Author: James Chen

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.

Jim's Corner Blog

Big, Bigger, and Biggest Backyard Telescopes

James Chen |

This month’s column is a cautionary discussion against aperture fever. Whenever you get the overwhelming desire for a bigger telescope, heed these warnings.

A look at the marketplace, major star parties (such as Stellafane, RTMC and the Winter Star Party) and in a small number of amateur astronomer’s backyards, there are telescopes ranging in size from 10” or larger, encompassing two families of large telescopes, the large Dobsonian and the SCT.

The most common large telescopes for visual use are the 10” to 12” Dobsonian telescopes and the 10” to 12” SCTs. 10” Dobs and 10”-11” SCTs are basically the practical upper limit to portable personal telescopes. Larger than these sizes results in telescopes that are difficult to transport and multiple people to assemble at a remote site.

Occasionally, a 16” Dobsonian shows up as a commercial product. There are off-the-shelf commercially available 13” or larger Dobs and 14”-16” Schmidt-Cassegrain telescopes that are available from both the “Big Two” manufacturers and some smaller suppliers. This category of large backyard telescopes are at best transportable, although the size and bulk makes them somewhat cumbersome and sometimes needing more than one person to set up. There are no commercially available refractors in this aperture grouping. For the record, any 6” or larger refractor becomes as cumbersome to transport as a 16” SCT because of the large mounting requirements.

Jim’s Celestron 11” SCT (James Chen)

Then there are the “Ferrari’” and “Rolls Royce” of visual use amateur telescopes. Included in this grouping are the truss construction large Dobsonians of apertures from 13” to 24” or larger. These telescopes are made by artisans at small telescope companies and are made to order. Again, there are no commercially available refractors in this aperture size grouping.

Briefly, there was an attempt by one telescope vendor, Orion, to market 36”, 42”, and 50” Dobsonians! Orion tried to premier their super colossal Dob at NEAF a few years ago, but were unable to get the telescope assembled for the expo. The plan to market these massive monster telescopes was eventually dropped.

7”, 24”, and under the cover a 30” Dobsonian at the Winter Star Party 2014 (James Chen)

Included in this super large telescope family are the 14” and 16” SCTs manufactured by the two major telescope manufacturers. The telescopes at Frederick County Middle School are 16” Meade LX-200 SCTs. One of our former members, John Hershey, is well remembered for his 14 inch Celestron SCT which he would bring to every public outreach that our club held. In order to assemble his telescope at a star party, he had a special mechanical lift that enabled him to hoist the OTA onto his mount. That’s dedication!!!

These large SCTs have all the bells-and-whistles, including GoTo capability and are astro-imaging capable with added accessories. Available with heavy duty field tripods, these superstars of the SCT world are best considered as transportable by a small number of people rather than portable and useable by a single person (with the exception of John). Ideally, these large SCTs lend themselves best to permanent setups in an observatory. There are some rare custom made refractors that equal or exceed 12”, and these are normally mounted under an observatory dome. These telescopes are generally not portable. In military terms, large SCTs are transportable (but only barely). They often require more than one person to set up and disassemble. Our club’s experience with the FCMS 16” SCTs is a perfect case study. We needed four people to lift the telescopes onto the mount drive assembly!

The views through these massive marvels are fantastic, definitely evoking a “WOW” factor through the eyepiece. The increased light gathering enables the observer to see fine tendrils and filaments of nebulosity in emission nebulae, extended views of galaxies and revealing galactic dust lanes, and layer upon layer of starry diamonds within globular clusters.

The largest current record holder for a large backyard telescope is a 70” Dob located in California, built from a military surplus spy satellite mirror. Historically, William Parsons, The Third Earl of Ross, hold the record for the largest amateur telescope, the 72 inch The Leviathan of Parsonstowm.

But there are downsides to the larger telescopes. Many are expensive, with the larger telescopes requiring the additional purchase of a truck or van in order to transport the telescope to a remote site. The larger Dobsonians require 8 foot or 10 foot ladders to reach the eyepiece. Many are difficult to setup on a routine basis. Most do not qualify as “grab-and-go” telescopes.

Always remember the telescope salesman’s Golden Rule, the smaller telescope that gets used a lot sees more than the bulkier, difficult to use larger telescope seldom used. That’s why the best selling sizes of telescopes are the 4” refractor, the 8” SCT, and the 10” Dob.

Jim's Corner Blog

M45 The Pleiades, Maia Nebula, and Merope Nebula

James Chen |

Unlike most of the other deep sky objects that are either faint or invisible to the naked eye, the Pleiades are a bright sight during the winter and can be seen without any optical aid.

An amateur astrophoto of M45 (John Livermore)

M45 The Pleiades
Alternative Nomenclature: Melotte 22, Seven Sisters, Subaru
Constellation: Taurus
Right Ascension: 03h 47m 24s
Declination: +24º 07′ 00”
Magnitude: 1.6

Maia Nebula
Alternative Nomenclature: NGC 1432
Constellation: Taurus
Right Ascension: 03h 47m 24s
Declination: +24º 07′ 00”
Magnitude:

Merope Nebula
Alternative Nomenclature: NGC 1435, containing Barnard’s Merope Nebula IC 349
Constellation: Taurus
Right Ascension: 03h 46m
Declination: +23º 54′ 00”
Magnitude: 13 (IC 349)

Hubble image of the Pleiades reflection nebula near Merope
(NASA and The Hubble Heritage Team (STScI/AURA)

This star cluster has been known to mankind since before the written word. Descriptions and mentions of the Pleiades permeate human culture more than any other deep sky object in this book.

The Pleiades are among the first stars mentioned in written form, with the Chinese writing about the star cluster in 2350 BC.

The Greek myth of the Pleiades tells the story of the Titan, Atlas, who was forced to carry the heavens on his shoulders. Orion began to pursue all of the Seven Sisters, and Zeus transformed them first into doves, and then into stars to comfort their father Atlas and the sea nymph Pleione. The constellation of Orion still pursues them across the night sky to this very day.

Any early mention of the Pleiades can be found in Homer’s Iliad, from about 750 B.C., and the Odyssey, about 720 B.C. From the Iliad:

He made the earth upon it, and the sky, and the sea’s water, and the tireless sun, and the moon waxing into her fulness, and on it all the constellations that festoon the heavens, the Pleiades and the Hyades and the strength of Orion and the Bear, whom men give also the name of the Wagon, who turns about in a fixed place and looks at Orion and she alone is never plunged in the wash of the Ocean.
Iliad 18. 483-89 (translated by R. Lattimore)
Besides the Chinese and the Greeks, the Pleiades appears in the legends and lore of cultures in every inhabited continent on Earth.

The storytelling of the Kiowa tribe in North America tells of the legend of seven maidens transported to the sky by the Great Spirit. The Navajo, the Western Mono Indians, the Inuit, Cheyenne, Cherokee, Hopi, Lakota, Nez Perce and Blackfoot tribes also told their stories relating to this star cluster.

The star cluster appears in the mythology and cultures of the Norse, Celtic, Ukraine, Incas, Aztecs, India, Indonesia, Japan, Philippines, Thailand, Australia, Hawaii, and New Zealand. The star cluster appears in the Swahili language and Sesotho language of Africa.

Even in today’s world, the Pleiades appears in song, literature, and more recently science fiction. Characters and plot lines in Isaac Asimov’s Foundation Trilogy and the television series Star Trek have referenced the Pleiades. A look at the emblem of a Subaru sports-utility vehicle is revealing. The Japanese name for the M45 star cluster is Subaru, and the emblem of the SUV is the Pleiades (or Subaru if you wish!).

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygete, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As seen in Fig. 8.8 and 8.9, nebulosity accompanies the stellar members of the cluster. American astronomer Vesto Slipher discovered the true nature of the nebulosity in 1912. He noted that the light from the nebulae around the Pleiades had the same features in its spectrum as the light from the Pleiades stars themselves. Hence, the Maia Nebula and the Merope Nebula are reflection nebulae, and the light was being reflected. The nebulae are blue in color because of the light scattering process, similar to the Rayleigh scattering that makes Earth’s sky blue.

Astronomers first believed the dust and gas was left over from the formation of the cluster. However, the proper motions of the stars and of the nebulosity are not the same. The Pleiades are actually moving through a cloud of interstellar dust. Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster.

The Pleiades open star cluster is estimated to be between 390 to 480 light years away from Earth. There has been much controversy over the distance measurement for the Pleiades. Using the long standard trigonometric parallax methods for determining astrometric distance measures for deep space objects less than 1,000 light-years away, the distance to the Pleiades was determined to be 135 parsecs or 440 light-years away. In 1989, the European Space Agency launched the Hipparcos satellite, designed to perform space distance measurements with great accuracy. The Hipparcos data established the distance as 118 parsecs. This measurement was in conflict with ground-based trigonometric parallax measurements, as well as with Hubble Space Telescope measurements of between 135 and 140 parsecs. The discrepancy has been argued in professional papers, with some resolution appearing after measurements taken with the Very Long Baseline Interfometry (VLBI) and data from the Gaia satellite, which has resulted in a value to 136.2 parsecs, or 444 light-years.

The Pleiades contains over 1,000 stars, many young, hot blue stars. The ages for the Pleiades stars range from 75 to 150 million years.

M45 is moving in the direction of the constellation Orion. Astronomers have determined that within 250 million years, the Pleiades cluster will disperse because of stars being ejected from the cluster due to close encounters with other member stars, tidal gravitational fields moving the stars outside the cluster, and eventual gravitational interactions with the Orion Molecular Cloud Complex.

The Pleiades is the easiest deep sky object to find with the unaided eye. M45 can be seen in almost any suburban location, barring standing in the middle of a brightly lit parking lot. It is an ideal object for binoculars. Use a low power, wide field eyepiece when using a telescope. To see the nebulosity of the Maia Nebula and Merope Nebula, dark skies and larger apertures are required. An eight inch telescope and a nebula filter is quite adequate for the task under suburban skies. The combination of a dark country sky, 4-inch refractor and a broadband nebula filter can also accomplish the task.