16 inch Dob project under way

[Joseki]

Ok. Personally I think spending the money to get the Kriege and Berry book saves money in the long run (I should have done this before I bought anything), but for the balance calculation I think all you need is it to approximately balance. For example, I'm slowly working on redoing my own ATM attempt (f6.4 10"). I've made the upper truss and weighed all the components. That comes to 2.1 kg. I've estimated the distance from the balance point (top of the mirror box since I'm mounting the bearings there) to the secondary as 1m. The weight of the poles is around another 2.1 kg and you can assume its centre of mass is around 0.5m.

So on one side of the pivot point you have a total torque of 2.1*1 + 2.1*0.5 = 3.15 kg m.

On the other side of the pivot point you have the mirror box, mirror cell and any counter weights. With an overly complicated spreadsheet (this is who I am ) and an iterative method I reckon I can get my telescope to balance roughly with around a 40cm high box. That works out at 5.7 kg for the sides (with a centre of mass of 20 cm from the pivot) and 1.5 kg for the base with a centre of mass 39 cm from the balance point (I'm using ~2cm thick plywood). The mirror and cell is about 4.0 kg and I estimate it's centre of mass is 35 cm from the pivot point.

.. so balancing the 3.36 kg m on one side of the pivot point you have 5.7*0.20 + 1.5*0.39 + 4.0*0.35 = 3.125 kg m

which is ever so slightly bottom heavy (room for a bigger finderscope). If you reduce the mirror box height you reduce the weight of the sides and also reduce the torque of any counter weight - so it's easy to end up doubling the weight of the whole OTA just to make a reduction of 10cm (or so my spreadsheet says!). If I set the mirror box height to say 10cm (i.e. distance from the pivot point to the bottom of the mirror box) I get a required counter weight of 37 kg.

[Joseki]

".. so balancing the 3.36 kg m on one side" should read ".. so balancing the 3.15 kg m on one side" 3.36 was from a calculation with a lower mirror box height, hence longer poles etc.

[estwing]

its 40mm left over from the floor in my obsy planning on boring out some holes in the circles to save weight and to match the rest of the scope, it will also give us a good fixing point for the spider. which we are yet to come up with an idea

..just a quickie..i too am strugglin' to get a spider..give Orion Optics uk a call they maybe able to help.

[Daniel-K]

Ahh great idea estwing

[estwing]

dont ring yet the "spider-mans" at astrofest....try next week-end i was told.

[Demon Barber]

Daniel,Any of the American bid dob builders can supply spiders to the atm`er,

Try here:http://www.astrosystems.biz/spiders.htm

Hope this helps.

ATB

Mike

[Daniel-K]

**UPDATE** been a while since any work has been done with starting my job at jaguar. just been to barrys to drill the holes for the lightweight truss i say lightweight but most of the wood is 40mm ply so the holes were necessary to save some weight. most of the wood is left over from the ROR OBS floor to save cost. planning on a 18 point mirror cell made from wood pretty basic really on each corner of the triangle will be a small supporting circle with cork or something on. hopefully get some nice dowel tomorrow and get the upper truss put together.

[Chris]

Glad to see you back underway with the Dob Dan I think with that 40mm ply you should nickname it the tank, I can tell its going to be very impressive when its all finsihed Did you use a book to teach you about the grind or was it pure internet research, the reason I ask is a mirror grind has been on my bucket list for quite some time as I first read a book on it a decade ago, think I've forgot 80% of it

I'll have a careful read back through the thread a think

Chris

[swamp thing]

Good to see you working on the Dob again Danny. Look forward to following your progress

[Daniel-K]

Chris at our society they have made a number if mirrors from 12-30"s they give advice when needed but most of the time we just gone for it learning by error having to make a new lap tomorrow as well last one broke may have to go down a grit to get 1 scratch out the mirror we shall see after an hour of polishing.

[Daniel-K]

Looking at rusty splits dob build we have to copy his mirror cell as its wicked desgin and simple to do aswell and would suit our scope

[Daniel-K]

emailed jon at OO UK had a ultra fast reply for a spider. their spider that fits there 16" VX will fit our upper truss so thats good that i can get that when the upper truss is put together £55+vat+shipping that aint too bad

[Daniel-K]

http://www.ebay.co.uk/itm/121149512543?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 i think i might get one of these to stick on the dob along side the telrad jus tpoped down to B&Q (hangs head in shame) and i was about to purchase some wooden 28mm dowel for the upper truss £13 2400m length when the wife points of some black coated ali round table legs £3.68 each 1000mm length with end plugs i knew it was a good idea taking her

[Daniel-K]

a little bit of progress got the upper truss tubes cut, maybe a little too big but i like it gives me more place's to add stuff to. total cost so far is less than £100 thats including all the grits and wood and mirror blanks

[Daniel-K]

time for some pitch pie not sure on how deep it should be so if doe not look enough i will have to redo it with a smaller lap, this is a 12" diameter lap made from 40mm kitchen worktop nice and heavy

[Chris]

What does your wife think of your cooking? Very productive day you're having there!

[Daniel-K]

before i worked at jaguar i got my city & guilds in catering and hospitality so its quite good i may even get the test the 9 inch scope tonight down as intermittent cloud so im fingers x'd

[Rustysplit]

Looking good Dan. The mirror cell on mine is dead simple as you say. Holds coli well and is easy to adjust. The 3 springs on each point keep it where it's put.

[Daniel-K]

This should be thick enough

[Rustysplit]

Blimey Dan, you must have money to burn. That is a lot of pitch

[Daniel-K]

It was donated by our society so never cost anything and there was some left in the pan!!! My damn wasn't deep enough to take it all !

[Astrobits]

We normally aim for about 1/8" ( 3mm ) thick pitch on a lap. Too thick and it flows too quickly, too thin and it doesn't flow fast enough to keep a good contact. Have to be careful that the kitchen worktop base does not swell when it gets wet, seal it with varnish to prevent this.

Nigel

[Daniel-K]

We normally aim for about 1/8" ( 3mm ) thick pitch on a lap. Too thick and it flows too quickly, too thin and it doesn't flow fast enough to keep a good contact. Have to be careful that the kitchen worktop base does not swell when it gets wet, seal it with varnish to prevent this.

Nigel

hi nigel its already varnished with yacht varnish

[Astrobits]

hi nigel its already varnished with yacht varnish

Excellent.

That pitch still looks to be too thick, although the pic has no reference dimension so difficult to tell exactly how thick it is. If it is too thick it will fill in the channels that you cut in the surface too quickly and you will be forever cutting new ones. Also it will slump quite quickly under it's own weight when you take it off to test the mirror, loosing the mirror profile and then needing much more cold pressing time between bouts of polishing.

Nigel

[Daniel-K]

Excellent.

That pitch still looks to be too thick, although the pic has no reference dimension so difficult to tell exactly how thick it is. If it is too thick it will fill in the channels that you cut in the surface too quickly and you will be forever cutting new ones. Also it will slump quite quickly under it's own weight when you take it off to test the mirror, loosing the mirror profile and then needing much more cold pressing time between bouts of polishing.

Nigel

thanks for the tips nigel whats the best place to store it?