Wednesday, November 24, 2021

Learning Lyra

My spatial reasoning scores are always lower than my verbal acuity grades. Consequently, the standard star charts for Lyra, a simple quadrilateral constellation marked by one of the brightest stars in the sky, has been oddly difficult to navigate. Therefore, I returned to basic methods. I have not yet found everything that I mapped out for discovery, but I am confident about the first half of the picture.

What it is supposed to look like

What I mapped in the sky

Although Lyra is supposed to be above Cygnus, clearly, to me, it is not. Of necessity, finding the double stars epsilon Lyrae and zeta Lyrae was a challenge. I have viewed and recorded the famous "double double" epsilon Lyrae several times. It is a target that I know. It is easily above and to the right of Vega.  ("Viewing Epsilon Lyrae" and "Binary Star Project" on this blog.) 

Three of Lyra's stars near Vega are interesting doubles. Barely above Vega is 4th-magnitude Epsilon Lyrae, the Double-Double. Epsilon forms one corner of a roughly equilateral triangle with Vega and Zeta Lyrae. The triangle is less than 2° on a side, hardly the width of your thumb at arm's length.

Binoculars easily resolve Epsilon. And a 4-inch telescope at 100× or more should, during good seeing, resolve each of Epsilon's wide components into a tight pair.

Zeta is also a double star for binoculars. It's much closer and tougher, but is plainly resolved in a small telescope.

And Delta Lyrae, upper left of Zeta by a similar distance, is a much wider and easier binocular pair. Its stars are reddish orange and blue.

https://skyandtelescope.org/astronomy-news/this-weeks-sky-at-a-glance-november-12-20/

At first, I tried following the guides in support of my direct views in the telescope. But I came to understand that the rich star field offered other objects that could match the verbal descriptions.

So, I lay down on a chaise longue with my binoculars and drew out the sky as it appeared to me. And while doing that, I also was able to identify the binary stars, zeta and (of course) epsilon.

  
The cellphone Compass app is handy and the app for Measure includes a Level which is accurate enough for my purposes.


I also made estimates of the separations based on the field-of-view (FOV) calculations for the 12x42 Buhnell binocular, as well as the telescope I was using, my Explore Scientific 102-mm f/6.47 achromatic refractor. 

That left me confident of those views though still open on the problem of locating delta Lyrae

Sunday, November 14, 2021

Discovering gamma Arietis

Having packed away the three 70-mm entry-level refractors I was enjoying (last week), I returned to my Explore Scientific First Light 102-mm doublet refractor but now using the better oculars, 82-degree sealed from Meade (14mm; Series 5000) and Nagler (7mm; Type 1), and a Vixen 25-mm SLV Plössl for wider views. The mount was an Explore Twilight I gear-driven Altitude-Azimuth.

10 November 2200 hours. Nominally “clear” but with not much sky, still clearing from rain with high overcast. In Aries only alpha (Hamal) and beta (Sheratan) were easily visible. So, I swept the area. Below Sheratan was another faint star and I found the double gamma Arietis (Mesarthim). Both were about equally bright. The next day, I checked An Anthology of Visual Double Stars by Argyle, Swan, and James (Cambridge, 2019). 

Gamma Arietis was found by Hooke (1664) and verified by Herschel (1779) as being “almost equal” in magnitude. 


Land-based estimates of magnitude for them is Gamma-1 = 4.58 and Gamma-2 = 4.52 while more recent estimates from Hipparcos are 4.75 and 4.64 respectively. Combined they appear as 3.86 from the ground. Their common orbital period is about 5000 years.

 

Two thousand years ago, the vernal equinox was in Aries. (It is now in Pisces and moving to Aquarius, which brings in very many cultural references to new ages.) The coordinates for Mesarthim are given as 

Right Ascension 01 hours 53 minutes 31.76 seconds; 

Declination +19 degrees 17 minutes 37 seconds. 

 

PREVIOUSLY ON NECESSARY FACTS

Newton versus the Counterfeiter

A Successful Imitation of Alan Turing

Coins and Stamps

Burnham's Celestial Handbook


Saturday, November 13, 2021

Book Review: The Science and Art of Using Telescopes

The essence of the book is given in the opening. Get past the beginner stage by finding new interests; specialize in subbranches of observational astronomy (page 2; page 4). After that, the writing devolves into a rambling monologue directed at knowledgeable amateurs. The information provided serves more as reminders of what we know, rather than providing new learning or directing us to important resources. Early on and throughout, the author tells us to find out about the current markets for instruments and accessories by referring to “monthly magazines” none of which he names. In point of fact, Popular Astronomy from the SPA appears bi-monthly, and the independent Amateur Astronomy comes out quarterly. The book offers no suggestions for websites, discussion boards, chatrooms such as Facebook and Reddit, or other online social media.

Author Philip Pugh does name his favorite brand of equipment, Sky-Watcher. He cites them 16 times (as “Skywatcher”), which is as often as he cites Meade (7), Celestron (6), Takahashi (2), Astro-Physics (1) , and Tele Vue (1) combined. Other labels are similarly passed over with brief mentions. 

The Science and Art of Using Telescopes
by Philip Pugh, Springer, 2009.
 

The references to Astro-Physics and Tele Vue underscore the fact that this book is poorly edited. The brand names are misspelled as Astro Physics (page 250) and indexed as Astro physics; and Televue. Takahashi is misspelled as Takashi (page 28) and Takahasi (page 250). Plossl (never, as proper, Plössl or Ploessl) appears in the index as two lists: Plossl and Plossl eyepiece

 

Those small errors reveal the lack of professional proofreading. That speaks to the painfully obvious fact that this book reads like a first draft. The author loves (even in parenthetical comments!) exclamation points! Pugh just wrote this off the top of his head and Springer accepted it uncritically.

 

As an indication that the author did not have his manuscript fact-checked by an independent reader or even check his own work, the definition of ED (extra-low dispersion glass) is wrong, and wrongly stated. He calls ED “extra dispersion” throughout the book, and in the Glossary: “An extra dispersion lens is an improvement in the achromatic objective lens theme where it uses extra dispersion flint glass to improve performance.” (page 368). 

ED Extra-low dispersion


"Nikon's original ED (Extra-low Dispersion) glass lenses effectively compensate for color fringing especially at high magnification." -- https://imaging.nikon.com/lineup/sportoptics/how_to/guide/fieldscopes/choosing/choosing_03.htm


"ED stands for "extra-low dispersion," which refers to the composition and optical properties of the glass used for the lenses.  ED glass is specially formulated and contains rare-earth compounds that greatly reduce a visual defect called chromatic aberration." -- https://www.celestron.com/pages/ed-glass


ED glass enhances apochromatic lens design by producing extra-low dispersion of the wavelengths of light passing through it thereby giving an even better apochromatic performance.  -- https://explorescientificusa.com/collections/apo-triplet


In this chart, standard glass is shown on the far left. To the right of it are two commonly used extra low dispersion (ED) glasses. -- https://www.stellarvue.com/optical-glass-types/

Perhaps the hallmark of his style is that he avoids unequivocal statements. Pugh cannot discuss telescopes (pages 6-8) without digressing to his preferred choices among binoculars, even though “Choosing Binoculars” is the next section after “Choosing a Telescope.” Paradoxically, that section is not at all about choosing a single telescope but argues very well that you need more than one. The author’s apparent fear of absolute statements results in meaningless advice. “While it is true that the Usual Suspects (see the appendix) can sometimes look better under clear conditions, some gems such as M81 in its full glory have to be enjoyed while the chance is there.” (page 41). That sounds like good advice: whatever your skies right now, take the opportunity to view what you can. But if you read the words carefully, Pugh is saying that M81 can be seen in its full glory even though not under clear conditions, which contradicts the opening clause. 

 

That example is from the section “Too Cloudy to Go Out?” (page 40-41) which is about why is it not really too cloudy to go out because telescopes can often cut through poor seeing conditions. Faint clusters, dim companion stars, and more can all be viewed under bad conditions. I get the point, but an editor would have retitled the heading. 

 

Aversion to unequivocal assertions delivers many instances of “however.”  

“Unguided exposures at long focal lengths can be troublesome on some mounts because of tracking errors. However, you will find that auto-guiding can compensate for this very well. Celestron offers a version of this mount. Moving up in quality and performance, companies such as Astro Physics, Takahasi [sic], and Software Bisque manufacture excellent mounts. However, considering the typical cost, they are not mounts for a beginner!” (page 250). 

Why are such mounts not suitable for a beginner who can afford them? He never says. 


It is disappointing in a book that recommends finding new interests that the author provided no pointers to the organizations that support the many sub-branches of observational astronomy and citizen science. On page 114, the author discusses occultations but only someone who knows the hobby well would know about the International Occultation Timing Association (IOTA) or the fact the the Society for Popular Astronomy has an Occultation section and that the chair of that committee will generate a spreadsheet for you of occultations predicted for your location. Similarly, for lunar, solar, and planetary viewing (Chapters 2, 3, 4), the Association of Lunar and Planetary Observers (ALPO) was founded in 1947 for amateurs, and the American Astronomical Society (open to amateurs) also has its Division for Planetary Sciences. For deep sky viewing, AAVSO (the American Association of Variable Star Observers) maintains a peer-reviewed archive of data, much of it provided by amateurs.


PREVIOUSLY ON NECESSARY FACTS

Seeing in the Dark: Your Front Row Seat to the Universe

Turn Left at Orion

Michael Shermer's Moral Arc

The Science of Liberty


Sunday, November 7, 2021

70-mm Shootout

Start with the mount. Just as the scabbard called “Avalon” was more powerful than “Excalibur” (the blade it sheathed), the positioning mechanism (“mount”) is more important than the telescope. In every case here, you could keep the telescope, but throw out the mount. But any telescope is better than no telescope. Given the limitations, if $159 is your price point, then these will show the stars you cannot see.  


Meade 70-mm StarPro Refractor

Celestron 70-mm AstroMaster Refractor

National Geographic 70-mm Refractor


Celestron AstroMaster 70 AZ Refractor Focal length=900mm; ratio f/13. $189.95 from Mile High Astro. Comes with two Kellner oculars, 20mm and 10mm.

 

Meade StarPro AZ 70mm Refractor Focal length=700mm; ratio f/10. $159.99 from Mile High Astro. Comes with three Kellner eyepieces, 26mm, 9mm, and 6.3mm oculars; and a 2X Barlow lens.


National Geographic Refractor Focal length=700; ratio f/10. $109.99 from Explore Scientific. Comes with two Ploessl oculars, 26mm and 9.7mm, and a 2X Barlow lens.

 

The Meade mount is not easy to use. Altitude and Azimuth clutches must be very, very tight and even then, the azimuth does not work well when it works at all. At best I can turn the handle and at the same time nudge the telescope from the strut. 
CELESTRON
PAN-TILT MOUNT

The Celestron pan-tilt mount is even worse. It is too tight even after disassembly, degreasing, and relubrication with WD-40 Silicon. In these tests, the Celestron was usually on an Explore Scientific First Light mount. In fact, after fighting with the mounts, the first night, I set them into an Explore Scientific Twilight I mount for its ease and stability. 


The mount and tripod of the National Geographic is a bad design. The altitude (or declination) is a twist knob screw drive and that works well. However, the thumbwheel for the right-left (azimuth or right ascension) was immovable with my left thumb, and jerky with my right.


Overall, the Meade is the best of three. I was disappointed in the Celestron because I had hoped to make good use of its longer 900 mm focal length. However, it did not stand up to simple backyard stargazing. The National Geographic was worst of the three. The objective lens was flawed, displaying ghost images from internal reflection. I already own a National Geographic that I was happy with and I bought this to test against it. The new one is not as good as the old one. The old one does not have ghosts.


From Star Ware by Philip S. Harrington
The Meade and Celestron come with Kellner eyepieces, a basic three-element design from the 19th century. Today, the baseline is the Ploessl design, also from the 19th century, but a more sophisticated four lens system that became popular in the 1980s. Kellners are considered to be cheap downgrades for these entry-level instrument packages. The National Geographic Ploessl eyepieces are Bresser branded. 


Overall, these do their best work with stars, not planets. With all three, among the first views were the Orion Nebula (Messier 42), resolving the interior Trapezium group, and then turning to the nearby open cluster Messier 41 (below Sirius). The last sessions were simple tours of the rich star field near Delta Cygni, the center of the Swan or Northern Cross. It was enjoyable because the area offers so many stars including red giants and binary stars. 


Jupiter, Saturn, and Venus were all challenging and disappointing in all three telescopes. I could identify the planets. The rings of Saturn were easy to define. However, all were plagued by cloudy, foggy glare from the bright disks. 


However, the Meade had a saving grace. It comes with an aperture reducer, a clever old hack from experienced observers, a mask--in this case a hole in the dust cover--which allows a limited cone of light increasing the focal ratio by reducing the aperture in order to bring out more details in planets, but at a cost of brightness.


After trying out all three in their "native mode" with the eyepieces provided in the package, I followed the advice offered by a customer service representative from Mile High Astro and tried them with my best oculars. 


06 November 2021 Testing with the best oculars: 

TeleVue Nagler-1 82-degree 7mm

Meade 82-degree 14mm

Vixen SLV 25mm. 

 

(discount priced)
Meade logo but no name
Series 5000 14 mm
82-degree field of view


1915 hours

Meade 70-mm StarPro with 7-mm. Nagler 

Saturn. 1 band north. 1 ring. Some hint of texture to the ring on the east side.

Titan? and Tethys? (It seemed so at the time, but the Sky & Telescope “Saturn” page (here) says no: these were just stars in the field.)

 

Jupiter. Hard to focus. Some chromatic aberration but north and south bands visible and some texture to equatorial region.

 

1938 hours

Celestron 70-mm AstroMaster with 7-mm Nagler.

Saturn. Hazy. Foggy. Titan (or star) discernable. Ring visible. 1 band north. Near limit of resolution. Image spotty and grainy.


1947 hours

Nagler 7mm Type 1
82-degree Field of View
12mm Eye Relief
7 element system

Jupiter. Hazy. Foggy. Near limit of resolution? (Inspected objective lens and diagonal prism for dew or condensation, but both were clear.)

 

1955 hours

Celestron 70-mm AstroMaster with 14-mm Meade.

Jupiter smaller (of course). Not as foggy. Still hints of diffraction rings. Two bands discernable. 

(Checked magnification: 900 / 14 = 64+. It should be a clear view.)

 

2005 hours

Meade 70-mm StarPro with Meade 82-degree 14mm.  (50X)

Jupiter. Nice view. 2 bands. Some chromatic aberration.  Not bad.

Used aperture reducer. Sharper image. More detail on planet. Not as bright. But more detail in the equatorial between the bands North and South.

 

2016

Meade 70-mm StarPro with Meade 82-degree 14mm.  (50X)

Saturn with aperture reducer.

Vixen SLV
Lanthanum coated
Twist-up eye cup
All metal body
Sharp image. No bands visible. Titan (?) discernable though faint, of course and one more moon (?) behind the planet. 

2032

Meade 70-mm StarPro with Vixen SLV 25-mm

Delta Cygni region. 

Lots of stars. Focus OK, not perfect, but close. Scanning area, settled on a red star at center and a discernable binary to the north or east. Lots of stars to look at in my Bortle 7-8 with no Milky Way. The telescope comes with a Modified Achromatic (Kellner) 26-mm, which is OK.


07 November 2036

Celestron 70-mm AstroMaster with Vixen SLV 25-mm Delta Cygni region. (see above: The area is rich with targets to observe, record, and enjoy. This telescope comes with two eyepieces, 10mm and 20mm, which magnify too much (45X and 90X) for broad and wide fields such as this, the Pleiades, and the Moon.  


Because the project began with the challenge to split the double-double of epsilon Lyrae with a 70-mm refractor, here are those notes.

 

01 November 2021

Celestron AstroMaster 70-mm refractor.


1948 Installed Celestron Lens&Filter kit Ploessl 6mm and kit 2X Barlow and successfully split the companions E Lyrae E1A and E Lyrae E2A. 

Smaller, of course,
but about like this








05 November 2021

1955 Chasing epsilon Lyrae.

2011 Chasing epsilon Lyrae.

Celestron 70-mm AstroMaster with 8mm Celestron Ploessl and 2X Barlow.











Note that northern members are touching. Western pair definitely separated. I tried the 6mm with 2X Barlow but it was just too hard to fight with the pan-tilt camera mount.


PREVIOUSLY ON NECESSARY FACTS

 

Focus on Georg-Simon Ploessl 

Product Review Celestron AVX Mount and Tripod 

Galileo and Saturn: Epistemology, not Optics 

Observing with NASA: An Open Platform for Citizen Science 


Below the fold.

Open the Jump to see all of the notes that I took 31 October to 05 November 2021. 

Tuesday, October 26, 2021

A Good 70-mm Refractor

In a kerfuffle on the Sky Searchers astronomy discussion board, a couple of the stalwarts insisted contrary to the experiential reports of a newbie that “a good 70-mm refractor” can split the popular double-double star system epsilon Lyrae. I could not with my National Geographic 70 mm. (Previously on Necessary Facts.) So, I decided to find out. 

I have two new telescopes on order now from Mile High Optics, a Meade 70 mm StarPro AZ and a Celestron 70 mm AZ AstroMaster. I also ordered a new National Geographic 70 mm refractor from Explore Scientific on the likelihood that mine failed for material reasons apart from design and manufacture. The key word there is “apart” because I took the objective apart and had to try a few times to get it back together right. I am still not confident about that.

The three telescopes that I have coming for testing are all achromatic doublets. The design consists of objective lenses of two different kinds of glass to minimize chromatic aberration caused by the refraction of different wavelengths of light by using two slightly different different media. This is a solution known in the 18th century. Better still would be an apochromatic triplet. I have one, an Astro-Tech 115-mm refractor (reported last October here) but that is not part of this experiment.

Meade Series 6000 at retail for $1339
for Optical Tube Assembly (OTA) only
in other words, just the telescope,
no diagonal, finder, lenses, etc.
Quadruple objective lens
for maximum chromatic correction
(Agena Astron Products, Cerritos, California)


My superpower is sleeping on a problem and waking up with the solution. (Sometimes, it takes a couple of days.) When I last viewed epsilon Lyrae with my Natl G 70, I was able to resolve the northeast pair, but not the southwest pair. (See also Viewing epsilon Lyrae here from last year.) The two couples are about the same visual distances, primaries from secondaries, and of about the same magnitudes. If one resolved then the other should have also. Based on that, I might take it apart again and rotate the two objective lenses by 90 degrees. 


Williams Optics, Gran Turismo. $933.
OTA only, though with Bahtinov mask
for precise focus adjustment

Anyway, the instruments that I ordered are all modest: under $200 for the Celestron and about $150 for the Meade, and just over $100 for the National Geographic. I did not buy the Svbony at $86 because it seems under-developed based on the reports from my colleague on The Sky Searchers and it is a “fast” scope with a short focal length, which comes with other problems, the touchy focusing being first. 


Sky-Watcher Evostar 72 mm "fast" Doublet. $490.
Rare earth (ED) glass meeting apochromatic standards.
(From Astronomics of Normal, Oklahoma.)
Made in Suzhou (Jiangsu), China, by 
Suzhou Synta Optical Technology Co., Ltd., of Taiwan.
OTA only, which is why the mount is in green
and no finder, diagonal and ocular are shown.
The focuser includes a fine adjustment.

Also, I did not buy the high-end 70-mm refractors because I accept as a matter of faith (for lack of a better word) that for the money, the optics would be good, and at the top of the line, fantastic. If the ones that I get cannot resolve the members of the quadruple system, then they fail the standard of “a good 70-mm refractor.”

 

Astro-Tech 70-mm $299 
Extremely-low Dispersion (ED) glass
considered "near apochromatic."
f/6 includes dual-speed focuser. 
OTA only from Astronomics of Norman, Oklahoma.

The reason why is that the arithmetic demands that standard. The formula for resolution has several expressions, depending on the chosen units, English versus metric, linear or circular.


Entry-level 70-mm refractors from 
Meade and Celestron 
$159 and $189 respectively.
Meade is f/10 F=700 mm.
Celestron is f/13 F=900 mm.
from Mile High Astronomy, Denver, Colorado.

  • Angular resolution in micrometers = 0.25 times [wavelength in micrometers / aperture in meters] (bringing the orders of magnitude into conformance.)
  • Angular resolution in radians = 1.22 times [wavelength in micrometers / aperture in micrometers]
  • Angular resolution in arc-seconds = 0.25 * [wavelength in micrometers / aperture in micrometers]

For D=70 mm 

Green = 1.9 arc-seconds

Violet = 1.38 arc-seconds

Red = 2.43 arc-seconds

Mean = 1.9 arc-seconds

(A Student’s Guide to the Mathematics of Astronomy by Daniel Fleisch and Julia Kregenow, Cambridge University Press, 2013, 2020.)

 


Actually, I ordered my new National G 70 from
Explore Scientific of Springdale, Arkansas,
a firm that I know and trust from interactions
within the hobby leadership community. 
Theirs on the top left was $119, not $99 as shown.
Note that the other two option packages
are both out of stock.
Above at $145 from Walmart.
I believe that this price will hold past the holidays.


Resolution = 5.45 / D inches

Resolution = 1.98 arc-seconds

(Star Ware: The Amateur Astronomer’s Guide to Choosing, Buying, and Using Telescopes and Accessories, 4thEdition, by Philip S. Harringon, John Wiley & Sons, Inc,, 2007, pages 6-9.

 

Smallest Resolvable Angle = [wavelength nanometers]/ [Diameter in nanometers] 

Smallest Resolvable Angle (arc-seconds) = [114/ Diameter in nanometers] 

for 70 mm = 1.6 arc-seconds

Observers Handbook 2021, Royal Astronomical Society of Canada, page 49.

 

The angular separations of the companion stars in epsilon Lyrae are given as 2.35 arc-seconds each or 2.4 and 2.5 arc-seconds respectively. "The component stars of ε1 have magnitudes of 4.7 and 6.2 separated by 2.6 (arc-seconds) ... Main components of ε2 have magnitudes 5.1 and 5.5 separated by 2.3 (arc-seconds) ...  " -- /https://en.wikipedia.org/wiki/Epsilon_Lyrae  


Therefore, based on the above, any 70-mm refractor ought to be sufficient. 


Svbony "fast" f/6 70-mm refractor. $82.90.
Comes with 5X finder scope, diagonal,
20 mm eyepiece, and tripod.
Made in Hong Kong, sold worldwide.

The Airy Disk

Astronomer Royal George Biddle Airy (1801-1892) modeled the visual telescopic image of a star in the course of his investigations of optics. The so-called “Airy disc” or diffraction disc is the small central portion of the false image of a star formed by a telescope at focus. Light not contained in the disc forms neat, concentric diffraction rings, or Fresnel rings surrounding the disc. The size of the star image is proportional to the wavelength of light, and inversely proportional to the aperture of a particular optical system. Thus, the larger the aperture, the smaller the Airy disc in stars of the same color.

            This ideal representation has confused some observers, who assume that their optics are flawed when stat images on the Airy model do not appear during routine sessions. … Due to atmospheric turbulence and “local seeing” disturbances in the air in and around the telescope, the model appearance is rarely glimpsed in the field. Typically, one sees an amorphous central discoid surrounded by a series of broken, shifting ring segments. […]

            In years of observing, the author has only experienced a handful of occasions when the atmosphere rendered a perfect Airy model visible—all in the wee hours of still, humid, subtropical mid-summer nights of marginal transparency. At times like this, planetary details stand out like the lines on a banknote, and doubles generally seen as barely split seem to have widened to admit an extra measure of black space between their components.” – Care of Astronomical Telescopes and Accessories by M. Barlow Pepin, Patrick Moore’s Practical Astronomy Series, Springer-Velag, London, 2005, page 24.

My goals for this project include keeping the Celestron 70 mm f/13 for its long focal length and deacquisitioning the two National Geographics and the Meade. I am also seeking a new Explore Scientific 102-mm f/9.8 F=1000 mm to replace my current one which is f/6.47 F=660 mm.


PREVIOUSLY ON NECESSARY FACTS

Reminders of Newtonmas Past 

Measuring Your Universe: Alan Hirshfeld's Astronomy Activity Manual 

Copernicus on the Revolution of Heavenly Bodies 

An Online Class in Astrophysics 


Saturday, October 23, 2021

National Geographic 70-mm Refractor Field Test

With a sturdy mount and tripod (see previous post), I was able to spend more time on targets that I know in order to determine the limits of this “hobby killer” department store telescope. My motivation was a discussion on The Sky Searchers board centered on another newbie who posted questions about the limits of his Svbony SV501 70mm f/6 (F=420 mm) refractor. Two stalwarts insisted that a “a good 70-mm” could split the famous double-double system epsilon Lyrae. He could not do that. I could not either with my 70 mm f/10 F=700 mm. So, the problem is: Is this “a good 70-mm”? My final judgement is that given the return-on-investment (ROI) for the price, it is good enough. 

[07 November. After working with two other 70-mm telescopes in the same price range, and after working with another, newly-purchased National Geographic, I believe that that product does not perform competitively.]

Three versions of the National Geographic 70 mm refractor
offered by Explore Scientific.
Walmart is not alone in pricing it at $145
and out-of-stock.

I started with two entry-level “First Scope” oculars from Celestron, a 12.5 mm and a 6 mm for 56X and 116.6+ respectively, both 50-degree field of view. I believe that these are Kellner designs, not Ploessl, (three lenses not four) just because they are inexpensive. I took one apart as far as I could conveniently, but never got down to the basic components and I left it at that. 

21 Oct 20:45 (CDT UT -5)

Venus. Noticeably just past quarter phase. Chromatic aberration (CA) red to right blue to left worse with these and less pronounced with Celestron 17-mm and 8-mm Ploessl. More CA with Celestron 2X Barlow and 17-mm. 

Jupiter. Less CA. Two bands. Four moons. No problems. But seeing is not good. Nominally clear but obviously poor sky high up. I quit early.


22 Oct 17:20 Forecast is clear through 03:00 hours.

Set up on Venus. CA depends on centering: blue to right, red to left; red to right purple to left. Worse with Kellners than Ploessls but both about the same and noticeably less than last night.


19:30 to 19:44. Jupiter.  12.5-mm Kellner. Planet and moons fill about one-third field of view (50 / 56 = 53.5 arc-min). Timed passages across FOV 2:01 min:sec and 1:55 min:sec. at 19:40 and 19:44 hours. Watching Jupiter, some detail in the south appears, another band, but broken, not distinct, comes and goes.

19:52 after some searching found Messier 22 in Sagittarius. Very faint. 

20:02 Albireo. Smaller blue on top of larger yellow. Yellow about twice the size of the blue. 


20:15 Test on epsilon Lyrae. No joy with both 12.5 mm and 6 mm oculars. I was able to see the first double, of course, but could not split their companions. 

20:29 switched to TeleVue Nagler 7 mm Series 1 eyepiece. This is a high value, wide view 82-degrees, that I acquired on close-out from Enerdyne in Suttons Bay. (It had been on the shelf unsold for four years and was now outdated by new designs from TeleVue.) No change. Same view as 6-mm Kellner.

(It is important to let your eyes left and right take long turns viewing as the sky changes, shimmering, clearing, worsening, etc.) 

Changed to 6-mm Kellner with 2X Barlow. (700/3 = 233.3+ X). 

Right pair is more pronounced. I can make out the companion above.

23:13 hours - Second companion is still not there. It should be to the left but is not.

 

23:13 Checked eta Cassiopeiai double star 12.5 mm (56X) with no problem. 

 

I spent the rest of the night not photographing the Moon and Jupiter with my Explore Scientific 102-mm refractor and my iPhone 11. I closed up shop at 02:00 on 23 October 2021. The ES 102 is a nice scope. It is my grab-n-go. But it has a design problem in that the draw tube does not allow a shorter focus with more than a basic lens in the diagonal. I had the same problem trying to view Venus with two filters to cut the glare. (The Natl G worked just fine.) So, none of the 50 snapshots was publishable. 

 

PREVIOUSLY ON NECESSARY FACTS

Four Books about Bad Science 

Science versus Common Sense 

The Philosophical Breakfast Club 

Science Fair: A National Geographic Film 

 

Thursday, October 21, 2021

DIY: Homebuilt Substitute for Vixen Mount

The Vixen Company, Ltd., of Saitama, Japan, (founded 1949) got the brilliant idea to make all of their telescopes compatible with all of their mounts. Today, the Vixen mount is a standard offered by most other makers. However, a new Vixen mount bar at $79.95 plus tax and shipping was wholly disproportionate to my needs. 

https://global.vixen.co.jp/en/


Four years ago, I bought a National Geographic 70-mm refractor used and abused from some kids down the street. Between Christmas and August they lost the eyepieces, center tray, guide handle, and cellphone attachment, and the dew shield was on backwards. (They said that they never had it outside.) I tested it with a 1.25-inch eyepiece against a traffic sign down the road and it was OK. So, I gave them $35 for it. 



For four years, I did what I could with the mount, taking it apart and putting it back together but degreasers and lubricants, rubber bands and hose clamps never solved anything. 

All-in-all it has been a good little viewer and given the 70-mm limit not half bad for backyard stargazing. At that time, I also owned a Celestron EQ-130 Newtonian which I donated to the Goodwill earlier this year. 



Meanwhile, in October 2020, I bought myself an Explore Scientific 102-mm refractor. It comes on a First Light mount to keep the price down and I was never happy with the mount. It is difficult to control and somewhat underweight even for the 102. I was able to afford a Twilight I mount from ES, used and reconditioned with warranty. That has gone well for me. Now, I wanted to put the National Geographic on the First Light mount.


The easiest solution would have been to buy a Vixen mount, $79.95 plus tax and shipping, which was wholly disproportionate to my needs. 

I am not a fabricator. I have very few tools here. So, I went to Home Depot and searched for wood. I bought a 24-inch slat of quarter-inch poplar. 

With the saw blade on my Gerber pocket tool, I cut it in thirds and glued two pieces together. (I learned about Elmer's in junior high wood shop class.) 

The attachment screws on the telescope are mounted from the inside and I did not want to take those screws out. So, I used them to attach the new plate. Lacking drill bits, I used wood screws to make the holes. (We have an electric hand drill that my wife got as a door prize at a computer user meeting.) I ran a thin one in first, then widened it. 

Even though I measured more than twice, I was off by an inch the first time. I also split the wood once with a screw too large. After gluing (and curing) that, I wided the holes with my Swiss Army Knife. 

I secured the slat to the telescope with Gorilla glue. 

The best telescope is the one that gets used. 

I recently bought entry-level Celestron "First Scope" oculars ("eyepieces") to go with this so-called "hobby killer" that my astronomy colleagues denigrate for being a "department store telescope." OK: it is not an apochromatic rare earth triplet; the aperture (diameter) is not four inches or eight or ten. Nonetheless, I have seen the Messier 22 globular cluster, the Messier 44 "Beehive" open cluster, the Andromeda Galaxy (Messier 31), and, of course, Venus, Mars, Jupiter, and Saturn. Now, the telescope has a mount worthy of its optics.

PREVIOUSLY ON NECESSARY FACTS

(Mostly, the addenda links can be complementary or contrasting or off-topic. These are about observing with the National Geographic 70-mm refractor.)

Viewing Mars

M44 Beehive Cluster First Sighting

Astrophotography and Me

Jupiter-Saturn Conjunction 2020