the curse of diy... serrurier truss and big dob

[Dave_D]

really getting to like this 3D program. It's called OpenSCAD on linux. you have to mathematically define each element so no clicky clicky draw a box etc etc but it's quite powerful.

very rough mirror cell and centre box with a simulated light cone for my 250mm f4.8 primary

springs... ain't they easy

module spring(r1 = 12, r2 = 3, h = 200, hr = 20)

{

stepsize = 1/8;

module segment(i1, i2) {

alpha1 = i1 * 360*r2/hr;

alpha2 = i2 * 360*r2/hr;

len1 = sin(acos(i1*2-1))*r2;

len2 = sin(acos(i2*2-1))*r2;

if (len1 < 0.01)

polygon([

[ cos(alpha1)*r1, sin(alpha1)*r1 ],

[ cos(alpha2)*(r1-len2), sin(alpha2)*(r1-len2) ],

[ cos(alpha2)*(r1+len2), sin(alpha2)*(r1+len2) ]

]);

if (len2 < 0.01)

polygon([

[ cos(alpha1)*(r1+len1), sin(alpha1)*(r1+len1) ],

[ cos(alpha1)*(r1-len1), sin(alpha1)*(r1-len1) ],

[ cos(alpha2)*r1, sin(alpha2)*r1 ],

]);

if (len1 >= 0.01 && len2 >= 0.01)

polygon([

[ cos(alpha1)*(r1+len1), sin(alpha1)*(r1+len1) ],

[ cos(alpha1)*(r1-len1), sin(alpha1)*(r1-len1) ],

[ cos(alpha2)*(r1-len2), sin(alpha2)*(r1-len2) ],

[ cos(alpha2)*(r1+len2), sin(alpha2)*(r1+len2) ]

]);

}

linear_extrude(height = 25, twist = 180*h/hr,

$fn = (hr/r2)/stepsize, convexity = 5) {

for (i = [ stepsize : stepsize : 1+stepsize/2 ])

segment(i-stepsize, min(i, 1));

}

}

best thing tho is that the main elements for the primary and secondary sections can be output for cnc cutting. not messing around doing stuff by hand like i originally planned.

[Dave_D]

found this site http://www.industrialgassprings.com/uk/products_end-fittings.asp  which has a good selection of end fittings for the truss tubes, not having the facilities to make my own. what would be a reasonable tube diameter to use for a 10" scope. was looking at 14mm inside diameter/16mm outer. any info would be gratefully accepted.

[Peter Drew]

Should be OK for a short focus 10", I've used 38mm o/d for my 30" F4.1. 

[Dave_D]

Should be OK for a short focus 10", I've used 38mm o/d for my 30" F4.1. 

thanks peter

[Dave_D]

really getting there now

[Davey-T]

Don't know if this is any use.

Dave

[Dave_D]

yeah i've seen that vid. looks promising. i was reasonably competent at brazing so i could save a good amount of money i guess by not having to pay for aluminium welding.

[Daniel-K]

forgive my ignorance but what advantages does this design have over a carbon solid tube? surely it would be easier and lighter to buy a carbon tube? 

[Dave_D]

3 reasons really... cost, cost and cost.

oh, and they look great

and i only have a vixen gp mount. any wind stronger than a gnats sneeze and i may as well not bother. open truss tube = less problems with wind.

i could list a few other reasons but y'know...

oh, did i mention serrurier trusses look great?

[Dave_D]

ok, opinions from more experience scope makers please.

i'm not convinced the top section is right... looks a bit 'bulky'. if i went down the carbon fibre route, what thickness would be needed for structural stiffness of the two rings? (they're currently drawn using 12mm ply)

for the sections that are definitely going to be wood, 18mm baltic birch ply is my first choice as from what i've read, it's one of the highest quality ply available.

or maybe i should just learn to make my own carbon fibre plates

[skybadger]

I would have said no more than 6mm if well made.

[RAC]

It looks good to me. You want the front section very stiff even more so if you were going to use a wire spider. Mine ended up very stiff with carbon rings but only because i have a carbon tube inside them. Without the carbon tube i would say it would not be good enough. Your center section looks like it will need to be closer to the front than in your sketch and that will make it look more even and not so bulky.

The main thing i found was where the three collimation bolts are mounted on the rear ring. Two can be close to the truss supports but one will not be. Even the smallest amount of flex in that rear ring will show up when you move the scope around the sky. Carbon over ply totally fixed that for me.

[Dave_D]

Your center section looks like it will need to be closer to the front than in your sketch and that will make it look more even and not so bulky.

Yeah, the positioning isn't correct at the moment until i can work out the weight of the cell and secondary cage so i can work out the strut lengths. changed the center section to one similar to yours, 2 x squares with 2 reinforcement lengths where the saddle is fixed. need to reduce as much weight as possible as a rough gestimation puts the weight similar to my current OTA which is defeating the purpose of making this. hopefully i can get the final version down to about 8kg which will be 25% less for the mount to handle... unless santa sends me an az eq6-gt (which i doubt)

[Peter Drew]

I have just one ring at the top end of my 30" truss, the focuser is attached to the "V"  of one pair of the truss tubes. The ring is made of rolled aluminium "T" section.    

[Dave_D]

I have just one ring at the top end of my 30" truss, the focuser is attached to the "V"  of one pair of the truss tubes. The ring is made of rolled aluminium "T" section.  

yeah they work well like that, my scope is solely for imaging though Peter, can't remember the last time i had an eyepiece in it so i want to get this right first time

[robjohnb]

Hello! Just read though your topic. Great project. Looks like you'll need a substantial mount for it.

Here's my effort...

http://stargazerslounge.com/topic/167567-10-f48-diy-ota/

The top section has been lengthend a few inches since these pictures.

Same size mirror as yours. OTA Weighing about 10kg. I have it fitted to an eq-5.

Regards, Rob

[Dave_D]

Ok, got almost all the materials i need specced out and sourced, ready for order. One question remains though.

In regard to the truss poles, currently, the poles sit at parallel to the optical path, the positions of the mirror cell, center box and spider frame brackets are in line with eachother. would moving the brackets on the mirror cell and spider cage inwards so the struts are angled slightly inwards from the centre box improve the stability of the design? i've found ball joints which could be used on the ends of the carbon fibre truss poles to allow the angles to be achieved. something similar to this:

[Dave_D]

Hello! Just read though your topic. Great project. Looks like you'll need a substantial mount for it.

I'm doing this to reduce the load on my Vixen GP, (the eq5 is basically a clone of this mount) and from my experience, far more capable than the eq5.  My current OTA+camera is about 11.5kg and my GP copes just fine but if i can get all this down to about 8kg, so much the better as I don't have (or likely will ever have) an obsy so portability is very high on the list of requirements.

[Dave_D]

So, after some serious contemplation of the secondary cage rigidity, i've opted for trusses to match the rest of the design and mounting the focuser on a curved section. only downside is i'll have to modify the focuser to remove one of the knobs, but as it's operated by an electronic focuser this isn't much of an issue. This *should* in theory at least make the secondary cage much more rigid than using rods running straight between the two cage rings. Anyone see any downside to this?

The truss rods are currently using 16mm dia carbon fibre but this could realisticly be reduced to 10mm dia as the load on the trusses in this area is reasonably light, just the focuser, 1100D dslr and off-axis guider.

The electronic focuser will be rehoused in a much smaller project box and sit neatly under the trusses.

[RAC]

That would make the front section mega strong! As for using the metal ball joints I think that would add too much weight with the amount you need.

[Dave_D]

it's very rare that i feel pleased with myself but i just totally reworked the 3D model to base all positioning off the focal length of the primary. i can now exactly measure the f/l of my supposedly f4.8 and put the proper figures into the model and have all offsets and strut lengths calculated exactly. just did a pic with the f/l set to 1200mm and then another at 900mm. everything works (using the exact same model)... the strut lengths are calculated using good old pythagoras and even the inward or outward focus can be set to take into account off-axis guiders and distances to ccd/cmos chips. could even get it to calculate the correct secondary size and offset for various focal lengths if i can be bothered lolol

i shall now go and have a wee snifter of southern comfort and be pleased at my own ingenuity

[Dave_D]

That would make the front section mega strong! As for using the metal ball joints I think that would add too much weight with the amount you need.

yeah i worked it out this morning... almost 1.6kg just in ball joints lol so forget that idea

[skybadger]

I think you are overegging it with a third layer of truss. Make the top ring stronger and hang the focuser off that to remove overall weight, I suggest.

Mike

[Dave_D]

I think you are overegging it with a third layer of truss. Make the top ring stronger and hang the focuser off that to remove overall weight, I suggest.

Mike

true, but i'd rather overdo it and get it right first time...

and it *does* look a bit nice IMHO  

[Dave_D]

hmmm and now a few 'tweaks'...

so i've been playing around with the scope designer at http://stellafane.org/tm/newt-web/newt-web.html

with my current optics, 250mm f4.8 and 66mm secondary and 270mm inside diameter of the secondary cage, i get a 100% illumination of only 11.5mm.

now, if i up the tube diameter to 290mm (which has zero impact on the current tube model) and use a 75mm secondary, i get 100% illumination of just over 20mm

as i'm using a dslr with a cmos size of 22.2 x 14.8mm, is  trying for a 100% illumination of around 20mm worth the expense of buying a new secondary?

keeping in mind i don't really do 'visual'