Alternate: Caldwell 30, Hickson Compact Group 92, Arp 319
RA 22h 35 m 51.9 s
Dec +33º56′ 42”
Alternate:Hickson Compact Group 92, Arp 319
RA 22h 35 m 56.7 s
Dec +33º57′ 56”
Alternate:Hickson Compact Group 92, Arp 319
RA 22h 35 m 58.4 s
Dec +33º57′ 57”
Alternate:Hickson Compact Group 92, Arp 319
RA 22h 36 m 03.5 s
Dec +33º58′ 33”
Alternate:Hickson Compact Group 92, Arp 319
RA 22h 36 m 20.4 s
Dec +33º59′ 06”
The Hickson Compact Group 92 was discovered by Edouard M. Stephan in 1877, during his tenure as the director of the Marseille Observatory in France.
The famous Stephan’s Quintet is the first compact galaxy group ever discovered. Stephan’s Quintet is a group of five galaxies, of which four members are interacting and one, NGC 7320c is a foreground galaxy.
Edouard M. Stephan was a French astronomer, and was the director of the Marseille Observatory from
1864 to 1907. Besides his discovery of Stephan’s Quintet, he was the discoverer of asteroid 89 Julia and galaxy NGC 6027.
NGC 7320c is about seven times closer to Earth than the rest of the group. Three of the galaxies have distorted shapes, elongated spiral arms, and long, gaseous tidal tails containing myriad star clusters, clearly the evidence of their close encounters. These interactions have sparked a frenzy of star birth in the central pair of galaxies.
NGC 7319, is an SB peculiar barred spiral with distinct spiral arms that follow nearly 180 degrees back to the bar.
NGC 7318A, an E2 peculiar galaxy, and NGC 7318B, an SB peculiar galaxy, are two galaxies appearing as one galaxy with two cores in both amateur and Hubble images. Encircling the galaxies are young, bright blue star clusters and pinkish clouds of glowing hydrogen where infant stars are being born. These stars are less than 10 million years old and have not yet blown away from their birth cloud.
NGC 7317 is an E4 elliptical galaxy that is less affected by the interactions.
Sharply contrasting with these galaxies is the dwarf SAB galaxy NGC 7320c. NGC 7320c is 40 million light-years from Earth. The other members of the quintet reside 290 million light-years away.
I observed Stephan’s Quintet in all its glory through both an 18” and a 24” Dobsonian telescope at the 2014 Winter Star Party in the Florida Keys. Stephan’s Quintet could be detected from my old 11” SCT from my backyard in the heart of the Cross Junction/Gore area Virginia, with four of its members popping into view and one member playing hide-and seek with averted vision. I have not attempted to view Stephon’s Quintet with my new 9.25” SCT. (For those readers who are wondering, I replaced my 11” with the 9.25” because of size and weight issues. I’m 68 years old and moving an 11” SCT was a strain.) Since the Hickson Compact Group 92 is so tightly concentrated, a medium to medium-high magnification with 60ºAFOV eyepiece is all that is necessary to view the galaxy group.
To all astro-imagers, a wide field image of The Deer Lick Group and Stephan’s Quintet can form a “mega-Cosmic Duet” photographically.
Many years ago, I attended a star party in Pennsylvania and overheard a heated discussion between to amateur astronomers over the merits of different ultra wide-angle eyepieces and Plossl eyepieces marketed by different manufacturers. Claims of superior or inferior glasses and different levels of quality control were bandied about as the discussion became more heated and started drawing a crowd.
I smiled as I was amused that these two guys really did not understand that both brands of telescope eyepieces actually came out of the factory in mainland China!
I interjected that fact into the discussion, and was told (in no uncertain terms) that I didn’t know what I was talking about! Just then, one of the vendors who was displaying and selling their products at the morning vendor show and swap meet happened by, defended my statement and left the verbal combatants in utter silence.
I tell this story to highlight the misconception held by many in the amateur astronomy world that every brand of eyepiece is uniquely designed and manufactured.
In reality, there are only a handful of optics manufacturers and many familiar brand names are just re-branded designs of these few optical houses.
Let us start with “Made in the USA” eyepieces. There are only two, Vernonscope Brandon eyepieces and Edmund Scientific RKE eyepieces. And the actual lenses for the Brandons aren’t even manufactured by Vernonscope, but by an optics house located in a southern state. I’ve been sworn to secrecy on the name of the optical company, but its a similar arrangement to Questar’s optics being made by Cumberland Optics in Maryland (oh, you didn’t know that? A subject for a future Jim’s Corner!).
All of the rest of the eyepieces that populate the advertisements in Sky and Telescope and Astronomy magazines are made in Japan, Taiwan, or mainland China. Yes, that includes most of the favorite brands around.
There are five Japanese companies manufacturing telescope optics including eyepieces. They are :
Carton Optical Industries
Mainland China is represented by:
Jinghua Optical Electronics Company
Kson Optics- Electronics Company
Suzhou Synta Optical Technology Company
The two Taiwan optical firms are Long-Perng Company and Guan Shang Optical..
Please note that Suzhou Synta Optical Technology Company are the current owners of Celestron and one of their subsidiaries operates Meade Corporation.
Occasionally, a handful of eastern European eyepieces are available in this country made by the Russian firms such as Lomo and Intes. And various versions of Zeiss othroscopics can be found on eBay or Astro-Mart.
By the way, the argument at the star party was over the merits of the Celestron Luminos eyepieces and the Meade Series 5000 UWA eyepieces. Both brands are made by Suzhou Synta Optical Technology Company, and are basically the same eyepieces except for the different housings and branding. Surprise!!!
The Rosette Nebula is a large Sharpless catalogued HII region located near one end of a giant molecular cloud in the Monoceros region. With multiple NGC designations, the Rosette qualifies as a Cosmic Duet.
The Rosette has several NGC designations:
NGC 2237 – Part of the nebulous region and also for the entire nebula
NGC 2238 – Part of the nebulous region
NGC 2239 – Part of the nebulous region that was first observed by John Herschel.
NGC 2244 – The open cluster within the nebula, discovered by John Flamsteed in 1690.
NGC 2246 – Part of the nebulous region
NGC 2252 – Part of the nebulous region
The cluster and nebula lie at a distance of some 5,000 light-years distant, and measures roughly 50 light -years in diameter. The illumination of the nebula comes from the ongoing star-formation activity that is occurring in the dense molecular cloud of the region.
The Rosette’s NGC 2244 star cluster can be seen with binoculars or telescopes of apertures up to 4”. The nebula itself is more difficult to spot visually and requires an 8” or greater aperture telescope with a low magnification and UHC filter. Dark skies are a must. The Rosette’s nebulosity is easier to image than to observe visually.
Orion Belt Region including the Flame Nebula and the Horsehead Nebula and M42/43(Jon Talbot)
The Horsehead Nebula
Alternative Nomenclature: Barnard 33
Right Ascension: 05h 40m 59s
Declination: -02º27′ 30”
Magnitude: dark nebula
The Flame Nebula
Alternative Nomenclature: NGC 2024, Sharpless 2-277
Right Ascension: 05h 41m 54s
Declination: -01º51′ 0”
Alternative Nomenclature: includes Barnard 33
Right Ascension: 05h40m59s
Declination: -02º27′ 30”
The Horsehead Nebula was first recorded on a photographic plate by Scottish astronomer Williamina Fleming in 1888, using the Harvard College Observatory. In describing the bright nebula IC 434 that surrounds the Horsehead, she described the nebula as having:
“a semicircular indentation 5 minutes in diameter 30 minutes south of Zeta Orionis.”
Williamina Fleming was the first member of what became known as human computers for the Harvard College Observatory. Working under the guidance of Edward Charles Pickering, an all-women team of women undertook the difficult tasks of analyzing and cataloging astronomical data. The Harvard human computers group of women, sometimes referred to as “Pickering’s Harem”, would also include Henrietta Swan Leavitt, Antonia Maury, and Annie Jump Cannon.
Some controversy surrounds the initial discovery of the Horsehead Nebula. Upon reviewing Fleming’s notes, William Henry Pickering, who had taken the photograph plate in which the Horsehead appeared, speculated that the spot was dark obscuring matter. Yet all subsequent articles and books denied both Williamina Fleming and W. H. Pickering credit. The compiler of the first Index Catalogue, J.L.E. Dreyer, eliminated Fleming’s name from the list of objects then discovered by Harvard, attributing them all instead merely to “Pickering”. This caused astronomers using the Index Catalog to assume the director of Harvard College Observatory, Edward Charles Pickering, to be the discoverer. By the release of the second Index Catalogue by Dreyer in 1908, Fleming, W. H. Pickering, and others at Harvard were recognized for later object discoveries, but not for the earlier discoveries of IC 434 and the Horsehead Nebula. The rightful credit for the discovery has now been established.
Located some 1,500 light years away, the Horsehead Nebula is a challenging object to observe visually.
The area where IC 434 and the Horsehead Nebula is perhaps the easiest to find Cosmic Duet while the most difficult to see visually. Locate the Belt of Orion, and locate Alnitak, the belt star on the left. And look slightly below Alnitak. The Flame Nebula will be on the opposite side of Alnitak.
The challenge is visually observing the Horsehead Nebula. It is basically a photographic object, but the Horsehead has been seen visually by amateur astronomers. However, it is a tough object to see. Although telescopes of at least 8-to-10 inches aperture have been used to observe faint hints of the Horsehead, apertures of 17 inches or greater are often necessary to see it successfully . Extremely dark, transparent skies are a must. A hydrogen-beta nebula filter is necessary to provide an increase in contrast. With all that in hand, the Horsehead Nebula will still be a challenge to view.
The observability of the Horsehead can change dramatically in the space of hours and also from individual to individual. It is a very difficult target in any scope under 16 inches in aperture. Observing the Flame Nebula, IC434, and the Horsehead Nebula simultaneously visually is especially difficult, and remains ideally suited as a photographic or astro-imaging target.
The ease with which it can be seen are dependent on many things:
The darkness of the skies. The darker, the better.
The transparency of the skies. Low humidity is a must. No high cirrus clouds.
Clean optics in the scope are a must.
A well baffled scope is a must. This has a major effect on contrast.
High grade optics yield better contrast than low grade optics, making it marginally easier to see.
Use an eyepiece that yields a suitable exit pupil. 3mm to 5mm is ideal as this effects contrast and target luminosity
Use a high quality eyepiece with good light throughput and contrast.
Use a nebula filter. A Hydrogen-Beta filter helps enormously and a narrowband or UHC filter helps a lot.
And most importantly, the sensitivity to red light are the individual observers eyes. A very significant portion of the light emitting from the background emission nebula IC434 is at the red end of the spectrum and observers with eyes that are less sensitive to red light will simply not see it irrespective of the conditions, because they cannot see the background emission nebula very well.
If and when the Horsehead is spotted, it is not in the upright position often seen in photographs. Since IC 434 is oriented downwards and slightly trending to the leftward direction, the Horsehead profile will be on its side, as if the horse was looking in the direction of Alnitak.
As with M42, Barnard 33, the Horsehead Nebula is part of the Orion Molecular Cloud Complex. It is one of the most identifiable nebulae because of the shape of its swirling cloud of dark dust and gases.
On the other side of the star Alnitak resides the Flame Nebula, NGC 2024. The gas that is energized by Alnitak to energize NGC 2024 and the nearby IC 434 is a part of the Orion Molecular Cloud Complex. At the center of the Flame Nebula is a cluster of newly formed stars. X-ray observations by the Chandra X-Ray Observatory show that 86% of the Flame Nebula’s 800 stars have circumstellar discs, indicating the presence dense gas and dust and the early formation of planetesimals leading to possible planet formation.
The glow of the Flame Nebula is as a result for the now-familiar forbidden transition and the hydrogen-alpha recombination line radiation, as described in the Orion Nebula section. Visually, a large aperture and an O III filter will reveal the Flame Nebula. But like its neighbor the Horsehead Nebula, the Flame Nebula is a more satisfying astro-imaging target.
My own experience of viewing the Horsehead Nebula and Flame Nebula was a the top of a ten foot ladder through an eyepiece on a 24” Obsession Dobsonian at the 2014 Winter Star Party (yes, the same telescope in which I viewed Thor’s Helmet from last month’s column.). A 20mm Nagler eyepiece and a 2” Lumicon Hydrogen-Beta filter on a dark, cloudless, transparent night facilitated the observation. Even with all the technology of large aperture, 82°AFOV eyepiece and Hydrogen-Beta filtering, viewing the Horsehead Nebula was not easy. The tendency for most backyard astronomers is look for patches of light and illumination. Viewing a dark nebula such as the Horsehead requires a mental adjustment and looking for where the light isn’t. I glimpsed it, but without a doubt in my mind, the Horsehead Nebula is at its best as a photograph. The choice of the Nagler 20mm eyepiece hindered the light transmission because of the light absorption of its nine element design. But the 24 inches of aperture from the Obsession Dobsonian helped overcome that shortcoming. However, balancing oneself on top of a ten foot ladder to peer through an eyepiece does require a little internal fortitude!
Thor’s Helmet, NGC 2359 a winter month object that deserves more attention from amateur astronomers. The nebula is approximately 11.96 thousand light years away and 30 light-years across.
The central star illuminating NGC 2359 is an extremely hot star thought to be in a pre-supernovastage of evolution, called a Wolf-Rayet star. Wolf-Rayet stars are the final stage of giant stars before going supernova. Their spectra shows broad emission lines of ionized helium and highly ionized nitrogen and carbon. The hydrogen to helium fusion that typifies our Sun has long since expired in a Wolf-Rayet. Helium fusion, followed by nitrogen, etc marks the end stage of these stars, ending with a massive supernova event. The properties of these stars were first described 1867 by Charles J. Wolf and Georges A. Rayet. There are 164 Wolf-Rayet stars are known in the Galaxy. These luminous Wolf-Rayet stars of spectral type O or B, and are hot stars, with effective temperatures between 30,000 and 50,000 K. Their very peculiar spectra show emission lines of a a stellar wind is ejecting matter into space at velocities between 1,000 and 3,000 kilometers per second, and indicate that an envelope of ejected matter exists around the star. The rate of mass loss by the stellar wind is significant, of the order of 10-4 solar masses per year. Wolf-Rayet stars are often seen surrounded by nebulosities as seen in NGC 2359.
Thor’s Helmet is similar to the Bubble Nebula. Interactions with a nearby large molecular cloud are thought to have contributed to the more complex shape and curved bow-shock structure of Thor’s Helmet.
NGC 2361 is a bright knot of nebulosity on one edge of the central ring of NGC 2359, thereby qualifying Thor’s Helmet as a cosmic duet.
My first view of NGC 2359 was at the 2014 Winter Star Party in the Florida Keys. I climbed to the top of a ten-foot ladder to reach the eyepiece of a 24” Dobsonian to get my first view of Thor’s Helmet. Definitely a memorable experience. Returning home from the trip, I was able to view NGC 2359 through my 11” SCT, as this deep space treasure can be seen through the more common 8” telescopes.
A nebula filter, such as an UHC or O-III filter, will be of benefit.
NGC 7752 and NGC 7753 are a set of galaxies approximately 272 million light-years away. Both were discovered by William Herschel on September 12, 1784.
NGC 7753 is the primary galaxy. It is a barred spiral galaxy with a small nucleus. NGC 7752 is the satellite galaxy of NGC 7753. It is a lenticular galaxy that is apparently attached to one of NGC 7753’sspiral arms, somewhat reminiscent to the famous M51 Whirlpool Galaxy.
In 1966, astronomer Halton Arp published a catalog of 338 galaxies entitled Atlas of Peculiar Galaxies.The main goal of the catalog was to present astro-photographs of many different kinds of peculiar structures of galaxies found among the various sky surveys. The reason why galaxies formed into spiral or elliptical shapes was not well understood, and he hoped that by publishing the atlas would stimulate the astronomy community into further study. He perceived peculiar galaxies as small “experiments” that astronomers could use to understand the physical processes that distort spiral or elliptical galaxies. With this atlas, astronomers had a focus group of peculiar galaxies that could be studied in detail. The atlas is a sampling of peculiar galaxies in the sky, each providing examples of the different phenomena as observed in galaxies.
One example of this stimulation of scientific inquiry is the following abstract written by Chandreye Sengupta, K.S. Dwarakanath, and D.J. Saikia of their study entitled “Hicontent and star formation in the interacting galaxy Arp86”:
“We present the results of Giant Metrewave Radio Telescope (GMRT) observations of the interacting system Arp86 in both neutral atomic hydrogen, Hi, and in radio continuum at 240 606 and 1394 MHz. In addition to Hi emission from the two dominant galaxies, NGC 7752 and NGC 7753, these observations show a complex distribution of Hi tails and bridges due to tidal interactions. The regions of highest column density appear related to the recent sites of intense star formation. Hi column densities ∼1–1.5 × 1021 cm−2have been detected in the tidal bridge which is bright in Spitzerimage as well. We also detect HI emission from the galaxy 2MASX J23470758+2926531, which is shown to be a part of this system. We discuss the possibility that this could be a tidal dwarf galaxy. The radio continuum observations show evidence of a non-thermal bridge between NGC 7752 and NGC 7753, and a radio source in the nuclear region of NGC 7753 consistent with it having a low-ionization nuclear emission region nucleus.”
NGC 7756 is the most likely member of Arp 86 to be seen by backyard telescopes at Magnitude 12.8. The fainter NGC 7752 is more likely picked up by astro-imagers than visually.
In my senior year of high school, 1969-1970, I ordered the parts to assemble a 3-1/4” f/15 refractor from the venerable A. Jaegers Co. of Lynbrook, N.Y. My old telescope is coming home to me, and here is its story.
For those veteran backyard observers of my generation, A. Jaegers and Edmund Scientific catalogs were in the possession of every amateur astronomer. World War II military surplus lenses, eyepieces, prisms, even the optical heads of Norden bombsights were available through the A. Jaegers optical catalog. Also available were the early beginnings of wide angle 2” eyepieces, the infamous radioactive 32mm or 38mm Erfle oculars salvaged from military gunsights and optical range finders.
Additionally, A. Jaegers manufactured for the amateur astronomy hobbyist air-spaced Fraunhofer refractor objectives, rack-and-pinion 1-1/4”focusers, and rudimentary (and by today’s standards primitive) equatorial mounts. Newtonian mirror grinding kits were available from 4-1/4” pyrex blanks to, what was considered the ultimate, a 12-1/2” mirror. The A. Jaegers optical catalog was a virtual cornucopia of optics that the amateur backyard astronomer would salivate over for endless hours.
In particular, the air-spaced Fraunhofer refractor objectives were special. Most of the telescope companies of the 1950’s-1960’s era manufactured Newtonian or classical Cassegrain designs, such Cave Optical, Criterion, and Optical Craftsman. The cheaper 50mm-60mm beginner refractor telescopes tended to come from overseas, under the nameplates such as Tasco, Jason, and Mayflower. Unitron was the only supplier of quality refractors. And the Ferrari of telescopes, the Questar was in a league of its own.
And then there was A. Jaegers with its line of refractor objectives, with aluminum tubes which the objective cells were claimed to be specifically machined to fit on (they didn’t), and rack-and-pinion focusers specifically machined for the other end of the tube (and did fit). The price difference of these A. Jaegers user-assembled refractors and the Unitron refractors was significant. Especially for a penny-pinching senior in high school.
After watching my good friend and astronomer buddy (the word nerd hadn’t been coined yet) grind his own 4-1/4” mirror, and test-and-polish-and-test-and-polish over and over again, I decided to buy the A.Jaegers parts to build the 3-1/4” f/15 refractor. I felt it would be significant upgrade to my 60mm Tasco.
The MgFl coated 3-1/4” f/15 was $36. The german equatorial mount with pedestal and the focuser brought the final order to around $130. A fortune for a high schooler and the sum of a summer’s lawn mowing.
The packages arrived from Lynbrook N.Y. A long aluminum tube suitable for a 48” focal length refractor telescope. One large box containing one refractor doublet in aluminum cell and one 1-1/4” focuser. Shipped separately and later in the week, the equatorial mount arrived with its pedestal.
Being in-between girlfriends that weekend (yes, I was a nerd, but I had dates. I wasn’t totally socially inept!), it was time to assemble my new astronomical weapon. The first task was to spray paint the outside of the tube the standard white that all telescopes of that era were painted. The future of orange tubed Celestrons and blue tubed Meade telescopes were not yet upon us. I then sprayed the interior with a flat black and positioned two flat black painted baffles into the tube for anti-reflection.
Next, I fitted the focuser on. A tight fit, and with three screws to hold it in place, that task was done.
Then there was mounting the objective cell onto the other end of the aluminum tube. This is where everything ground to a halt. The objective didn’t fit. Tight wasn’t the problem. It did not fit. Calling my friend over to help, we decided to file the end of the tube down to force the fit. After what seemed like hours, but less than 45 minutes, the objective finally slid on. Three set screws and the telescope was done.
The equatorial mount went together without a hitch. Two adjustable metal straps and felt was glued to the saddle, and the telescope was ready for use.
Except, no finder scope! I had forgotten to mount a finder scope on my new telescopic beast! In the corner of my bedroom sat my forlorn and now abandoned Tasco. I had an inspiration. I had seen in Unitron catalogs pictures of Unitron telescopes with smaller aperture refractors mounted piggyback as guide-scopes. With the weather being cloudy for the next few nights, my new telescope wasn’t going to see first light anyway. So I mailed an order off to A. Jaegers for some mounting rings. The rings eventually arrived (quick service!) and my Tasco had a new home atop my A. Jaegers f/15 refractor. For the rest of that winter, I braved the freezing cold and enjoyed my new telescope.
In my freshman year at the University of Maryland, with an assist from my old high school physics teacher now an astronomy department faculty member, I, joined by my astronomy telescope building buddy, used an expensive research grade laser on an optical bench to collimate my telescope. The collimation was not that far off, but now it was perfect. All was right with the world.
The quality of the Jaegers refractor objectives have been debated over the past decades. The pre-1970’s lenses were of high quality, some being measured to 1/10thwave smoothness, very impressive. After a turnover of opticians at A. Jaegers, the quality suffered, with many examples being as bad a ½ to 1 wave, not so good.
Although I have never had it tested, my objective was a 1969 manufacture, so I believe it is at least ¼ wave and maybe better.
The real test numbers doesn’t matter, I got great high-contrast images of Jupiter, Saturn,and Mars at opposition with this telescope. Of particular memory was the redness of Jupiter’s Great Red Spot. M42 with its Trapezium was spectacular to my teenage eyes. The Ring Nebula was ghostly and beautiful.
This telescope stayed with me until my senior year at the University of Maryland. I brought it to campus because I had met my now best friend who shared my love of astronomy. I had my refractor, and he had a 10” Cave Newtonian. We went out into the fields between dorms to compare our telescopes, and I was blown away by the Cave 10”. M13 globular cluster wasn’t a fuzzy ball, but a concentration of individual stars that resolved to its center!
Later that fall semester, one of the guys in my dorm was peering through his binoculars out his window. His roommate then setup his small Jason telescope to get a better look. I mentioned my telescope, and was convinced to bring it to the room to get a better look. To make a long story short, the guys were so impressed with my telescope that someone on the spot pulled out $350 cash to buy my telescope! Doing quick math in my head, I was quickly overwhelmed by the profit motive and that was the last time I saw this telescope. At sometime after, I acquired a 6” Newtonian on an equatorial clock driven mount, a true classic of the times, a Criterion RV-6.
My long-departed A.Jaegers refractor was just a memory until September 7, 2020. My best friend was searching through Craigslist when he found an ad for an old refractor selling for $100. It was very familiar to him, and he called me. The picture, the description and the eyepieces that were supplied confirmed this was my old telescope! So my old telescope is finding its way home!
As for A. Jaegers, the optical company was not so fortunate. During the 1970’s, there was a high turnover of opticians and the production of the Fraunhofer objectives suffered as the telescope market changed. Al Jaegers no longer took an active role in the production of the refractor optics. By the 1980’s, A. Jaegers suffered a disastrous fire that destroyed the whole facility.An attempt to revive the company occurred 20 years later under the moniker A. Jaegers, Jr. Optics. But time had past it by, and the inventory of the failed revival was sold to Surplus Shed.
But for those of a certain age, the A. Jaegers catalogs were the stuff that telescope optics dreams were made of.
For many amateur astronomers, these are common place telescope formulas that we learned early in our astronomy careers.
But to many, some of these formulas are unfamiliar, need to be looked up in a book, or are listed under the category of “How do you figure that out?”.
Telescope Magnification, or What power am I using?
The most common question when someone looks through a telescope.
M = focal length of telescope/ focal length of eyepiece
where the focal lengths of both telescope and eyepiece are in the same units.
M = f.l. telescope in mm/ f.l. Eyepiece in mm
M = f.l.telescope in inches/ f.l. in inches
Exit Pupil, or Am I seeing all the light?
A young person’s pupil can open to 7mm in dark conditions. Older eyes aren’t so night friendly, with pupils opening to 5.0mm to 5.5mm. So a eyepiece-telescope combination that yields an exit pupil of 8mm means you’ve wasted your money on equipment because the extra light is unseen and wasted.
Exit Pupil = D/M
D = the diameter of the telescope’s objective lens or primary mirror in millimeters
M = magnification = focal length of telescope/focal length of eyepiece
or Exit Pupil = F/f
F = the focal length of the eyepiece in millimeters
f= the telescope’s focal ratio ( the f-number)
True Field of View, or If my eyepiece has an apparent field-of-view, what is my real field?
If you can get 4° or 5° or more of True Field at low power with your telescope, you probably don’t need you finderscope anymore. Just a low power, wide field eyepiece. And the advantage is using a much larger aperture of the main telescope instead of a smaller aperture finderscope.
True Field = AFOV/M
AFOV = the apparent field of view of the eyepiece in degrees
M = magnification
Focal Ratio, or Why is my telescope so long or short?
A focal ratio of f/12 or more means easily attainable high power but a limited field-of-view. A focal ration of f/7 or even the shorter f/5 or less means low power wide field-of-view.
Focal Ratio = f.l./D
f.l. = focal length of the telescope
D = diameter of the telescope objective
Dawes Limit or Resolving Power, or What’s the smallest thing that I can see?
A double star or lunar observer is interested in this figure to determine the resolving limits of their telescope.
Estimate Resolving Power = 4.56/D in inches
Estimate Resolving Power = 116/D in mm
D = diameter of the telescope objective
Estimating Residual false color in Achromatic Telescopes, or Do I really need to spend $$$ to get a refractor that don’t show secondary color?
FromTelescope Optics: Evaluation and Design, by Harrie Rutten and Martin van Venrooij:
focal length >0.122D
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.
M35 is the only Messier open cluster in Gemini. It was discovered byJean-Philippe Loys de Chéseaux in 1745 and independently discovered by John Bevis before 1750. This star cluster is large and covers an area the approximate size of thefull Moon. M35 is located 2,200 light-years from Earth. M35’s Trumpler classification as III,3,r according to all reference sources.
With an age of approximately 100 million years, M35 is an intermediate age open cluster, and contains several yellow and orange giants of spectral type late G to early K. Its hottest main sequence star is given as of spectral class B3. With a blue Doppler shift, it is approaching Earth at a rate of 5 km/sec.
The compact open cluster NGC 2158 is directly southwest of M35. NGC 2158 is also an open cluster in Gemini.
The two clusters are unrelated, as NGC 2158 is around 9,000light years further away than M35.
Observing this Cosmic Duet definitely benefits from a dark and moonless night, where M35 is easily seen, but the dimmer NGC 2158 appears as a faint smudge. Although within reach and observable through my 4” apochromat refractor, the dimmer NGC 2158 was more apparent and observable in the my 130 mm apochromat refractor and my 9.25” SCT.
This cosmic duet is not only a binocular pair, but under dark skies can be seen with the naked eye.
Both M6 and M7 are open clusters in the Scorpius. The M7 is detectable without optical aid and is located near to the tail stinger of Scorpius, the Scorpion. M7 is the southernmost object in the Messier catalog. M6, being slightly dimmer, can also be seen with the naked eye nearby. Dark country locations will enable naked eye sittings of M6 and M7. Suburban and urban observers need not apply.
The ancient Greeks knew of M7. It was first recorded by the 2nd-century Greek-Roman astronomer Ptolemy, who described it as a nebula in 130 AD.This is how M7 gained the nickname of Ptolemy’s Cluster. Some astronomy historians think that Ptolemy may have also observed M6, but he did not record that observation.
Italian astronomer Giovanni Batista Hodierna observed M7 before 1654 and recorded it containing 30 stars. In 1654, Hodierna observed and recorded the Butterfly Cluster. Hodierna published in 1654 a book that contained his catalog of non-stellar objects, but this work apparently was unknown to Charles Messier. In 1764, the Butterfly Cluster and the Ptolemy Cluster became the sixth and seventh members of Charles Messier’s famous catalog.
There is some disagreement on the Trumpler classification of M6. Trumpler has classified M6 as II,3,m, while the Sky Catalog 2000.0 gives its Trumpler type as III,2,p. Other sources have listed M6 as II,3,r.
M6 is estimated to be around 100,000 million years old. Most of the bright, visible stars in it are hot, young, blue stars belonging to the spectral class B4-B5. However, the brightest star in the cluster is an orange giant belonging to the spectral class K.
The brightest star in M7 is a yellow G8-type giant with an apparent magnitude of 5.6.
Observations of M7 reveal about 80 stars within a field of view of 1.3° across. M7’s estimated distance of 980 light-years, and approximately 25 light years across. The age of the cluster is around 200million years while the brightest member star is of 5.6 magnitude. M7 is classified as of Trumpler type I,3,m or I,3,r.
The two Messier clusters can be easily observed through 2.1×42, 7×50, and 10×50 binoculars, even under slightly light-polluted suburban skies. M6 and M7 are separated by 6º, making telescopic duet observing virtually impossible.
As a special added attraction, a dark nebula can be detected under dark skies in the M6/M7 region. The famous Pipe Nebula on the Scorpius / Ophiuchus border, lies north of M6 and M7. The Pipe Nebula is also known as Barnard 59, 65–67, and 78. The Pipe Nebula is a dark nebula of light obscuring dust and gases in the Ophiuchus and is part of the larger Dark Horse Nebula. It is a large but readily apparent pipe shaped dust lane that obscures the Milky Way star clouds behind it. Clearly visible to the naked eye in the southern United States under clear dark skies, it is best viewed with 7×50 or 10×50 binoculars. Observing dark nebulae requires the special skill that is opposite of a sky observer’s normal observing process. Instead of recognizing where stars and nebulosity is, the goal is recognizing where stars aren’t.