#1  
Old 11-11-2019, 07:02 PM
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GrahamL
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12 "" f5 rebuild

I have started to rebuild my 12 "scope to something a little more manageable weight wise , my knees dont like lots of weight any more and i cant place the scope somewhere easy to get at so as a consequence rarely use it



mirror box loaded 24 kgs

truss poles 3 kg

uta around 3 kgs 3.5 probably with a pan 35

ground board 13 kgs

for 44 kgs all up .


I can shave quite a bit off the bottom by cutting down the mirror box and redoing the ground board a little differant and have increased alt bearings to 23 " almost double what i had before , the balance point with the increased bearing size rises 100 mm

My truss poles will increase in weight being longer

moonlite 1 " connecters still working well

(is CF an option ? )
could try and turn down the moonlites to 19 mm if CF is going to end in tears price wise .
Who sells Cf tube in + metre lengths ?



My uta with a K2 focuser I can drop a fair bit of weight by rebuilding

Single or double ring up top ?
stiffening up to a single ring or lightening to a double as far as can tell are going to be similar weights .


any input most welcome
graham
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  #2  
Old 25-11-2019, 06:49 AM
hamishbarker
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Reiner Vogel built a lightweight 22" and saved a lot of weight by making the mirror and rocker boxes with a sandwich of thin ply and foam. Structurally makes a lot of sense to put the stiff thing on the outside to take bending stresses while the inside foam only needs to transfer shear stress from one skin to the other. Full thickness parts are needed where point loads impinge on the thin skins, can be wooden blocks.


edit: just checked his website and couldn't see anything re foam sandwich. Must have been someone else. anyway lots of lightweighting and EQ platform construction tips. A great website

http://www.reinervogel.net/index_e.html


When I rebuild my large dob (after finishing the figuring!) I hope to follow this path.

Last edited by hamishbarker; 25-11-2019 at 06:54 AM. Reason: checked information
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Old 26-11-2019, 06:01 PM
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Thats a nice site , I'll have a good look in time



Two days of work and sometimes deep contemplation and sometimes even deeper contemplation , and sometime wondering what the ,, have i done with something i just had in my hand has me here I have most of the marine ply (18 MM) to finish $150 i think give or take a few screws and bolts etc ,my workshop 10 years on is still sitting on the concrete in the driveway ,,sigh



I'm not overly happy with the old mirror box ( i should of rebuilt ) and the new ground board sit together , a small error i must of built around originally is a little more obvious now the box not square by a couple of mm .


A couple of days later maybe i'm to hard on this like it runs well on the bearings and isn't dragging , bearings arnt clean and laminate not applied yet and i dont have weight up top to balance things out a little and see


The enlarged bearings are differant to how they move weight around i think , looking forward to the ongoing process , atm is sometimes about

making things right then better it never ends . Peter Read once again most helpfull with some tube inserts .
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Last edited by GrahamL; 26-11-2019 at 06:25 PM.
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Old 26-11-2019, 06:29 PM
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GUS.K (Ivan)
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Good luck with the project graham, will be following your progress.
Ivan
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  #5  
Old 02-12-2019, 04:18 PM
sfarndell (Scott)
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It's always going to be a tradeoff, but ultimately a well-balanced scope can be any reasonable weight without being difficult to operate (there are some large scopes in excess of 200kg which are not motorised and work well).

Is weight the factor? Do you have a lot of stairs? This could also impact use if you have to make multiple trips up and down to fetch/store parts.

With the UTA it's unlikely to be heavy enough to be much of an issue and it comes at the expense of larger bearings (which can be made quite light by removing bulk) but you'll have shorter truss poles. The bearings don't have to be permanently attached so the extra bulk unlikely to be an issue.

As to single vs double ring: I've built a 16" ultralight dob with a single ring UTA then remade it in a more traditional way with a double ring as I had too much vibration/issues. Double ring UTA is much easier overall (IMHO) as it's easier to attach the spider (I use a wire spider), baffles and the focuser. It also reduces vibration attributable to these components due to how they attach on the double ring UTA. As an added benefit, you also don't have to constantly worry as much about scratching your secondary when you dismantle, etc. Extra weight can be saved as you can also cut a lot of wood out of the rings to reduce weight without reducing structural integrity (same thing for your mirror box, but you'll need to add some sort of dust cover/protection).

I think your major issues lie in your mirror and rocker boxes. Have you considered a flex rocker? They can be made relatively light, but quite stiff, provided you don't mind cutting a lot of holes! Should be a lot lighter than 13kg!

Def some room to shave weight on your mirror box. How heavy is your mirror? Can you post pics of your current setup?

On Carbon fiber. I'm in the process of making a 14" binodob and had the *great* idea to make the upper assembly from CF. It's expensive. Very very expensive. You need a vacuum pump, gauge and catch pot as a minimum, plus consumables. CF cloth is around $30/m but you will require multiple layers over a core material to make it stiff enough so it'll use a lot. Cheap core was $70 for a 2400 x 1200 x 20mm sheet. AA grade Marine ply not much more expensive, easier to work and more readily available.

My only output so far is a 10mm thick of CF which I hope to turn into helical focusers. Unless I stuff it up, which is also consuming a lot of thinking time.

CF requires a lot more effort and much more difficult to work with than plywood so I'd recommend you use plywood and then skin it in CF if you want the look, but at the cost of extra weight.

That being said, I'd love to see a CF build

Hope that helps

Scott
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Old 08-12-2019, 02:20 PM
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It will be a double ring 9 mm MP



I think the box will be 15 kg or a little less fully loaded which im happy enough with 40 % reduction all round mostly the uta will be the hardest i think I'm 3 kg now need to be around 2 Kg when i pull my current one down illknow a little more.


ground board finished working my way up
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Old 30-12-2019, 03:46 PM
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A little time challenged of late


mirrox box lid with tektite (asia) weird shape almost completely circular east west fracture , preasure /impact, seems to of split it clean from original.
most of the bottom end is close to finished mirror box will come in at 15.kg
I think , lighter would be better but it much less than before .
ebony star to bottom of rocker and laminate to bearings tommorow


About $250 spent so far , (45 hours total so far ) uta left to cut and fit out making it more compact and a few little things to tie in hopefully end of the weekend .


Somethings dragging az , not sure yet will look at bolt and sleaves tommorow, maybe the teflon has deformed a little being not sercured for a couple of months .


the bigger bearings need some sort of keeper each side in the mix somewhere like a lot of atmers seem to do and likely a strut out front still not laminated yet



not sure I want my argo fitted to this scope yet Have used the encoder az centre bolt again if i change my mind i only have to centre the alt axis and attach something in regard .



Had a local aluminium supplier who was really good , new owner dosnt stock tube at all by the looks and is rarely open , answer to what about 20 mm tube ?,,,"I wouldnt know where to get that ".. Will have to travel I guess.


Onwards through the fog !
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Last edited by GrahamL; 31-12-2019 at 07:56 PM.
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Old 31-12-2019, 10:44 PM
sharpiel (Les)
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Had a local aluminium supplier who was really good , new owner dosnt stock tube at all by the looks and is rarely open , answer to what about 20 mm tube ?,,,"I wouldnt know where to get that ".. Will have to travel I guess.


Bunnings,,?
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Old 01-01-2020, 10:48 AM
Saturnine (Jeff)
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My local hardware stores, Bunnings and Hardware Man both stock 20mm x 1.6mm alum. tube in a few different lengths. Have been taking notice for future reference because of an upcoming strut newt project.
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Old 01-01-2020, 03:54 PM
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I never thought to ask bunnings they dont have it on the shelf , will check it out .



I believe theres some some tubes sizes with a 1.2 mm wall .


My ground board wasnt running freely , Ive been looking at few things and couldnt see anything wrong over and over .
I came back to the teflon for some reason I thought backing off the torque on my drill was the go when mounting it again instead of picking up a screwdriver. Though i couldnt see it as the screw bit i think it must of distorted the teflon slightly on one or two pads . , I drilled out the the area around the mounting holes and put in clean ones and its working well now .I will chase up some 1/4 inch instead of the 1/8 I have now .
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Old 02-01-2020, 10:10 PM
ausastronomer (John Bambury)
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I would use at least 25mm diameter aluminium poles on a 12" scope.

25mm x 1.2mm poles are a lot stronger and will give greater stability at the top end than 20mm x 1.6mm. 25mm x 1.6mm would be the best choice IMO.

My 10"/F5.3 has 20mm poles and in hindsight I wish I specked it with 25mm poles.

Cheers
John B

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Old 03-01-2020, 10:40 AM
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Thanks John



My moonlite inserts would have to be turned down for a smaller
(D) tube which is already fitted for 25 x1.6 so i cant go to 1.2



Your FL is 1345 & pole length 1070 or so ?
Mine would be a little longer and with the smaller mirror box
and more compact uta .


Havn't commited to anything yet .
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Old 03-01-2020, 01:08 PM
ausastronomer (John Bambury)
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Hi Graham,

If your pole inserts are already made for 25mm x 1.6mm then that would work well.

Increasing the diameter of the poles has a much greater effect on their strength and rigidity than does increasing the thickness of the aluminium.

The modern trend from those who want bragging rights is to use carbon fibre poles, but to be honest it gains very little on a telescope other than a slight weight saving and I wouldn't consider the truss poles to be the heavy part of any telescope. If you use the right sized aluminium poles they will do everything a carbon fibre pole will. They have served the telescope making community very well for about the last 50 years.

Cheers
John B
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Old 05-01-2020, 08:01 PM
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Mostly finished UTA today 9 mm marine ply



Theres is some flex in the rings themselves but not through the optical train as far as i can see (my older 1/2" rings also are similar ) , I messed with a single ring for a while back and on a smaller scope build and couldn't get that bit right .
Focuser board through the truss axis again and I will glue the ply light

shield this time its seems to offer some pretty handy lateral strength to the whole exersise . Moonlites when mounted will also add a little more strength



Protostar double pinned spider tensioned up ties in everything nice , I had issues with dusty threads in most all mounting pins but was lucky enough to pick up (first go ) a tap in my whats in the box to clean things up, puling apart 14 year old hardware has been the major PITA.
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Old 06-01-2020, 09:44 AM
sfarndell (Scott)
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Looking good!
Is it intentional that your astro-dog looks like an inverted-colour starfield?
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Old 18-01-2020, 07:37 PM
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He likes that rug usually our grandaughter is on it with him .


Uta mostly good to go , pulling old hardware apart again a major pain

focuser board needs a sand and maybe a little drilling on rear to shave a little more weight .


adjustment knobs need shortening



The protostar 4 vane I like a lot

The original set up is grub screws sunk down into the hub looks tidy But I never liked messing with a hex key above my primary so added these knobs , also one night I travelled with my scope and one of those grub screws fell out had no idea when and where and obviously had to pack up and go home This works much better .


Back at work now so things have slowed a bit I dont seem to have much time spare of late so this last part will likely drag on a bit
$350 down maybe a little more or less ,(50 hours time wise ) , truss tubes left to buy and measure out the FL and adjust to my eyepieces , I think I'm mostly ok these days I remember when i had the pretoria and a few other odd ones the H2 travel was really tight to get all to focus , a few little things to work through , secondary to work out not much else i can think of .
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Last edited by GrahamL; 18-01-2020 at 07:58 PM.
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Old 26-01-2020, 01:57 PM
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focuser attached



ply for light shield cut down sanded and used again
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