April 13 2018
In a previous communication, I described my acquisition of a Revelation 12″ f/5 Dobsonian telescope, which delivered fine images of selected double stars and which was used to establish the Taylor hypothesis, namely, that if the seeing conditions are fair to average or better, sub–arc second pairs can be readily observed at appropriately high magnifications using the generous aperture of this telescope.
After conducting this body of work, I have had little time to enjoy the considerable benefits of observing with a big Newtonian system such as this, but in this blog I hope to report still more observations with the same telescope and improve its optical and thermal performance in the field.
During the course of that previous communication, I described the optics in this telescope as being unreasonably good. Indeed, they far exceeded my expectations given the very modest cost of the telescope! I also described some modifications I made to the instrument, including the re–coating of the secondary mirror (just 70mm semi major diameter giving a 23 per cent central obstruction) with super–high (97 per cent) reflectivity coatings as well as the procuring of Bob’s Knob’s to assist precise collimation in the field, but I postponed some other aspects of this project. In particular, I wished to also have the coatings on the large primary mirror similarly upgraded in order that it would increase light throughput to the eye as well as improving the overall contrast of the images so rendered.
Today, I endeavoured to resume work on the telescope and that meant removing the primary mirror from its cell in order that I could despatch it to the mirror coaters located south of the border in England. The mirrors for this telescope were sourced from GSO and seem to have been more or less consistently good, as judged by other experienced amateurs who had taken the time to assess one or more units of the same product. One such assessment is documented here and I would heartily agree with the conclusions of Mr. Stoitsis.
After removing the mirror cell from the rear of the optical tube, I was able to accurately measure its thickness, as well as assess the design of the accompanying cell. The thickness of the mirror was measured to be 36mm (so 1.5 inches), yielding a mirror thickness to aperture ratio of 1:8. This result is consistent with this author’s finding in respect of the ability of the telescope to acclimate adequately to ambient outdoor temperatures, allowing him to make those important observations with regard to resolving sub–arc second pairs.
The mirror cell is housed in an open cell. It appears to be well designed and incorporates a 9–point floatation system; just about perfect for a mirror with these dimensions.
The mirror was then carefully packed away for safe passage to the mirror coaters.
I hope to despatch the mirror early next week, so hopefully I will have it back in a few weeks; just in time to explore the deep sky glories of spring!
April 14 2018
Almost forgot to tell you: several weeks after exchanging resources for the foot ‘scope, I received an envelope in the post. Curiously, I prized it open, and there it was! A small, battery powered fan; the same one that originally came with the telescope! As I explained in the opening blog, I wasn’t too bothered about not having it, nor did I really need it. But it was a warm gesture from the original owner of the telescope to send it on; something I appreciated!
Time: 4:20 pm
As described in previous blogs, I also wish to line the inside of the 1450mm long tube with cork and overlay this with flocking material. Materials were ordered yesterday and should keep me busy until the primary returns home.
April 21 2018
With a decent spell of settled weather now upon us, I spent the afternoon lining the inside of the optical tube with a thin layer of cork. The product I chose has an adhesive layer on the underside of the cork substrate and adheres to the rolled aluminium tube fairly easily. The cork itself is very delicate though, and so some care must be taken not to tear it while preparing the strips. 3 full 100 x 45 cm rolls were used up today, but I needed to order up a couple of extra rolls to complete the job. These arrive on Monday so I can complete the task then. The flocking material has already arrived so that will be overlaid on the cork. This afternoon, I only flocked the inside of the focuser draw tube.
The mirror arrived safely at the coaters and on the invoice they noted its diameter to be 303mm. Hoping to have it back in a week or so.
Date: April 24 2018
Well, I finally managed to complete the cork lining as well as covering it over with flocking material. Since the rolls are the same size ( i.e. 45 x 100cm), I can report that it will take 4 complete rolls of each material to insulate and flock the optical tube assembly of the 30.3cm F/5 Newtonian. I’m pleased with the result. All I need do now is wait for the arrival of the primary mirror and I’m back in business.
Date: May 8 2018
Well, the primary arrived back safely from the coaters this morning. I was busy with a few other things so only did a quick check to see that all was well. It was exceptionally well packed for transit. Later on, I gave it a good look over and can say that the firm did another excellent job applying the high reflectivity coating to the 30.3cm mirror, and they centre spotted it, as requested.
It was then carefully placed back inside its cell, making extra sure that the clips holding it in place were not overly tightened to avoid pinching of the optics.
After putting the mirror back inside the tube, I left it outside to cool in the bright evening sunshine.
Following a few hours of rain, it cleared up in the late afternoon and stayed that way well into the evening. So, all I need to do is align the optical train before sunset and I can take it for a spin under the stars.
Date: May 9 2018
What a glorious night! Telescope performed flawlessly!
Boys oh Boys!
Will tell you about it later today.
Starting at about 11:30pm local time, as the twilight had all but abated, the fully acclimated and carefully collimated telescope rendered excellent star tests on a few third magnitude stars, with a very high degree of symmetry in the intra- and extra-focal images at 250x. I could however detect a trace of under-correction, as I reported before, but it was not enough to bother the image in any significant way. In focus, the same stellar targets rendered hard, round Airy disks with no astigmatism or coma at the centre of the field at powers up to 450x (the highest tested this evening). No evidence of pinched optics was manifest.
My first proper target was Izar(Epsilon Bootis) now sitauted high up the southeast. At 250x, the image was remarkable! Both the primary and the secondary were beautifully resolved and intensely bright! Bright targets like this would actually benefit from filtering in this large telescope with a neutral density filter or polariser. As the sky darkened further, I turned the telescope on two stellar targets with very faint companions in comparison to their primaries. Keeping the magnification at 250x, Polaris B stood out wonderfully well; far more ‘in your face’ than the images garnered by even the 8 inch f/6 Newtonian. Even more striking was the very faint and close-in companion to Alula Borealis. The ruddy primary here shines at magnitude +3.5, and the secondary, a feeble +10.1. The 12 inch cleanly resolved the pair separated by about 7.4″ of dark sky, with the companion very obvious and much more easily seen than with any of my smaller telescopes. Where even the 8 inch requires some degree of concentration to pick off the secondary, the 12 inch made it very easy to see at a glance. Indeed, I can’t recall seeing it so well and so easily!
As the sky became maximally dark between midnight and 1 am local time, I turned my attention to a variety of deep sky objects to assess both the light gathering and defining power of the 12″ f/5. And here again, I was not disappointed!
I first turned the telescope on M 51 in Canes Venatici, now very well placed high overhead. Even with the 32mm Plossl I used to centre the galaxy in the field, it was strikingly bright and obvious in the expansive, low power portal. Inserting my 7.5mm Park Gold ocular delivering 200x, I was able to discern far more of the spiral structural details in the galaxy’s spiral arms of this amazing target. Indeed, it was in a completely different league to the views I have recently been enjoying with my 8″ f/6 Newtonian. A very enjoyable experience!
Next, I moved into Hercules, which by this time was well placed high in the eastern sky. Excitedly, I turned the 12″ on its deep sky jewel; Messier 13, the great globular cluster some 23,000 light years away. The instrument served up an amazing image at 200x( 7.5mm Parks Gold), but it was even better at 250x (6mm Baader ortho). The storm of stars resolved in this large aperture telescope was simpy mesmerising and again in a completely different league to that rendered in any of my smaller instruments. For kicks, I cranked up the power to 450x (Parks Gold 7.5mm with a 2.25x Baader shorty Barlow), refocused and sat back to enjoy a field of view littered with innumerable faint, round stars, all finely resolved right down to the core. Lesser instruments just run out of light at these very high powers but not so with the 12″!
Not too far away and significantly higher up in the sky was Messier 92, and once the telescope was centred on it, it produced a wonderfully sharp and well resolved globular cluster, with many hundreds of individual stars clearly seen at 250x (Baader 6mm ortho). As before, while the view in my 8″ f/6 was rewarding, the 12 inch takes you to a whole new level of visual experience!
My final targets were located in Lyra, which I visted shortly before 1am local time. First I turned the telescope on the famous Double Double (Epsilon 1 & 2 Lyrae). At 250x, all four components were beautifully resolved and intensely bright, more like distant coach lamps etched onto the sky than anything else. Slightly lower in altitude was M57, the famous Ring Nebula. At 250x, the image of this planetary nebula, more like a luminous smoke ring, was big and bright and easy to study. Generous amounts of structure were delineated at 450x along its southern border, with many gradations of brightness, mottling etc observed in the brighter outer annulus. Central star not seen (of course!) but many more stars observed in its immediate hinterland than that presented in the 8 inch instrument.
Conclusions: The telescope delivered great images, fully in keeping with its large, high quality optics. I am adequately convinced that the 12″ gains that extra (approximately) one magnitude over the 8 inch telescope, allowing many deep sky objects and faint stellar companions to be more easily studied. Throughout last night’s vigil, the stellar images at very powers remained tight and calm, suggesting that the insulating cork lining was doing its job (no fan used). All images were presented with very good contrast and with little in the way of stray light drowning out the faintest details. If anything, the experience in the field induced strong desires for even more aperture. But it is reassuring to realise that in order to gain yet another magnitude in light grasp, I’d have to move up to an 18 inch!
I am over the Moon with the performance of this telescope, which was purchased for just a few hundred pounds. And even with the additional costs of the modifications, the overall financial outlay came in at about half the price of my most expensive telescope; a diminutive but very fine 5″ f/12 refractor. Heck, that telescope would make a good finder on the 12 inch behemoth lol.
Date: May 10 2018
Time: 20:15 UT
The forecast looks good again this evening and so I can spend more time with the foot ‘scope. I leave it set up in my back garden with the optics capped and let it cool naturally to ambient temperature for a couple of hours before commencing observations. That way I can return to my writing committments and/or other things. Although one can have the instrument set up permanently in a cool outhouse, I want the coatings I invested in to last as long as possible. Water vapour and its condensations are the enemies of all optical coatings, so that’s why I store all of my Newtonians indoors when not in active use. My Newtonians are all long term committments; parts of my family, as it were. And like everything else that is valuable in life, it pays to give your instruments a bit of TLC if they are to deliver top perfomance time and time again.
Date: May 11 2018
Time: 23:00 to 00:30 UT
I enjoyed another hour or so of good dark skies last night, visiting a whole suite of double stars in Coma Berenices and Bootes, which were well placed near the meridian at this time. Conditions were a little hazier this evening though, reducing transparency.
There are two lovely doubles in Coma worth visiting; 24 Com, which consists of a gorgeous colour contrast double, the primary shines with a ruddy complexion at magnitude 5 while the ‘secondary’ has a gorgeous blue white hue some 1.3 magnitudes fainter. The system is orientated roughly east to west and very nicely framed at 200x. I say ‘secondary’ as it is unknown whether this is a true binary system and at this time, the consensus appears that they are unrelated. The second system visited was 35 Coma, with the telescope easily resolving this tight duo of yellow and yellow white components with a striking 9th magnitude outlier. I believe this system is a little over 1 arc second at the present time but is slowly converging over the next few decades. A power of 450x was found to be optimal to splice this puppy, the components of which are 5.1 and 7.1. The orbital period of this system is about three and a half centuries.
Then into Bootes; Iota, Epsilon,Kappa, Mu, Pi and Xi were beautifully rendered at 200x to 250x. With the generous aperture of this telescope, the stellar members of these systems were very brilliant and colour faithful. Seeing was not as good last night as on the previous night, but I was extremely impressed at how the instrument maintained high quality images over periods of a few minutes, as each system in turn was studied at leisure.
Yet again, I was very impressed with the optical prowess of this large telescope. It holds collimation well (I checked it at various times and in various orientations throughout the vigil), perhaps a tad better than my other Newtonians. It also appears to be behaving itself thermally, which is a great relief. Again, no active cooling was employed.
As I was packing up my gear, I realised that I had not named this telescope properly. I can’t just call it the ‘foot ‘scope’ forever now can I? But what shall I call it? Not ‘Alexa’ to be sure; far too creepy for my liking. I need to spend more time with the instrument before I decide.
Time: 13:15 UT
I am thoroughly enjoying the sturdy dual speed focuser on the 12″ f/5. All my other Newtonians have simple, singe speed focusers but having the ability to very finely adjust the focus position during high power applications is a great bonus. Frankly I’m amazed that the manufacturers were able to offer this feature as standard equipment with the telescope. A super nice touch!
Well, though the morning was quite cloudy, with the rain arrving on schedule this afternoon, the forecast says that it will quickly pass through, leaving the evening clear once again. I would like to return to the realm of the globular clusters; but not in Hercules. There can’t be many dark nights left what with summer twilight knocking on the door.
The sky has not yet cleared up. Up early tomorrow so need to call it a night anyway.
Date: May 14 2018.
Time: 23:00 UT
After a cloudy and wet morning, the skies cleared to give a beautiful sunny afternoon. Those conditions gave way to a good clear night, but even at midnight local time, the twilight has noticeably brightened in the last few days, especially towards the north. I set the foot ‘scope out about 10pm local time to fully acclimate prior to resuming observations. Tonight I wish to begin with another celebrated globular cluster, Messier 3, conveniently located almost exactly half way between Arcturus and Cor Caroli.
Another great night, although the seeing was a bit rough. Nonetheless, some great sights captured in the foot ‘scope. Will tell you about what I got up to later today. Nite nite.
Starting about local midnight, I trained the 12 inch Dob on Messier 3 in Canes Venatici. Centring the object in the low power (47x), approximately 1 degree field of my 32mm SkyWatcher Plossl, I noted a fairly bright field star just west of M3 which provided a means to carefully focus the image. Messier 3 has a very condensed core with quite a few stars being resolved at its periphery. Both my 5.1″ and 8″ Newtonians are well able to resolve the outer parts of this globular cluster but the core remains stubbornly unresolved even at higher powers. Not so with the 12″ instrument, which opened up this globular at powers of 200x and 250x, showing many more stars resolved to its core. The instrument works well with simple, short focal length eyepieces. Indeed, my 4mm Revelation Plossl, which delivers a power of 375x, produced a truly wonderful view, where its constituent stars filled the 0.14 degree field of view. Tracking the instrument was never a problem owing to the smooth azimuth and altitude bearings on the Lazy Susan mount, which enables me to nudge the telescope along as the object drifts through the field from east to west.
M 3 is noticeably smaller than M 13. But it is also located some 10,000 light years further away from the solar system than the latter, explaining its smaller angular size at equivalent magnifications.
This telescope is worth every penny spent on it just for the quality of the views of globular clusters alone. The foot ‘scope is a veritable “glob buster.”
I recorded 4 bright telescopic meteors during my 75 minute vigil.
Keen to get a look at some other showpieces before the worst of the twilight returns, I turned the telescope on the bright galaxy pair, Messier 81 & 82 in Ursa Major, and my trusty 32mm Plossl framed both galaxies in the same field of view. Even at this low power, the generous light gathering power and resolution of the telescope displayed these galaxies extraordinarily well, with prominent mottling in M82 evident at a glance along its major axis. Cranking up the power to 200x using my 7.5mm Parks Gold, clear spiral structure in M81 could be made out without much effort. Again, the level of detail seen in these galaxies is a very significant increase over my next most powerful telescope; the 8″ f/6 Newtonian.
Time: 21:15 UT
Another clear night beckons, so more tests can be made on the foot ‘scope.
Date: May 15 2018
Time: 00:20 UT
Tonight I enjoyed a stellar extravaganza, cruising at 200x through a wilderness of light and colour. More info later today.
Last night’s spell with the telescope took me to a variety of colour contrast double stars. First came 30 and 31 Cygni, arguably one of the most comely binocular doubles in the entire sky, the brilliancy of the stars and their colours; orange and turquoise were beautifully framed in the 0.25 degree field of my 7.5mm Parks Gold eyepiece delivering 200x (a rather pedestrian magnification for this large telescope). While smaller ‘scopes certainly present this vista well, the view is truly transformational in this large aperture telescope owing to its superior light grasp and defining power. I then moved south to Beta Cygni (Albireo) and enjoyed a wonderful view of the marmalade orange primary and blue –green secondary at the same power. It was just a joy to see them so faithfully rendered in their true colours and shining so intensely in the telescope.
Following this, I ventured into the large and sprawling constellation of Hercules, starting with Rasalgethi(Alpha Herculis) with its red giant primary and bluish companion perfectly framed in the eyepiece. Then I threw caution to the wind and moved higher up the sky visting each star brighter than 5th magnitude within the constellation, examining their hinterland in the 200x portal. As I moved from star to star using the 8 x 50mm finder, I was amazed at the sheer light gathering power of this telescope and the number of extremely faint stellar ‘companions’ which attended many of the brighter stars. Where my smaller telescopes only revealed the brighter members or none at all, the 12″ f/5 pulled in many more! I was reminded of the work of the English amateur astronomer, the Reverend T.H.E.C Espin of Tow Law, Northumbria, who used a 17.25 inch equatorially mounted Calver reflector at the turn of the 19th century to discover a sizeable tally of these faint and wide companions strewn all across the northern sky. It seems that good reflecting telescopes are ideally suited for such work. Indeed, they can hardly be beaten in these pursuits!
With every increase in magnitude, there is a corresponding increase in stellar number, but there is no fixed power law that might enable us to compute how many more stars there might be as the magnitude is increased. The distinguished 19th century German astronomer, F. W. Argelander, estimated that each magnitude exhibits a rise of about 300 per cent. Indeed, in data presented on page 294 of W.F. Denning’s masterful tome, Telescopic Work for Starlight Evenings (1891), he provides these figures, collated from a survey between 2 degrees south of the equator all the way to the north pole:
Having spent some time traversing the stars of Hercules, I can definitely see that Denning was on the right track. Indeed, I would say that the 12 inch instrument does yield an approximately three fold (maybe more) increase in star numbers over my optically excellent 8 inch f/6 Newtonian; and in many cases that is sufficient to change the visual perspective of each telescopic field by a considerable degree. In small telescopes, many star fields can present as rather bland and uninteresting. Patently not so with the 12″ f/5!
Although the sky is clearing up as I speak, I have decided to take a break tonight as I’m knackered and need to recharge the ‘batteries’. More to come soon.
Date: May 16 2018
I love the low tech approach of the Dobsonian. No electronics to fiddle with, no star alignments to perform. Just mount it on its lazy suzan and you’re off to the races. But this simple approach doesn’t suit everyone. In particular, many prefer driven mounts that keep objects centred in the field while making observations. If that’s your forte then there’s a solution; enter the driven Dob mount. Many such equatorial platforms are available for purchase today and they vary quite a bit in price, but these days you don’t have to spend a fortune acquiring one. Have a look at this one, for example. All you need to do is adjust the inclination of the platform to coincide with your latitude(in my case it’s 56 degrees North), power it up and you can enjoy an hour of active tracking before you have to reset it. If you do decide on one, you need to ensure it can be adjusted to your latitude angle. That’s why a variety of them can be purchased to suit your precise location, either south or north of the equator.
The good Lord has granted us a spell of settled weather here and tonight looks very good to go. Where will my foot ‘scope carry me off to this evening?
Date: May 17 2018
Tonight I stuck mainly to low power, wide–field viewing. When coupled to a good 2 inch wide–angle eyepiece, the foot ‘scope serves up some spectacular views! More later.
Last night the foot ‘scope did experience some thermal issues as it struggled to follow the rather large temperature differential between daytime highs and night time lows. This time of year, we tend to experience larger temperature swings than in other months where it is not especially unusual for the diurnal temperature variation to exceed 15C (as opposed to about 5 or 6 degrees which is normal). As a result, high magnification images of stars were quite swollen and, in the absence of any breeze, I considered using the battery–powered fan. In the end, I changed strategy and decided to explore the wide field sky around Lyra and Cygnus.
The telescope came with two eyepieces from the former owner; a Revelation 30mm 70 degree Superview 2 inch ocular, as well as a 9mm Revelation Plossl. The former unit was quickly found to be adequate on axis but rather poor off axis, with coma, distortion and astigmatism being in evidence. To enjoy a much more immersive view, one needs to invest in a higher quality wide angle ocular. The 32mm Plossl gives a significantly smaller true field but off axis aberrations are still manifest. To this end, I removed the 1.25″ adapter on the focuser and reached for my 40mm Explore Scientific Maxvision 68 degree eyepiece, which has been used extensively over the last few years with my 8 inch f/6 Newtonian.
While the exit pupil was pushing 8mm on the 12″ f/5, I was delighted with how well it performed. Yes, there was some minor light loss but otherwise it served up excellent, bright and sharp images of star fields out to about 90 per cent of the way to the field stop. And even then, the distortions were more than tolerable. The Maxvision eyepieces are clones of the longer established Meade Super Wide Angle(SWA) and Tele Vue Panoptic oculars, but are offered at significantly lower prices.
To best match the faster f/5 system of the foot ‘scope with my 49–year–old eyes, I ordered up the 34mm unit, which will deliver an excellent 1.5 degree true field, a magnification of 44x and sub–7mm exit pupil. This will be an excellent eyepiece for sweeping the heavens for comets, faint nebulae and open clusters on the precipice of visibility. A large, high–quality telescope like this deserves a good, wide angle eyepiece. The Maxvision range offer this quality at very attractive price points (£114 plus postage). I’m hoping to receive the unit by the middle or end of next week. Of course, there are other options for money conscious amateurs; the second hand market is likely to have something suitable come up from time to time and with patience and discernment, good deals can be had.
This evening looks good to go again, but I would like to take a break from the foot ‘scope for a few days and feed some starlight to my smaller instruments. More soon.
Date: May 18 2018
Last night I enjoyed a wonderful evening of double star observing with my 130mm f/5 Newtonian.The sky was very tranquil, allowing almost textbook perfect images to be generated on a variety of systems. The good weather remains with us again today and this evening I noticed a beautiful crescent Moon hanging in the western sky. I just couldn’t resist the chance to observe it in the 12″ f/5. My 32mm SkyWatcher Plossl delivered a jaw dropping view of the crescent at 47x in a one degree field and the wondrous earthshine enveloping its darkside. Reaching for my 7.5mm Parks Gold delivering 200x, I was delighted with the razor sharp views of the lunar regolith, especially considering its fairly low altitude at the time of observation. Though not a lunar observer per se, I look forward to observing this magnificent world with the foot ‘scope as it rises higher in the sky in the coming days.
Date: May 21 2018
The new 34mm 68 degree Maxvision eyepiece arrives tomorrow, so not as long a wait as I anticipated.
Date: May 22 2018
I set the foot ‘scope out late this evening so that I could get a quick look at a fairly well placed first quarter Moon, with the express intention of testing the high magnification images garnered by the telescope. In the end, it proved an exercise in frustration more than anything else, as the haar (low altitude cloud and mist) came rolling in off the North Sea from the east as soon as the Sun’s rays became weakened by its falling altitude in the northwestern sky. I did however manage to get some quick peeks at the lunar surface at powers of 250x (6mm Baader ortho) and at 375x (4mm Revelation Plossl). I can report that both ‘high power’ oculars delivered very sharp and detailed images of the lunar regolith in twilight, demonstrating that these powers can be productively used on extended objects like the Moon and the planets in the foot ‘scope. That said, I’ve used nearly double these powers on sub–arc second double stars in work conducted during the summer and autumn of last year.
Alas, the courier never showed up today with the eyepiece……ho hum.
Maybe tomorrow lol!
Date: May 23 2018.
Time: 12:20 UT
Well, the new 34mm eyepiece has finally arrived.
It’s a scaled down version of the venerable 40mm.
What I especially like about this series of wide angle eyepiece is that they provide excellent performance in a no–frills package. It came in a plain box with no colourful logos and no advertising brochures. Compare them to the now discontunued Meade SWA incarnations;
I seem to recall that the Meade 34mm SWA retailed for £249, but were reduced in price for clearance after they were discontinued. Many retailers do not offer the Maxvision series but appear to be selling a hermetically sealed (argon purged) product with a re–designed body for £219.
So, I was able to purchase essentially the same eyepiece for a little over half the price of the latter. That’s what I call a bargain!
You might need to shop around to get your hands on these eyepieces, but I was able to secure this one from Rother Valley Optics.
Of course, I could have gone for some 82 degree wide angle eyepieces which would provide a slightly larger true field (approximately 1.6/7 degrees), but my eyes seem to prefer 68 to 70 degree fields.I appreciate that this is a highly personal choice though.
The Maxvisions seem to be enjoyed by many amateurs. See this thread, for example. Some amateurs prefer to de–cloak them for some reason, perhaps to pare down their weight or to make them look more appealing, but I have never seen the need to do so.
The stubborn haar is still with us and though my skies are currently blue, it will likely roll in off the North Sea later this evening.
No matter, I’ve done good!
I couldn’t wait to see how it performed, so I mustered my 80mm f/5 achromatic telescope and inserted my Televue 2 inch Everbrite diagonal and in went the 34mm Maxvision.
The eyepiece delivers a wonderful, wide field at 12x giving a 5.75 degree true field. The field stop is hard and well defined and contrast appears to be excellent. With these eyepieces one can readily adjust the distance of the large eye lens by rotating the upper section to provide the optimal level of viewing comfort.
I’m currently writing a book that is wholly dedicated to the ShortTube 80 achromat and one of those chapters will de devoted to choosing eyepieces for use with it. I hope to perform more critical tests on the night sky in due course.
The 34mm will also serve Octavius, my 8 inch f/6 Newtonian, by delivering a power of 35x in a field just under 2 degrees in extent.
Time: 21:45 UT
Contrary to what I expected, the haar did not roll in in the late evening, and it is still clear just now. I set the foot ‘scope out again at 19:00UT and was able to continue my high magnification testing of the early gibbous Moon. I began observing at about 20:40 UT when the Moon had past the meridian and was located at an estimated altitude of about 37 degrees. This was a good altitude to test the defining power of the 12 inch. On fine days like this, the air can be very tranquil in the minutes immediately after sunset and so I began with 200x (Parks Gold 7.5mm). Eventhough the sky was far from dark at this time, the lunar regolith presented with razor sharpness and very good steadiness. Ditto at 250x. Then I attached my Baader 2.25x Barlow to the 7.5mm Paks ocular which delivered a power of 450x. Carefully focusing, I was delighted to obtain a wonderfully sharp and (still) fairly stable image. Having the facility to use the microfocuser proved very useful. Looking at the ragged crater walls, I could see details which I do not recall observing in my 8 inch at high powers.
This was a most satisfying result! This instrument is indeed capable of very high powers on extended objects like the Moon and a solid indicator that the optics are of high quality, but I am also fortunate enough to live in a place where the seeing will allow a 12 inch to work beyond the remit of my 8 inch telescope.
I’m away out again to conduct some further observatons and to test the 34mm Maxvision ocular. Shall report back later.
Time: 22:45 UT
Well, the haar has come back lol. It was rolling in as I was writing the last section of the blog. But there was a few suckerholes in the sky and I naturally took advantage of them. My first target was Capella now located quite low down in the north northwest. Charging the foot ‘scope with the 34mm Maxvision I can report excellent results. The bright first magnitude star remained pinpoint sharp across the vast majority of the field while wearing my eyeglasses (which correct for the astigmatism in my eyes). Only at the extreme edge of the relatively massive field did the star show signs of field curvature, coma and astigmatism. The same was true of Polaris located at an altitude of 56 degrees above my northern horizon.
Intriguigingly, I also tested the same eyepiece on my 80mm f/5 achromat. The results were broadly the same but I would say that there was slightly less aberration at the extreme edge of the field of view! It had field curvature and astigmatism but not much in the way of coma. Thinking about this for a few moments, I figured that this slightly better result is due to the fact that even at f/5 an achromatic doublet has very little coma inherent in the design. This is a relatively unsung virtue of modern refractor optics.
So a very good result, which left me very satisfied indeed. The eyepiece does exactly what it says on the tin. No hyperbole; just great performance at an excellent price!
Need to pack up all the toys now and return to barracks.
Date: May 25 2018
Time: 00:05 UT
Just a quick report from tonight. I only fielded Octavius, my 8 inch f/6 Newtonian this evening to test out the 34mm Maxvision ocular. True to form, it delivered fantastic views and just that little bit better correction right at the edge of the field. I’m going to have an absolute ball with this when dark skies return later in the summer!
My main high resolution target was 78 UMa, conveniently located very near Alioth (Epsilon UMa), one of the stars that form the handle of the Ploughshare. My notes inform me that I’ve not revisited this system for close on three years. And it’s got tougher! The separation was about 0.84″ in 2015 but judging by observations conducted tonight using a power of 500x in good seeing conditions (II) at its currently high altitude above the horizon, I would say that the companion is nearer 0.7″. It was spotted roughly east of the primary and pretty much kissing it but I need to make more observations, especially with the foot ‘scope. The primary is magnitude +5.02 and the secondary +7.88.
Time: 10:25 UT
Setting up the 8 inch is easier on the back than doing the same with the 12 inch. There is quite a step up in mass and bulk volume as the image below suggests;
Still, I am more confident than ever that this will also be matched in terms of performance.
I found out that the companion to 78 UMa orbits the primary in just over a century and it’s currently rapidly closing, reaching its minimum separation in 2026 with a separation of 0.48.” Dynamically, this will be an excellent subarcsecond system to study over the next few years and both the 8 inch and 12 inch Newtonians will be pressed into service monitoring its movements. More details here.
This is just one of many fascinating high resolution targets that you can study using telescopes of this size.
Right folks, that’s your lot for this blog. I hope you have enjoyed its content and that it gives you some encouragement to get out there and enjoy the glories of the night sky.
Update: June 4 2018
Since I intend to get a lot of use out of the foot ‘scope, I invested in a small trolley that can help lighten the load of carrying the large optical tube from idoors to the outdoors and back. All I have to do is place the optical tube on the platform and secure it in place using two ropes (supplied with the trolley). It is thereby easily moved from place to place using a couple of small ramps.
I attach a couple of pictures of the ‘scope on the trolley, for interest;
The trolley can also be folded down to a neat size for easy storage.
Neil English is author of Choosing and Using a Dobsonian Telescope.