Category: Jim’s Corner Blog

Jim's Corner Blog

Deep Sky Challenge: NGC 3166 and NGC 3169

James Chen |

NGC 3169, NGC 3166, and a small faint NGC 3165 (Jon Talbot)

 

NGC 3166

Alternate:PGC 29814

Sextans

RA 10h 13 m 44.9 s

Dec +03º25′ 31”

Magnitude 11.3

 

NGC 3169

Alternate: UGC 5525

Sextans

RA 10h 14 m 15.1 s

Dec +03º27′ 58”

Magnitude 11.5

 

NGC 3165

Alternate:

Sextans

RA 10h 13 m 31.4 s

Dec +03º22′ 30”

Magnitude 13.9

For this month, I am offering a challenge to those with larger telescopes. A ten-inch or greater telescope is needed, with those with eight-inch telescopes needing optimal dark conditions clean optics, and averted vision to make out this pair.

NGC 3166 and 3169 in Sextans are a gravitationally bound galactic pair, discovered by the great William Herschel in 1783. The smaller fainter NGC 3165 was discovered by William Parsons, the 3rdEarl of Rosse in 1856, using the famous Leviathan of Parsonstown 72-inch telescope.

NGC 3166 is an Sb spiral galaxy with a very small, very bright nucleus in a short smooth bar. The spiral pattern is only in the center, and there is a large very faint extended envelope.

NGC 3169 is an SaHubble classification spiral galaxy about 75 millionlight-years distant.It is an unbarred spiral galaxy with tightly-wound arms, but with a peculiar feature. There is an asymmetrical spiral arm and an extended halo around the galaxy.

NGC 3169 is located in close physical proximity to NGC 3166, and the two have an estimated separation of around 160,000 light-years. Their interaction has created a gravitational distortion that has left the disk of NGC 3166 warped, in addition to NGC 3169’s asymmetrical arm. Combined withNGC 3165, the three galaxies form a small group within the larger Leo I Group. The three galaxies are embedded within an extended ring of neutral hydrogen that is centered on NGC 3169. Astronomers feel that the interacting galaxies will eventually merge to form a single galaxy in the distant future.

For the backyard astronomer, this Cosmic trio is well within the capabilities of my 11” SCT and the contributing astrophotographer’s 130 mm apochromat refractor. As always, a dark moonless night is required, especially in locating the dimmer NGC 3165, as seen in the accompanying photo.

 

 

Jim's Corner Blog

NGC 7000 (The North America Nebula) and NGC 5070 and IC 5087 (The Pelican Nebula)

James Chen |
The North America Nebula and the Pelican Nebula (Illustration courtesy of Jon Talbot. Used with permission.)

Often referred to erroneously as the North American Nebula, the North America Nebula is an emission nebula in the Cygnus, close to the bright star Deneb. The Pelican Nebula is an emission nebula located with the North America Nebula. The NGC 7000 shape resembles that of the continent of North America, complete with a Florida peninsula and Mexico forming a prominent Gulf of Mexico shape.

NGC 7000 is visible from Spring through Summer into the Fall. As long as an observer can see the Summer Triangle of Deneb, Vega, and Altair, the North America Nebula and its companion Pelican Nebula can be seen.

The gaseous contortions of the Pelican Nebula bear a resemblance to its namesake bird. The Pelican Nebula is also located near Deneb, and is divided from its more prominent neighbor, the North America Nebula, by a dark dust cloud.

NGC 7000 was discovered by William Herschel on October 24, 1786. The Pelican Nebula, also designated by two Index Catalogue of Nebulae numbers IC 5070 and IC 5067, was discovered by the Reverend Thomas Espin in 1899, with its discovery announced in 1900. Both nebulae are part of a large Hydrogen-II emission region in Cygnus, with both the North America Nebula and the Pelican Nebula serving as a massive nursery for newly formed stars. The distance of the nebula complex is not precisely known, nor is the star responsible for ionizing the hydrogen so that it emits light. If the star inducing the ionization is Deneb, as some sources say, the nebula complex would be about 1800 light-years away, and the size of the North America Nebula would be 100 light years across. The Pelican Nebula is estimated to be 30 light years across.

Observing the North America and Pelican nebulas requires medium-to-large binoculars and the help of (UHC) filters. This is a challenging, but rewarding pair to observe. 12X60, or better yet 15×70, binoculars in a dark country sky are needed to adequately observe this Cosmic Duet. With the aid of UHC filters to enhance the viewing and allowing only Oxygen-III and Hydrogen-beta emission lines to pass, the dimmer Pelican Nebula can be detected along with details of the North America Nebula.

Without the UHC filters, a 15×70 binocular will pick up the entire Florida / Gulf of Mexico / Mexico / Central America region of NGC 7000. Just the slightest amount of ambient background light from light pollution or bright moonlight will render the nebulosity invisible.

To observe both the North America and Pelican, seek out the darkest of sky locations and use UHC filters. Use a tripod, if necessary, since 12×60 or 15×70 binoculars are difficult to handhold steadily for any length of time. Or lay back into a comfy bean bag chair!

Jim's Corner Blog

The Secret to Apochromat performance at Achromat prices

James Chen |

Anyone who knows me knows I love refractors. Inch-for-inch, refractors are the best performing telescopes. Pinpoint star images, high resolution, and high contrast images are their hallmark.

In today’s world of refractor telescopes, there are achromats and apochromats, terms created to differentiate the levels of color correction of the respective lens systems. The classic achromatic refractor design uses a two lens objective, with one lens made of crown glass, and the other lens made of flint glass. With the lenses ground with proper curves and using glasses with different refractive indices, the result is a telescope that can bring to focus two of the three prime colors of light, typically red and blue wavelengths.

In the 1980’s, the apochromatic refractor became commercially available to the amateur astronomy market. Sophisticated designs appeared using combinations of two, three, or four lenses. Experienced telescope users found themselves learning a new vocabulary, with terms such as fluoro-crown, lanthanum, fluorite, FPL-51 or -53, APO-triplet, and Petzval. Apochromats offered improved sharpness, contrast, resolution, and color correction over the classic achromat refractors, while at the same time creating a more portable telescope with f-ratios of f/8, f/7, or less. Make no mistake, apochromatic refractors are the high performance Ferraris of the telescope world. But this optical perfection comes at a price. Apochromats are up to 10 times more expensive than an equivalent aperture achromatic refractor. Some of the exotic glasses are difficult to work with, and the production yield is lower than more conventional materials. For example, one telescope manufacturer has stated that even with their most advanced manufacturing processes, only one out of every ten fluorite lens sets is good enough for use in their telescopes. The cost of an apochromat refractor can get expensive very quickly as the size increases. A typical 80-mm apochromat retails in the $700 range versus an achromat of $250.. A 102-mm triplet can easily cost over $2,500 compared to an achromat costing $450.. The 130-mm apochromats fall into the $4,000-$5,000 realm, and prices exceeding $12,000 and higher are not unusual for larger apochromats.

The old telescope salesman explanation of an achromat refractor is a “false color-free image,” meaning there is no primary color fringing in the telescopic image. The telescope salesman pitch for an apochromat is “this time, I’m serious! It’s really false color-free!” This time, the salesman is pitching that the apochromat has no primary color nor secondary color fringing. Depending on the observer, the image improvement of an apochromat over an achromat can be described as either a slight aberration or a day-night difference. A lot depends on the sensitivity and sensibility of the refractor owner. From a technical viewpoint, there is no question that the apochromatic refractor offers better telescopic images over an achromatic refractor.

A tip to those buyers with apochromat tastes but are on an achromat budget, the following formula should be applied:

FromTelescope Optics: Evaluation and Design, by Harrie Rutten and Martin van Venrooij:

focal length >0.122D

where

D = diameter of the telescope objective in mm

If the focal length of an achromatic refractor is equal to or greater than this calculation, the residual chromatic aberration will not be a factor.

For example, a well-known refractor manufacturer (telescopes with stellar performance that give a great view!) at one time manufactured an 80mm refractor with a 750mm focal length. It was advertised as an f/9.4 (although mathematically it computes to an f/9.375). Using the above formula:

focal length >0.122D

>0.122 (80) = 9.76

which means the 80mm f/9.4 achromat refractor had very little residual false color and almost apochromatic performance! The optical designer has taken advantage of our light-polluted skies by using a combination of ED glasses, altered lens curves, and adjusting the air spacing between the two lenses of the achromat doublet. The color error is moved into the violet wavelengths to match the not-quite-black-really-deep-purple light-polluted background sky, resulting in an excellent performing telescope that produces sharp and crisp images with the color error conveniently and cleverly hidden in the light-polluted background. Hence, the telescope has an underground following as a hidden gem of telescopes for its near-apochromatic performance at achromatic refractor price!

So, if you have apochromat tastes in a refractor, but have only an achromat budget, remember this formula and apply it to the achromatic refractor you are interested in. There is a tradeoff in in size of the field-of-view, but if your primary interest is a refractor view without false color, keep this formula in mind and you might find another underground gem!

Jim's Corner Blog

The Whale and The Hockey Stick – NGC 4631 and NGC 4656/57

James Chen |

NGC 4631, NGC 4627 and NGC 4656/57 (Jon Talbot)

NGC 4631 is a SB class barred edge-on spiral galaxy witha slightly distorted wedge shape giving it the appearance of a whale, thus giving the galaxy its nickname.

The Whale Galaxy was discovered in 1787 by William Herschel. Astronomers have estimated that NGC 4631 is only 25 million light-years awayand is of similar size to our Milky Way galaxy. NGC 4631 has a recessional velocity of 605 km/sec that is too small to be a reliable indicator of distance, because of the possibility of significant peculiar (non-Hubble-expansion) velocities. However, its distance based on that recessional velocity (about 27 million light years away) is in reasonable agreement with redshift-independent distance estimates of 18 to 24 million light years. In larger telescopes than an 8”, a small companion E4 elliptical galaxy NGC 4627 can be seen nearby the Whale Galaxy, making this a Cosmic Duet for larger (16”+) telescopes.

NGC 4656 is a large Sb spiral galaxy discovered by William Herschel in 1787.Why the two NGC designations? The bright knot on the East of this galaxy has been assigned the separate NGC number NGC 4657, since William Herschel had cataloged it separately. Some astronomers believe NGC 4657 is a companion to the galaxy.

The Hockey Stick galaxy upward curve is a result of distortion by the interaction with The Whale Galaxy and its small elliptical NGC 4627 companion. A bridge of hydrogen gas is connecting both galaxies.

This cosmic duet of galaxies are best seen with telescopes of at least 8” (or 200 mm) of aperture.

Jim's Corner Blog

M4 and NGC 6144

James Chen |

M4 and NGC 6144 (Jon Talbot)

In a rare pairing of two globular clusters, M4 and NGC 6144 are separated by 1°. To observe this pair of globulars, the observer should try to frame the eyepiece image to exclude the glaringly bright Antares. By doing this, NGC 6144 is more readily identified.

M4 was discovered byJean-Philippe Loy de Chéseauxin 1745 and catalogued by Messier in 1764. It was the first globular cluster in which individual stars were resolved. M4 is a Class IX globular cluster and is described as a large loose globular cluster is only 1.3° west of the bright 1st-magnitude red star Antares in Scorpius. At a distance of only 7,000 light-years, it is considered the nearest of all globular clusters and is easily resolved by a 4” telescope.

Observations of M4 have yielded 43 variable stars, and Hubble Space Telescope photographs and data have shown the presence of some of the oldest white dwarf stars , at an approximate age of 13 billion years, known in the Milky Way. One white dwarf star has been identified as a binary with a pulsar as a companion, and a super Jupiter-sized planet that is 2.5x the size of our solar system’s largest planet.

Discovered by William Herschel on May 22, 1784, NGC 6144 is located between M4 and Antares. NGC 6144 is a class XI globular cluster and is visually a smaller 9th-magnitude globular. NGC 6144 lies 33,000 light years distant and is over three times farther away than M4. At that distance it also appears through the telescope eyepiece as over three times smaller; a mere 6.2′ in diameter.

Using a 102 mm f/7 apochromat refractor with a medium magnification wide field eyepiece, M4 and NGC 6144 were easily observed under dark moonless conditions.

Jim's Corner Blog

M84 and M86

James Chen |
M84 and M86 (
includes not only M84 and M86, but also M88, M89, M90, NGC 4478, and many other Virgo Cluster galaxies.)
(Jon Talbot)

Charles Messier discovered M 84 and M86 on March 18, 1781 and subsequently added both to his catalog.

Describing M84, Messier noted:

“Nebula without star, inVirgo; the center is a bit brilliant, surrounded with a slight nebulosity: its brightness & its appearance resemble that of those in this Catalogue, No.s 59 & 60.”

M86 was described by Messier as:

“nebula without star, inVirgo, on the parallel & very near to the nebula above, No. 84: their appearances are the same, & both appear together in the same field of the telescope.”

John Herschel catalogued M84 as h 1237 and later added it to the General Catalogue as GC 2930, describing it as:

“Very bright; pretty large; round; pretty suddenly brighter toward the middle; mottled.”

M84 is an E1 Hubble elliptical galaxy and M86 is an E3 elliptical galaxy.

M84 is situated in the inner core of the Virgo Cluster, a region populated with galaxies, including M49, M60, M86, M87. the edge-on spirals NGC 4388 and NGC 4402, the elliptical galaxy NGC 4387, the barred spirals NGC 4413 and NGC 4425, and the interacting galaxies NGC 4438 and NGC 4435. M84 is part of Markarian’s Chain, a stretch of galaxies that appear in a curved line when seen from Earth and the subject of the next Cosmic Duets writeup. Named after Armenian astrophysicist Benjamin E. Markarian, who discovered the galaxies’ common motion in the early 1960s. Some of the galaxies are superimposed, but others appear to move coherently.

Studies of radio wavelengths and images taken by the Hubble Space Telescope in 1997 have revealed two jets of matter emanating from the core of M84 and a disk of rapidly rotating stars and gas, which indicates that the galaxy contains a supermassive black hole at its centre. The black hole has an estimated mass of 1.5 billion solar masses.

M86 is also a member of the inner core Virgo Cluster. M86 is experiencing ram pressure stripping and losing gas as a result of moving through the Virgo Cluster at such a high velocity. The galaxy is leaving behind an enormous trail of hot gas, visible in images taken by the Chandra X-ray Observatory.

M86 is approaching the Milky Way at 244 km/s (419 km/s according to some sources), which is unusual for a member of the Virgo Cluster because the entire cluster is receding from Earth at an average velocity of 1,000 km/s. M86 displays the highest blueshift (fastest approaching velocity) of all Messier objects. It seems to be moving toward Earth because it is really moving towards the center of the Virgo Cluster rom the opposite side, thus explaining the blueshift. Because of its enormous mass, the cluster’s core has a strong gravitational field, one that can accelerate any galaxy’s velocity to that observed in M86. For example, IC 3258, another member of the cluster, is approaching us at 517 km/s.

Although this Cosmic Duet can be observed through a 4” refractor in dark skies, this author has found this pair much more satisfying to observe through a 130 mm apochromat refractor or an 8” SCT. The extra aperture over 4” helps to spot additional Virgo cluster objects in the field-of-view. Medium-low to medium magnifications are appropriate.

Jim's Corner Blog

The Best Worst Telescope: The Crossley

James Chen |

The historic Crossley reflector has been described as “the worst best telescope”. Its history dates back to 1879 where it was located in the backyard of Andrew Ainslie Common’s home in the Ealing district of west London, England. Common was an English amateur astronomer, who perhaps like many of amateur astronomers, suffered from a severe case of aperture fever.

For many years, Common used an 18 inch Newtonian reflector mounted in a shed built in his backyard in Ealing. Eventually, aperture fever took over and he commissioned the building of a 36 inch reflector of his own design and implementation, also installed in his backyard in Ealing, England. The optics of this telescope were outstanding, however the mechanical design showed a lack of engineering skill by Common. The primary mirror was mounted ahead of the vertical axis which caused a balance issue with the telescope, necessitating the use of counterweights. Strangely, Common used a polar axis design that used liquid mercury in its bearing mechanism, causing a mechanical and now recognized environmental issue. Mechanically, the telescope was awful. But Commons was able to perfect astrophotography techniques with this telescope.

The telescope was sold to British politician Edward Crossley in 1886, who operated the telescope until 1895. Crossley built a new dome enclosure to protect the telescope and observers from the harsh Halifax, England weather. But this climate was far from ideal for observation.

After about 10 years, Crossley donated both telescope and dome to Lick Observatory, where it was put into operation in 1896. Lick Observatory director James Keeler had the Crossley re-engineered mechanically by replacing the original mercury bearing mount with a conventional equatorial cross-axis mount. Director Keeler then put the Crossley to good use by producing early astro-photographs of “nebulae”, those fuzzy-looking areas in the night sky, not knowing that the future 100 inch Hooker telescope at Mt. Wilson would be used to discover many “nebulae” that were actually galaxies.

An example of the discoveries by the Crossley is Arp 148 was discovered by American astronomer Nicholas U. Mayall of the Lick Observatory, using the Crossley reflector. Arp 148, nicknamed “Mayall’s object” and is located in the constellation of Ursa Major, the Great Bear, approximately 500 million light-years away. Arp 148 is the aftermath of an encounter between two galaxies, resulting in a ring-shaped galaxy and a long-tailed companion. The collision between the two parent galaxies produced a shockwave effect that first drew matter into the centre and then caused it to propagate outwards in a ring. The elongated companion perpendicular to the ring suggests that Arp 148 is a unique snapshot of an ongoing collision. Infrared observations reveal a strong obscuration region that appears as a dark dust lane across the nucleus in optical light.

 

Arp 148 (NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), K. Noll (STScI), and J. Westphal (Caltech)
Jim's Corner Blog

Using Cepheid Variables to Measure Distance

James Chen |
Period versus Luminosity chart for Type I and Type II Cepheid Variables ( Las Cumbres Observatory)

Understanding the use of Cepheid variables to determine distance is often glossed over in many astronomy books. The following is an attempt to address the use of Cepheid Variables in distance calculations.

By definition, a Cepheid Variable is a star that pulsates radially, varying in both diameter and temperature, thus producing changes in brightness with a well-defined stable period and amplitude. Or, in another way to describe it, Cepheid Variable stars are intrinsic variables which pulsate in a predictable way. In addition, how often a Cepheid star pulsates is directly related to its luminosity or brightness.

The discovery of the relationship between luminosity and period was made by Henrietta Swan Leavitt, during her tenure as one of the “Harvard Computers”. Beginning in 1893, Edward Charles Pickering, the Director of the Harvard College Observatory, hired a group of highly educated and skilled women to help him process astronomical data and data analysis using the large number of photographic plates that had been accumulated by the observatory. The “human computers” became known as the “Harvard Computers”. Joining Leavitt were Williamina Fleming, Annie Jump Cannon, Antonia Maury, and many more.

Two types of Cepheids are used for distance measurement. The Type I Cepheid variables is also known as the Classical Cepheid. Type I Cepheids are yellow bright giants and supergiants that undergo pulsations with very regular periods on the order of days to months.

Type II Cepheids variables are old, metal-poor stars that have periods of 1 to 50 days. The physical characteristics of all the type II Cepheid variables are very poorly known, but the longer period Type II Cepheids are more luminous and have proved useful in establishing distance.

Why does a Cepheid variable pulsate and vary in luminosity? A current thought is based on how the charge state of helium ions are produced in the fusion process within a Cepheid star. Cepheid variable stars are giant and supergiant stars 30 to 100 times or more the mass of the Sun. The process for the pulsation and varying luminosity is thought to be:

  1. Helium is formed at the core from the star’s fusion reactions.
  2. Due to the extreme size of the star, pressures and heat build up within the star.
  3. The heat and pressures causes He to lose an electron, turning He to He+. In this state, He+ is transparent.
  4. Heat continues to build, expanding the starand causing He+ to lose another electron, turning He+ to He++. He++ becomes opaque, thus dimming the star’s luminosity.
  5. The star’s expansion reach a point where the He++ cools, enabling the re-combination with electrons and returning the He++ to He+ and He atoms. As a result, the He becomes transparent again.
  6. The star shrinks to its starting point size.
  7. The process repeats with a period of 1 day to several months, depending on the size of the star.
  8. See step 1.

Cepheid variables are extremely luminous and very distant ones can be observed and measured because of this extreme luminosity. Once the period of a distant Cepheid has been measured, its luminosity can be determined from the known behavior of Cepheid variables. Then its absolute magnitude and apparent magnitude can be related by the distance modulation equation, Cepheid variables can be used to measure distances from about 1kilo-parsec to 50 mega-parsec.

The distance modulation equation is:

m –M = 5 log d– 5

where

m is the apparent magnitude of the object

M is the absolute magnitude of the object

d is the distance to the object in parsecs

The equation rearranged to determine distance results in:

d = 10(m – M + 5)/5 parsecs

If an astronomer observed a Cepheid star with period of 34 days, comparing to previously measured Cepheids, its absolute magnitude is -5.65. If its apparent magnitude was +23.0, the astronomer could use the distance modulus equation to find the distance to the Cepheid:

d = 10(23 – -5.65 + 5)/5 parsecs

d = 106.73 parsecs

where a parsec is equal to 3.26 light-years

d = 106.73 x 3.26 = 347.94 light-years

I hope this demystify’s the use of Cepheids for determining astronomical distances. Remember, the Cepheid variable technique is only one element of the astronomical distance ladder that I described in an earlier Jim’s Corner.

Jim's Corner Blog

M20 The Trifid Nebula with Barnard 85

James Chen |

As the last glimpses of the summer Sagittarius constellation gives way to the autumn deep sky delights, we still will have a chance to view one of the classic Messier objects to observe, the Trifid Nebula M20.

The Trifid Nebula was discovered by Charles Messier on June 5, 1764.

The Trifid Nebula, in the constellation Sagittarius, is a fun object to observe, since it is a combination of an emission nebula, a reflection nebula, and a dark nebula. (see NASA Hubble figure below)

Hubble Image of M20 (NASA, ESA, and STSci)

The name Trifid means “divided into three lobes”. For those readers who are hopping around this book, here are some of the historic details you may have missed:

Charles Messier wrote:

“June 5, 1764. 20. 17h 48m 16s (267d 04′ 05″) -22d 59′ 10″
Cluster of stars, a little above the Ecliptic, between the bow of Sagittarius & the right foot of Ophiuchus. Seen again March 22, 1781.”

John Herschel was the first to use the term Trifid in describing M20, taken from his notes:

“Sweep 30 (July 1, 1826) 
RA 17h 51m 64.3s, NPD 113d 0m 6s (1830.0) 
vL; trifid, three nebulae with a vacuity in the midst, in which is centrally situated the double star Sh 379, neb == 7′ in extent. A most remarkable object. 
very large; trifid, three nebulae with a vacuity in the midst, in which is centrally situated the double star Sh 379, the nebula is 7′ in extent. A most remarkable object.”

M20 is an easy 2 degrees northwest of M8, the Lagoon Nebula. At magnitude 9, M20 does not reveal its details for binoculars and small telescopes. The true nature of M20 is revealed with an 8 inch or larger aperture telescope. Aperture is your friend in showing details of M20, especially Barnard 85, although a high quality apochromat refractor is quite capable for this object. Using either a UHC or O-III nebula filter is a great aid in observing the Trifid. (see M8 and M20 figure below)

M8 and M20 (Jon Talbot)

Few backyard astronomers realize that M20’s dark lanes are catalogued as Barnard 85, from E.E. Barnard’s unique list of dark nebulae.. The dark nebula veins can be seen in a 5” apochromat refractor telescope at 65x-to-104x, and details appear more prominent with 8 inches or more aperture. Sharp eyed observers may grasp a hint of Barnard 85 using 4” telescopes with great optics on clear steady nights.

Edward Emerson (E.E.) Barnard was one of the greatest astronomers of the 19th century. His exceptional eyesight aided him in the discovery of the fifth moon of Jupiter, approximately thirty comets, and numerous bright and dark nebulae. In 1919, he published his initial catalog of dark nebulae in the Astrophysical Journal under the title “On the Dark Markings of the Sky with a Catalogue of 182 Objects.” The catalog eventually expanded to 370.

Astronomers have estimated the age of the Trifid Nebula to be approximately 300,000 years old, making it one of the youngest nebulae of its kind known.

The emission nebula portion of M20 is illuminated by the light of massive young hot class O stars that form the core of the emission nebula. The star cluster is partially obscured by the thick dust lanes of Barnard 85. Barnard 85 consists of dust clouds that absorb and block light from the bright star cluster behind them. It is responsible for the apparent gaps in the larger emission nebula that give M20 its trifurcated look.

The reflection nebula portion of M20 is not physically associated with the emission or dark nebula portions of the Trifid, and is in the line-of-sight to the Earth observers.

Jim's Corner Blog

Binoculars and Backyard Astronomy

James Chen |

An important piece of equipment for observing stars and deep sky objects in the amateur astronomer’s arsenal is the binocular. Armed with star atlases, or for the more technically oriented a smartphone or tablet app, and binoculars, the stars and deep sky objects can be hunted down among the constellations, the Milky Way, and star fields. The binocular offers the widest field of view available to the backyard astronomer, with lightweight convenience and ease-of-use.

For experienced observers, the binocular is an optical tool that allows for wide-angle viewing of star fields, up-to-6º or 7º of field-of-view. An amazing amount of night sky observing can be observed with binoculars, particularly when searching and discovering stars and deep sky objects that can only be seen with the wide field-of-view that a pair of binoculars can provide. Many of deep sky objects can be easily scooped up by a binocular sweep. Seeing the stars and deep sky objects in relationship to the sky around them will put that object in its proper context in the sky.

Most backyard astronomers hand hold their binoculars for observing. To obtain the steadiest images, the preferred technique for hand holding binoculars is to hold the ends of the binocular barrels, instead of gripping them around the binocular body where the prisms are located. This technique works well for both 7×50 or 10×50 binoculars. This handheld method is especially effective with the larger 12×60, 15×70 or 16×70 binoculars.

The larger 80-mm or 100-mm binoculars require the steadier support of a tripod or parallelogram binocular mount.

Observing in a bean bag chair is a comfortable and steady way to observe the night sky with hand-held binoculars. Nestle into the bean bag with binoculars in hand, with the elbows resting on the bean bag sides as they puff up from your weight . The binocular images will be steady from the support! This is an easy and comfortable way to observe the night sky with binoculars, and cheaper than some of the parallelogram binocular mounts or other forms of binocular mounts. This technique has been used by me successfully with binoculars up to 15×70’s.

Diametrically opposite from the aforementioned 50-mm, 60-mm and larger binoculars are the new pocket-sized low power 2.1×42 binoculars. Some of our members have looked through my pair of 2.1×42 at some of our outreach star parties. The 2.1×42 binoculars are small, lightweight, and provide an extremely wide field of 25º field-of-view. Easily carried for backpacking adventures or just a quick peek at the sky in the backyard, these binoculars can be used to scan constellations and large star fields. The night sky seems to come to life with the 2.1×42, giving the user the feeling of having 42mm aperture eyeballs. Don’t let the very slight field curvature bother you, the wide-open expanse of view easily outweighs this slight aberration. stars and deep sky objects comprising of open clusters or nebula clouds appear small due to the low magnification, but are still easily seen and identifiable, and are set in their stellar surroundings. stars and deep sky objects separated by tens of degrees can be captured by this diminutive optical tool. Although slightly aperture-lite, these are fantastic for hunting down stars and deep sky objects in clear Shenandoah country skies with little or no light pollution.

On the other end of the scale are the giant binoculars with apertures of 80 to 100 mm, with the rare 125-mm behemoths. The larger apertures of these giant binoculars do enhance light-gathering ability, but at the cost of convenience, portability, and wide-field-of-view. These large prism binoculars encroach upon the territory occupied by the 80-to-102 mm telescope. With a limited field-of-view of sometimes less than 2.5˚ and needing a sturdy tripod or binocular mount, the wide-field advantage, portability, and ease-of-use of binoculars is lost by using these giant binoculars.

Also, I would be remiss in failing to mention the rare binoculars that use interchangeable 1-1/4” eyepieces. I own two of these bad boys, and will bring them out to one of outings in the future.