A work begun September 18 2020.
As I have explained in previous blogs, I am a Newtonian convert, after spending more than a decade promoting small aperture and way over priced refractors. It was in January 2015 that I finally set out on a journey of transformation that gradually convinced me that for serious amateur astronomy, where high resolution targets were concerned, Newtonian reflectors offered much greater bang for buck. Indeed, a humble SkyWatcher 8″ f/6 Dobsonian costing less than £300 completely out-performed state of the art refractors costing £1500 and upwards I had used in the 5 and 6-inch aperture range. After I had convinced myself of the truth of this revelation, I began to communicate my ideas in a series of observation reports, much to the chagrin of the “refractor nuts” who I believed(and still believe) had deluded themselves for years and decades. Furthermore, I stated that one of the principal reasons for the popularity of refractors in the amateur community pertained to their lack of maintenance, as well as their rapid acclimation owing to their small apertures. Furthermore, I attributed the decline of the Newtonian reflector, at least in part, to an unwillingness of amateurs to learn how to properly collimate and acclimate their telescopes. Blinded by the instant gratification of small, unobstructed apertures, they foolishly forsake the feral but oh so sweet charms of a well-tuned Newtonian. Had they learned how to adequately set up their Newts, they would not have joined the rat race, as I once did, to buy ever larger and unwieldy instruments that unnecessarily drain amateurs of resources. To my mind, as an observer who chooses horses for courses, refractors are beginner telescopes that really lack the aperture needed to see the creation in all its detail…. warts an’ all.
Over a period of time, I embarked on a number of projects that first improved the performance of the 8-inch(Octavius) and my smaller ‘grab ‘n’ go’ instrument; a 5.1 inch(a.k.a. Plotina) f/5 reflector, which included buying in higher quality optical flats and treating the mirrors to the highest quality coatings money could buy(but still very economical in the scheme of things) as well as learning the art of precise collimation and acclimation. I also studied the problem of tube currents and how insulating the tubes greatly reduced these problems. These telescopes gave me a great deal of pleasure in pursuing the entire panoply of astronomical targets, and in my specialist area of double star observing, their fine, sharp and colour-pure images were nothing short of revelatory! And once I began exploring the long and rich history of the Newtonian reflector in the hands of highly skilled observers, I discovered that my sentiments toward these wonderful telescopes were shared by many of them. You can explore a lot of these stories in my large historical work, Chronicling the Golden Age of Astronomy. Failing that, take a long, hard look at the hundreds (thousands?) of testimonies about Newtonian reflectors in this ongoing blog.
In the summer of 2017, I added an even larger Newtonian telescope to my arsenal, a Revelation-branded 12″ f/5 Newtonian reflector (Duodecim). The instrument was outfitted with a GSO primary and secondary mirror. My star testing of the instrument showed that the optics were very good indeed, especially when one considers the very modest price I paid for it second hand( ~£400 as I recall) and I enjoyed many evenings of double star and deep sky observing with it. I did not elect to upgrade the 70mm secondary of this telescope unlike my smaller instruments, but only to treat both mirrors to the same state-of-the art coatings I had also applied to my other reflectors.
Despite owning this large 12 inch instrument for over three years now, I have never subjected it to serious testing on planets. This was not due to any lack of enthusiasm on my part, but simply reflected the fact that visually interesting worlds like Jupiter, Saturn and Mars were not favourably placed at my observing location to warrant any serious study of them. They were simply too low in my local skies and obstructed by trees to give the telescope a fair chance of showing off its powers. But all that changed in the autumn of 2020, when the planet Mars presented itself as a bright, morning object, rising to very decent altitudes in my sky to finally enable me to assess its performance in this regard.
So, in this blog, I wish to offer my opinions on how well it performs on the Red Planet in comparison to my smaller, but optically excellent, 8-inch reflector. The results were a long time in coming, that’s for sure, but I now have reached a very clear and unambiguous conclusion. To find out the details, read on.
Beginning my observations at the end of the first week in September, by God’s grace I was treated to a long spell of settled weather, which still persists to this day (September 18). I usually began my observations starting at around 23:45 UT and ended them about an hour later at 00:45 UT(add an hour for BST) by which time the planet had attained a very decent height above my south-eastern horizon but still someway from its highest altitude when it culminates in the south.
Both instruments are mounted on simple, non-motorised, lazy-Suzan Dob mounts and were fully acclimated and precisely collimated prior to making any planetary observation. The reader will also note that I do not employ any active cooling(electric fans) on either instrument, in keeping with my desire to preserve my style of observing, which is ostensibly low-tech, and in keeping with the methods employed by my astronomical forebears.
Optimal Magnifications Employed
I employed a good but very simple Orion 10mm Sirius Plossl eyepiece coupled to Barlow lenses on both instruments. The Plossl is a superb planetary ocular, owing to its technological refinement over more than a century, its small number of glass elements, and though eye relief is tight, it is considerably improved by adding a Barlow lens, which makes the viewing more comfortable and immersive!. In my testing, conducted over several nights, I gravitated toward an optimal power of 192x for the 8″ f/6 instrument and 244x on the 12″ f/5 instrument. These powers were obtained by coupling the 10mm Plossl to a 1.6x Barlow in both instruments. The reader will note however that by employing a 2x Barlow with the same eyepiece, I was also able to get very satisfactory results with the higher powers it delivered(240x and 310x for the 8- and 12-inch, respectively). A common mistake made by novice observers is to try to coax very high powers on planets to obtain a greater disk enlargement but I have found by experimentation that finer details are often gleaned by backing down the power a bit so that image sharpness is optimised over apparent disk size.
As well as observing the planets as presented by the Plossl and Barlow combination, I also studied the contrast enhancing effects of several colour filters, which included simple, inexpensive Wrattens, but also a number of interference-based filters marketed by Baader Planetarium and Tele Vue. In the next section, I will outline the results I obtained.
Results: The telescopes were set up next to each other and experienced nights of good seeing (Antoniadi II or less) as well as average seeing( Ant III) during the wee hours of the morning. The image in the 8-inch was very bright, but the 12-inch presented intensely bright images with its attendant diffraction spikes. That said, after a few minutes, one’s eye adjusts and more details pop out of the image. Both telescopes showed impressive levels of detail; a small south polar ice cap, a more extensive northern ice cap, very distinctly resolved darker areas and limb mist. At a glance, the 12-inch reflector showed more detail regardless of whether the conditions were above average or just average. A feature merely hinted at in the 8-inch was unambiguously discerned in the larger, 12-inch instrument. The 8 inch reflector showed less atmospheric turbulence than the 12-inch but the increase in turbulence was less than I had anticipated. I concluded that the 12-inch could be used productively as a powerful planetary telescope which came as a great relief to me.
A variety of filters were employed to assess their contrast enhancing effects. An orange Wratten #21 proved especially good for bringing out surface details and proved equally good on both instruments. A Baader green long pass filter also proved very effective, especially in the 12-inch, showing up surface details complementary to green. The Tele Vue Bandmate Planetary filter was also excellent in both telescopes. For enhancing atmospheric phenomena, a blue Wratten # 38 A really enhanced morning limb mist. For a minimalist effect though, I tried the Baader single polarising filter, which did an excellent job increasing contrast and reducing telescopic glare without imparting any colour shift. I intend to use the single polariser in routine work on this planet as it approaches its mid-October opposition and beyond.
To get an idea of the kind of detail I could discern through the 12-inch, have a look at some sketches shown here and here, made by experienced UK-based observers employing 12-inch reflecting telescopes during the current Martian apparition.
Conclusions: Some UK-based observers in the modern era have claimed that a 12-inch is too large to use productively as a planetary instrument but I must respectfully disagree with that conclusion. Under the conditions in which I routinely observe the 12-inch proved the superior instrument. So aperture wins, though the 8-inch reflector is much easier to use because it has very smooth motions in both azimuth and altitude axes. My 12-inch Dob base moves far less smoothly but the results convinced me that I should improve its azimuth bearings or acquire a better quality base for the telescope. Such a modification will go some way to increasing both the efficacy and enjoyability of this large instrument. Suffice it to say that I am very much looking forward to observing Jupiter and Saturn at higher altitudes with my 12-inch over the coming years, God willing.
Neil English is the author of several books in amateur and professional astronomy. He is currently writing a book on how to improve the performance of budget Newtonian reflectors of various sizes, which is due out in 2021. Thanks for reading.