Constellation of the Month - Auriga

Albert Brakel

Auriga can be found between Taurus and the northern horizon on December to February evenings. Its brightest star, the yellow Capella, is the sixth brightest star in the sky, and culminates at an elevation of only about 10 degrees at our latitude. In mythology, Auriga is portrayed as a charioteer holding a goat and kids, hence the name of Capella, meaning "little she-goat". The brightest stars of Auriga form a pentagon, if we include Beta Tauri which was formerly called Gamma Aurigae. The constellation lies in the northern Milky Way, and so contains the usual retinue of objects befitting such a position.

M37 (5h 52.4m, +32 33'), lying a bit SE of midway between Beta Tau and Theta Aur, is a rich open cluster, and the prettiest one in the constellation. 19th Century observers, as usual, tried to outdo each other in their descriptions. C.E. Barns called it "a diamond sunburst", while W.H. Smyth wrote of the "whole field strewn as if it were sparkling with gold dust". Superlatives aside, it is a densely-packed group of stars (150 brighter than mag. 12) with a sprinkling of medium-bright ones. One of the latter is an orange star dramatically placed at the centre of the cluster, and the brightest in the group, making all the difference to the cluster's appearance. I tried a couple of magnifications. Although filling more of the field at 117x, the cluster looked better at 77x, because the contrast between the rich cluster and the darker sky beyond is more obvious.

Four degrees to the NW, M36 (5h 36.1m, +34 08') is a more loosely-spaced cluster of mainly medium-bright stars up to mag. 9, with some fainter ones, when seen at 77x. One pair of medium stars is located on the SE side of the central region. 117x discloses more faint stars. The group is easily seen in finder scopes.

Further NW, M38 (5h 28.7m, +35 50') is larger and brighter than M36, and consists of medium-bright and faint stars, with most of the brightest being arranged in the form of a rough cross with the E-W arms longer than the N-S arms. The centre is empty except for a star at the intersection of the arms of the cross. Some references described the stars as being arranged roughly in the shape of the Greek letter pi, and while it's possible to imagine this shape, I didn't find it obvious.

About halfway between M36 and Phi Aurigae is the small (about 3') emission nebula NGC 1931 (5h 31.4m, +34 15'). It looks vaguely like an elliptical galaxy with some stars involved. Observers with average eyesight will need at least an 8" scope and a dark sky. Large scopes should be able to see it as a double-lobed shape.

Go about the same distance west of Phi Aur as M36 is to the east, and you will come to the variable star AE Aurigae (5h 16.3m, +34 19'). This is a flare star that is moving rapidly away from its point of origin in Orion. On the way it encountered by chance a cloud of gas and dust that it is at present faintly illuminating as the Flaming Star Nebula (IC 405). Photographs show that this 18' diameter nebulosity contains filaments similar to the Veil Nebula in Cygnus.

SE of AE Aur the fairly sparse open cluster NGC 1893 (5h 22.7m, +33 24') contains about 20 stars between mag. 9 and 12 in an irregular area 12' across. It is enveloped in the very faint haze of the emission nebula IC 410, visible only in a very dark sky. The 2nd edition of Hartung says that a H-Beta filter gives a marked improvement, especially to the east and south of the cluster.

The only bright planetary nebula in Auriga is NGC 2149 (5h 56.3m, +46 07'), located north of Beta Aur and west of Pi Aur. It is a bluish-green disk about 10' across, and as usual is enhanced by an O-III filter.

If you're starting to get impatient for some double stars, yes, Auriga does have some, like all constellations.

Omega Aur (4h 59.3m, +37 53') is a good-looking mag. 5.0 and 8.0 white and yellowish pair, 5.2" apart.

Struve 644 (5h 10.3m, +37 18) can be split in good conditions, as the separation is 1.6" and the magnitudes are equal (6.7 and 6.8). In average conditions, even the unresolved elongate object is interesting, because one end looks white and the other orange.

Theta Aur (5h 59.7m, +37 13') is 3.4" apart with a large magnitude difference (2.6 and 7.2). At this low elevation, small apertures will often have an advantage over larger ones, because they are less sensitive to poor seeing.

41 Aur (6h 11.6m, +48 43') - yes, I know it's getting close to the horizon, but if you have no obstructions, this nearly equal pair (6.3 and 7.0, separation 7.6") is very easy even with small scopes, and is a good one to finish with.

Beta Phe as a Binary Star

Ross Gould

I recently [As at 1997-12 Ed] had a phone enquiry from John Morland regarding the supposedly fairly easy double star Beta Phoenicis, which he and an observing friend had found very difficult to separate. John had succeded with a 12-inch scope at fairly high power, his friend with an 8-inch did not split the pair.

Beta Phe is found at 01h06.1m, -46d43’. There is a wide companion of 11th magnitude noticed by John Herschel - the main pair was discovered in 1891 by RP Sellors at Sydney Observatory, consisting of two yellow stars of mags 4.0 and 4.2.

Hartung indicated the pair was not particularly difficult, “clearly divided with 15cm”. At that time he suggested the separation had been increasing. The measure he quoted (1.4”) suggests a good 4-inch scope should split it. So I went to check the measures, including recent ones.

Sure enough, Beta Phe is now closing, after a period of widening separation. To give a list of the measures I found -

As you can see, the position angle has been decreasing, while the separation gradually increased until 1954, slowly decreased until 1982, then rapidly closed. It is now quite difficult. Notice that the change in PA, like the change in separation, has become quicker recently - from about 1/2 degree per year early this century, to nearly 2 degrees per year from 1982 to 1991. The pair is in the periastron section of its orbit, and after some time can be expected to widen again.

The period of this binary is not yet determined, but cannot be short given the arc traversed in the last 100 years. Also, we shall have to wait to see what is the minimum separation - and for how long the pair stays very close (or closer).

I then looked through my observing notes - last December I observed Beta Phe, and found it only elongated with the 18cm refractor at 270x.

I observed it again recently with 18cm at 330x, and found much the same - a definitely elongated but not split image, in pa estimated perhaps a little under 300. It requires good seeing, and a telescope above 20cm is probably needed for a clear split now.

Telescopes for Comet Hunting

Ross Gould

Visual comet hunters have used quite a variety of equipment in their pursuit of the elusive fuzzies, with some observers using reflectors, others refractors, and some big binoculars. There is unexpected equipment too - long focus telescopes not able to give the very wide fields and low power expected.

Here I shall not talk about search strategies, the need for good charts, nor the best times and sky areas to favour. For that Read Vello’s accounts (in past issues of Southern Cross) of his discoveries, and items by Bill Bradfield and others. Instead I shall look at some notable comet discoverers, and the kind of equipment they used. First, to some history.

Early Days

Charles Messier had a variety of telescopes, but for comet-hunting often used a small telescope of very low power - a 2-inch refractor of only 5x magnification. This is an example of overdoing the low power approach - but it would certainly have given a large field of view. His 13 discoveries were spread over a long period - from 1759 to 1801. Messier these days is remembered mostly for his catalogue of deep-sky objects, compiled as “non-comets”.

In the same period Caroline Herschel found 8 comets between 1786 and 1797 using a telescope made by her brother William Herschel. Caroline’s telescope was a 4-inch f/6.7 Newtonian reflector, with a power of 20x, giving a 2deg 12’ field. An unusual point is that this ‘scope was mounted with a pivot opposite the eyepiece, so that the eyepiece had a fixed position.

Although the f-ratio is middling by today’s standards, it should be remembered that William Herschel’s reflectors were normally of long ratio - around f/12 or f/13 was common. The common Huygens eyepieces of the time are reasonable at f/12 or longer, but very poor at short ratios. Herschel seldom used Huygens eyepieces even in his long reflectors - he experimented with eyepieces of his own design, preferring for high power single-lens oculars. Where a large field was wanted, he reverted to double-lens designs.

One suspects a Ramsden style eyepiece would be preferable in the comet seeker, because it would give much sharper images at f/6.7 than a Huygens. Jesse Ramsden had described his new eyepiece design in a 1779 paper to the Royal Society. This period is long before orthoscopics or plossls became an option.

The Prolific 19th Century

Jean-Louis Pons set the record for visual discoveries of comets by an individual. Originally appointed concierge at the Marseilles observatory in 1789, he was given astronomy instruction from the observers, and soon allowed to use the telescopes. He took to comet hunting, his favourite telescope one that gave a 3-degree field of view (this suggests a magnification around 15x). He apparently had a remarkable ability to remember star fields, and therefore to recognize changes.

His first comet was found in July 1801, a discovery shared with Messier who found it the following day. Pons found 35 comets, but only 27 are credited to his name - his last was in 1827, giving a rate of 1 per year. Not all these were found from Marseilles - he became director of Marlia Observatory in 1819, and of Florence Observatory in 1825, at both of which he made discoveries.

Ernst Wilhelm Tempel found 17 comets, for 13 of which he had priority. He found his first comet in 1859 with his 4-inch refractor while living in Venice. A lithographer by trade, in 1860 he moved to Marseilles, from where he discovered 8 more comets. Because of the Franco-Prussian war, Tempel as a German had to leave France in 1871, and went to Milan, and there found several more comets. In 1874 he moved to the Arcetri Observatory of Florence, where for the first time he had access to larger telescopes. Consequently he turned to studies of nebulae, and found only one more comet. His success in comet seeking had been achieved with an ordinary 4-inch refractor.

EE Barnard’s 16 comets were found between 1881 and 1892. He had bought a 5-inch refractor in 1876, and later began to use it for comet-hunting. He was very successful, and because a wealthy astronomical patron, HH Warner of Rochester NY, had offered a reward of $200 (at that time a fairly large sum) for each comet discovered, Barnard was able to build a small house from the proceeds - he referred to this as his “comet house”. In 1883 he moved to Vanderbilt University where he had the use of a 6-inch refractor. By the time of his graduation in 1887 he had discovered 8 comets. Others were found during his time at Lick Observatory in California where he moved in 1887, staying until 1895. Barnard’s success showed that one didn’t have to use a short-focus telescope to find comets.

William R. Brooks, second only to Pons in visual comet discoveries, found 21 (possibly 22) comets between 1883 and 1911. 13 of these were discovered while he was at the Smith Observatory, Geneva NY, which had a 10.5-inch refractor of only 8-foot focal length. This quite short focal length for such a refractor (the traditional f/15 would give over 13 foot focal length) would enable an eyepiece with a large field lens to give a 1 degree field.

Lewis Swift was another who used a long-focus telescopes in discovering his 12 comets in the late 19th century. He used telescopes as large as a 16-inch refractor.

The English amateur observer WF Denning, noted for planetary observing and as a pioneer of meteor studies, also found 5 comets with the 10-inch f/8 Newtonian he used for planetary studies. Denning commented that many telescopic comets are so small, almost starlike, that too low a magnification will result in some being missed. So he usually swept with 40x, examining suspects at 60x. His book, Telescopic Work for Starlight Evenings (1891), is still very readable on observing techniques, though hard to find.

Some Past Australian Observers

John Tebbutt was a great observer of comets, but discovered only two. His main contribution was in measuring positions, the better to establish orbits. He had a high interest in comets from an early age, reinforced by the appearance of Donati’s Comet of 1858, and another bright comet in 1860. He measured the positions of these comets with a sextant (!!), and calculated their orbits. In May 1861 while searching the western sky for comets with a small marine telescope he found what was to become the Great Comet of 1861. This brought him conspicuously to the notice of the astronomical world.

Later in 1861 he bought his first real astronomical telescope, a 3.25-inch refractor, to which he soon added a position micrometer. His second comet discovery did not happen until 1881, when he noticed a “hazy looking object” with the naked eye, confirmed as a comet by the telescope. This also became a very bright object. Later that year he discovered a third comet, using the 3.25-inch telescope, but it had been first observed two months earlier from America. Tebbutt is thus not a helpful model for comet hunters - his contribution lies mostly in astrometry of those already found.

WF Gale was a notable Australian amateur. Among his observing achievements was the independent discovery of 7 comets, 3 of which received his name. He had a variety of telescopes - in the 1890s he was using a 6 1/4 inch long focus refractor and an 8 1/2 inch reflector. By 1902 he had added a 10 inch reflector. His first “Comet Gale” was in 1894; the two later named comets were in 1912 and 1927. He continued to sweep for comets up to the time of his death in 1945. Again he is an example of using “non-special” equipment for comet hunting.

20th Century Notables

The American Leslie Peltier was a very prolific variable star observer, who also discovered 12 comets, 5 of which bear his name. Peltier appears only once to have bought a telescope - a 2-inch refractor, for observing variables. The AAVSO then lent him a 4-inch refractor; and soon after Princeton Observatory sent him - again on loan - a 6-inch f/8 refractor, in 1922. Later still he was lent a 12-inch Clark refractor, in 1959. His comet discoveries were made with the 6-inch, which he records had a 2-degree field with its low power eyepiece. The 6-inch had been designed for comet hunting - and before Peltier had it Zaccheus Daniel early this century discovered 3 comets with this telescope.

The Czech Skalnate Pleso observatory, once famous for producing very good star atlases in the 1950s and 60s, also had a group of comet hunters who found 19 comets between 1948 and 1960. Large binoculars were the instrument of choice there. The most successful of this Czech group was Antonin Mrkos, who found 11 comets in 11 years.

In England George Alcock, discoverer of novae and comets, traces the beginnings of his effective observing to acquiring a pair of WW II binoculars, 25x105, in 1959. Four of his comets were discovered with these binoculars, the fifth was found with 15x80 glasses during a nova search. A good effort given the English climate.

Bill Bradfield has been the 20th century’s most successful visual comet discoverer - at last count 17. Bradfield began in 1971 with a 6-inch f5.5 refractor, cobbled together by a fellow amateur in the South Australian society. It has proved a very useful instrument. Later, this was supplemented with a 10-inch Newtonian, but the 6-inch remained the prime instrument for comet searches. Bradfield also achieved a fast discovery rate in his earlier period - his first 11 comets were found in twelve years.

Further Recent Observers

To diverge for a moment from visual discoverers - the Shoemakers team effort used an 18-inch Schmidt, and comets found were typically around mags 13 to 17 - too faint for the visual seekers, whose comets average 10th mag at discovery.

David Levy had been quite successful in comet hunting before he joined the Shoemaker team - previously he had found several comets with a 16-inch Newtonian. Other American observers have also used 16-inch Newtonians. Both Hale and Bopp found their joint comet with 16-inch Newtonians, though for both of them the discovery was fortuitous, as the comet was in the field of a Messier object.

Vello Tabur, our local comet discoverer, uses an 8-inch short focus Newtonian, near f4.7, with a 30mm Vixen Lanthanum eyepiece, hence about 32x with a 1.6 degree field. This is similar to a number of other successful hunters, including Bradfield.

It is usual for the telescopes or binoculars to be alt-azimuth mounted - horizontal sweeps are the usual manner of sweeping. Many equatorial telescopes can have their mountings reset to an alt-azimuth (or Dobsonian) configuration, which means scopes not specifically set up in “comet mode” can still be used in this way. Peltier, for example, describes adapting an equatorial to altazimuth use.

David Sergeant, another Australian comet discoverer, discusses binoculars - his own, and larger ones. He points out that bigger isn’t always better - due to increased sky brigtness (presumably in twilight sky in particular). He relates this to exit pupil - as one might expect. Hence bigger binocs, like bigger scopes, should sacrifice some field to obtain a smaller exit pupil (and higher magnification) to darken the sky, thereby improving contrast to take advantage of the brightness gain. Brightness without contrast is unhelpful for fuzzy objects, as experienced observers know.

To give some personal examples. The effect showed up earlier this year with Comet Hale-Bopp - seen low in a twilight sky, not fully dark, it had better contrast in 15x80 binoculars than in a C14 at 70x. Given the similar exit pupils (just over 5mm in each), this was apparently an effect of increased light gathering, making the semi-dark sky too bright in the large aperture. Of course the small field at 70x didn’t help either. Back in 1986, one of the best views I had of Comet Halley, when it was still brightening, was from southern Sydney in 25x125 binoculars. Despite a mediocre suburban sky at the Sutherland Astronomical Society’s site, the comet showed well including its tail - an image of better contrast than I had with other instruments. And, yes, two eyes also helps.

Current Equipment

In recent times various Japanese observers have been successful with large binoculars, some as large as 150mm aperture. Some observers prefer a two-eye view for hunting comets. Of course, big binoculars cost a lot, as you have two complete optical systems. Notable Japanese observers using big binoculars have included Seki, Honda, and Ikeya.

Fujinon advertise their 25x150 binoculars as the instrument which allowed Hyakutake to find his bright comet, 1996 B2. Now Vixen have produced a slightly less expensive (though not cheap) 125mm binocular telescope, available with either 20x or 30x oculars. Altazimuth mounted, it appears suitable for comet hunting, as well as general low-power touring of the sky. Regrettably it appears not to allow interchangeable eyepieces, to judge from the description by Orion (of San Francisco) in their on-line catalogue. The US price starts at $3400, so don’t expect it to be cheap on the Oz market.

One eye - with a home-made scope like Vello’s - is a much more affordable way into comet searches. An affordable example of the two-eye approach is by the American observer Don Maccholz, of California. Maccholz built 125mm binoculars, based around two old aerial telephoto lenses. This heavy device has a power of 27x, using two 30mm Plossl eyepieces - Maccholz quotes the total price as under $300 in the 1980s. His other instrument, used more often, is a 10-inch f/3.8 reflector (coma, anyone?) used at 32x, with a 1.6 degree field.

Brightness of Comets at Discovery

Bill Liller has done a study of the brightness of comets at discovery, during the 1970s and 1980s. Of the 60 comets in his table, 23 were discovered photographically (the Shoemaker photographic team at Mount Palomar). All but 2 of these comets were in the magnitude range 13 to 17 at discovery. The visually discovered comets were of mag 6 to 12 when found, plus one of mag 13.

This suggests that for succesful hunting, a telescope able to see fuzzies to mag 12 is the basic requirement. Combine with this a fairly large field, and sufficient magnification to show smaller comets, and you have the basic requirement - probably a fairly short-focus 15-25 cm telescope, used in dark-sky conditions, with an eyepiece giving an exit pupil of about 5mm (for better contrast). With 15cm this gives a minimum power of 30x, with 20cm 40x.

A Plossl-type eyepiece at 30x gives a 1 2/3 degree field, at 40x a 1 1/4 degree field. Therefore more magnification is possible - without diminishing field size - by going to wider-field eyepieces. Various models are available, with field angles around 65-68 degrees, or 82-84 degrees. Eye-relief is of significance - short eye relief eyepieces are unpleasant for lengthy observing. At present there are two types of eyepiece offering long (20mm) eye relief - the Vixen Lanthanums and the Pentax XL series. The Vixens have a 50 degree field angle, the Pentaxes 65 degrees.

Take as an example a 20cm f5 telescope. A 21mm Pentax XL gives 48x, and a 1 1/3 degree field. The exit pupil is 4.2mm, and eye relief is 20mm. This should give better contrast than lower powers, and enough magnification to make small comets more likely to be noticed. Obviously, other eyepieces should be on hand to examine suspicious objects.

The above ramble may be of some interest in indicating that approaches to successful comet hunting have been more varied than one might expect.