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  1. #1
    rmollise's Avatar
    rmollise Guest

    Default Telescope tube weight.

    On Mar 19, 7:42 am, "Chris.B" <> wrote:

    With today's relatively short f/l refractors, there is no advantage.
    Back in the day of f/15 + achomats, I reckon stiffness was a big help.

  2. #2's Avatar Guest

    Default Telescope tube weight.

    Hi: The reason tubes are not as straight as one might hope is
    normally the manufacturing process used, one can certainly buy tubes
    that are quite straight, DOM (Drawn over a mandrel) hydraulic tubing
    can be very straight in small diameters and long lengths. I have a
    barrel made from 1.5 inch OD DOM steel tubing that is 16 feet long,
    it's plenty straight. I would suspect that what you are seeing when
    the tube is lifted is purely elastic deformation and would not result
    in the actual plastic deformation unless the tube were very small in
    diameter. (Note that from this standpoint, the thinner the wall, the
    less likely it is to bend..)

    Another result of using thin wall tubing analysis is that the
    advantage of aluminum over steel from a mechanical standpoint is
    simply that the aluminum is less likely to locally buckle or "beer
    can." Since the stiffness of steel is approximately three times that
    of aluminum and it's density is also three times greater, that means
    that equal diameters tubes of equal weight have approximately the same
    bending stiffness. But since the local buckling is a higher order
    function of the wall thickness and since equal weight aluminum tube
    will have a wall three times as thick, it is much less likely to dent
    or beer can...

    Another advantage of aluminum is it's heat conduction...


    On Mar 19, 4:19 pm, "Chris.Bee" <> wrote:

  3. #3's Avatar Guest

    Default Telescope tube weight.

    On Mar 20, 2:24 am, "Chris.Bee" <> wrote:

    This is the way I look at it...

    I am reasonably sure there are aluminum tubes thre are sufficiently
    straight over a 6 foot section to avoid measureable vignetting but
    they may be rather thick walled. In general tubes can have a run out
    of fractions of millimeters. The bending deflections depends on the
    weight of the objective and the geometry of the tubing, on the tubing,
    diameter and thickness, a 6 or 7 inch aluminum tube is stiff...
    Obviously aluminum is a viable option...

    A good machinist would setup the tube in a lathe and when he or she
    were done, the ends would be centered and square.


  4. #4's Avatar Guest

    Default Telescope tube weight.

    On Mar 20, 2:00 pm, "Chris.Bee" <> wrote:

    Chris: I don't know where you live but around here my first stop
    would be Tube Service Co. They have a lot of stock and in the past
    I have gone to their place and selected my tubing.

    On the downside, I was talking to one of the machinists yesterday, he
    was not as optimistic as I about finding a straight piece...

    If I were you, I would head down to my local Tube Service Company and
    see what they had to offer.


  5. #5
    Brian Tung's Avatar
    Brian Tung Guest

    Default Telescope tube weight.

    Rod Mollise wrote:

    No question.

    Although, when I first read this, I read "f/l" (focal length) as "f/1."
    That is, "f-one." I was thinking, that's a bit more than "relatively"

    Brian Tung <>
    The Astronomy Corner moved to
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