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#11
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Observatory Mounts
Macro writes:
What observatory mounts could be used for a Dobsdonian that could be computer controlled? One solution to computer control a Dobsonian telescope: http://www.bbastrodesigns.com/BBAstroDesigns.html pej -- Per Erik Jorde |
#12
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Observatory Mounts
Jan Owen wrote:
Today's "Dobsonian" telescopes have EVOLVED into relatively short focal length instruments. But most of the originals were far from Rich Field, short focal ratio scopes... Even in large aperture Newtonians, f/8 was not an unusual focal ratio in the dim times... Hi, Jan, and given the general topic "Observatory Mounts," maybe it would be interesting, as your analysis suggests, to consider some of the trends quite apart from the question of defining a "Dobsonian." An interesting question is how far back the "dim times" go.. or up until how recently? Certainly I'd agree that large aperture Newtonians traditionally could be f/8 or quite slower. Some of William Herschel's Newts are the first examples that occur to me. Thus his famous 18.7" scope had a focal length of 20 feet (by which it was mainly known at that time), or about f/12.8, compare to a typical SCT nowadays. His 48" aperture scope, really pushing the envelope on aperture fever, had a 40-foot focal length, or f/10. But improvements in mirror making and mirror mounting, have resulted in evolving mount designs that have drifted toward shorter and shorter focal ratios to allow more and more aperture in smaller and smaller packages, without sacrificing structural rigidity or operational smoothness, without backlash, in both axes... In some respects, the newer scopes are better, but in some cases they are not. Yet, most of them ARE different, while sharing some common basics... This is what might called an interesting "synergy" in design: the practicality of faster optics, as you explain, which could make the Dobsonian type of alt-az mount indeed a way of getting "more and more aperture in smaller and smaller packages." Maybe it might be interesting to survey how f-ratios for Newts of different apertures evolved, say, over the period of 1960-1985 as reflected in telescope ads in the leading amateur magazines. Personally, I think we spend WAY too much time worrying about the minute details of what to call them and why. It's a design many folks have found to be very useful, and many of THOSE folks have found things they can do to make it work for THEM, even better. That's evolution... The fact that it's based on the mechanics of old cannons just further emphasizes the evolution of the design... Please let me admit that I have a certain weakness for semantic and specifically definitional minutiae grin, what are sometimes called "goat's wool matters" -- not inappropriate, now that Capricorn is so prominent in the night sky. Does this imply that s.a.a., or Usenet generally, is mostly woven out of mohair? However, if these discussions can lead to a broader and better understanding of design and how it evolves, they have enhanced value. Thanks for inviting me to consider this larger perspective. Not to worry... We'll likely still be calling scopes of this basic design Dobsonians long after most folks have ANY idea where the name came from... By the way, this also raises a question for me as to whether the term "Dobsonian" might now be associated, certainly with the kind of design we're discussing, but also a kind of "observational lifestyle" not necessarily specific to Newts. I might describe that lifestyle as mostly DSO observing with free manual pointing of the scope and starhopping as the strategy. For example, a rich-field refractor around f/6 on an alt-az mount might favor a similar lifestyle, albeit with more weight for the same amount of aperture. Most appreciatively, Margo Schulter Lat. 38.566 Long. -121.430 |
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