10" f/8 A planetary Newtonian?

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The need for such a tall pier, to raise a refractor mounting high above my intended platform, raised real doubts as to its practicality. The cost of a large achromat is closely matched by that of the high quality optical flats needed to fold the instrument. Folding makes real sense when dealing with a large refractor. How else is one to shelter the beast? Let alone provide any shelter from the wind and cold for the observer.

The giraffe may by tamed by folding but gains considerably more weight and girth. Folded refractors can be bulky and awkward in both use and appearance. The folded refractor also falls foul of thermal effects unknown to the pure, long tube refractor. Even baffling it against stray light can be a nightmare with some optical layouts. The difficulties involved suggested an alternative instrument be sought. One which would still offer high quality images of the planets and the moon.

Commerce and amateur astronomy folklore would have you believe that refractors leave all other telescope alternatives for dead on planetary detail. A Newtonian only provides crude and cheap aperture to gather the light for deep sky viewing.  Doesn't it? While the much more refined refractor is vitally necessary for the planets and Moon. Even better (allegedly) was an Apo (at even greater cost for even smaller apertures) for the study of exquisitely fine detail.

After all, "everybody" knows that Newtonians are crippled by the large secondary mirror slap bang in the middle of the light path to the primary mirror. The spider introduced diffraction effects which must surely destroy all chance of a sharp image of the planets. Newts can't even hold collimation for more than a few minutes at a time. All reflectors are total martyrs to thermal currents and cooling down issues. All reflectors suffer from the ill effects of needing those dreadful reflective coatings. With all the "well known" deleterious "glare" and "scattering" this must have a deadly effect on contrast and resolution. The Newtonian is a complete dinosaur. An instrumental throwback. Incapable of evolution or even subtle improvement. Or so "they" say.

I began searching online for possible alternatives to a large refractor. Hardly expecting anything fruitful to pop up. However, before very long I was deeply absorbed in an archived thread on Cloudy Nights forums: Cloudy Nights Shoot-out.. The thread discussed a comparison between some apparently humble Newtonians and the completely unaffordable toys of the comfortably rich, American amateur astronomer. These 'toys' were incredibly costly, large Apos (a 10"!) and very large aperture Dobsonians approaching 30"! All with premium optics by legendary opticians. Yet none of these mouthwatering instruments were able to match the remarkable planetary images provided by these carefully prepared, fully optimised, but far cheaper Newtonians.

These rather special Newtonians didn't look very special. They were of quite modest aperture but enjoyed very high quality, longer focal length mirrors. They used curved secondary spider supports, small secondary mirrors and forced ventilation from behind their plywood mirror cells.

Their tubes were made from lengths of that rather down market material called Sonotube. A disposable, cardboard former used for casting round concrete pillars and piles on building sites. The name Sonotube has become popularised amongst optical and audio hobbyists. Though are there are many other makers of these relatively cheap, spiral wound, paper tubes. Quality, thickness and even the different waxy/plastic coatings (or lack of) vary between manufacturers. None of them shout "quality" in the context of multi-thousand dollar telescopes!

The vital point about cardboard tubes is the lack of thermal capacity and quite reasonable insulation. In fact the cardboard is rather thermally (and acoustically) dead as far as telescope use goes. The draught from small fans, placed down at the bottom, pushes heat out of the mirror blank and on up the chimney formed by the cardboard tube. Clearing away all the usual heat waves clinging to both mirrors, the walls of the tube and the spider.

Moreover, the tubes of these very special Newtonians were deliberately oversized to place the telescope tube walls well outside the light path. The tubes were also made over-long to ensure a lack of stray light entering the field of view. Yet natural sky light, bright moonlight and street lights were cheerfully tolerated to ensure the observers' eyes did not dark adapt. The eye being much more sensitive to fine detail, colour nuances and subtle contrast when behaving normally. As they can only do in reasonable light. Rather than the coarse, grey, monochromatic view seen by the fully dark adapted eye.

The combination of all these simple "tricks" applied to these Newtonians produced planetary images which competed well against the finest instruments available to amateurs, at (almost literally) any price. Often beating those of twice the aperture.

Telescope mirrors are notoriously difficult to match thermally to the air in the tube with falling ambient temperatures. The thick mirror blank has considerable heat capacity and lags behind in heat loss to the air as temperatures fall. Most observing is done in the evening when temperatures are falling most quickly.

Serious amateur astronomers often put their instruments outside to cool down before they start observing. Despite this preparation, many mirrors never achieve thermal equilibrium in active use. Particularly those with full thickness mirror blanks. Not to mention closed off mirror cells when the telescope is brought out from a warm house, or vehicle, to a cool and cooling outdoor environment.

Boundary layers and rising heat currents exist with very low temperature differentials between the mirror and the surrounding air. These often rob a telescope of resolution, detail and contrast. Cooling mirrors often lose the near perfect figure placed there by highly skilled opticians. The telescope owner never enjoys the views which their large financial investment promised.

Could any amateur enjoy similar "giant killer" views with their own "humble" reflector? Well, the basic commercial Newtonian often comes in a thin, rather flexible and highly resonant metal tube. A tube which is often made rather small in diameter to avoid having a very large instrument to house indoors. So the standard tube handicaps the commercial telescope before it has even left the factory!

One can slide a flocked and insulated sleeve inside the original metal tube but it is unlikely to help much. Insulation has little effect where the tube is radiating to the night sky. The metal becomes supercooled relative to the air. Insulation further reduces the clearance between the light path and the cold metal. It is the differential temperature between air, metal and glass which wrecks the view. The metal tube amplifies these differences.

Ideally, you want an inch or more clearance between mirror diameter and the inside wall of the tube. Even more clearance with mirrors over 10" in diameter.

The major problem here is that few amateurs would really want a larger, spiral wound paper tube for cosmetic reasons. It doesn't look remotely posh or "techy" enough to show the large expenditure invested in their instrument. It simply does not impress (anybody) like a high gloss metal tube. Not unless a great deal of time is wasted on making the cheap, cardboard tube look prettier. Usually involving endless filling, laminating with glass fibre mat or cloth, more filling and spray painting.

All completely unnecessary tasks in improving telescope performance. Not unless the changes add extra stiffness to the finished tube. Wrapping the cardboard tube in a door skin veneer will be far more beneficial and will greatly improve the appearance. Thin, birch, aircraft ply would be an alternative but require great skill to hide the joints between the usually small sheets. Though some plywood stockists can offer much larger sheets, far more cheaply, than those usually found (at a very high price) in most model shops.

A cooling fan fixed directly to the mirror cell might well introduce vibration and this is usually exacerbated by the thin metal tube. Many thin metal tubes flex where they are fixed to the mounting. Large mounting rings will undo a lot of flexure associated with mounting small dovetails.

Those all-conquering Newtonians of the CN thread sometimes used a heavy plywood cell in the cardboard tube. Which would be naturally dead to such vibrations despite having holes to allow the fan to push air at the back of the mirror. Sometimes they used a very exposed mirror, in its skeleton cell, cooled by a large fan just sitting on the ground underneath the telescope.

Which raises another important point. The surface on which the telescope rests in normal use. Concrete and tarmac absorb the sun's heat and stores it in vast quantities. Releasing it later, so that it produces heat currents for hours after dusk. The same is true of roofs of course. Many surfaces and roofs are far too hot to touch during the day. They are usually quite massive, with a huge capacity for absorbing and releasing heat. Observing from such pre-warmed sites can be like looking over a garden bonfire. The built up heat rises, causing tremors and waves easily visible in the telescope eyepiece.

A lawn or a field is far more forgiving and can provide a  much more stable and suitable base for the instrument. The heating problem and temperature lag also exists with concrete bases and supporting walls to observatories. Not to mention the heat waves emanating from human beings standing near truss telescope tubes. Or the warmth of the observer being carried across the top of the tube by a breeze. Why handicap yourself and your instrument without a second thought? A timber building hardly absorbs any heat. While metal and roofing felt will soak up heat like a sponge! Foliage will help to shelter walls subject to direct solar heating. Though it can take a long time to establish a dense enough covering to completely exclude the burning heat of the sun.

Then there is the problem of the popular Dobsonian mounting commonly used for Newtonians today. The altazimuth Dobsonian mounting is either standing still. With the object sailing rapidly across the field of view. Or wobbling jerkily to follow an object. Neither allows the observer to relax and stare continuously at the object centred in the sweet spot of the eyepiece. Particularly at very high powers while waiting for those all too rare rare moments of exceptional seeing. So a really steady, driven mounting, or driven equatorial platform, is also essential to the careful study of fine planetary or lunar detail. Can you use powers of 400x-500x on Saturn, Mars or the Moon with your present set-up? It seems that the humble Newtonians in the linked CN thread could.

A longer focal length mirror is useful for reasons which may not even occur to those who choose faster optics. The slower optics have a far greater depth of field. Requiring a much more relaxed focus setting immune to change or endless fiddling with slow motion focusers. Atmospheric thermal effects, which defocus the image, are ignored by the longer focus telescope.

The longer instrument also suffers from very little visible coma and astigmatism compared with a faster instrument. Coma correctors introduce more glass reducing contrast and detail. Slower mirrors are far easier to figure to a very high level of accuracy compared with faster optics. Ask any optician. Slower mirrors favour a huge range of inexpensive eyepieces. Offering lots of eye clearance and high powers from simple, relatively low powered eyepieces. Less glass is more detail and contrast. On axis is where the detail lies and where complex eyepieces offer no advantage provided the telescope is driven to follow the stars.

What is more, a carefully optimised and thoughtfully situated reflector can often dwarf the aperture of most Apochromatic refracting telescopes. Or even many high quality refractors costing many times as much as a "humble" Newtonian. Aperture provides greater resolution and light gathering power. Smaller apertures soon run out of useful magnification.

An 8" or 10" long focus Newtonian, with careful attention to all of the above, can often maximise the seeing under poorer conditions. Atmospheric convection cells have specific average dimensions. Too large an aperture will be hampered in all but the best seeing conditions. So the 8-10" long focus instrument offers even more bangs for your bucks/dead squid. Provided, of course, that you carefully optimise your Newtonian for observing. Rather than lavishing your telescope funds only on its decorative appearance. Or on expensive fixes for the instrument's own, designed-in faults.

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