Structure a Dobson

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  • #10513
    Stefanosky
    Stefanosky
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    I am approaching the stage where, completed the mirror, I'll have to design a structure able to welcome him to the best.
    I will open a specific discussion on the choice that I will, where will document the whole process from design to actual construction.
    By nature I am more inclined to believe this is the way that technological solutions and will be percorrò however I'd like to know your opinion about the various types of structure and materials used. Wood, metal, light type, heavy, with carbon truss, aluminum, etc
    Specifically, considering the MY project, What are the pros and cons of various ways?

    #10516

    Giulio TiberinI
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    Hello Stephen.
    On the structure I tell you my thoughts and attention to ask, the whole in a very general.
    I would say first that since your telescope will be a F6, so it will be quite "long", This requires attention to tradeoffs to choose, starting from the type of telescope, that is, if the so-called normal or light dobson.

    it is'’ clear that the normal classic lattice type (I'm talking like my 360f5 and the American Obsession) remarkable ease of use has advantages, maintenance of collimation and ease of balancing, relying on the greater distance between the bottom of the primary case design and the center of rotation lateral Crescent bearing, that are smaller than those of a light type, until they disappear by becoming a simple circle, in telescopes not trade trellis, that are constructively similar to original dobsonian telescope created by John Dobson.

    Greater distance than in the classic but has the disadvantage that dobson the same case and the rest of the ground part of dosbon, are larger than those that would be in the case of the construction of light type…
    But on the flip side of this coin is that vice versa in a lightweight dobson, to get close to balancing grade’ equivalence of the classic, You must specify the radius of the two Crescent side bearings, and attach them to the primary case centered, but moved to wire cash back same, in order to ensure that when the angle of the telescope, as the virtual telescope tube exits the verticality and grows the cosine of the angle of inclination, There is an automated "load" counterweight that grows with the growth of the lever arm , which in this case is equal to the projection on the ground of distance between Center of curvature and the bottom of the primary case.
    The disadvantage is then having to build such efficient and large half moons (that would be as seen in the discussion at the following link) so that they can be folded to reduce transport problems created by them, in a telescope lightened just to facilitate its handling:
    http://dobsoniani.forumfree.it/?t=69692325&st=75#entry589150091

    WOOD OR ALUMINUM
    The type of material with which to build the telescope is subjective. I prefer the plywood of poplar, being still a lightweight composite material has the advantage of being a heat insulator. And this in the use of the telescope has the considerable advantage of avoiding the rapid conduct of room temperature mirrors subject to fogging. it is'’ the fact remains that these are avoidable with thermal bands, but I think their use is an absurd contradiction astronomical forces you to heat up what you need to store at room temperature to avoid turbulent thermal currents inside the telescope, It also involves the expansion of the hardware to carry around, with the use of a power supply.
    (But that my opinion comes from my idea that technical solutions the most beautiful, are those that maximize the combination of simplicity and efficiency).

    As for lightness, however, aluminum (specific gravity 2,6) It would be on a par with plywood (specific gravity 0,45) If you think the thickness of use which makes them equivalent amounts to 2.6/0.45 = 5,7; that is 1 mm thick aluminium is equivalent to 5, 7 mm thick plywood…In other words, the structure of a 2 mm thick aluminium telescope, has the same weight as that achieved in 12 mm plywood, which is definitely less flexible and less dancer with thin aluminium.
    In other Word: Since the structure of dobson must be sturdy and rigid, It's easier to get that great result (wanting to, pure high-tech), building in wood rather than aluminium, and painting with waterproof and wearproof poliretanica par excellence, available in DIY stores for boats.

    MIRROR CELL
    Possibly avoiding that "belt" classic made so: http://www.webstertelescopes.com/18_cell.jpg in favor of a much better supporting the mirror with a triangle structure made so:
    https://www.bresser.de/out/pictures/generated/product/7/460_460_70/4e92ce651b9a2a4deab59da15f893ecf_0116940_d5_0817.jpg , which allows attaching mirror backing triangles and their eventual barbells, and also the convenient collimation made both in a traditional way on the back side of the mirror, that from front to rear, While maybe the eyepiece (and using the Allen key inserted into a tube long enough).

    Also the mirror stops because you don't come out of his cell, in this cell type are in solidarity with the movement of the mirror collimation, and not attached to a separate facility and fixed (as with the classical cell with the support belt mirror) at the risk of being loose or too much in contact, Depending on the variable primary collimation movement.

    THE TRELLIS
    I expressed the following questions in detail in the article:

    The truss tube for Dobsonian telescope… and surroundings


    The building in my opinion better is one that combines the tubes "compass point" two by two, for example by means of a piece of angular momentum, that with three or four pairs of tubes United States, provides a trellis which has a secondary case support, that's perfectly perpendicular to the optical axis, that case relies simply and comfortably to the mating is screwed angular tubes.
    This fact of Union “in higer place” couple tubes, lets not need down (that is the primary case), trellis Panel clamps that have the drive to facilitate the subsequent fastening secondary case, But enough 4 simple bolts outgoing from it.

    The total advantage, however small or short focal length telescopes, When the frame in one piece opening and lockable "accordion" , as I did in my 250F5 tool kit.

    SECONDARY SUPPORT
    Can be easily obtained for turning from black nylon round cheapish, always on the ground to avoid the cold reach the mirror from "spider" support, which is normally aluminium.
    The "joint" secondary "tilting adjustable stand that allows the, operable with the usual three collimation screws being, It can also be just a nylon threaded rod M6, It is flexible and robust, without presenting problems of "brittle failure" even at low ambient temperatures. Nevertheless, I should I put the classic lanyard anti-fall of the secondary, glued behind the mirror with a scoop of silicone and linked to spider vane.
    another (usually) constructive advice to avoid condensation at the secondary, you paste it with three balls of silicone to its support, mettendo 3 fiammiferi come distanziale (da togliere a incollaggio avvenuto), in modo che fra lo specchio e il supporto ci sia uno spessore di 2mm si silicone.
    Tutto ciò è visibile in una foto presente nella galleria di immagini in fondo a questo articolo:

    Dobson “light” 300F6 – Phase 2: Transformation into “Low-Riding”

    Queste elucubrazioni rappresentano il mio pensiero che si è evoluto nella sequenza di realizzazione dei miei 4 dobson. Il mio pensiero non ha quindi nessuna pretesa di perfezione, ma rappresenta solo le scelte che ho fatto, ligio al mio pensierosemplice e funzionale = bello”, nell’intento di migliorare quel che strada facendo mi pareva migliorabile con una soluzione più semplice, e più (o quantomeno altrettanto) efficiente.

    Hello- Julius

    #10518
    Stefanosky
    Stefanosky
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    Grandissimo Giulio, molto esauriente come sempre!

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