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Newtonian Reflections


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Newtonian Reflections

 

Hi all.

 

My name is Malcom Bird and I have been in/out and around astronomy since I’ve been a teen. (couple of years at least). I have owned, enjoyed and maligned many telescopes over the years. I have definitely formed preferences during this time, but they seem to be driven more by changing lifestyles and physical capabilities than anything else. Most good telescopes are, well, good. But some are better if they’re not ‘good’, and that comes down to how complicated they are to begin with.

 

(note: Newtonian, Newt and Reflector are used interchangeably in this article)

 

The Newtonian reflector was what most of us either started with and/or grew up with. But somewhere along the way, it got replaced by sexier SCTs and refractors… and seems to have fallen out of favor, and perhaps for all the wrong reasons.

 

I suppose that before we consider the reflector’s virtues, we have to consider why it fell from grace.

 

  1. Well, first and foremost, I think it is regarded as ‘old tech’ – after all, the Newtonian design has been with us virtually unchanged since Newton’s day. In a world that is constantly running towards the newest shiny thing -  a Newt is an anachronism to many.

    I mean, back in the day, you could actually grind and polish your own mirror (and still can), make the necessary mechanicals and in essence, make your own precision telescope from scratch. Many of us considered it a rite of passage.  Not so with a refractor or SCT.
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  3. The market has been flooded with cheap reflectors, that while not necessarily bad optically, have been handicapped by poor mounts/accessories and hyped beyond their capabilities.
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  5. Newtonians are comparatively inexpensive but have many of the same components as much more expensive designs, so there is little incentive for growing optical companies to make Newts when they could be selling SCTs and refractors for many times more.
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  7. Newt OTA’s are bulky (but not necessarily heavy) for their aperture, and in this day of weighing and cubing shipments, freight becomes a critical component of the selling price for companies looking to corner market share. This has pushed manufacturers towards faster systems to keep the bulk down. I’m currently building an 8” F7.5 with a focal length of 60” and it’s all you can do to keep the focuser at eyelevel pointed at the zenith.  Any more, and you’d need steps or ladders… The tube is ~6 feet long and weighs about the same as an 8”F4.5 - but would cost disproportionately more to ship because of the length.
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  9. Lastly, collimation of reflectors has been discussed to death to the point where it is considered a black art that only practicing shamans can fully understand. In reality, collimation on any Newt F6 or above is not overly critical, and once done, rarely needs attention. But because of the aforementioned perceptions and market considerations, the market is flooded with fast F4.5-F5 systems that require much more critical collimation, which may be rarely done or misunderstood – to the instruments’ detriment.

 

As a result of the above, Manufacturers have been focused on Catadioptric (SCTs and Maks) or APO refractor designs to make them the next GOTO (pun intended) marketing darling – all at the expense of reflectors.

 

So does any of this mean that the Newtonian reflector is a bad design?

 

On the contrary, the basic Newtonian has many advantages inherent in its design.

 

  1. A Newtonian reflector only has two optical surfaces – the main mirror and the secondary mirror. Given the same attention to accuracy as the multiple elements of their competition, Newts have less light losses and are capable of excellent optical accuracy.
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  3. Coupled with the smaller secondary possible in longer focal ratio’s, the effects of the secondary obstruction all but disappear, and a longer focal ratio reflector will give any telescope design a serious run for its money. Again, the larger secondaries dictated by today’s faster systems handicap this optical performance.
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  5. The tube is open, so cool-down is faster.
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  7. The mirrors are located inside the tube, so dewing is greatly reduced if not eliminated.
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  9. It is apochromatic because it is a mirror only telescope.
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  11. The viewing position is more comfortable – particularly when pointing directly overhead (where the good stuff invariably is) When looking at elevations over 45*, the eyepiece on a Newt is at easy bend/stand height. I cringe every time I see a large refractor pointing impressively at high elevations and you realize that the eyepiece is only about 24” off the ground.  SCTs are better in this respect because of their short stubby tubes.
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  13. In longer focal ratio’s (F6 and higher) collimation is not critical despite websites being dedicated to nothing but..
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  15. Lastly, they provide the most bang for the aperture buck. People looking to get into the hobby can get an excellent 6-8” reflector on a good mount for peanuts compared to what they would pay for an SCT or refractor of similar capabilities.

 

Well – that all sounds great doesn’t it? – But nothing is without caveats.

 

The downside to reflectors are:

 

  1. Because they have a fixed focal length depending on the primary diameter and focal ratio – and if you don’t want to start using stepladders to access the focuser, you have to keep the focal length less than about 60”. This is easy enough to do in anything up to about an 8”.
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  3. 2. Apertures more than this require that you compromise on the focal ratio to keep the overall length of the optical tube within manageable dimensions, both from a shipping and ease-of-use perspective. A rule of thumb is that a Newtonian OTA will be the same overall length as the focal length. This is what has driven the short F4-F5 trend more than anything else.([edit] There has since been a resurgence of a pseudo Bird-Jones design which seeks to pair a fast spherical primary with a corrector to deliver the aforementioned results.)
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  5. Newtonians do have an optical obstruction in the form of the secondary mirror. However, by keeping the focal length above F6 – this can be held to less than 20% by diameter. As a comparison, your typical SCT has a secondary obstruction of 30% or more – naturally, refractors have no obstruction.

    So bigger mirrors require shorter focal lengths, which in turn requires larger secondary mirrors, which in turn increases the secondary obstruction. An inescapable consequence of this is that the effects of diffraction increase, and collimation becomes more critical - a double whammy.
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  7. The nature of equatorial mounts is that apart from the heft required to support the OTA weight, this has to be further increased due to the necessity of balancing the OTA on the opposite side of the Dec axis with heavy counterweights (about half of the OTA weight). This dramatically increases the total weight. So large reflectors on equatorial mounts quickly become non-portable (which is one reason why Dobsonians became so popular)

 

Check out any ads for an ‘old-school’ Cave or Edmund equatorially mounted reflectors to see what proportions these scopes can assume even for modest apertures. This naturally has an adverse effect on shipping costs and portability.

 

Back when we all got started, the magic ticket was a 6”F8 reflector which, given even middling attention to optical accuracy, could still provide great images due to the inherent advantages of the design. Mounted on a sturdy, basic equatorial mount, it gave most of us our first taste of the universe.

 

But somewhere along the way, we got seduced by sexier SCTs and refractors on computerized mounts (reflectors can be had on these mounts too).

 

And I think that’s a shame because the qualities of a good reflector haven’t changed since Newton’s day.

 


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