp, B. Madore, and W. Roberton) as AM 1806-852, in the category 2b: E+S interaction. In this sub-category an E-like galaxy is interacting with another galaxy which is classifiable as a disk or spiral galaxy. The sub-category contains some very disrupted pairs shown in the middle of the section, and spirals with long, open arms at the end.

Figure 2. The interacting system NGC 6438 (upper panel) and the fainter 

components of the system NGC 2573A & B (lower panel).
Photo by STSCI DSS.

At low magnification (42x), the brightest star in the 1° field of view is the 6.5 magnitude star HD 159517, situated just on the border of the field when the galactic pair is centered on it (see Figure 1). A very subtle nebulosity is visible through this kind of telescope at low power. The fact that the system lies between some faint stars makes me think that we are observing both the system and the faint nearby stars so higher magnification is necessary to try to discern the galaxies from the surrounding field. From the beginning, I recommend covering your head with a black blanket to avoid any unwanted surrounding light, however weak it may be. Also, try to use your averted vision as help as this object is definitively faint for a telescope of this size. Observing carefully, some of those stars could be identified. Applying averted vision a small hazy patch could be glimpsed, without a doubt the brighter part of the system that, according to Svend Laustsen, Claus Madsen, and Richard M. West in their book "Exploring the Southern Sky: A Pictorial Atlas from the European Southern Observatory (ESO), lies at 110 million light-years from us.

At 78x the view improves a little, the group of stars and the galaxy itself can be identified, but it is not an easy view. The shape of the very faint and smooth nebulosity looks, for moments, irregular (maybe a suggestion of the fainter companion NGC 6438A, a type Ring B galaxy with a visual magnitude of 11.6 and a surface brightness 12.5 (mag per square arcmin) according to the Wolfgang Steinicke's Revised NGC and IC Catalog web page).  

106x is a good magnification. Observing carefully and always with averted vision as help NGC 6438 appears, for moments, round and easier to see (or less hard to detect) than its elongated companion, NGC 6438A, that seems to be visible because a very faint, elongated East-West hazy structure reaches the star GSC-9527-1716, a faint 12.8 magnitude star (see upper panel on Figure 2). The whole systems look rather smooth in brightness, situated close to the chain of stars labeled with blue circles in Figure 2. All those stars could be glimpsed as very faint points of lights that night. The stars of the tight pair at one of the ends have magnitudes of 12.9 and 14. The star at the opposite end has a visual magnitude of 14.7 and it could be barely glimpsed through the 8-inch telescope, indicative of a dark, clean, and steady night for deep-sky observations.

196x was, maybe, the magnification that made it possible to see the galaxy pair more clearly, being always a challenging picture for an 8-inch. Taking advantage of the excellent seeing and very good transparency that night NGC 6438 was seen, at this power, round and with a slightly brighter core. An elongated hazy structure can be glimpsed roughly East-West as was aforementioned. Remember, an appropriate dark adaptation and the use of averted vision is a must to see this obscure target. A little worse view was obtained at even higher magnification (266x), with NGC 6438 barely glimpsed in the field of view, and with NGC 6438A not clearly identified.

A close encounter seems to take place in the Universe, not far from the southernmost point where you can aim a telescope from Earth, the south celestial pole.

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* NGC 6438A is not an NGC object but it is sometimes called in that way. This is the galaxy PGC 61793.

Remote Globular Clusters

Some of these swarms of old stars are Outer-Halo members of the Large Magellanic Cloud and are far away from it in the southern sky, appearing as "isolated" objects.

The Large Magellanic Cloud and its remote globular clusters.

A conspicuous "nebulosity" shows up high in the sky as soon as it becomes dark, after sunset, on any summer day in the Southern Hemisphere. That "nebulosity", actually a nearby galaxy, the Large Magellanic Cloud (LMC), is surrounded by some faint constellations, namely Dorado, Reticulum, Hydrus, and Mensa. You can enjoy the view of these constellations from a dark sky site. Now, did you know that those constellations host globular clusters that are members of our satellite galaxy? Of course you will need a telescope, at least an 8-inch diameter mirror, to see some of them.

This article deals with 5 globular clusters that are situated far from the central bar of the Large Magellanic Cloud, thus appearing as isolated stellar systems in the southern sky, in the south celestial pole neighborhood.

From any place on planet Earth south of -31◦ of latitude, this five objects never set, being thus what is known in astronomy as circumpolar objects. However, the summer months in the Southern Hemisphere (i.e. December, January, and February) is the optimum season to see them at their highest.

The Large Magellanic Cloud (LMC) is unique in containing massive star clusters at all stages of evolution.  Reticulum, NGC 1841, and NGC 1466, three of the objects included in this article, have at times been considered to be Milky Way globulars (e.g. Webbink 1985). Bengt E. Westerlund, in his book "The Magellanic Clouds" (1997), states that these clusters are all outside the sky-projected tidal radius, but still considered to be LMC members.

NGC 1841
R.A. 04 45 23.4  Dec. -83 59 56.6 (J2000.0)

This is an extragalactic globular cluster situated in constellation Mensa, discovered in 1836 by John Frederick William Herschel. In projection on the sky, this cluster is situated 14◦5 south of the center of the Large Magellanic Cloud, close to the border with the southernmost constellation Octans (see map above). In fact, NGC 1841 is not too far from the south polar star Sigma (σ) Octantis (roughly 6◦5). NGC 1841 could have been formed in a relatively isolated fragment of the proto-LMC, or it could have been part of an independent system now disrupted. Further support to the latter hypothesis could be the fact that NGC 1841 is the farthest cluster from the LMC center (∼ 10 kpc) (paper "The relative ages of LMC old clusters, and the case of NGC 1841" by  Ivo Saviane, Alfred Rosenberg, Giampaolo Piotto, & Antonio Aparicio, 2002). Evidence is provided that NGC 1841 is younger than the rest of LMC globular clusters. 

Figure 1

I observed this remote globular cluster from a dark sky site, with a clear and steady sky that night. Although several sources give a magnitude of 14.1 for this object (which would make it a target out of reach of an 8-inch telescope), it could be glimpsed through an 8-inch telescope working at 42x like a very faint and hazy patch of light showing a smooth brightness. This supports my opinion that this cluster should be brighter than that magnitude. The value of 11.4 given by the Simbad Astronomical Database  seems to be much more accurate. It is visible through an 8-inch (20cm.) telescope according with Ernst Johannes Hartung`s book "Astronomical Objects for Southern Telescopes: With an Addendum for Northern observatories". Averted vision slightly improves the detection of it at low magnification. The star HD 34017 (visual magnitude 9.1, spectral type F3V) is, along with HD 31132 (magnitude 8.8), one of the brightest stars in the 1◦ degree field of view. I used HD 34017 and the patterns of stars encircled with blue ellipses as guides to find the exact place where NGC 1841 lies (Figure 1).

At 78x, NGC 1841 appears in the field of view relatively big, round, faint, and smooth in brightness. As with lower magnification, averted vision helps for a better detection of this elusive object for an 8-inch telescope. It was not possible to resolve the cluster in some of its members.

John Frederick William Herschel

106x is a good power to observe this cluster. It looks pretty big, faint, and smooth without resolved members. Even higher magnification (148x) makes the view of NGC 1841 not very good, appearing very faint and being difficult to see because of the low contrast. Optimum Detection Methods give, if we consider a visual magnitude of 11.4 and angular dimension of 0.9x0.9 arc min for this cluster, an optimum magnification of 80x under a 6.3 or 6.4 limiting magnitude sky, and 75x under a 6.5 sky.

Remember, beyond all the theory and methods for improving the detection of any deep sky object, the best you can do is to go to the mountains, or a place with dark skies, and have your own experience observing the targets in your observing programs. It is observing how you can draw your own conclusions about how difficult is to detect an object through the telescope that you have. But do not stop there, try to observe the same object in different nights, so you can compare the results. Every observing night is a unique experience that enriches your passion for stargazing.

Reticulum cluster 
R.A. 04 36 09.0 Dec. -58 51 30.0 (J2000.0)

Figure 2. Position of the Reticulum Cluster in the sky

In 1974, Sèrsic discovered an object in constellation Reticulum on plates taken with the 0.7m Maksutov telescope at Cerro El Roble Observatory. At first, he catalogued it as a probable dwarf galaxy and member of the Local Group. More recently, Demers & Kunkel (1976), and Gratton y Ortolani (1987) suggest that "Reticulum system" is a globular cluster of the Large Magellanic Cloud. Reticulum is an old and sparsely populated globular cluster that is located ≈ 11◦ from the center of the LMC (Demers & Kunkel 1976). Paper "Variable Stars in Large Magellanic Cloud Globular Clusters III: Reticulum" by Charles A. Kuehn et. al. 2013. Reticulum is classified as an OoI type1 cluster (Oosterhoff classification). It is located at approximately 11◦ from the LMC bar (l =269◦ , b=-40◦). 

This cluster resides in constellation Reticulum, very close to the border with Dorado (see Figure 2. Enlarge for a more detailed view). A way to find it is to use the stars Alpha (α) Reticuli, Alpha (α) Doradus, and Zeta Doradus that form a triangulum in the sky. The cluster lies roughly in the spot where bisectors of that triangle cross each other.

Figure 3

Once there, I observed the field at low magnification, knowing in advance that this is a faint globular for this kind of telescope. After identifying the star GSC-8515-1355 (magnitude 12) and the pair TYC 8515-1611-1 and GSC-8515-0765 (visual magnitudes 10.7 and 11.8 respectively) I focused on the small region between that stars (center of the field of view on Figure 3) to try to glimpse this extragalactic globular. Only a few faint stars are visible there at this magnification, surely foreground stars of our Milky Way galaxy.

At higher magnification (63x), the view is similar. The same stars are visible (labelled with blue circles in Figure 3). Even higher power (118x) did not show more than that faint stars scattered in the area. The faint star labelled with a red circle could be very barely glimpsed using averted vision at this last magnification.

As a final remark, this globular cluster is a faint object for 8-inch telescope. Owners of bigger mirrors should be glimpse this member of the Large Magellanic Cloud when observing from a dark sky site.

ESO 121-SC03
R.A. 06 02 02.50 Dec. -60 31 25.5 (J2000.0)

ESO 121-SC03. Picture from AladinLite view

ESO 121-SC03 lies at a projected angular separation of ∼ 10◦ from the LMC centre. It was first studied in detail by Mateo et. al. (1986). David Stevenson in his book "The Complex Lives of Star Clusters" (2015) estimates the age of this globular in around 9 billions years.

At 42x, the field where this object, also known as KMHK2 1591, is situated looks interesting with some stars forming fine shapes. The brightest star in the 1◦ field of view is the 6.4 magnitude SW Pictoris. The chain of four stars (indicated with blue lines in Figure 4), which contains the 6.9 magnitude star HD 41451 at its eastern end, and the stars forming a sort of crown (center of the field in Figure 4) helped to find the zone where this faint and obscure member of the Large Magellanic Cloud lies. This 14 magnitude cluster with angular dimension 2.0 x 2.0 arc minutes is, without a doubt, a faint object for an 8-inch telescope. Obviously at this low magnification nothing was visible in the field.

Figure 4

According with the paper "Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - II. The unique LMC cluster ESO 121-SC03" by A. D. Mackey et al. (2006), this cluster may lie 20 per cent closer to us than does the centre of the LMC.

At 78x, ESO 121-SC03 is not visible after observing carefully under a dark and steady sky. I observed it again from another place, higher in Los Andes mountains, but I had the same results after using 118x. In both nights and from both sites, something seems to be there when observing with averted vision, but it is hard to assure that the cluster can be glimpsed. As I always say, if you are not sure you saw any object, you must say "I did not see it". 

The brightness of this extragalactic object reserves the view only for bigger instruments. Higher magnification (e.g. 160x, 222x) gave the same results, no object visible in the field.

NGC 1466
R.A. 03 44 32.4 Dec. -71 40 16 (J2000.0)

Figure 5

John Frederick William Herschel discovered NGC 1466 in 1834, a globular cluster with a magnitude of 11.4 situated in constellation Hydrus. It is listed, along with NGC 1841, 2257, and Reticulum, as one of the oldest generation globular (10 Gyr) in our nearby galaxy according to B. E. Westerlund in his book "The Magellanic Clouds" (Cambridge University Press, 1997). 

With roughly 8◦ of separation with the Large Magellanic Cloud in the sky, this member of our satellite galaxy is visible at 42x in a field that offers a beautiful image. The 6.3 magnitude HD 24188 is, by far, the brightest star in the 1◦ field, visible close to the cluster. This makes easy to find NGC 1466. At this low power, NGC 1466 looks like a slightly defocused star situated between the stars HD 24115 (visual magnitude 9) and the fainter GSC-9156-0534 (visual magnitude 12). At this magnification, the globular cluster reminds me a round planetary nebula. With averted vision, the core of NGC 1466 appears somewhat brighter.

NGC 1466. ESO Online DSS

At 78x it is possible to get a good view of the cluster and the stars surrounding it. NGC 1466 appears obvious in the field of view of the telescope. Even with direct vision the cluster shows some "granularity". Applying averted vision, at least two stars can be discerned.

106x seems to be a good magnification to observe NGC 1466, appearing very clear in the field of view as a round hazy patch. Some members of this cluster are easily discerned.

Even higher magnification, like 148x, is a good power to see this cluster under a very dark sky. You can see a nebulous patch and some stars within when using averted vision..

NGC 2257
R.A. 06 30 13.86 Dec. -64 19 32.2 (J2000.0)

Figure 6

NGC 2257 (also ESO 87-24, KMHK 1756) was discovered in 1834 by John Frederick William Herschel. NGC 2257 appears of particular interest since according to Stryker 1983, Nemec Hesser and Ugarte 1985 (NHU) and Walker 1989 (W89) it is probably the oldest LMC cluster. This object is situated about 8◦5 northeast of the LMC bar in projection on the sky.

I observed this LMC cluster when it was about 59◦ in the sky, just a few minutes after its transit by the local meridian, so that the altitude was the best to try to catch this old member of our satellite galaxy. At 48x, I could identify an asterism which I indicate with lines in Figure 6. This asterism was very userful to focus the atention on the spot where NGC 2257 should appear. This cluster can be glimpsed at this magnification, appearing as a very faint, round, and smooth patch of light immediately to the West of the stars TYC 8902-1763-1 and GSC-8902-0650 (visual magnitudes 10.9 and 12.4 respectively). Averted vision sligthly improves the view of it.

At 63x the view of the cluster is similar to that at lower power. Without a doubt, a "nebulosity" with smooth brightness and without any discerned star can be seen in the field. It looks round in shape. It is a faint object (magnitude 12.6) so averted vision helps for a little sharper image of the cluster against the starry field.

Figure 7

118x shows this object faint, a ghostly image of a globular cluster. Even with averted vision the view is hard and elusive. No stars are discerned in the cluster at this power. The angular dimension of NGC 2257 looks similar to the distance between the aforementioned stars TYC 8902-1763-1 and GSC 8902-0650 (see Figure 7).

There were not better results applying higher magnification (namely 222x). Again, a faint and featureless image of the cluster showed up in the eyepiece field. Even if NGC 2257 is visible through an 8-inch telescope, the view through bigger scopes should be more interesting and fruitful.

“Astronomy, as nothing else can do, teaches men humility.”

Arthur C. Clarke (1917-2008)

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1_ The Dutch astronomer Pieter Oosterhoff noticed that there appear to be two populations of globular clusters, which became known as Oosterhoff groups. The second group has a slightly longer period of RR Lyrae variable stars.^[39] Both groups have weak lines of metallic elements. But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII).^[39] Hence type I are referred to as "metal-rich" (e.g. Terzan 7^[40]) while type II are "metal-poor" (e.g.ESO 280-SC06^[41]).

2_ KMHK refers to a list of LMC clusters published in 1990 by M. Kontizas, D.H. Morgan, D. Hatzidimitriou and E. Kontizas (Astronomy and Astrophysics Suppl. Series, Vol. 84, p. 257).

Planetary Nebulae - Part 2

Some Fascinating Planetaries Inhabit 

the Constellations of the Winter Southern Skies 

Part II. Ring planetary nebulae

A winter starry night anywhere in the Southern Hemisphere shows a region in the sky, between Right Ascension 12 and 19hs, where some constellations like Crux, Centaurus, Norma, Ara, and Sagittarius host some interesting planetary nebulae. The four objects included in part I of this article are bipolar planetaries, one of the five types in the morphological classification by Schwars et al. (1992), i.e. Elliptical (e), Bipolar (b), Point symmetrical (p). Irregular (i), Stellar (st). Part II of "Intriguing Planetary Nebulae" is devoted to the so-called ring or annular planetaries. Below there are three good examples of this kind of planetary nebula. As an observer, you surely know the Vorontsov-Velyaminov classification which classifies ring-shaped objects as type 4.

If you have at least an 8-inch telescope, all of the planetaries mentioned in this article are visible. Of course, you will be able to find more features or analyze their morphologies in more detail if you observe them through bigger instruments. 

Shapley 1

The observing site in Pampa El Leoncito. The dome of the 2.15m telescope 
of CASLEO observatory is visible in the middle of the photo.
CASLEO  is situated about 14kms from this site in a straight line.

In the inconspicuous constellation Norma, in the southern sky, a fine and faint planetary nebula resides. I am talking about Shapley 1 which shows a nice annular shape at least in astrophotographies. Shapley 1 (α = 15h 51m41s , δ = −51◦ 31′ 23′′, J2000), discovered by H. Shapley (1936), was described as “nearly perfectly circular” in appearance by Bond & Livio (1990), see the paper "The morphology and kinematics of the Fine Ring Nebula, planetary nebula Sp 1, and the shaping influence of its binary central star⋆" by D. Jones et al. (2011). A hypothesis states that Shapley 1 is actually an axisymmetric nebula viewed almost pole-on. It is also classified as type 4 in the Vorontsov-Velyaminov classification scheme. Sources give a visual magnitude of 12.6 for this planetary that is also named PK 329+2.1.

I observed this planetary in two nights from a very dark site just a few kilometers north of CASLEO observatory in San Juan province, Argentina, in the majestic Andes mountain range. The first night was good enough to carry out deep-sky observation but the seeing was not as good as the second night. Even a cold wind bothered most of the night. 

At low galactic latitude, immerse in the band of the Milky Way, we can find this fine annular shape planetary nebula, a target for an 8-inch telescope if you know where to aim it in the sky.

Picture taken using the Photopic Sky Survey ©Nick Singer.

After aiming the telescope to the zone where Shapley 1 lies (see map above) I could found the 1-degree field of view surrounding the planetary. Shapley 1 is situated between the stars labeled with circles in Figure 1. The three stars linked with a red line helped for an accurate search of the position of the planetary as it forms a "rhombus" with them. 

Figure 1. 1-degree field of view with Shapley 1 at the center. North is up.

At 63x without a nebular filter, the planetary is not visible (at least that night under the aforementioned conditions). However, a UHC filter helps to detect the ghostly image of Shapley 1. It can be barely glimpsed using averted vision appearing like a very faint nebulosity rather smooth in brightness. The target was low in the sky for a more detailed observation that night (around 25°), so I decided to make another observation the following night. 

A new observation earlier in the second night (at the end of the local astronomical twilight) made possible to see the planetary higher in the southwest sky (37°). This higher altitude plus a more steady sky made possible a better detection and view of this Milky Way object. At 63x, Shapley 1 could be glimpsed even without a nebular filter, looking like a very faint, roundish, and hazy smooth patch. Applying a UHC filter the view definitively improves. Through this kind of filter, the object is undoubtedly visible. Averted vision makes possible to detect, for brief moments, the west edge appearing slightly brighter than the rest of the planetary (indicated with A in Figure 2).

At higher magnification (119x) Shapley 1 can be barely seen using averted vision. Now some faint stars can be identified in the zone, like those indicated with arrows on Figure 2 that is very elusive through an 8-inch telescope and you need averted vision to barely glimpse them. One of them is the central star of the planetary which has a visual magnitude of 14 according to several sources. That central star seems to be a close-binary central star system according to D. Jones's paper. At this magnification, it is visible with averted vision appearing very faint. Moreover, other faint stars are detected in the area. At this magnification, the detection of the planetary is faint through a UHC filter. However, this power makes it possible to detect the inner central part of Shapley 1 darker, a suggestion of its annular morphology. Again, the west rim of the planetary seems to appear slightly brighter when saw it with averted vision.

Figure 2. DSS image of Shapley 1 showing its annular shape.

North is up. East to the left.

At 162x the planetary appears very faint at the eyepiece. The faint stars aforementioned look a little easier, but they are still elusive. Because of their positions, that stars help to delineate the shape and angular size of Shapley 1. UHC filter helps but the view at this higher magnification is ghostly and challenging.                                                                  

Shapley 3

Position of the peculiar planetary nebula Shapley 3 in the winter southern sky.

Picture taken using the Photopic Sky Survey ©Nick Singer.

Figure 1. DSS image of Shapley 3. The gray circles on the image indicate areas of

brighter stars

Late August is a good moment to observe another remarkable planetary nebula, Shapley 3, which was first reported by Harlow Shapley in 1936. According to the book "Hartung`s Astronomical Objects for Southern Telescopes 2nd. Edition" by David Malin and David J. Frew, this planetary nebula is visible in an 8-inch telescope as a faint circular glow 30" wide. This object, also known as Hen2-341 and PK342-14.1, has a magnitude of 11.9 and a diameter of 36". To find this planetary is easy because it lies only 56 arc minutes to the south of the naked eye star Theta (θ) Arae (visual magnitude 3.7). If you can find NGC 6397 and enjoy the view of this nearby globular cluster, it can be useful to know that the planetary lies about 4.8 degrees northeast of it. 

Using the Optimum Magnification Methods and considering that a visual magnitude of 11.9 and an angular size of 0.6 arcmin are accurate values, the optimum magnification to detect this Milky way´s nebula is 100x for an 8-inch telescope working under a 6.2 limiting magnitude sky. So, let`s see what I could see at different magnifications from a site that offers not exceptionally dark skies but dark enough to carry out this kind of observation.

Figure 2. DSS image of Shapley 3 and its surrounding stars

At low magnification (42x) the field where Shapley 3 lies is rich in stars. Several of them are bright (around magnitude 9) for a telescope. In Figure 1 I indicate with circles the zones where the brighter stars lie. The zone between those mentioned areas shows not too many stars and all of them are faint. At this power, the stars linked with a blue line were used as a starting point to find the accurate position of Shapley 3. Without a doubt, this planetary is a challenging target for an 8-inch telescope. In spite of that, it can be barely glimpsed using averted vision as a quasi-stellar object, very small in apparent size. The very faint stars immediately surrounding this object can be also detected using averted vision, like the one indicated with the arrow in Figure 2 that was useful to find the accurate position of the planetary nebula. Using the Orion Ultrablock filter the planetary looks like a very small hazy spot. Averted vision is necessary to glimpse this faint object. With UHC the view is slightly improved. However, at this low magnification, the view is difficult appearing as a subtle and small hazy dot.

At 78x the faint neighboring stars are better viewed so it is easier to identify Shapley 3 in the field. Now the planetary nebula looks, even with direct vision, like a faint and small nebulosity surrounding a central star, which jumps to the view more easily when applying averted vision. With a UHC filter, the view is very different. It improves the view of the planetary a lot, appearing still faint but with a clear nebular nature, round, and rather smooth in brightness. For moments the central region appears brighter because of the presence of the central star. The view through the Orion Ultrablock filter is not so useful as that with the UHC filter. Shapley 3 does not appear so detached as with UHC. The central star is better viewed, always surrounded by subtle nebulosity.

Shapley 3. Image from Simbad database

Higher power, like 106x, allows you to see the central star which is better detected using averted vision. This 12 magnitude star is surrounded by a very subtle nebulosity. UHC filter improves the view of this planetary nebula appearing as a round hazy spot of smooth brightness that can be identified from the surrounding stars. Dark adaptation is a must for an optimum view of this faint object. Using the Orion Ultrablock filter at this magnification the view did not depart from that without a filter. It was not as useful as the UHC in order to get a better contrast.

I got similar results after observing Shapley 3 with a little higher power (148x). However, the planetary appears even fainter, especially through the UHC filter. Orion Ultrablock offers a not so detached view of this object when comparing with UHC.

IC 4642

IC 4642 is a faint planetary nebula situated in constellation Ara, not so far from NGC 6397, one of the nearest globular clusters.
Picture taken using the Photopic Sky Survey ©Nick Singer.

IC 4642 ©R. Corradi et al.

For those observers who want to look for more challenging planetary nebulae with an 8-inch telescope, this one is an interesting target situated at the core of constellation Ara. IC 4642 (also PK 334-9.1) was discovered by Williamina Fleming in 1901. Some planetarium software, like Skymap for example, classifies this object as type 4 (ring shape in the Vorontsov-Velyaminov scheme). This planetary nebula is listed in the paper "Precessing Jets and Point-Symmetric Nebulae" by J.A. Cliffe et. al (1995) as having a bipolar/point symmetric symmetry. 

This 15" size object is immersed in a field that shows several faint stars. Some brighter stars are visible in the east half of the field of a telescope working at low magnification. In a 1-degree field, the brightest star is the 8.3 magnitude HD 154970. The stars linked with a blue line in Figure 1 are useful to find the planetary which forms a sort of "question mark" shape along with four faint stars of 11 and 12 magnitude (linked with pale blue lines on Figure 2 below). The 12 visual magnitude variable star V788 Arae, a semi-regular pulsating star according to the Simbad Database, is indicated in Figure 2 with a blue arrow. To use that asterism makes the identification of IC 4642 very easy. At low magnification (63x) and without any nebular filter IC 4642 is visible, appearing like a very small hazy disc that can be glimpsed even with a direct vision.

Figure 1

Averted vision, on the other hand, allows seeing a more detached view. A UHC filter improves the contrast at this power.

Jumping to a higher magnification (118x) the view of IC 4642 is more obvious at the eyepiece, clearly looking like a small, round nebulosity among the surrounding stars. Through a UHC filter, the view is definitively beautiful with the planetary detached against the background sky, rather smooth in brightness, and circular in shape. An Orion Ultrablock filter also improves the view, which is pretty similar to that obtained with the UHC filter but somewhat brighter through an 8-inch telescope.

This power (160x) makes the planetary to appear round and smooth in brightness. With averted vision the core of this object appears to look a little darker for moments, suggesting (in a rough way of course) a ring-type structure. Seeing conditions at the moment of this analysis was not the best, so it would be good to make another observation under more optimum conditions to compare results. The observation using the Orion Ultrablock shows a round and smooth nebula. Averted vision makes possible to detect what appears to be a point-like feature, slightly brighter, on its west side (indicated with a white arrow in Figure 2). It is important to say that this feature was very difficult to confirm.

Figure 2

At 222x IC 4642, round in shape, seems to show a somewhat darker core. However, this view is very subtle. Nebular filters, as in other cases, helped to show a more detached view of this planetary.