A Work Commenced June 21 2025

Back in 2015, I parted company with refractors as my main astronomical viewing instruments, having ‘seen the light,’ as it were, when I acquired a brilliantly simple 200mm F/5.9 Dobsonian from Skywatcher. Once properly collimated and acclimated under good seeing conditions, it significantly out-performed a top-rated 5 inch F/12 refractor on pretty much everything.  The experience radically changed my perception of the true value of refractors, particularly when scaled up beyond 5 inches. 

A year ago, I was delighted to see an astro imager had performed a shootout between a Skywatcher Explorer P 200mm F/5 Newtonian and a high-end 5 inch F/7.8 ED refractor on a high resolution lunar target, showing clearly the former’s superiority despite a nearly five-fold price differential. 

Over the last decade, I’ve made a series of modifications to this 200mm Newtonian, called Octavius, owing to its 8 inches of aperture. The primary mirror from Skywatcher was good but I decided to replace it with a true 8-inch (203mm) F/6 Ultra Grade mirror with an accuracy exceeding 1/10th wave PV from Orion Optics UK, which also came with state of the art reflective coatings – HiLux enhanced aluminium – increasing the mirror’s reflectivity from 85 per cent to over 97 percent across the visible spectrum. 

I then upgraded the secondary mirror by ordering up a smaller(1.52 inches) and more optically flat Antares unit with a 1/15th wave PV surface accuracy, whilst also reducing the central obstruction to less than 20 per cent by aperture.

Like the primary, the small secondary mirror came with enhanced aluminium coatings as standard, to maximise image brightness. Because it’s such a small and lightweight mirror, I could attach it to the stalk using double-sided tape.

My own personal experience with enhanced aluminium coatings is that they have much greater longevity compared with standard coatings. Indeed I have older mirrors treated with HiLux that are 10 years old and show no signs of deterioration despite regular use in my damp Scottish climate. 
The somewhat flimsy secondary housing on the original SkyWatcher 200P was removed and replaced by a much more rigid and solidly-built structure manufactured by the German company, BackYard Universe.

The secondary holder has a diameter of 40mm – slightly larger, in fact, than the 38.6mm elliptical flat mirror – giving a minimum central obstruction of 19.7 per cent. 
Of course, If I so desired, I could have installed a low-profile focuser and decrease the secondary obstruction to just 10 per cent using a even smaller secondary, not to mention the need to redesign the spider structure. But in the end I decided against it, as I wanted to create a more all-purpose visual instrument rather than a specialised lunar and planet killer. And in any case, optical theory informs me that once the central obstruction falls below 20 per cent or so, the effects are almost indiscernible from a non obstructed optic of the same aperture.

I did however replace the original single-speed rack and pinion focuser on the SkyWatcher Explorer P after ten years of heavy use, with SkyWatcher’s dual-speed Crayford  focuser, which allows me to achieve smooth and precise micro-focusing on high resolution targets.  

I also attached an inexpensive USB type cooling fan to the rear of the primary mirror which helps remove the boundary layer of warm air accumulating above the surface of the mirror, to help keep the images as crisp and detailed as possible. 

The interior of the tube is lined with cork and overlaid by matt black flocking material to further enhance its thermal properties, as well as maximising image contrast.

The telescope sits in the original lazy susan altazimuth mount with the stalks under its base fitting into some pre-cut holes  I made in an inexpensive plastic water butt. This raises the height of the telescope further above the ground providing a supremely comfortable standing height – I’m a six footer – for all my astronomical observations. And that brings me to the second reason for abandoning refractors- I hate sitting down, huddling over an eyepiece, straining my back while trying to study a target located high in the sky. Those kinds of observations are much more comfortably executed while standing up. Indeed, I even use my travel scope- a high quality 130mm F/5 Newtonian placed on an altazimuth mount- in exactly the same way i.e. standing up. Once I got used to standing with my Newtonians, there was no going back to refractors. 

Just as any fine musical instrument benefits from occasional tuning up, so too does Octavius. At F/6 collimation is very easy, so easy in fact that it only requires a simple collimation cap to get you 90 per cent of the way there, and quite often better. The final tweaking is done manually by performing a star test at high power and making those final adjustments using the collimation screws on the primary mirror. It’s quick and easy, taking only a couple of minutes to get perfect alignment of the optics.

The more rigid secondary structure has greatly helped to maintain precise collimation from night to night, so much so that the instrument only needs occasional tweaking when moved from its indoor storage space to the outside air. That’s how it should be!

So how good are the optics? In a word- superb! But to elaborate: star testing at 200x under good conditions shows identical diffraction patterns inside and outside focus. Stars focus down to tiny, round Airy disks. By far the biggest benefit has come from the primary mirror. On the night the new primary arrived back in January, I was immediately able to discern the improvement on Jupiter. The new mirror snapped to focus better, showed more contrast and routinely revealed subtle details on the Giant planet that I had only vaguely discerned using the original mirror. Magnifying the first quarter Moon up to well beyond 400x showed no image breakdown whatsoever. The image below is a single shot image of the first quarter Moon taken with the Svbony SC001 2 Megapixel imaging camera, on the evening of May 3 2025, showing some very fine details.

Know Thy Telescope

Modern Newtonian primary mirrors are figured into the shape of a parabola. But why a parabola? Some analytical geometry can help us understand why: 

In contrast, a spherical mirror has no fixed focal point but can approximate a parabolic shape if the focal ratio is made very large. Unfortunately, as the aperture increases, the focal lengths needed for spherical mirrors to work well very quickly
become impractical.

Optimum Aperture for Resolving Binary Stars.

Octavius is used mainly as a lunar, planetary and double star scope, although I also use it for deep sky observing too. One of my passions is double star observing, at which Octavius excels. Indeed, with a decade of experience with this telescope, I’ve come to accept that it’s very close to ideal for observing tricky binary systems reaching pairs less than 1” separation. 

The following analysis is based on the work of Michael Greaney from chapter 25 of the book, Observing and Measuring Double Stars(2nd edition) which was incorrectly printed( the errors from which were reported by yours truly ,to the editor, Bob Argyle, at the Institute of Astronomy, Cambridge, UK).

According to Sidgwick, the maximum useful magnification of a telescope is approximately 28D^0.5 where D is the aperture of the telescope in millimetres. This was empirically derived, so has taken the effects of atmospheric seeing under British skies into account.

On the other hand, according to the work of the late professional double star astronomer, Dr Paul Couteau, the minimum useful magnification for resolving close binary stars is twice the resolving magnification of the telescope, which turns out to be D in millimetres.

There must exist an optimum value of D between the maximum useful magnification and the minimum useful magnification. If we express this as f(D) = 28D^0.5 – D

Then differentiating with respect to D gives:

f’(D) =14D^-0.5 -1 = 0 for the optimum, which turns out to be D =14^2 =196mm. 

This result is just shy of 8 inches, the same aperture as Octavius.

Octavius, Optimus!

A Mid-Summer Night’s Dream

I’ve noted the efficiency of the 8-inch F/6 Newtonian many times over the years, but as a case in point, I used the instrument on two nights just after the summer solstice to resolve some tricky double stars at high power. First on the night of June 24-25 and again on June 28-29 2025. My workhorse eyepiece for this kind of project is my trusty Svbony SV 215 3-8mm planetary zoom eyepiece yielding magnifications of 152x – 406x. As the experienced observer, Alan Dyer, of Sky & Telescope concluded in his review, this is an excellent eyepiece, fully the equal of the Nagler zooms but offered at a much more attractive price. Additionally, I usually have 1.6x, 2x and 3x Barlow lenses handy to boost the magnification as high as 1218x. 

This time of year, the twilight limits my observations to a short period of time between 11:30pm and about 1:00am local time, as any later and the sky begins to brighten again. I usually ‘warm up,’ starting with easier pairs such as Epsilon 1 & 2 Lyrae, all four components of which are easily split at 152x in the 8-inch. Moving on to Delta Cygni, which has a faint close companion, Octavius made light work of resolving this system at powers upward of 300x. One invaluable tool I use to enhance the detection of these close, unequal magnitude pairs is a polarising filter, which helps to cut off some of the diffraction ‘gunk’ between the components, in much the same way as an apodising filter works.

From there, I moved over to Aquila, low in the east, and tracked down the sixth magnitude star Pi Aquilae, centered it in my 8 x 50 finder scope and then cranked up the power to 406x, which is sufficient to tease both components – a pair of white sixth magnitude stars separated by just 1.4 arc seconds – and orientated roughly east-to-west.

Confident of good, steady skies, I next trained the instrument on Lambda Cygni – by now high in the eastern sky. Starting at 406x, I leave the star drift through the field, watching with intensity what the image might show me. Sure enough, I began to glimpse the secondary star (magnitude 6.3) almost precisely north of the primary(magnitude 4.7) and separated by a sliver of twilit space merely 0.9 arc seconds apart. Checking Stelledoppie afterwards, I was delighted to see its estimated position angle to be 01 degree, so almost immediately due north of its primary! Cranking up the power to 812x using a 2x Barlow lens gave an even more impressive view: the two stars cleanly resolved, and racing through the field of my un-driven telescope at breakneck speed! How swiftly the Earth moves! Time waits for no one.

At their distance from the solar system, the pair orbit their common centre of gravity every 391 years. 

The above was recorded after midnight on June 25 but I was able to repeat the split just a few nights later on June 29. So, two clear nights just after the solstice when the telescope was able to resolve this sub arc-second pair! This might sound unusual, but it’s actually quite typical of our summer sky seeing – and, truth be told, all over the British Isles(having surveyed its suitability across Ireland as well),- here in rural central Scotland!

Unequal Pairs

The Dawes Limit has long remained the empirical limit on the resolution of double stars. Formulated in the 19th century by the celebrated Victorian observer, William Rutter Dawes, it states that the resolution limit for a telescope in arc seconds is given by 4.56/D where D is the aperture in inches. The formula Dawes arrived at was for equal brightness 6th magnitude stars. Thus, Octavius ought to be able to resolve equal magnitude pairs as close as 0.57 arc seconds. But things get more challenging for pairs of unequal brightness. In short the bigger the brightness differential between the stars the harder they are to tease apart. 

Of the closer pairs already discussed, the easiest is Pi Aquilae(1.4”), since they only differ in brightness by 0.41 stellar magnitudes. More challenging yet is Lambda Cygni (0.9”) with a brightness differential of 1.53 stellar magnitudes. But at 00.02h on the night of July 17, during a spell of excellent astronomical seeing,  I trained Octavius on Zeta Herculis, a yellow-white sun-like star still high in the summer sky just past meridian passage. The secondary is separated from its primary by 1.53” but the magnitude differential is 2.45. Charging the telescope with a power of 650x using my polarising filter, the 8-inch Newtonian was easily able to resolve the secondary immediately due east of its primary( confirmed by consulting Stelledoppie). Another similar test of this kind can be conducted on Propus(Eta Geminorum)during the winter months. With a separation of 1.83” but a magnitude differential of 2.63, this is considerably easier to tease apart.

Why mention close, unequal pairs like these? Well, commonly accepted wisdom holds that Newtonians work best for equal magnitude pairs, but I’ve found this to be another myth promulgated by some members of the amateur community. After studying the work of Christopher Taylor, a theoretical  physicist by training, and keen double star observer with a particular interest in resolving sub-arc second pairs using a 12.5 inch F/7 Calver reflector erected on a massive, un-driven altazimuth mount down at Oxford(discussed in Chapter 11 of Argyle’s book previously mentioned), it became clear to me why Octavius does so well on unequal double stars like Zeta Herculis. It has a small central obstruction and vanishingly low spherical aberration-  both of which result in transferring substantially less energy from the Airy disk into the diffraction rings, thereby making a faint companion more easily resolvable from its primary. Indeed, Taylor admits as much in his book chapter, by making the point that his century-old Calver mirror was deliberated left under-corrected to help keep track with falling temperatures.

Examining the Airy disks of second and third magnitude stars in Octavius on nights of good seeing, reveals tiny pinpoint stars with a singular ring of moderate size and brightness. In other words: the images are very refractor-like. 
Perhaps most fascinating of all is the sheer dynamism of pairs like Zeta Herculis. Located a few tens of light years away, the companion to Zeta Herculis orbits its primary in just 34.5 years. That means that its position angle relative to its primary changes significantly in just a few years! Having carried out observations of this system regularly over the last decade I’ve witnessed its rapid movements from one year to the next. How cool is that?

For the record, Tau Cygni, easy to pick up even in summer twilight, and currently separated from its primary by 1.1” but with a 2.75 magnitude difference, presents a similar type of challenge.

A New and Improved Primary Mirror Fan

I’ve dithered for years about whether a primary mirror cooling fan would be worth installing. I’ve generally not encountered much in the way of thermal issues using Octavius, as we experience only small diurnal temperature swings for much of the year, though April and May have brought their problems, with warm sunny days and chilly nights. I generally store the telescope in a dry, unheated shed, just a few degrees warmer than the outside air. However, during the cold winter of 2024-25,  I bought a small USB desk fan and affixed it to the rear of my primary mirror using a few layers of double-sided tape. It was powered by a small portable power pack and after several trials,I became convinced that it did clean up the images of Jupiter and Mars in a subtle but consistent way. However, I did perceive some vibrations at the high powers I use for close double stars. I attributed this to the direct attachment of the fan to the mirror. 

So I decided to splash out on a dedicated mirror fan manufactured by Austral Solutions, based in Spain. This new fan attaches to the outer tube of the telescope in such a way that it doesn’t touch the primary directly, but blows ambient air onto to the centre of the primary mirror. It’s powered by a 12v DC step-down transformer (user provided)powered from a mains (AC) source. Although this requirement necessitates having a constant supply of electricity, it’s more reliable than the portable power pack I had used with the original fan, which often cut out, especially on colder evenings, over the winter and spring. My results this summer, examining high-resolution double stars, testing various permutations (turning the fan off and on and critically examining the seeing disks) has convinced me that keeping the fan running constantly significantly increases the stability of the Newtonian image. 

Finding Fainter Pairs

By late July, the skies above me become noticeably darker, allowing me to better track down fainter binary star systems. One example is Mu Cygni, which can now be tracked down using my finder scope on the eastern flank of this large, summer constellation. My notes show that I had not visited this system for about six years, so I was excited to pay a visit to this old friend. On the night of July 23-24, I trained Octavius on Mu, which is easily identified by virtue of a wide, unrelated 7th magnitude star off to the east of it in the finder telescope. Charging Octavius with a power of 406x(fan running), I was greeted by a beautiful sight: a pair of white suns separated by 1.6”, the primary having a magnitude of 4.8 and the secondary- 6.2-roughly due north of it. I don’t remember this system to be much of a challenge for a good 4- or 5-inch refractor employing high powers back in the day. Clearly the secondary is closing in on the primary, making it more challenging than I remember. 

In addition, on the same night, I trained Octavius on Iota Cassiopeiae, now at a decent altitude off to my northeast, and cranked the power up to about 200x to reveal this beautiful triple system for the first time this season. A nice way to end my vigil on this rather cool July night!

A Return to Dark Nights

By the last week of July, the skies are turning noticeably darker after sunset here in rural central Scotland. On the night of July 29-30, I enjoyed a reasonably calm, clear sky and by half past midnight local time, I could once again see the faint glow of the summer Milky Way coursing its way through Cygnus and Cassiopeia. Seeing was only average though, so no high-resolution double star observing was conducted. Instead I turned Octavius on some familiar summer deep sky objects. To view some of the larger objects on display, I employ longer focal length 1.25” eyepieces like my 22mm (70 degree AFOV) yielding a low power of 55x in a true field of 1.26 degrees and a medium power from my 15mm (68-degree AFOV) which yields a power of 81x in a 0.84 degree field. The reader will note that I do not employ 2” eyepieces for lower power wider FOV viewing for two principal reasons: I strongly dislike having to switch between the 1.25” and 2” formats, and the lower powers of these longer local length oculars shows up the astigmatism in my right observing eye which I find distracting when used without eye glasses. That said a 1.26 degree field is more than enough to enjoy pretty much all of the objects I visit with Octavius. If I want larger fields, I turn to my large, 15 x 56 binoculars.

I first turned Octavius on the Ring Nebula(M57) in Lyra, its ghostly glow coming through nicely at 152x. Then moving into Hercules, I quickly located both M13 and M92, both globular clusters located far out in the halo of our galaxy. Although these are best resolved at powers of 150x to 200x, tonight I was happy to admire these beautiful baubles of ancient starlight at 81x using my 15mm eyepiece.

After having quick looks at easy double stars such as Gamma Delphini and the colourful, O1 and O2 Cygni and the famous 61 Cygni, I trained the telescope on the wide but rich open cluster M39 at the head of Cygnus, enjoying the dozens of pinpoint stars strewn across the field of the 22mm eyepiece, and set against a rich Milky Way hinterland. By 1:00 AM local time, Cassiopeia was just clearing my rooftop and I had hoped to catch a glimpse of the famous Double Cluster in Perseus. Alas it was still too low for me to access it. It will have to wait a wee while longer before I can view this spectacular object in my trusty 8 inch Newtonian.

The Assault on Tau Cygni

The night of August 9-10 was cool and breezy, but the sky was clear, with relatively calm twinkling stars..The full Moon culminating low in the south. I fielded Octavius at around 10:30pm local time and tested sky conditions by looking at a few binary systems of increasing difficulty. First I targeted Delta Cygni and was delighted to get a  very clean split at 244x. Then I moved over to Lambda Cygni, cranked the power up to 650x and once again managed to split this system with relative ease. Confident that this would be a good night to try my hand at Tau Cygni, I began by observing the star at powers up to 406x but with not a lot of success. Patience is a virtue, however. The system had a few hours to go before it reached its highest altitude crossing the meridian. Leaving the fan running continuously, I covered the optics and ventured inside for a while before returning to the telescope every half hour or so. With each attempt, I could see that the appearance of the Airy disk of the 4th magnitude yellow white primary  looked better and better as I was looking through less atmosphere. After midnight, using a power of 812x , I got my first glimpse of the secondary with a current separation of about 1” off to the SSE of the primary but it was only tentative at this stage. By about 1.00AM local time, I ventured out again, charging the instrument with a power of 812x and positioned the star on the east side of the telescopic field. In moments of superior seeing, I could make out a faint disk in the same position I had glimpsed earlier, but now it was more steadily held. Reaching for my 3x Barlow and switching it out with my 2x Barlow, the telescope could now deliver a power of 1218x – crazy I know! To see this system at such high power on an un-driven mount, you start at the lowest zoom setting (8mm with a 3x magnification boost), placing the star at the eastern edge of the field, focus as finely as possible and then crank the power upwards toward 1218x. The strategy paid off! At these extremely high powers, I was better able to tease the components apart and record its position relative to the direction of drift. After I had made a sketch, I consulted Stelledoppie and was delighted to see that the companion was in just the right position it should be in August 2025. I note the last measure was made back in 2017 when the estimated position angle was 190 degrees. Since then the 7th magnitude companion had moved eastward in its orbit round the primary. Back in 2017, it was located off to the SSW but now it was in a SSE direction. 

This was a thrilling observation: one of the most difficult I’ve tried in quite some time: a companion separated by just 1” of sky but fully 2.74 magnitudes fainter! This would be a severe challenge for a 6-or 7-inch refractor but it was quite doable on a good night here in rural central Scotland, using a fine-tuned 8-inch Newtonian with a sub-20% central obstruction. The sun-like star Tau Cygni B, orbits its primary every 49.5 years. 

A Visit to 61 Cygni:

In the same telescopic field as Tau Cygni my 8 x 50 finder scope picks up the lovely pair of orange stars known as 61 Cygni. Even the smallest telescope is capable of resolving these stars and even steadily held 10x binoculars should tease these apart. At 153x Octavius frames these stars perfectly with a nice Milky Way background. The brighter of the two has a magnitude of 5.2 and its companion nearly a full visual magnitude fainter. To my eye both stars exhibit the same orange hue and that’s not surprising as they are both K spectral class dwarf stars with temperatures a little over 4000K. 61 Cygni was all the rage in the first few decades of the 19th century, when astronomers noted their large proper motion, a good sign that these stars were relatively close to the solar system. As recounted in great detail in Chapter 11 of my large historical work, Chronicling the Golden Age of Astronomy, it was the great German astronomer, Friedrich Wilhelm Bessel, who, by 1840, first measured their distance from the solar system to be 10.3 light years away: only 10 percent lower than the modern accepted value. 61 Cygni represents a true binary star system, the components of which orbiting their common centre of gravity every 680 years. 

Astronomers have yet to identify planets orbiting these cool, long-lived stars, but in general, unless their planets orbit very close to their parent stars, the chaotic orbital motions of close binary star systems like this generally cause their planets to be ejected from their system over time. Still, in the mind’s eye, I can imagine being on the surface of one such world, watching two golden suns rising and setting in a clear sky. How wonderful it would be to witness such a sight!

Two Golden Binary Stars in Draco:

On the lovely evening of August 26, a light southerly wind kept the dew at bay and the sky above was glorious, with the Milky Way snaking its way from southwest to northeast. I trained Octavius on Eta Draconis: a lovely golden sun easily visible to the eye at magnitude 2.7.  This was my third observation of this system in as many weeks.  Seeing was excellent tonight as is often the case in August, so I cranked the power up to 406x and watched it drift through the field. It wasn’t long before I glimpsed its very faint secondary close in at 4.7” from the primary, and off to the southeast. The challenge here, once again, is not the angular separation per se, but rather the magnitude differential. The companion is 5.4 magnitudes fainter but still quite easy for Octavius. 

Returning to the lowest power setting on my zoom eyepiece yielding 152x, brought another star of the 6th magnitude of glory into the same field of view as Eta. Indeed it is quite easily seen in my 8 x 50 finder. This star, known prosaically as STF 2054, also took on a golden hue but after I increased the power to 406x I could easily resolve its companion just 0.93” away from the primary and orientated north-south( the companion is due north). The fainter star is about a magnitude dimmer than its primary. Using my 2x Barlow lens provided 812x which made seeing the components that little bit easier again. Intriguingly, after consulting the astrophysical data on both Eta Draconis and STF 2054, I discovered that both stars have exactly the same spectral class – G8III – making them about 700 degrees cooler than our Sun but also explaining why their colours  looked similar in the eyepiece.These two stars visible in the same field will make for an excellent test of resolution for moderate-aperture backyard telescopes under good seeing conditions.

To be Continued……

De Fideli.

A Work Commenced April 14 2025

When it comes to introducing the wonders of the night sky to beginners, it’s very hard  to beat a small refractor. Praised for their crisp, high-contrast views and almost complete lack of maintenance, even absolute newcomers can quickly engage with the night sky. Over the last few years, the newly established company, Svbony, has brought to market very high quality astronomical gear at prices that were considered unthinkable only a few years ago. As a seasoned amateur astronomer, and experienced reviewer of telescopes, I have used Svbony eyepieces and other accessories with my own telescopes and can vouch for their great bang for buck.

Svbony reached out to me to request an honest review of their new entry-level telescope; the SV520 telescope: a 90mm air spaced F/8.9 achromatic refractor on a dedicated altazimuth mount with slow motion controls on both axes.  I agreed to do so, and what follows is a fairly detailed overview of its quality and capabilities.

The SV520 telescope, mount and accessories arrived in a well packed double box. Though the instruction manual is clear and comprehensive, I found it easy to assemble the mount and attach the telescope in just a few minutes. 

Let’s take a closer look at the telescope mount first. Having used a Vixen Porta II altazimuth mount for nearly 15 years, I was delighted to see that the SV520 came with a very similarly designed unit. It can accept a wide range of small telescopes using a regular dovetail plate. Slow motion controls provide smooth and precise tracking on both axes. The legs of the tripod seem to be made of stainless steel and can be extended to increase the overall height of the telescope for taller users. The legs can be securely locked in place using three large thumbscrews. The accessory tray screws onto the long vertical bar that mates the mount to the tripod legs. 

The set up is quite stable with the telescope mounted. After giving it a hefty shake, I found the dampening times to last for just a few seconds, which is more than adequate even when using high power eyepieces. Incidentally the mount can accommodate loads of up to 5 kilos.

Two eyepieces are provided with the optical rube assembly: a 25mm and 10mm Kelner delivering powers of 32x and  80x, respectively. A decent mirror diagonal is also included for comfortable 90 degree viewing. It is not threaded to accept filters though.

The finder was arguably the biggest disappointment in the SV 520 package: a very plasticky 5 x 24mm specification with a singlet objective lens. As you can imagine, it produces very strong colour fringing owing to its short and stubby focal length. But I was able to align it easily with the main telescope and it did serve its purposes as a basic finder scope. Indeed as bad as it is, I think it’s still infinitely better than a generic red dot finder all too often seen on amateur scopes these days. 

The SV520 comes with a removable plastic dew shield, which reveals the lens cell underneath. The focuser is a single-speed rack and pinion design, which moves very smoothly and has enough focus travel to use quite a lot of accessories including diagonals, or mounting photographic gear. It can also be rigidly locked in place if need be.

Optical Testing

To be honest, I was expecting good performance from this 90mm f8.9 achromat based on many years experience of long focus 80mm refractors I’ve owned and used over the years. The colour correction ought to be decent: as good in fact as a 4 inch F/10. The two eyepieces supplied with the telescope – a 25mm and 10mm Kellner –  deliver powers of 32x and 80x, respectively. The 25x views were very nice. Good on the Moon, star clusters and the brighter nebulae etc. The 10mm Kellner also provided nice views of the Sun using a full-aperture white light solar filter. The slow motion controls on the mount allowed trouble free tracking with dampening times of just a few seconds.

But when I turned the telescope on the stars at 80x, I could immediately see that something was amiss. They were astigmatic. Defocusing the image of Capella both inside and outside focus confirmed this clearly. I was unable to get a good image of Jupiter and double stars were horrid. Clearly there was something wrong with the telescope. Taking out the star diagonal and peering down the tube showed the focuser rail was not parallel with the optical axis. 
I did attempt a ‘home fix’ of sorts by removing the dew shield, loosening the retainer ring holding the objective lenses in place and gently wiggling the lenses to see if they would settle into the cell better. Unfortunately, there was no improvement.

Disappointed, I dispatched the telescope to Es Reid, a professional optical engineer based in Cambridge, to see if he could get the optics working better. He confirmed the astigmatism and also noted the objective’s over correction. He added thicker lens spacers and got the focuser to move parallel with the optical tube. When I got the telescope back, I conducted more observations: first a star test at 180x under good seeing conditions. The astigmatism was gone and the intra- and extra-focal Airy disk looked a lot better. Turning the telescope on Jupiter, by now well past its best seasonal position in the sky,  showed a much improved image: a few bands were clearly seen. Turning the scope onto Castor in Gemini showed the A and B components with excellent definition at 200x. 

Upgrading the diagonal to a dielectric type and using better eyepieces significantly increased the quality of the images of both deep sky objects and higher resolution targets, like the Moon and  double stars. Indeed I was now able to press the magnification to 200x or more with good results.

Conclusions: 
I understand that Svbony marketed this telescope for beginners, and to some degree, it performed quite well so long as the magnification was not pushed higher than 80x. But even beginners appreciate decent views of the Moon at high powers and for stars to look like pinpoints of light at magnifications above 100x. I don’t know if I received a bad sample or whether this was just a typical example, but Svbony are capable of much better than this. If you’re willing to live with low powers than this would be an OK telescope but, as it was presented to me, I cannot recommend it for those who wish to push the envelope a little to tease out powers of 200x or a little higher. 

Neil English is author of Choosing and Using a Refracting Telescope.

De Fideli.

Unlike telescopes, which are mainly used by dedicated amateur astronomers, binoculars, for obvious reasons, are owned and used by a much broader cross section of the general population. When my students get to know me, they will inevitably have to endure my unbridled enthusiasm for optical devices of all kinds lol, and that includes binoculars. One of my mathematics students, Sandy, expressed an unusual interest in some of my instruments, and he further informed me that his parents, who run a small ferrying business at Balmaha, on the shores of nearby Loch Lomond, used several binoculars in their everyday work. My interest was further piqued when Sandy told me that his grandfather owned a big Zeiss binocular, which was inherited by his father and would eventually be passed on to him in the goodness of time. I asked Sandy whether he would be willing to bring the Zeiss binocular by so that I could have a look at it. After checking with his parents, Sandy agreed and kindly allowed me to use it for a week in order that I could assess it and give it a good clean. Naturally enough, I jumped at the opportunity!

The instrument, a Carl Zeiss Jenoptem 10 x 50W porro prism binocular, came in a lovely leather case; a far cry form anything made in this era.

The instrument had no lens caps and so had accumulated quite a bit of grime on both the ocular and objective lenses over the years. The Jenoptem, which was manufactured in East Germany(DDR), featured a Zeiss multi-coating, which helped me to date it to after 1978, when the company apparently began to apply their anti-reflection coatings to all the lenses and prisms in the optical train. So my guess is that it was probably acquired in the early 1980s. I believe Zeiss Jena offered a higher quality porro 10 x 50 in the Decarem line around the same period, but I have not had the pleasure of testing one of these units out.

The instrument has a very Spartan look and feel about it. Weighing in at about 1 kilogram, the Jenoptem is built like a proverbial tank, with a central focusing wheel and right eye dioptre.Turning the nicely machined metal focusing wheel first clockwise, and then anti-clockwise, all the way through its trave,l showed that it was still in excellent working condition, with zero backlash and bumping that one usually encounters with cheaper porro prism binoculars.

To begin the cleaning process, I unscrewed the objective housings from the front of the binocular in order to get at the inside surface of the objective lenses, which had a significant amount of grime as well as a small amount of fungal growth. Using a good quality lens brush, I carefully removed much of the dust before using a microfibre lens cleaning cloth soaked in a little Baader Optical Wonder fluid. In just a few minutes I was able to remove the remaining grime on both the outer and inner surfaces of the binocular objectives, as well as the surfaces of the prisms in the rear module of the instrument. The ocular lenses were also given a good cleaning.

I was able to verify that the prisms were indeed coated in the same way as the objectives, although I also discovered that the steel clips holding the prisms in place had rusted significantly over time. I did not attempt to clean the clips, as I judged that doing so might throw the instrument out of collimation.

Seen in broad daylight, I was able to verify that the lens coatings had not suffered much in the way of wearing, looking smooth and evenly applied, giving a bluish or purple cast, depending on the angle of view.

 

Optical tests:

After screwing the objective modules back into place, I was now ready to begin my optical tests of this older Zeiss binocular. I compared the views served up by this instrument with those garnered by my Barr & Stroud 10 x 50 Sierra roof prism binocular that I use almost exclusively for astronomical viewing. After setting the right eye dioptre on the Zeiss to suit my own eyes, I started with an iphone torch test to assess how the instruments fared in suppressing glare and internal reflections.

Because the Zeiss does not have the same close focus (~2m) performance as my Barr & Stroud, I had to place my iphone torch several metres away in my hallway in order to get the Zeiss to focus on its light. As usual, the torch was adjusted to its highest (read brightest) setting. Comparing the two in-focus images, I could see that the Zeiss fared considerably worse than the Barr & Stroud. Specifically, it picked up two fairly bright internal reflections, as well as quite a lot of contrast-robbing diffused light, which rendered the Zeiss image considerably less clean and contrasted in comparison to my control binocular. The difference was quite striking!

After dark, I aimed the binoculars at a bright sodium street lamp and again compared the images served up in both instruments. As expected, the Zeiss showed much more in the way of internal reflections, with a lot of diffused light that produced a fog-like veil around the street lamp. The Sierra 10 x 50 in comparison served up a much more ‘punchy’ image with much better control of internal reflections and far less of the foggy, diffused light evidenced in the Zeiss.

Next, I compared the Zeiss and the Barr & Stroud Sierra on a daylight test, examining a tree trunk in the swing park about 80 yards from my front door. Again, the difference between both instruments was striking! Although the image was very sharp in the Zeiss at the centre of the field, it was noticeably dimmer than the Sierra. That diffused light I picked up in the iphone torch test created a foggy veil that significantly reduced its contrast in comparison to the control binocular. I was also able to discern many more low contrast details in the Sierra owing to its ability to gather significantly more light than the older Zeiss. The colour cast presented by both binoculars was also noteworthy. The Zeiss threw up quite a strong yellowish colour cast  to the Sierra, which showed a much more neutral cast in comparison.

Examining the periphery of the same field also showed that the Sierra was exhibiting a larger depth of focus than the Zeiss, which was quite unexpected, as I had been given to understand that porro prism binoculars in general show more depth of focus than their roof prism counterparts. In addition, the Zeiss showed more distortion at the edges of the field than the control binocular.

The Zeiss Jenoptem has very tight eye relief, which I estimated to be just 10mm. The Barr & Stroud Sierra, in contrast, has much more generous eye relief in comparison- 17mm – making it significantly more suitable for eye glass wearers. Indeed, I found it difficult to image the entire field in the Zeiss, having to move my eyeball around to see the field stops.

In summary, these daylight tests clearly showed that the venerable Zeiss was no match optically for the Barr & Stroud 10 x 50 roof prism I had tested it against. The latter was simply in a different league to the former, no question about it!

Handling in the Field:

The Zeiss is rather big and clunky in my small hands and is more difficult to find that optimal position while viewing for extended periods. Weighing more than 200g more than the Sierra, it is also harder to hold steady. The significantly smaller frame of the Sierra roof prism binocular is much easier to negotiate, and is simply more comfortable to use. In addition, the Zeiss has no provision to mount it on a lightweight tripod or monopod, but the Sierra, like most other modern binoculars, does.

Astronomical tests:

Though the weather proved quite unsettled during the week that I tested the Zeiss, I did get a few opportunities to test it out on the night sky. Once again, I used my Barr & Stroud Sierra 10x 50 roof prism as a suitable control. My first target was a bright, waxing gibbous Moon fairly low in the southern sky. The Zeiss threw up more in the way of internal reflections than the Sierra. The colour cast of the lunar surface appeared more yellow in  the Zeiss compared with the cleaner images of the Sierra. As I expected from my iphone torch tests, the sky immediately arround the Moon was also brighter in the Zeiss, with noticeably lower contrast than the Sierra. Moving the Moon to the edge of the field also showed that the Zeiss threw up more distortions than the Sierra control binocular.

Turning to Vega high in the northwest after sunset produced good on-axis images in both binoculars, but when moved to the edge of the field, the Zeiss threw up that little bit more distortion than the Barr & Stroud Sierra. The same was true when I examined the Pleaides and the Hyades in Taurus.

Conclusions and Implications:

The Zeiss Jenoptem was a good binocular in its day but is clearly inferior in almost every sense to the Barr & Stroud roof binocular used in comparison. 40 years ago, the Zenoptem would have set the average factory worker a whole month’s salary to acquire new. In contrast, the Barr & Stroud Sierra can be had for between £100 and £120 in today’s market.  The value of waterproofing was made manifest in the observation of rusting of some of the metal internal components of the Zeiss. The Sierra, in contrast, is fully waterproof, o-ring sealed and purged with dry nitrogen gas to inhibit internal fogging and corrosion of any metallic components used in its construction.

Enormous advances in optical technology over the last four decades, particularly full broadband multi-coatings applied to all lens and prism surfaces, higher quality optical glass, as well as phase coated prisms on the roof binocular, collectively allow very efficient light transmissions to the eye. This is all the more remarkable since roof prism designs usually have many more optical components than their porro prism counterparts.

Better eregonomics in modern roof prism binoculars as well the employment of strong, low mass polycarbonate housings in their design make them lighter and easier to use than their porro prism counterparts from a generation ago. All of these add to the comfort of using them either during the day or at night when looking at the heavens.

I had a look on ebay to see what these old Jenoptems were being offered for. I found quite a few of them selling for between £150 and £200, so not the high prices demanded by other classic binoculars.

This comparison test must have implications for many people who already own or use older binoculars and who have not compared them to modern incarnations. And that’s as true for Zeiss as with any other manufacturer. Indeed, I was quite shocked at how much better my first quality roof prism 8 x 42 roof prism binocular fared compared to an old 7x 50 porro I was gifted back in the early 1990s. Technology has well and truly marched on! And while I like classic instruments just as much as the next guy, I see little point in using any when even modest instruments created in the modern age are likely to perform better than similar instruments made a generation ago. It’s just a hard fact of life.

The technology of the past is certainly interesting but it would be daft to neglect the advances offered in the modern era.

 

I would like to extend my thanks to Sandy and his parents for allowing me to test drive these old binoculars. I will be advising him to use lens caps on the optics when not in use and have also provided a sachet of silica gel desiccant to minimise moisture-induced corrosion of the optic.

 

Neil English discusses all manner of classic telescope technology in his 650+ page historical work, Chronicling the Golden Age of Astronomy(Springer-Nature).

 

De Fideli.

but test everything; hold fast what is good.

                                                                           1 Thessalonians 5:21

 

My Local Weather

 

Jet Stream Data

Introduction:  One of the statements that is oft quoted by observers, particularly in the UK, is that the meteorological phenomenon known as the Jet Stream seriously affects the quality of high resolution telescopic targets. I have decided to investigate these claims to determine to what extent they are true or not, as the case may be. These data will also provide the reader with an idea of the frequency of nights that are available for this kind of testing over the time period the study is to be conducted.

Method: For simplicity, I shall confine my studies to just four double stars that have long been considered reasonably tricky targets for telescopists. To begin with, my targets will include systems of varying difficulty, ranging from 2.5″ to 1.5″ separation, and the aim is to establish whether or not I can resolve the components at high magnification. These systems include *:

Epsilon 1 & 2 Lyrae

Epsilon Bootis

Delta Cygni

Pi Aquilae

* These systems were chosen for their easy location in my current skies, but may be subject to change as the season(s) progress.

Viewers are warmy welcomed to conduct their own set of observations to compare and contrast results in due course.

Instrument Choice & Magnifications Employed:

 

A high-performance 130mm (5.1″) f/5 Newtonian reflector was employed to investigate the effects of this phenomenon, as this is an aperture regularly quoted as being sensitive to the vagaries of the atmosphere. Magnifications employed were 260x or 354x (they can however be resolved with less power). The instrument at all times was adequately acclimated to ambient temperatures and care was taken to ensure good collimation of the optical train. No cooling fans used on any of my instruments.

Results;

Date: August 17 2018

Time: 21:20 to 21:35 UT

Location of Jet Stream: Currently over Scotland

Conditions: Mild, 14C, very breezy, mostly cloudy with occasional clear spells, frequent light drizzle.

Observations: Power employed at the telescope 354x

Epsilon 1 & 2 Lyrae: all four components cleanly resolved.

Delta Cygni: Faint companion clearly observed during calmer moments

Epsilon Bootis: Both components clearly resolved during calmer moments.

Pi Aquilae: Slightly mushier view, but both components resolved momentarily during calmer spells.

 

Date: August 19 2018

Time: 20:30 – 21:50 UT

Location of Jet Stream: Currently over Scotland.

Conditions: Mild, 13C, mostly cloudy and damp all day but a clear spell occurred during the times stated above, no wind, heavy dew at end of vigil.

Observations: Seeing excellent this evening (Antoniadi I-II); textbook perfect images of all four test systems at 354x and 260x.

Nota bene: A 12″ f/5 Newtonian was also fielded to test collimation techniques and I was greeted with a magnificent split of Lambda Cygni (0.94″) at 663X. Little in the way of turbulence experienced even at these ultra-high powers. Did not test this system on the 130mm f/5.

Clouded up again shortly before 11pm local time, when the vigil was ended.

Date: August 22 2018

Time: 23:30-40 UT

Location of Jet Stream: Currently over Scotland

Conditions: Very mild (15C), breezy, predominantly cloudy with some heavy rain showers interspersed by some brief, patchy clearings.

Observations: Just two test systems examined tonight owing to extremely limited accessibility; Epsilon 1 & 2 Lyrae and Delta Cygni. Both resolved well at 260x.

 

Date: August 22 2018

Time: 21:00-21:25UT

Location of Jet Stream: Currently over Scotland

Conditions; partially cloudy, brisk southwesterly wind, bright Gibbous Moon culminating in the south, +10C, rather cool, transparency poor away from zenith.

Observations: The telescope was uncapped and aimed straight into the prevailing SW wind, as is my custom.

All four systems well resolved at 354x, although visibility of Pi Aql was poor owing to thin cloud covering.

 

Date: August 23 2018

Time: 20:30-45 UT

Location of Jet Stream: Moved well south of Scotland

Conditions: Mostly clear this evening, after enduring heavy showers all day; cool, 10C, fresh westerly breeze, good transparency.

Observations:  All four test systems beautifully resolved this evening (seeing Ant II) at 354x. Just slightly more turbulent than the excellent night of August 19 last.

 

Date: August 24 2018

Time: 20:30-45 UT

Location of Jet Stream: Just west of my observing site.

Conditions: Almost a carbon copy of last night, light westerly winds, cool (9C), good transparency and almost no cloud cover. Very low full Moon in south-southeast.

Observations: All four system resolved at 260x, but less well at 354x owing to slightly deteriorated seeing ( II-III). Delta Cygni seems especially sensitive to seeing.

Nota bene: Epsilon Bootis now sinking fast into the western sky. This test system will soon be replaced by a tougher target, located higher up in my skies; Mu Cygni.

 

Date: August 25 2018

Time: 20:20-21:00 UT

Location of Jet Stream: Right over Scotland.

Conditions: Very hazy, calm, poor transparency, cool (9C), seeing excellent (I-II)

Observations: Just three of the four systems examined tonight owing to very poor transparency. Only Pi Aquilae could not be examined. All three were beautifully resolved at 354x.

 

Date: August 26 2018

Time: 22:30-23:05 UT

Location of Jet Stream: Well south of Scotland.

Conditions: After a day of heavy rain, the skies cleared partially around 11pm local time. Fresh westerly breeze, fairly mild (12C), bright full Moon low in the south.

Observations: Mu Cygni observed instead of Epsilon Bootis owing to the latter’s sinking low into the western sky at the rather late time the observations were made.

Three systems well resolved ( Mu Cygni, Pi Aquliae and Epsilon 1 & 2 Lyrae) in only fair seeing, with Delta Cygni B only spotted sporadically in moments of better seeing. This system is very sensitive to atmospheric turbulence due to a large magnitude difference between components, as opposed to their angular separation. 260x used throughout.

Nota bene: Readers will take note of the frequency of observations thus far made.

Date: August 27 2018

Time: 20:30-21:05 UT

Location of Jet Stream: West of the Scottish mainland.

Conditions: Mostly cloudy, mild, 13C, light westerly breeze.

Observations: I took advantage of a few brief clear spells this evening to target my systems(including Epsilon Bootis). Seeing very good despite the cloud cover (II). All four systems easily resolved tonight at both 354x and 260x.

Date: August 29 2018

Time: 20:25-40UT

Location of Jet Stream: Not over Scotland.

Conditions: Mostly clear, occasional light shower, cool (11C), light westerly breeze, seeing and transparencyvery good (II).

Observations: Mu Cygni now replaces Epsilon Bootis.

All systems very cleanly resolved at 354x and 260x.

Nb. All systems also beautifully resolved in a 12″ f/5 Newtonian at 277x, set up alongside the 130mm f/5.

 

Date: August 30 2018

Time: 20:45- 21:00 UT

Location of the Jet Stream:  Not over Scotland.

Conditions: Partially cloudy with some good clear spells, cool (9C), very little breeze.

Observations: Seeing good tonight (II). All  four systems nicely resolved at 260x and 354x.

Note added in proof: Local seeing deteriorated (III-IV) somewhat between 21:00 and 22:00 UT, so much so that Delta Cygni B could no longer be seen.

 

Date: 31 August 2018

Time: 20:30-22:00UT

Location of Jet Stream: North of the British Isles

Conditions: Partly cloudy and becoming progressively more hazy as the vigil progressed. Mild, 12C, very light westerly breeze.

Observations: Seeing only fair this evning (II-III), all four systems resolved at 260x and 354x, though Delta Cygni B visibility was variable.

 

Date: September 1 2018

Time: 20:30-50UT

Location of Jet Stream: to the northwest of the Scottish Mainland.

Conditions: Partially clear, very mild (16C), light southerly breeze, good transparency.

Observations: Seeing quite good (II).  All four systems resolved at 260x and even better delineated at 354x under these clement conditions.

 

Date: September 4 2018

Time: 19:55-20:20UT

Location of Jet Stream: Not over Scotland.

Conditions: Cool (10C), mostly clear, light westerly breeze, good transparency.

Observations: Seeing very good (II).  All four test systems well resolved at 260x and 354x this evening.

 

Date: September 5 2018

Time: 20:35-20:55UT

Location of Jet Stream: Not over Scotland.

Conditions: Very unsettled with frequent squally rain showers driven in by fresh westerly winds. Good clear spells appearing between showers. Transparency very good. 12C

Observations: All four test systems resolved under good seeing conditions (II) at 260x and 354x.

 

Date: September 6 2018

Time: 20:00-25 UT

Location of Jet Stream: Not over Scotland.

Conditions: Cool (8C), little in the way of a breeze, mostly clear, excellent transparency.

Observations: Seeing good (II). All four test systems well resolved at 260x and 354x.

 

Date: September 7 2018

Time: 20:25-40UT

Location of Jet Stream: Not over Scotland.

Conditions: A capital evening in the glen; 11C, good clear sky, brisk westerly breeze, excellent transparency.

Observations: Seeing very good (I-II).  All four test systems beautifully resolved in the 130mm f/5 using powers of 260x and 354x

Nota bene:

Know thine history!

Any serious student of the history of astronomy will likely be acquainted with the early work of Sir William Herschel (Bath, southwest England), who employed extremely high powers (up to 2000x usually but actually he went as high as 6,000x on occasion) productively in his fine 6.3-inch Newtonian reflector with its speculum metal mirrors. The high powers employed by this author are thus fairly modest in comparison to those used by his great predecessor. Check out the author’s new book; Chronicling the Golden Age of Astronomy, due out in October/November 2018, for more details.

Note added in proof:

With the excellent conditions maintained well after midnight, I ventured out at about 00:00 UT,  September 8, and noted Andromeda had attained a decent altitude in the eastern sky. At 00:10UT I trained the 130mm f/5 Newtonian on 36 Andromedae for the first time this season and charged the instrument with a power of 406x. Carefully focusing, I was treated to a textbook-perfect split of the 6th magnitude Dawes classic pair that are ~1.0″ apart. It was very easy on this clement  night. The pair look decidely yellow in the little Newtonian reflector. I made a sketch of their orientation relative to the drift of the field; shown below.

 

If you have a well collimated 130P kicking about why not give this system a try over the coming weeks?

 

Date: September 9 2018

Time: 21:10-25UT

Location of Jet Stream: Currently over Scotland

Conditions: Frequent heavy showers driven in from the Atlantic with strong gusts, 11C, some intermittent clear spells.

Observations: Seeing III. 3 systems fairly well resolved this evening. Delta Cygni B only seen intermittently. Magnification held at 260x owing to blustery conditions.

Date: September 12 2018

Time: 00:10-20UT

Location of Jet Stream: Currently over Scotland

Conditions: Very wet, windy with some sporadic clear spells, good transparency once the clouds move out of the way. 10C.

Observations: Seeing (II-III). Just three systems examined tonight; the exception being Pi Aquliae, which was not in a suitable position to observe. All three were well resolved at 260x. Did not attempt 354x owing to prevailing blustery conditions.

 

Date: September 12 2018

Time: 21:40-55 UT

Location of Jet Stream: Not over Scotland

Conditions: Still unsettled, blustery light drizzle and mostly cloudy with some clear spells. 10C.

Observations: Seeing (III), three systems resolved well, Delta Cygni B not seen cleanly at 260x under these conditions.

 

Date: September 14 2018

Time: 19:30-50UT

Location of Jet Stream: Currently over Scotland.

Conditions: Rather cool, (9C), very little breeze, rain cleared to give a calm, clear sky.

Observations: Seeing II. All four systems cleanly resolved at 260x and 354x

 

Date: September 16 2018

Time: 19:20-40UT

Location of Jet Stream: Currently over Scotland

Conditions: Mild (12C), fresh south-westerly breeze, some occasional clear spells.

Observations: Seeing very good (II), all four systems cleanly resolved at 260x and 354x.

 

Overall Results & Conclusions:

This study was conducted over the course of one month, from mid-August to mid-September 2018, a period covering 31 days.

The number of days where observations could be conducted was 21, or ~68% of the available nights.

No link was found between the presence of the Jet Stream and the inability to resolve four double star systems with angular separations ranging from ~2.5-1.5″. Indeed, many good nights of seeing were reported whilst the Jet Stream was over my observing location. In contrast, some of the worst conditions of seeing occurred on evenings when the Jet Stream was not situated over my observing site.

There is, however, a very strong correlation between the number of nights available for these observations and the efforts of the observer.

Many of the nights the Jet Stream was located over my observing site were windy, but this was not found to affect seeing. While the wind certainly makes observations more challenging, it is not an indicator of astronomical seeing per se. That said, no east or northeast airflows were experienced during the spell these observations were conducted. At my observing site, such airflows often bring poor seeing.

The archived data (from January 16 2014) on the Jet Sream site linked to above provide many more data points which affirm the above conclusions.

I have no reason to believe that my site is especially favoured to conduct such observations. What occurred here must be generally true at many other locations.

These results are wholly consistent with the available archives from keen observers observing from the UK in the historical past. This author knows of at least two (or possibly three) historically significant visual observers who enjoyed and documented a very high frequency of suitable observing evenings in the UK.

Contemporary observers are best advised to take Jet Stream data with a pinch of salt. It ought not deter a determined individual to carry out astronomical obervations. Perpetuating such myths does the hobby no good.

Post Scriptum:

June 18 2019: Irish imager, Kevin Breen, used his C11 to obtain decent images of Jupiter under a very active Jet Stream. Details here.

July 2 2019: Another testimony of “good seeing” under Jet Stream here

 

Neil English debunks many more observing myths using historical data in his new book, Chronicling the Golden Age of Astronomy.

 

De Fideli.