Structuring a Dobson

[Giulio TiberinI]

Hi Stefano.
I am also on holiday without computers. Looking at the CAD design are parties can immediately find out if I was wrong to attach a dimension line.
I think it is easier that the error is mine, crafts, compared to the professional response of Atmos.

[Stefanosky]

Hi everyone, I started the journey again! I'm designing the structure, once completed I will return to take care of the mirror. In the end I opted for a traditional structure, non light, that I will make in birch wood.
At the moment I have designed cell a 18 points and the relative steel structure that will be positioned inside the box. (I put a printed sheet on the fly with two handwritten notes just to make people understand)

I have practically all the measures I need, sizing of the secondary, of his cage, indicative length of pipes etc..
The question I ask myself is this, not knowing in advance the weights of the various components and their distribution along the axis, how I calculate the height of the box to stay within a range that allows me not to have any nasty balancing surprises in creating adequate crescents?
I followed documents that explain how to have an indicative balance a priori but they assume all the weights involved are known, of all the materials used etc.. There is a more approximate method for dimensioning the height of the box?

[Giulio TiberinI]

Hi Stefano.
Happy Easter!!….I am happy to reread!

Since yours is a 300F6 while my biggest construction is the same as why the 300F6 has a 1800mm focal length equal to that of my 360×5, I can as an alternative, just give the measures of my calculations to see if you'll make the height from the ground and are acceptable dell'equlibratura.

Unfortunately, the mechanical moment resulting for each component element of the balancing lever dobson is very variable depending on the focal length, that is the one that determines the length of the arm of the weight forces; and the weight itself of the constructive elements, which it depends on the type of material and its thickness chosen or available.
So for the equilibrium calculation I do not know a different method from identifying a hypothetical fulcrum, do the calculation according to what, and then adjust the backward calculations by finding the equilibrium point that feels good.

Beware, however, that the calculations should also include the focuser; il cercatore ottico e quello Telrad o meglio il leggero quickfinder, the cloth….(But no more scrambling too because the correction can be done with the counterweights).

Since the relationship 360mm diameter mirror of my 360F5 (calculated in proportion to the book table Kriege) mi dava 380 depth of the primary case (and because of my experience of installation and testing, into the structure of my 360F5, 300F6 of my mirror as your, first to build him the kind of light structure, without focusing problems), I would try starting with calculations assuming a depth case the primary by just under my…. that is, for example, 350 o 360mm, doing account that the fulcrum of rotation falls to the centerline of the upper part of the case.

All this if you have not built the case of the secondary…(because it would go wide as the primary case for convenience of a straight truss. And a large 350mm case would make you a bit lower and perhaps more comfortable ground clearance eyepiece).

also calculates that for example in my 360, the distance from the ground of the primary case is 130mm…(then the fulcrum of my crescents diameter 520, I know also located at 520mm from the ground).

On the basis of the depth that you choose, You should try to calculate and sum the products of the weight force of each element acting on each of the two sides of the fulcrum, multiplied by its arm, to find what is the total from both sides, and for comparison correct the position of the fulcrum by lengthening or shortening the depth of the primary case, calculating backwards which is its equivalent value to the other side of the lever, to find the balance.

(I do not know if with my chatter I managed well to confuse the ideas)

[Giulio TiberinI]

If it is worthwhile to calculate the weight of the truss in aluminum tubes, you can download the catalog PDF aluminum with meter weights, This from my supplier site.
https://www.comefimetalli.it/

If you can be of interest to get an idea: My Case secondary 360, full of seekers and eye, weighs a lot…or about 5 kg.
Which they require a balancing of 25kg, of which 20 are balanced by the balancing and the 5 remaining from 5 by counterweights (one of which hung behind the counter of the primary, is the field lighthouse which I converted into power supply 6 – 12 volts of my fans).

It lightens the case of the secondary, whether to join the two discs of wood of its structure, using the wooden framework SAMBA 30x30mm (which is located in brico), it weighs 350 grams per meter.
Mine is heavy because I have to make recycled as spacers 4 pieces of aluminum tube Ø25×1,5mm, surplus trellis, joining them with M6 threaded rod and cap nuts.

[Stefanosky]

Thanks Giulio, maybe i found a solution a little bit “advanced” which if I could master it would be a masterpiece. I use a software called Solidworks for 3d prototyping which I found to have a function (among the thousand thousand) very interesting, created all components in 3d (mirror, cell, box, cage, secondary, spider, seeker, etc) and assign him the materials (consequently the weights) the assembly is then assembled as if it were the finished telescope. At that point, the function is launched that calculates the center of mass of the whole and should return the exact fulcrum. Then eventually changing the box you would have in real time the variation of the center of mass. Found that, it is still not clear to me how to calculate the radius of the crescents

[Giulio TiberinI]

Sorry I'm late Stefano (I'm abroad).
The solution with Solidworks is definitely the most comfortable. And having the ability and knowledge, should certainly use it.

As for the diameter of the crescents, a method would be proportional to the data of the other telescopes considered from the book of Kriege.
I remember that in the book there is a table of diameter 15″ in sù..ma I am out of the house and do not remember the proportions

[Giulio TiberinI]

The fact is that for a F6, which is “long”, It should be a primary cash more “high” to have greater margin of balance with the trestle along; which is counterproductive to the portion of altitude you will encounter the eyepiece , rising to about five 80. I for my 300F6″ lightened (where the impossible to balance the experienced disc brake), then commuted the low-riding for the comfortable to the eye 1,75 meters of my eye.

But in a lightweight construction it does not help much (…but without powerful technological means it is difficult) find the best compromise between increasing the height of the primary case; Ocular final height from the ground; and diameter crescents.

Increasing the diameter of the crescents increases the height of the tele, but it has the benefit of balanced given by the movement of the breech of the telescope, which should aim to achieve an increase in the weight of the primary mirror, exactly as it increases the cosine of the angle of inclination of the cosine puntamento..Questa is the rule that eliminates any additional counterweight….and no technology is not easy to calcolo..se not for experimentation and approximation.

[Stefanosky]

Very well, I will see the point where the theoretical center of gravity will mark me and I will try to size the crescents. So now I'm designing the various parts and I've already bought the steel for the cell (which I will build first) so as to check if the weights correspond to those calculated by the software.
I'll update you on the matter soon

[Giulio TiberinI]

You will see that the advantages of a traditional type dobson (I mean model obsession) They are enormous in terms of movement and openness without inpuntamenti shots; How tight collimation and a ragolazione of insensitivity to spilling a variation range of weights to change eye / accessories, given by the possibilities of mutually alienate 2 Teflon plugs of the sliding crescents.

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