Resolving Close Double Stars in a 12″ f/5 Dob during Cold Winter Weather.

Doudecim: the author’s 12″ f/5 Dob with a 8 x 50 achromatic finder astride.


Description of the Instrument:

Revelation Dobsonian, 305mm aperture with a focal length of 1525mm ( f/5)

Secondary mirror: 70mm minor axis, corresponding to a central obstruction of 22.9%

Primary and secondary mirror origin:  GSO

Primary mirror thickness(measured): 36mm (1.5″)

Mirror cell design: 9 point floatation system.

Both primary and secondary mirrors re-coated with enhanced HiLux coatings each with 97 per cent reflectivity.

Tube: Rollled aluminium, internally lined with a thin layer of cork and overlaid with standard flocking material.

Focuser: 2 speed Crayford style

Length of acclimation from room temperature to ambient: ~2 hours.

Fans used: none.

Finder: standard, straight-through  Revelation 8 x 50 model.

Method of collimation: collimation cap and Chesire eyepiece, verified with Hotech laser collimator.

Time required for precise collimation: 2 minutes.


The reflecting telescope reigns supreme in the 21st century as the telescope of choice for serious students of nearly all disciplines of amateur and professional astronomy. The reason is simple; in the case of Newtonians in particular,  they are, far easier to fabricate and mass produce than any other kind of telescope on the market, and are offered at sensible prices, allowing many people to fruitfully engage with the hobby. Today, an amateur can acquire a truly large telescope thanks to great advances in material science, where a thin layer of aluminium is vacuum deposited onto a carefully figured parabolic glass substrate, avoiding the problem of chromatic and spherical aberrations almost completely.

These days, it is not uncommon for an amateur to own extremely large aperture reflectors – in the 12 to 30 inch aperture class – at prices that don’t cost the earth. The development of the refractor, in contrast, has been severely retarded by the great difficulty and huge expense in producing optics of even moderate aperture. How many amateurs own an 8, 10 or 12 inch refractor? A little researching will soon reveal that very few of us would be willing to shell out the relatively enormous sums of money demanded by the few opticians willing to make them and even professional astronomers would laugh at the suggestion of replacing a large Newtonian or Cassegrain reflector to fund the installation of say a 10 or 12-inch ‘state-of-the art’ apochromat. As a case in point, this author vividly remembers an especially zealous refractor nut who openly called for the alumni of Universities to fund the installation of one such refractor. Not surprisingly, his clarion call fell on deaf ears.

Folk ain’t stupid.

While the Newtonian reflector has deservedly enjoyed a huge following from dedicated planetary and deep sky observers, their use in double star study has been artificially stunted by three decades of nefarious propaganda, instigated by individuals who clearly have little clue about how they behave when properly adjusted and acclimated. Like I said before, it’s easy to get a small refractor to ‘beat’ a large Newtonian if the latter is not adjusted correctly or has not been given enough time to cool down to ambient temperatures, but that hardly constitutes a fair test. Worse still, a subsection of the amateur community dismiss the Newtonian merely because they don’t look as ‘sexy’ as a long, slender refractor. Such individuals have reduced the hobby to something more akin to pornography than anything else. And you don’t have to look long to find it; check out the hysteria over this peashooter, for example. Has anyone not told them that a good 130mm f/5 or 6″ f/8 Newtonian would leave it in the dust, and for about half the price? Like moths to a lamp, they always seem to attract the same motley crew.

As a keen student of the history of astronomy, I have come across many cases where large Newtonian reflectors were used productively to pleasurably observe, or in some cases, even discover new double and multiple star systems. One need only look at the work of such luminaries as Sir William Herschel, Warren De la Rue, Sir William Lassell, the Reverend T.W Webb, William Denning and T.H.E.C. Espin, to name but a few, to see that the Newtonian reflector has been a very productive telescope in the divination of double stars, both for pleasure and for scientific gain. The archives of these historical figures reveal many extraordinary feats of resolution achieved with the Newtonian telescope.  And it is to these individuals that this author has turned to for guidance and inspiration.

Having left the world of small refractors behind, apart from my fabulous achromatic binos and my little ShortTube 80, I set out on an ambitious program of field testing Newtonian reflectors of ever increasing size, having gained intimate experience with the behaviour of the said telescopes in the 3, 5 and 8 inch range, firmly establishing that they are excellent double star instruments. What is more, this author has found through his own experience that a good 8 inch f/6 Newtonian reflector is a much better double star telescope than an optically excellent long focus classical refractor of 5 inch aperture and f/12 relative aperture.

These personal discoveries have led me to more closely investigate the performance of an even larger Newtonian reflector, a standard 12-inch f/5 Dobsonian, which has not enjoyed nearly as much field use than it should have since it was first acquired a few years ago. The limelight was firmly stolen by the smaller instruments, which are easier to deploy in the field, acclimate faster and are less sensitive to the vagaries of the atmosphere, but no more so than with a refractor, or indeed, any other kind of telescope, of the same size. That being said, I have enjoyed many wonderful nights where the traditional bugs that attend Newtonians were either minimal or non-existent, allowing me to obtain truly spectacular views of a suite of double stars traditionally considered ‘tricky’ by my astronomical peers.

In this capacity, this short blog will exemplify the techniques used and the results obtained in observing double stars with such a large, fast telescope.


What follows is a report of one night- January 8 2019 – between 22:00 and 23:10 UT

Ambient temperature: 0C at 22:00 UT falling to -1C by the end of the vigil at 23:10UT

Conditions: Slightly hazy, no wind, steady atmosphere (Antoniadi II), very cold.

The telescope was brought out from a warm, dry, indoor environment and left to passively cool for 2 hours before the commencement of observations. Though it is fully acknowledged that cooling fans can accelerate the acclimation process and help scrub away the boundary layer on the primary mirror, no such fans were used in keeping with the procedures of the aforementioned historical figures, none of whom had access to (or knowledge of) such fans.

Observer; suitably attired for cold weather observing; several layers of clothing; vest, jumper, overcoat, hand gloves, hat( me ole beanie). No dew shield used.

The following systems were examined in the 12″ f/5 Dob:

Theta Aurigae; a greatly unequal double, primary (A) , magnitude 2.6, secondary (B) magnitude 7.2. Separation 4″.

Iota Cassiopeiae: Triple system: AB- magnitudes 4.6 and 4.9 separated by 2.6″

AC: magnitudes 4.6 & 9, separation 7.1″

Eta Orionis: AB: magntidues 3.6 and 4.9; separation: 1.6″

Eta Geminorum (Propus): AB: magntidues 3.1 and 6, separation ~1.7″

52 Orionis: Classic Dawes pairing; AB equal 6th magnitude components, 1″ separation.

All systems observed either naturally or using a Baader single polarizing filter which cuts glare and increases contrast without imparting a colour shift to the stellar components.

Power employed: 277x throughout (Meade Series 5000 5.5mm UWA ocular).


The telescope resolved all 5 systems beautifully. The enormous resolving power and light gathering capability of the 30.5cm Newtonian made observing them especially thrilling. While there were many moments during the observations where the stellar images broke up or swelled slightly, there were also many opportunities where the images came together, producing text-book perfect results. As aperture increases, the size of the Airy disk shrinks, allowing the faint, close in companions to be more readily seen.

Iota Cassiopiae was quite simply stunning! All three stars appeared very bright and round as buttons in the 12 inch telescope at 277x. I have not personally experienced a better view of this triple system. Unequal pairs were always more challenging owing to the glare of the primary components, but still readily observed.

Propus(Eta Geminorum); is a particularly difficult system to crack, but on this evening the faint close-in companion presented better than I have ever seen it in my 8″ f/6 Newtonian at any power; the sheer resolving superiority of the 12 inch instrument clearly strutting its stuff.

A polarizing filter screwed into the bottom of the 1.25″ ocular significantly improved the aesthetic of the images by reducing glare and increasing contrast (read darker sky hinterland) very reminscent of a large refractor. Indeed, the filter made it significantly easier to prize the faint companion of Propus and Theta Aurigae in the same instrument.

The closest pairing observed on this evening – 52 Orionis – produced an image that was in a completely different league to any of the smaller Newtonians I have enjoyed testing. The components were much brighter, easier to see with lots of dark space between the components.

All in all; a very good short session under the starry heaven!


The Newtonian reflector has clearly come along way since it was first conceived of by Isaac Newton in the late 1600s. And while speculum metal allowed great gains to be made in both amateur and professional astronomy circles, it may surprise some readers just how far the reflector has been intelligently re-designed ever since those glory days of the past. Did you know, for example, that according to the studies of Dr. Wolfgang Steinicke, an expert on the telescopes at Birr Castle, that the 72 inch Leviathan of Parsonstown had a light gathering power equivalent to a modern, state-of-the-art 25 inch Newtonian reflector [see my book Chronicling the Golden Age of Astronomy,  as well as chapter 7 of William Parsons, 3rd Earl of Rosse: Astronomy and the Castle in Nineteenth-Century Ireland (Charles Mullan ed. 2016)]. Seen in this light, this author considers himself very fortunate indeed to have acquired such a great telescope as the 12 inch f/5 Dob for just a few hundred pounds( it was second hand).

The reader should note that these results are not at all exceptional. Indeed, the author has clocked up many similar nights where the full power of the 12 inch can be pressed into service. Neither is the author out to set any records; the closest pair thus far resolved has only been ~ 0.7″ but the same instrument is fully capable of resolving significantly closer pairs should he wish to pursue them. These results are wholly consistent with the historical archives of many of the early observers mentioned in the introduction, who managed to split extraordinarily close pairs with telescopes of broadly equivalent aperture. For example, T.W. Webb was able to resolve the components of Eta Coronae Borealis using his silver-on-glass 9.5 inch f/8 With-Berthon Newtonian, which had a separation of 0.55″ in the 1870s. Such results make it patently clear that such work can be acheived with larger Newtonians, with excellent results.

Focusing is very challenging in such a fast telescope as the f/5 Reveation Dob. The author very much appreciates the fine focus on the dual-speed Crayford that came with the ‘scope. At f/5 it is a very worthwhile tool in attaining the most precise focus possible.

Filters, such as the Baader single polarizer, are very useful in attaining the right balance between image brightness and contrast. The advantage of using the latter filter is that it does not impart a colour shift to the stellar images, allowing the observer to record the pure colours of the component stars. Using a blue filter will also help resolve the very tightest pairs, as the resolving power of the telescope scales inversely as the wavelength of light used to probe the system. When you have access to such a large telescope, it collects so much light that productive work with even deeply coloured(read low transmittance) filters becomes very worthwhile.

As always, the author would encourage others who have access to such a large Dobsonian to give such systems a try. When careful attention is givien to both collimation and acclimation, the sky’s really the limit!

Thanks for reading and keep looking up!


If you liked this blog and wish to support the author, please consider buying a copy of his new book: Chronicling the Golden Age of Astronomy, newly published by Springer-Nature(available in both hardback or electronic formats).


De Fideli.



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