Saturday July 8 2017
I took a chance on a large reflecting telescope; a Dobsonian with a 305mm (12”) mirror and focal length of 1525mm (so f/5 relative aperture). The price was just too good to pass on so I contacted the seller and it transpired that he was a professor of optics from a Welsh University. I asked him about the telescope and in particular, about its optics. He assured me that they were good from his limited use of it, a tad over corrected but otherwise fine. When I enquired as to why he was selling the telescope he simply admitted that he didn’t have much time to use it and would rather see it collecting starlight than sitting in his garden shed. Although based near Chester, England, the owner kindly agreed to drop off the instrument in person (with an appropriate reimbursement for pertrol) and so early on Saturday morning, he and his wife set out on the road north to my home here in rural central Scotland. The day was bright and sunny, good for a car trip and they arrived in the mid afternoon.
Branded as a ‘Revelation’ 12” f/5, the instrument is fitted with GSO mirrors, which, as I have shown in previous blogs, has now established itself as a manufacturer of high quality optics at very reasonable prices. A few years back, I had actually tested one of these units but, unfortunately, found the mirror to be astigmatic; a real show stopper for me as I like to push my instruments on high resolution targets.
The instrument came with all the usual accessories; a 9mm Plossl (1.25” fitting), a 30mm wideangle ocular (2” format), an extension tube and a ‘straight through’ 8 x 50mm achromatic finder ‘scope. And while the original instrument came with a built in fan, the owner apologised that he had taken it off whilst installing a set of stronger springs to support the primary mirror and had forgotten to reinstall it. He was relieved to see that I wasn’t in the least bit bothered about it and reminded him that many of the great observers of yesteryear never used fans on their reflectors and that this didn’t seem to hinder their fine work in any significant way. He gave me a wide smile. I handed over the cash and treated the couple to high tea, where we discussed, among other things, the many virtues of the Newtonian telescope. Suitably refreshed, my wife and I bid them farewell and they began their long car journey south of the border.
No sooner as they hit the road, I visually inspected the primary mirror and found that it had accumulated quite a bit of dust, which imparted a milky appearance to its surface. There was no evidence of coating deterioration on the primary mirror, but it would require a good cleaning, so I removed it from the tube, gave it a ‘finger tip’ cleaning, described previously in this short blog, using a few drops of washing up liquid added to a bowl of lukewarm water, before hosing it down with my garden hose, whereafter it was dried out indoors and reinstalled in the tube.
The mirror was centre spotted (which came as a relief) and I noted its thickness to be about an inch Thus, it is acceptably thin and should cool off in a reasonable amount of time.
The telescope came with a very smooth dual speed (10:1) Crayford style focuser, a feature I was to learn to greatly appreciate, as the reader will discover shortly.
Collimation was very easy to perform using the oversized knobs on the rear of the primary cell, as well as using a small Philips screwdriver to make the appropriate adjustments to the secondary.
An inexpensive SkyWatcher laser collimator whipped the optics into line in a matter of seconds. Later I removed the secondary mirror for inspection and noted no coating issues with it either. I measured its minor axis to be approximately 70mm so a very respectable linear obstruction of the order of 23 per cent. Not bad at all!
Movements on the lazy suzan mount were smooth and easy to execute, both in azimuth and altitude; certainly adequate to the intended purposes I wishes to use the telescope for.
I fitted the instrument with my right angled 8 x 50mm finder borrowwed from ‘Octavius’, my 8 ” f/6 Newtonian reflector. Whatever others claim concerning the curse of acquiring a new telescope to play with, the weather remained fair for me that evening, with the promise of a clear sky, albeit in summer twilight. So I readied the instrument and set it outside to acclimate as sunset slowly approached.
At 23:00 BST, I managed to track down the bright summer luminary, Vega, in bright twilight and excitedly aimed the telescope at it, using a power of about 190x initially. To my great relief, the star focused down to a very tight disk with intense diffraction spikes from the secondary support. The image was remarkably calm and I then racked the eyepiece inside and outside focus. Again, I was very impressed at how the defocused Franhofer diffraction pattern presented itself. The optics looked nice and smooth, being virtually identical inside and outside focus. Inserting a quality, 6mm orthoscopic yielding 254x, I re-examined the star in the same way. Again, I was very impressed at how the star test was shaping up. I noted a touch of overcorrection; but it was very minor. Spherical aberration was wonderfully well controlled and there was no visible sign of astigmatism (phew!) or ugly zones. This was indeed a good, large, mirror; no, a very good mirror; in fact, as I recall, absolutely astonishing for the low price I paid for it!
As the sky grew a bit darker, I swung the telescope over to Epsilon 1 & 2 Lyrae, which the finder was just able to pick up in the bright, twlit sky. Leaving the 6mm orthoscopic in situ, I was delighted to see that it rendered a textbook perfect split of all four components. From there, I moved the instrument westward into Bootes and centred Epsilon Bootis (Izar) in the field. Refocusing slightly (it has a lower altitude afterall), I was again rewarded with an excellent image. This tricky, colour contrast binary system can elude telescopes if the seeing is not up to scratch. It was around this time that I reached for my trusty Baader single polarising filter, which imparts a slight darkening of the twilit sky and increased the contrast of the stellar components without imparting a colour shift. The filter improved the image still further by reducing irradiance, I thought. Indeed, I recalled that the previous owner had also learned this trick whilst observing Jupiter with the telescope, so he was a convert to using filters on bright objects too!
Feeling more ambitious, I turned the 12” reflector on Pi Aquilae now positioned low in the south southeast; an interesting binary system with roughly equal magnitude components separated by about 1.5”. Well, the instrument made light work of the system; both stars focusing down to beautiful, round buttons at 254x. Finally, at around 00:15 BST (Sunday), I swung the telescope up the sky once again, where the difficult Delta Cygni was now showing in a darker sky. Again: astonishing! The telescope made very light work of picking off the faint companion set beside the comparatively bright primary. The image was wonderful and calm; a fine night to begin my adventures with this large telescope. Alas, there it had to end, as I had to rise early the next morning, but I retired from the field confident that I had an excellent telescope and one that would assuredly be put to good use in the coming days.
Monday July 10 2017
Time: 16:30 BST
After an afternoon of drizzly rain, it stopped for a wee while allowing me to perform a high power daylight test to the telescope. With a very overcast sky, the temperatures had stablished so I was assured of a good image of a relatively nearby terrestrial target. Coupling a 2.25x Barlow lens to the 6mm ortho yielding 570 diameters, I focused on the topmost boughs of a horse chestnut tree adorned in its full summer foliage and located approximately 80 yards distant. Focusing carefully with the fine focuser of the telescope, I was greeted with a beautifully sharp image of the leaves, with all their minute imperfections being made manifest before my eye. 570x is very close on 50 x per inch of aperture and it passed this daylight test with flying colours. Still, I was quite unprepared for what I experienced the following evening.
Tuesday, July 11 2017
Time: 22:50 BST
Seeing: very good (II), partially clouded over again after a rather clear evening.
I began the evening with a star test on Vega as before at 254x, the result of which was as good (if not better) than the results obtained on Saturday last. Very slight overcorrection noted. Nothing else to report. Polariser used to darken the sky a bit.
23:15 BST: Epsilon 1 & 2 Lyrae perfectly split at 253x. Cranking up the power to 570x yielded similar results. Stars tiny and round; very well corrected optics.
23:20: Ditto for Epsilon Bootis at 570x. Excellent split. Beautiful stellar diffraction images at these uber high powers. Amazing!
23:40: Delta Cygni. Wonderful split at 570x. More turbulence witnessed at this power but the image was mighty impressive. Stars remaining, tiny and round.
Wednesday July 12 2017
Time: 00:10 to 00:20 BST
Seeing: remaining excellent (I/II)
Moved the telescope over to Lambda Cygni, a 0.9” pairing of white stars well positioned very high in the sky. At 254x, the system was well resolved. Then the power was increased to 570x and the image rexamined. Wow! I had never seen the system so clearly and so easily at a glance. The stars are orientated north to south, effectively perpendicular to the direction of drift!
The experience affirms the superiority of aperture under good seeing conditions. What a magnificent telescope I have in my possession!
The following evening was also clear. Details below:
Time: 23:45 to 00:15 BST (July 14)
Temperature: 13C easily noted as the midgees were more numerous tonight than the cooler night of last night.
Seeing: III/IV much more turbulent than the perfection of last night
Instruments: 130mm f/5 Newtonian & 305mm f/5 Newtonian
Targets: Epsilon 1 and 2 Lyrae: split in both instruments but aesthetically more pleasing in the smaller instrument (powers employed 185x and 253x, respectively)
Epsilon Bootis: Not reliably seen in the larger instrument at 250x. Only a swollen seeing disk with maybe a hint of a companion under brief moments of improved seeing.
Companion was clearly seen in the 130mm instrument and aesthetically more pleasing at 185x
All absolutely normal.
What a difference a night maketh!
After a week’s vacation to the southwest of Scotland, I was able to resume my testing at home.
Sunday July 23 2017
Time: 23:45 BST
Instrument: 12″ f/5 Newtonian
Seeing: Appears excellent once again (I/II), quite a bit of cloud, clearing slowly.
00:00 BST (local midnight July 24)
Had a quick look at 78 UMa at 570x but failed to resolve this 0.8″ system now about half way up the northwestern sky. It is considerably lower in altitude than Lambda Cygni though. Will stick to Lambda Cygni.
00:18: Finally cleared up, but still quite hazy. Managed another excellent split of this system this evening. Execllent definition at 570x. System followed through several fields. The addition of the fine focuser is a godsend. Very useful addition!
This is the second time in 12 days that I’ve managed this subarcsecond pair!
Therein lies a project; to ascertain, through actual observation, the frequency of such nights that are available for the 12 ” to do this kind of high resolution work at this location.
00:50; went out to reexamine the system and still very well resolved!
As it so happens, Lambda Cygni is Astronomy Now magazine’s Double Star of the Month (August 2017 issue pp 86). This is what astronomer, Bob Argyle, says about the system;
In 1842, using the new 15 inch refractor at Pulkowa, Russia, Otto Struve noted that the star was a close and very unequal double star. The companion (B), some 1.5 magnitudes fainter than the primary, was found in position angle (PA) 107 degrees (east southeast) and at a distance of 0.7 arcsecond. Since then, the companion has moved retrograde to PA 5 degrees (almost due north) and the separation is near one arcsecond………B is easily seen though not so easily measured with the 20cm refractor at Cambridge; a measurement last year put B about 5 degrees behind the ephemeris, which is derived from the 391 year orbit of Wilhelm F. Rabe(1893–1958).
My observations match Argyle’s very well. That it can be monitored with a 12 inch f/5 Newtonian shouldn’t surprise those who have followed my work over the years. Having enjoyed tremendous success with both a 20.4cm f/6 Dob (which has split this system but not nearly as convincingly) and a smaller, 13cm f/5 ultraportable Newtonian (pictured above), I was never in any doubt that the larger instrument would deliver. But to my knowledge no one has addressed the frequency with which this observation can be made with a 30cm aperture at this location. My hypothesis is that it is significantly more frequent than is commonly reported in the literature(or on forums). I do not, however, consider my location to be special in any particular way.
While I would concede that a large aperture classical refractor, mounted on a good equatorial mount, would be the ideal instrument for measuring this system, my observations suggest that the 12 inch reflector shows it more easily, supporting my previously stated maxim: ‘eye seeth afore I measureth.’
Monday, July 24 2017
A set of Bob Knob’s was fitted to the secondary of the 12 inch, thus facilitating quicker collimation in the field.
Looking good tonight for clear spells.Fingers crossed!
Time: 23:20 BST
Seeing: A notch or two down on last night. (Ant III), Clear sky, very little cloud.Midgees legion.
Marginal split reported at 570x (with polariser) but seeing disks swollen due to atmospheric turbulence. To get some perspective on this, Ant III seeing allows a good split of Epsilon 1 & 2 Lyrae in the 12 inch, but less aesthetically pleasing splits of Delta Cygni and Epsilon Bootis; again with slightly swollen seeing disks.
Will have another look shortly after midnight.
Tuesday July 25 2017
Time: 00:10 BST
System slightly better resolved; certainly to be counted as a split at 570x. Very impressive!
An Aside: 00:30BST
Had my first look at M13 with the telescope. Sky not fully dark yet but boy is it impressive at 254x! Easily superior to the best dark sky images with the 8 inch. Physics is physics afterall!
Some Implications of the ‘Foot’ ‘Scope Project
One of the most important things a tester of telescopes must do is to work in an environment that allows such instruments to be properly assessed. For example, if one lives where the seeing is continually lousy, or at best, mediocre, one will consistently report that small, high quality optics beats everything else, especially on planets and double stars. This is, of course, the mantra of the refractor nut, and it is true only insofar as what their local seeing can establish. Having said that, find out where your favourite telescope testers live, and what the seeing is generally like there. Chances are you’ll see a pattern in their reports. It is anomalies like these that lead to much heated (and needless) debates on forums, where the sincere convictions of one individual conflicts with others, only because the air under which they live puts a ceiling on what can be achieved.
For example, the long time reviewer of telescopes, Ed Ting, provided an in depth review of the Discovery 12.5 inch f/5 Dob in the November 2003 issue of Sky and Telescope Magazine. In that review he states that big Dobs are not always the best choice for double star observing. He writes:
A large Dobsonian is not always the first choice for double star observing; the scope is often too bulky to track at the high powers necessary to split close doubles. Nevertheless, I ran some tests with the 12.5 inch PDHQ. The well known stars Mizar and Polaris were easy, as were the Double Double in Lyra. I was also able to split Alpha Herculis, which has a relatively easy separation of 4.6 arcseconds but is made somewhat difficult by the mgnitude difference (3.5 and 5.4) between its components. But despite repeated tries, I never did split Delta Cygni, an even tougher target with 2.2 arcseconds separating its 2.9 and 6.3 magnitude components.
While there is no other indication from the same review that there was anything untoward about the optics in this telescope, what does seem odd is that Ting never managed to split the relatively easy pair, Delta Cygni. This would indicate to me that his seeing conditions were just not up to scratch for this task. The 12.5 inch ought to have done far better if his conditions were in any way decent. Thus, and with all due respect to Mr. Ting, reports like this must be taken with a pinch of salt.
Time: 22:45 BST: Looking promising again tonight! Yeehaw! I have just come in from a quick test on Epsilon Bootis with the 12 inch telescope and managed an excellent split at 254x. Midgees biting like mad though. More cloud tonight but some good clear spells presenting themselves.
Spent some time earlier dipping into the interesting chapter 11 of the 2nd edition of Observing and Measuring Double Stars (Argyle R.W, ed). I have mentioned the author’s work before; a one Christopher Taylor, who has been observing ultra tight binary stars from central England since the 1960s using a 12.5 inch F/7.04 Calver reflector. On page 135 Taylor mentions something of great interest to me;
It is probably in part the lack of such training and consequent failure to distinguish the seeing blur( the gross image outline) from the still visible Airy nucleus which is responsible for the persistent myth that seeing limits ground level resolution to 1 arc second at best, and is certainly the origin of some of the more spectacularly absurd figures one sees quoted for alleged image size.This author’s experience of typical conditions at a very typical lowland site may be of some interest in this context: using a 12.5 in Newtonian at 400 feet elevation (130m) in central England, an equal 0.75 arc second pair (such as η CrB in May 2000) is steadily separated by a clear space of dark sky at 238x in seeing of only III to II (Ant.)
You don’t say!
Time: 23:41 BST: After waiting for a suckerhole to appear in the clouds, I finally got a glimpse of the system (it’s dead easy to find!) for about 90 seconds, but enough to witness yet another clean split of the stars. Power 570x. Much better than last night. Steady image with plenty of dark sky separating them.
n = 4
I believe Taylor is telling the truth. My observations so far match his.
I would warmly encourage others to participate in this high resolution experiment. A 10 or 12 inch Dob should do the trick, and don’t worry about how much the instrument set you back. As far as I’m concerned the cheaper the better!
Vere dignum et iustum est.
If you’ve been following my blogs, you’ll be well aware that I’ve been chasing Lambda Cygni for a few years now. The 8 inch f/6 Newtonian has resolved the pair well, but only on the best nights (Ant I), but the 12 inch f/5 seems to show this spectacle of nature routinely at Ant III or better. I think Taylor is on to something here. Allow me to couch what I think he’s saying in the simplest possible terms:
With a modest gain in aperture, separating these stars is easier because there will always be moments in fair to average seeing conditions for the larger aperture to split them. It’s pretty much analogous to the idea that a larger aperture shows planetary details better even in fairly rough seeing because nature provides opportunities to let those details pop into view, if only for a few fleeting moments. Shimples!
To be continued……………………