Optimising a 8-inch Newtonian for Visual Use

Based on an article which originally appeared in the peer-reviewed  Astronomy Now magazine (August 2015).

Telescopes 101

200POver the last decade or so, amateur astronomers have become increasingly obsessed with acquiring very expensive apochromatic refractors that offer near optically perfect views under good conditions, but are limited by their restrictive aperture. I have heard people claim that a 4- or 5-inch Apo refractor gives ‘better ‘images than an 8-inch reflector on planets for example. This is patently nonsense, as the 8 inch Newtonian has twice the aperture and much more light gathering power than any 4 inch refractor, no matter what its pedigree. So what is going on here? As I said elsewhere, this is a pleasant fiction.  The view through the smaller ‘scope might look nice from moment to moment but that’s only because it can’t resolve finer detail that the larger telescope can, but at the expense of being more sensitive to the vagaries of the atmosphere.  In other words, the smaller ‘scope conceals far more than it displays; its beauty merely skin deep. Rest assured though, when the Newtonian is working optimally it will not only yield ‘prettier’ images than the refractor but they will be a whole lot more detailed too. That’s just physics.

After many years of testing telescopes of every conceivable size and genre, I have come to the conclusion that a good 8-inch F/6 Newtonian provides the biggest bang for buck in today’s market. It offers decent aperture for both planetary work and deep sky observing, with a generous 2.25 degree field. It is portable and acclimates quickly, often without the need for cooling fans. With a focal ratio of f/6 it works quite well with even budget wide-angle eyepieces and is capable of being accurately collimated during daylight hours. One of the best examples comes from the SkyWatcher Skyliner range of Dobsonians, which can be purchased as an entire package for less than £300, including delivery to your door. Having purchased this telescope, I wanted to demonstrate ways in which a very good instrument can be further improved to give the best possible images – improvements that do not incur a large outlay of additional funds.

Tube modifications
Refractors tend to have very well baffled tubes that stop stray light from flooding into the optical train especially in comparison to economically priced Newtonians. But through some simple measures, you can help control this stray light reaching the eyepiece. One of the most important things that needs to be done is to flock the region of the tube immediately opposite the telescope focuser. Many astronomy retailers sell rolls of flocking material costing just a few pounds. I simply cut off a piece of this material measuring 6 x 8 inches and stuck it onto the inside of the tube immediately opposite the focuser. In addition, the drawtube of the focuser was similarly flocked.

Flocking the tube opposite the focuser is a good move.

Flocking the tube opposite the focuser is a good move.

Mirror Modifications
The secondary mirror in the Newtonian is usually elliptical in shape and is orientated such that its minor axis minimizes the size of the central obstruction that is all too important in producing images rich in contrast. The mirror supplied with the Skywatcher Skyliner 200P has a minor axis diameter of 50mm, thus providing a central obstruction of 25% by aperture. And while this is perfectly acceptable for all round use, a number of alterations can be made to the secondary to improve the telescope’s overall performance.

A new 44mm flat with edges blackened with matt black paint.

A new 44mm flat with edges blackened with matt black paint.

 

 

 

 

 

 

 

 

 

 I contacted Orion Optics UK, based at Newcastle Under Lyme, Staffordshire, who have a long-standing expertise in delivering quality Newtonian and Maksutov Cassegrain optics to discriminating observers. In particular, they have developed their highly regarded Hilux enhanced coatings with 97 per cent reflectivity and were also able to make to order any secondary size I wanted. At first, I had intended to get the existing SkyWatcher secondary and primary Hilux coated and to purchase an additional secondary with a 36mm diameter for high resolution work. But in the end I decided to settle on a single flat with a diameter of 44mm, thus providing a very modest 22 per cent central obstruction. This size of flat also means that I can employ wide-angle two inch eyepieces without imparting too much in the way of vignetting at the edge of the field. Finally, before mounting the new secondary, I blackened its edges with matt black blackboard paint.

Inserting a cooling fan to blow cool air over the surface area of the primary mirror would help to accelerate the telescope’s acclimation but, truth be told, I haven’t found the need for one. The telescope will acclimate in about 40 minutes if taken from a warm indoor room to the cool of the night air. What’s more, if the instrument is left in a dry, unheated shed, it will be in a permanent, ‘grab ‘n’ go state.

Performance in the field
The telescope was mounted atop an inexpensive water butt with the mushroom knobs on the base of the lazy Suzan mount slotted directly into two pre-drilled holes of the butt. Such a measure raised the telescope to a decent height off the ground and kept the base free from dirt and grime. The modified 8 inch Newtonian has the same contrast transfer as 6-inch refractor (200-44mm) and, owing to its ultra-high reflectivity coatings considerably greater light gathering power. All in, the telescope and its modifications came to less than £550. How does it perform? In a word; splendidly! But to elaborate, I’ve enjoyed some of my very best views of Jupiter with this telescope. Indeed, they are every bit as good as a 6 inch apochromatic refractor costing five to ten times more! And contrary to popular belief, a 8” f/6 Newtonian is no slouch on double stars. You just have to look at the superlative work done by astronomical artist, Jeremey Perez, who uses a similar telescope to see why. During a spell of good, clear weather I was able to cleanly resolve the tricky pairs , Iota Leonis, Mu Cygni and Eta Geminorum – systems that are more challenging with an excellent 12.7 cm  f/12 refractor and 17cm f/16 Maksutov. Subsequent work has shown that the same telescope can resolve sub arcsecond pairs, again, within the remit of its aperture.

Deep sky objects really come alive in an 8-inch telescope. I have enjoyed some beautifully crisp views of the Double Cluster (Caldwell 14) and its star-studded hinterland at 30x. Spring galaxies like M81, M82 in Ursa Major and M51 in Canes Venatici are very well presented and a joy to study at medium and high powers. All in all, this was an enjoyable and worthwhile project to undertake and has transformed a good telescope into a great one!

Octavius: instrument of change.

Octavius: on solid ground.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For further details see my related articles here and here.

Two more threads that you might find interesting can be viewed here and here.

In this thread, the poster uses a 12.5″ f/6.5 Newtonian to resolve pairs down to 0.5- or 0.6″, as well as posting actual images of other pairs here.

Post scriptum: The Premo-Dob manufacturer Teeter’s Telescopes are now using GSO mirrors in their Dobsonian line. As Rob Teeter openly acknowledges, the optical quality of these mirrors is generally excellent. These are the same quality mirrors that went into the telescope highlighted above. So, like I said elsewhere, I wouldn’t trade my 8-inch Newtonian for any 6-inch refractor on Earth! Why would I?

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

De Fideli

8 thoughts on “Optimising a 8-inch Newtonian for Visual Use

  1. Your astronomical journey is coming full circle! I had an 8-inch f6 for many years and made all your modifications – and more – to it, including upgrading the focuser and adding a curved secondary support. This will “virtually” eliminate the undesirable diffraction spikes… or not. Actually the diffused light around the target object is still diffused, but it doesn’t present to the eye visually as spikes, which can be a distraction, especially on planetary observing. It instead shows as a very faint soft glow, almost indiscernible, which leaves planetary detail unaffected.

    Unfortunately for me, an 8-inch Dob does not fit into a 2-seater sporty car very well…;(

  2. Hello Antony,

    Thanks for your message.

    I like the diffraction spikes and see no need to use a curved secondary support. For planetary work, they have no effect on the underlying image. Experienced folk learn to ignore them.

    Re: the vehicle: no one can see your sporty two seater in the dark; better to use a no-frills, less expensive, four-seater vehicle in this case eh?

    Cheers,

    Neil.

  3. As much as I like my 6″ Mak, there is truly some poetic justice in the performance of a Newt!

  4. Hello again after a while.
    I think what you have done is just right. With such a versatile newtonian with good mirror a slightly smaller and brighter secondary along with the slightly darker sky background better stray light control will give there’s no other design that’s so quick and easy to set up. The optics have quick cooldown too.

    There would have been one other step down this route for the ultimate in planetary views however, which would have been to get orion optics to make you an 8″ f8 dobsonian with 44mm secondary, all hilux coated and with their highest mirror tolerance spec.
    Myself, I still have my 8″ f6 intes deluxe mak-newt (35mm secondary) which gives views totally indistinguishable to a 6″ or so apo refractor on stars whilst being able to push up the power on planets a lot more than with my 5″ apo!

    My 9″ f8 achro refractor project is nearly finished -lens is in tube securely fixed…
    Now all I need is a dark sky.

    • Hello Alex,

      Thanks for your comments. I agree entirely with you. It would be great to have a 8″ f/8 or install a smaller secondary to go below 20%. I could indeed employ a mirror with a higher spec etc. And all these modifications could be done at reasonable cost. I completely understand why some folk might want to do one or more of these things; whatever makes you happy.

      In my own case, I have not seen the need to improve the telescope any further. It already has a very good mirror, gives excellent views of every kind of astronomical target and I am entirely satisfied with it.

      I suppose you could say that I’m tired of chasing the wind. I just want to enjoy what I have.

      Your 9″ achromat project sounds very interesting. I wish you every success with it.

      With best wishes,

      Neil.

  5. Thank you, Neil.
    Seems like it has a very good mirror indeed. Even so, I myself am unlikely to tire of chasing the wind as long as there’s wind in me. Even if I am chasing my tail so to speak most of the time.

  6. Hello Alex,

    A few internet rogues, out of nothing more than sheer spite for my work, have claimed that Newtonians are not good for double stars. I have claimed otherwise and provided as much evidence as I could humanly do.

    Thank goodness other observers have taken up the gauntlet and provided further evidence that bolsters my claims in this regard:

    See the results of this man’s efforts:

    http://www.cloudynights.com/topic/518744-some-images-with-the-125-lockwoodteeter/#entry6916109

    I have split some of these myself with my 8-inch Dob and many tighter pairs besides.

    The same gentleman has reported visual observations showing the 12.5 inch instrument could resolve pairs as tight as 0.5-0.6″

    See this link for details:

    http://www.cloudynights.com/topic/517640-125-f65-teeter-dob-with-lockwood-glass/

    I sent him a PM to personally thank him for making these reports and to encourage him to keep going.

    Regards,

    Neil.

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