Structuring a Dobson

[Stefanosky]

I'm getting closer to the stage where, finished the mirror, I will have to design a structure capable of welcoming the best.
Then I'll open a specific discussion on the choice that I will, which I will document the entire process from design to actual construction.
By nature they are more inclined to technology solutions and I think this is the way I walk but I'd love to know your opinion about the various types of structure and materials used. Wood, metal, type light, heavy, with carbon truss, aluminum, etc
In particular, considering the MY project, what are the pros and cons of the various possible ways?

[Giulio TiberinI]

Hi Stefano.
On the structure I tell you my thoughts and attentions to Ask, the tuttto in a very general.
I would say first that since your telescope will be an F6, so it will be quite "long", This requires attention to compromise to choose, starting from the type of telescope, ie whether it will be the so-called normal or lightweight Dobsonian.

Is’ Clearly, the type classic trellis NORMAL (I talk about what I like 360f5 and American all'Obsession) It has considerable advantages of simplicity of use, maintaining the collimation and ease of balancing, relying on the greater design distance between the bottom of the primary case and the center of rotation of the lateral crescent bearings, which they are smaller than those of a type LIGHT, until it disappears becoming a simple circle, in telescopes trellis trade, that they are constructively similar to the original telescope Dobson created by John Dobson.

Greater distance but in the classic dobson has the disadvantage that the same case and the rest of the ground part of the dosbon, They are larger than those that would be in the case of lightweight type of construction…
But the flip side of this coin is that vice versa in a Dobsonian of light type, to achieve a degree of balance close to’ equivalence of the classic, necessary to oversize the radius of the two lateral bearings crescent, and also secure them to the case not centered primary, but moved flush with the rear part of the case itself, so as to ensure that during the inclination of the telescope, as the "pipe" virtual telescope exits the verticality and grows the cosine of the angle of inclination, there is an automatic "load" of the counterweight that increases with the increase of the lever arm , that in that case is equal to the projection of the distance between the ground in the center of curvature and the bottom of the primary case.
The disadvantage is then having to build such efficient and large crescents (that would be as shown in the discussion at the following link) in such a way that they can be made collapsible to reduce transportation problems created by them, It lightened in a telescope precisely 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 wood also compensated only poplar, however being a lightweight composite material has the advantage of being a thermal insulator. And this in the use of the telescope has the considerable advantage of avoiding the fastest run of the ambient temperature subject to fogging mirrors. Is’ true that these can be avoided with the thermal bands, but I think their use is an absurd astronomical contradiction that forces you to warm what is necessary to maintain the ambient temperature to avoid turbulent thermal currents within the telescope, and also it involves the expansion of hardware to carry around, with the use of a power supply.
(But I think this comes from my idea that the best technical solutions in absolute, They are those that maximize the combination of simplicity and efficiency).

As for lightness, however, the aluminum (specific weight 2,6) It would be on a par with offset (specific weight 0,45) if you think that the thickness of use which makes them equivalent is equal to 2.6 / 0.45 = 5,7; ie aluminum in 1mm thickness is equivalent to plywood in 5.7mm thickness…In other words, the structure of a telescope in 2mm thick aluminum, It has the same weight as that achieved in offset by 12mm, which it is certainly less flexible and less dancing than with thin aluminum.
In otHer word: Given that the structure of the dobson must be sturdy and rigid, it is easier to get that excellent results (wanting, pure high-tech), constructing wooden instead of aluminum, and painting with waterproof and wear poliretanica par excellence, sold in DIY stores for boats.

CELL MIRROR
If possible, avoid the "belt" classic made so: http://www.webstertelescopes.com/18_cell.jpg in favor of a much better which supports the mirror with a triangular structure made so:
https://www.bresser.de/out/pictures/generated/product/7/460_460_70/4e92ce651b9a2a4deab59da15f893ecf_0116940_d5_0817.jpg , which allows the attachment of the rear support of the mirror triangles and their eventual rockers, and also the convenient collimation is made in the traditional way from the rear side of the mirror, which from front to rear, being perhaps the eyepiece (and maneuvering the Allen key inserted in a long enough tube).

Furthermore, the latches of the mirror because he does not go out of his cell, in this kind of work cell they are integral with the mirror in collimation movement, and not fixed to a separate and fixed structure (as in the case of classical cell with the mirror support belt) the risk of being too loose or contact, depending on the movement of the primary variable in collimation.

THE TRUSS
I have expressed in detail in the article the following considerations:

The truss tube for Dobsonian telescope… and surroundings

The construction in my best opinion is the one that combines the "compass" pipes in pairs, for example by means of a piece of angular, that with three or four pairs of joined tubes, It provides a trellis which has a supporting surface for the case of the secondary, which it is perfectly perpendicular to the optical axis, in which case rests simply and conveniently screwed to the angular pairing tubes.
This fact of the union “in higer place” of the tubes in pairs, It allows not to need down (ie to the primary case), to have the truss clamps that guide him to facilitate the subsequent attachment of the secondary case, but suffice 4 simple bolts outgoing from it.

The total benefit is however in small telescopes or short throw, when one realizes the trellis in a single openable and closable piece to "accordion" , as I did in my case type 250F5.

THE SUPPORT OF SECONDARY
It can be easily obtained by turning from solid cheaply round black nylon, always on the ground to avoid the conduction of cold flow in the mirror from the "spider" support, which it is normally aluminum.
The "coupling" swivel that allows the slap of the secondary support, manoeuvrable with the usual three screws in phase collimation, It can also be just a threaded rod M6 nylon, that is flexible and robust, without presenting problems of "brittle fracture" even at low ambient temperatures. Nevertheless, I precaution I put the classic anti-fall of the secondary lanyard, glued behind the mirror with a silicone ball and linked to the spider vane.
Another (usual) constructive advice to avoid condensation to secondary, is silicone paste with three balls to its support, putting 3 matches as a spacer (as to remove bonding occurred), so that between the mirror and the support there is a 2mm thickness is silicone.
All this is visible in this picture in the gallery at the bottom of this article:

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

These ruminations represent my thinking that has evolved in the construction sequence of my 4 dobson. My thoughts therefore has no claim to perfection, but it is only the choices I made, loyal to my thinking “Simple and functional = beautiful”, in order to improve the road doing what I seemed to be improved with a simpler solution, it's more (or at least equally) efficient.

Ciao- Giulio

[Stefanosky]

Very Giulio, very comprehensive as ever!

[Stefanosky]

Giulio I then decided to wood as you consigliatomi, allow me more generous machining tolerances and any changes or additions in the course of work but on occasions there was a need.
Now I need to figure out how to calculate the size of the secondary cash and obviously this dimension.
I currently do not will opt for a low riding so we say that will be a configuration “academic” but having not yet focusers and eye I have difficulty using the different formulas.
You advise me to be an effective method to decide:
-inner diameter of the secondary cage
-axis diameter smaller secondary mirror
doing so precisely to use a secondary “quite right” in a cage “right” which allows me to use a low profile focuser maybe able to exploit most of the eyepieces (I plan to take such a great zoom 8-24) and having to need a full field light enough to use my camera for some shots “on the fly” (sporadic use and limited by the nature of a dobson).
Thank you

Edit: I remind you that we start from a 300mm f6

[Giulio TiberinI]

Ok.
If you give me a little time (one or two days) I make one each sketch, which it is the best way to figure out how to get by in the dimensions.

My outline sketch “A” normal road to get to a size which is always a function of the full light field and the extraction of the desired fire, hence the extent of the secondary.

And last, the length of the truss will “agree” the whole, bringing into focus the most critical eye, namely that of your group of eyepieces that requires greater integration “inside” the focuser.

It is clear that if you will want to use a camera to take some pictures, the full light field will be resized to the diagonal of the sensor format, or shooting film. And so it will be great also the secondary and obstruction, but the trellis will be shorter.

[Stefanosky]

All the time you need Giulio! There is no rush
Now that I still think I would not go to increase the obstruction to the habit to attack an SLR for which Dobson is not the right tool. It should then also calculated the offset of the secondary real? Oh well, see if you can figure out farmici then something will adapt to the concepts needed

[Giulio TiberinI]

Hi Stefano.

I start by sending you e-mail the little drawing reference CAD.

I insert here also two catches of the screen JPG concerning the design, one of which is set , while the other is a magnification for readability of the secondary mirror part, where various reasonings:
direct connection 1 – MUCH BETTER than the preview LEGIBLE:
https://s8.postimg.cc/x3bsk7y45/300_F6.jpg
forum Preview 1:
[/url]

direct connection 2 – MUCH BETTER than the preview LEGIBLE:
https://s8.postimg.cc/bto69hcph/300_F6_zona_secondario.jpg
forum Preview 2:
[/url]

—————

Now I explain the details of the design:

1) The drawing on the left side, to represent the mirror as a stretch of long blue vertical line 300mm from the middle part of which a second dash orthogonal to the left, long 3,12mm that represents the depth of the central arrow mirror 300F6.

2) From that extreme of the mirror recess part and then the optical axis (in dash-dot) long (300×6) = 1800mm rightward.

3) The cone of light is indicated by two yellow lines that converge from the edge of the mirror to the focal point on the optical axis of the top.

4) It is time to choose the full light field CPL, for visual purposes, normally it has a value ranging from 10 a 20mm, and I imposed 20mm; Account "round" that, understood the cabin operation, It will be modified at will.

(In this regard you can parallel deepening / clear your head, leggendoti the following short but old argument, always very valid, in “Dobsoniani” ):

https://dobsoniani.forumfree.it/?t=54081380

5) So I draw a new cone of light, this time in green, which always starts from the edge of the primary mirror, and ends at the ends of my dash-amplitude range of Piana Light 20mm CPL.

6) Now with CAD it is easy to quote me the two corners that underlie the primary from mid of the light cone yellow (see 85.23 ° altitude close to the primary), and those relating to green CPL (see adjacent share 85.54 °).

7) Now, subtracting the two measurements find that the amplitude of the sky that depict just in full light will be 2 times (85,54-85,23)= 0,31°; that is, 3,31*2 = 0,62°

(otherwise I would find equally dell'ampiezzaa the value of half of the sky framed, knowing that half of the CPL (10mm) divided by the focal length 1800mm, is the tangent arc with CAD that I find much more easily).

8) Before rotating the axis to simulate the reflection of the secondary, I know the radius distance from the optical axis, which will be in the field diaphragm of the eyepiece.

Given that an empirical rule establishes that the opening diameter of the secondary case must be equal to the diameter of the primary, added to one hundredth of the focal length, I find that this value is 300+(1800/100)= 318mm,
then the beam will be =(318/2)=159mm.

At that radius I have to add the dimension of the thickness focheggisatore. That is the point at which it will be the field diaphragm of the eyepiece inserted into the focuser. Thickness which is usually for focusers Newton type or INTES JIMI, and of 90 mm, that will add to that ray: That is, the field diaphragm of the eyepiece will be located at a distance from the optical axis equal to the radius of (159+90)= 249mm (see picture)

At that distance from the axis orìttico, then draw a parallel, at the bottom of the drawing, I will serve as a reference to draw a tangent to it circumference (circumference in dashed and dotted lines).

Then starting from the two ends of the dash of the CPL which is at the top of the optical axis, I draw two straight lines that come back in the cone of light of the CPL, up to intercept, and even at that point I draw the dash 20mm CPL.

Now pointing as center of the circle on the center line of the dash, Draw a circumference 249mm diameter.

So with CAD I make a copy of the cone green light, and the optical axis of the dash with its dimension from 20mm, and it rotates the copy 90 degrees clockwise, finding the position of the eyepiece (at the bottom in the drawing).

Now looking at the intersections of the two copies of the cone of light green, Trace the blue diagonal 83,66mm, that shows me the extension of the longer side of the elliptical secondary mirror.

From that measurement divided by the root of 2 (that is 1,41), I find the shorter side of the secondary mirror, which corresponds to the diameter of 59,3mm.

Then, to avoid subjecting to the reflection edge of the secondary mirror, which it is the most critical point of quality…(ie seat of the major defects of entry and exit of the tool that generated the plane of that mirror), I ordain a secondary mirror with commercial size greater than a few mm .. for example 60mm.. (see the blue circle diameter 60mm).

———————–

Now as for the OFFSET, I find if I go to dimension the two distances from the optical axis of the two edges of my secondary (lower and upper in the drawing).

I'll find that the secondary edge which is at the top in the drawing, over the optical axis (corresponding to the opposite side of the focuser) It is from the optical axis 31,88mm (see blue share) ; while the lower edge (that is toward the focuser) is only 27,28mm.

By subtracting the two measurements to intercept and I find that entirely reflect the light cone of my CPL coming from the primary mirror, I have to move "back" (ie upwards in the drawing) the secondary (31,88-27,28)= 4,6mm

END OF SARABANDA

[Stefanosky]

What about Julius, a show… Thank you!
So starting from the desired cpl are pulled out all sizes,. For example, if I was interested doing photography with my camera which has a 22.3 sensor×14,9 mm should calcolarmi the diagonals doing square root of (According to the second more height to the second) and I would get 26,81 mm cpl would be the need to cover the sensor (In this case, the calculations should also take into account the sensor away from the focuser to be added to 90mm). Since I would start them with the other calculations, am I right?
Taking into account that it is not my primary interest to take pictures I would say that a cpl 20 mm would allow me a little all options including any pictures maybe croppando in post production.
For curiosity, leaving out the photographic part and then choosing a cpl 10 mm, in the face of minor obstruction which would have disadvantages regarding the eyepieces used?

[Giulio TiberinI]

Hi Stefano.
To see what happens with a CPL halved, you should change the design.
But I find it very convenient to just simulate the loss of light at the edges, inputting the diameter of a smaller diagonal in the on-line program from which the following link.

You can see it to have a pretty good idea of ​​the lighting decline'd get to the edge of your eye, entering the following data in millimeters; 300mm diameter mirror, 1800mm focal; of 249mm away from the optical axis to the plane; m36mm diameter of the field diaphragm or film;.

Put that data leaving aside the list of diagonal including the software will automatically take into account the best , also by showing the two adjacent a little’ underperforming.
Or secondly you put the size of the commercial diagonal that you want, to see what would be the bad habit depending on the diagonal no longer by 36mm from what you want.

http://www.bbastrodesigns.com/diagonal.htm

[Stefanosky]

All clear! I'll get right to work and when I understand the situation I'll open a thread on the design of what will be (let's hope) my first homebuilt tool

[Giulio TiberinI]

All right, unhurried!

You'll see that with your own “cloth” technique that we now know, will be an exceptional telescope.

[Giulio TiberinI]

Bear in mind, however, that I am not a
expert…esolo but a curious fact that few times “roads” of which describe the transit impressions.

For which I am one that normally is technically outclassed by those who have accompanied for those brief initial stretches of road that I know, because already trails. And this is the thing I like best.

[Stefanosky]

Bah, experienced or not your luggage for people like me is pure gold…
Speaking of the focuser, of which models could direct me? So out of hand I would prefer a crayford. Those low profile ingolosiscono me for the possibility of a smaller secondary, minor obstruction and lower weight, But I'm afraid that the blanket is short with certain ocular or other accessories.
Of those “normal” I saw some models but I think they have a height less than that indicated by you of 90 mm (the data to be taken for the calculations is that its just the minimum height?). Then there is the choice of the available travel, some 40mm, 35mm, other 15mm and so forth.
What do you think of this? I've seen more serious stuff like Feather Touch but have very high prices.
https://www.baader-planetarium.com/en/accessories/telescope-accessories/focusers/2%22-bds-nt-baader-diamond-steeltrack.html
Pair this with a Baader Zoom 8-24 Mark IV (that I read a lot of good). It might be a good choice?

[Giulio TiberinI]

Uno zoom 8-24 It is a good choice because it will be useful in a future when you have maybe a telescope 250F5 suitcase travel.

The focuser nice but too expensive.

Because crayford are simple, robust, and a life on the market, I think there will be much of differenza..se technology is not the production costs of living produttori..quindi countries GSO crayford low profile in China costs about 160 euro.
My top is a 90mm Intes Crayfotd (russo) of 2″ and flat base, that in 2002 Pagai 125 euro.
Today there are many brands, Also the low profile high half of my. with demoltiplica 1:10 (that it did not, and of which I have never felt the need).

The run of mine is 40mm, and with the trellis cut to bring into focus the eyepiece which requires the insertion of the most fok, They are going to focus even more eye of different brands 10vecchi, including the three I always use, which are from Explore Scientific 14, from 8,8 e 4,7mm e 82 degrees, and which they are mutually parfocal (that is, change them without the need to refocus)., More 2 Former curious microscope, with grid, requiring a longer barrel to stay behind.

The board on the eye and then take the series and sressa Parfocal.

The other helical focusers (stepwise advancement screw 3 two or three principles, for advancement 6 or 9mm lap), weight of 31,7mm 40 grams, I did turning on my design from solid black nylon, for small telescopes travel, have 40mm stroke.

[Stefanosky]

Having these days of problems using Cad on my PC I tried to Use clean Atmos for the design of measures.
I entered the same data, mirror 300, focus 1800, cpl 20, distance from the optical axis 249 mm (Data from within 318 mm of the 90mm Inner Case foc) but I get different values.
Negligible with regard to the size of the secondary but almost halved as regards the offset. How is it possible?

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