20" f/4 Dob build

[swamp thing]

Well I finally had some time to make a start on my next scope. Shouldn't complain been busy with work recently, and just having work is pretty good for a builder in these times of tightened purse strings.

Started by laying out the mirror cell in cardboard to check I've got the sizes right.

This will have to be a 27 pointer to give the proper support.

The coin is a 10p for scale.

Then using a plasma cutter, cut out the support triangles.

These are cut from 1/4" aluminium. The larger connecting triangles underneath will be cut from 1/2" aluminium. which only arrived late today.

Unfortunately the plasma cutter only cuts up to 1/4" so the rest will have to be done on the band saw.

Depending how busy I am, I hope to get these cut in a day or two.

Regards Steve

Ps anyone that spots the deliberate mistake doesn't win a cookie. Sorry:)

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[steelfixer]

Steve looking good.

Can you give me some pointers on how you work out the configuration of a mirror cell.

I will need to know for when I start mine.

Many thanks

Graham

[swamp thing]

Cheers Graham.

First thing I did was buy this book.

The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes: Amazon.co.uk: David Kriege, Richard Berry: Books

This gives you nearly all the sizes you need for cell building. I had to scale mine off the measurements provided as they didn't cover a 27 point cell for a 20" mirror and asked Moonshane to check my numbers (cheers Shane). I wasn't sure the 18 point would provide enough support for my mirror which is lightweight and thin.

There is a program (plop, I think it's called) that apparently will help you design your own mirror cell. Unfortunately I couldn't get it to work for Mac.

HTH

Regards Steve

Ps you may well get the book cheaper elsewhere

[steelfixer]

cheers Steve I will try and get hold of a copy.

[Glasspusher]

Great project Steve. The mirror cell designs given in Kriege's book will differ from those produced by PLOP. The 'old' method used in the book is based on the equal area principle, each support point being allocated an equal area of the mirror. PLOP uses a different method based FEA (finite element analysis), I won't pretend that I understand it though! Both designs seem to work well enough, PLOP based designs seem to be popular at the moment. I would be interested to hear more about your mirror, did you make it yourself?

John

[swamp thing]

Hi John

It does seem strange that they dont allow the PLOP to run on a mac. But it seems that the Kriege Berry system is tried and tested. So should be ok.

The mirror is out of a Beacon hill open tube Dob that was a bit of a dinosaur TBH.

Not sure for certain who actually ground it.

It does need a recoat as the surface is beginning to let light shine through it.

I will build the scope first though.

Regards Steve

[Merlin66]

A hint to allow the support plates to self centre and "float"

Use a 8mm ball bearing semi recessed in the cross member and the underside of each plate; a good coating of silicon in each recess..

[swamp thing]

A hint to allow the support plates to self centre and "float"

Use a 8mm ball bearing semi recessed in the cross member and the underside of each plate; a good coating of silicon in each recess..

Excuse my stupidity.

But do you mean between the crossmember and the connecting triangles or between the connecting triangles and the support triangles, or both?

Not sure I've even got the names of the triangles right.

Cheers

Regards Steve

[Zul]

Good luck with it

[Merlin66]

The final triangular plates plates which have the final 27 points, to the support triangles underneath; if the "top" triangles float on the ball bearings between, then you get a far more even support.

HTH

[ollypenrice]

Hi Steve,

My 20 inch Dob, Sir Isaac, was also built by a builder (Dennis Mien from Oldham) from the same book. He's a mighty fine scope. I did once have to speak quietly to a group of young guests who had started addressing him as 'Isaac.' Not the way to keep his collimation sweet...

Best of luck with the project and keep us posted.

Olly

[swamp thing]

Good luck with it

Cheers

The final triangular plates plates which have the final 27 points, to the support triangles underneath; if the "top" triangles float on the ball bearings between, then you get a far more even support.

HTH

Thanks for that. Great tip.

Hi Steve,

My 20 inch Dob, Sir Isaac, was also built by a builder (Dennis Mien from Oldham) from the same book. He's a mighty fine scope. I did once have to speak quietly to a group of young guests who had started addressing him as 'Isaac.' Not the way to keep his collimation sweet...

Best of luck with the project and keep us posted.

Olly

Cheers Olly. Will keep all posted.

[fwm891]

Hi Steve, I used the K/B book to work out the 27-point cell for my 22-inch scope.

One thing I found useful was to put springs between the various plates to keep them in correct orientation especially when placing/removing the mirror.

My main frame differs from the K/B book as my tube was round rather than the normal square mirror housing.

Good luck

[swamp thing]

Thanks for that Francis. Did you use ally too, or is that SS?

Was hoping to push on with cutting the last triangles today but just checked the bandsaw and it needs a new blade.

Typical let someone else use your tools and they mess em up.

Steve

[swamp thing]

Hi All

Managed to pick up a new bandsaw blade today. Fitted that in the saw then noticed that there was very little room inside the garage to work so I had to cut em whilst standing outside, brrrr! Holding onto freezing cold ally awash with cutting fluid, made the hands a bit chilly. Still, being a brand new blade it cut them very nicely.

Darkness came before I could finish the edges so hopefully I can get out at the weekend and do this.

Here are all the triangles cut and mocked up.

The half inch ally support triangles sure look beefy. 1p piece to show thickness.

Starting to look a bit like a cell now.

Next I will make up a drilling rig for drilling the triangles for necessary hardware.

Regards Steve

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[fwm891]

Steve, Triangles are 8 mm ali and the subframe 1 1/2" x 3/4" steel tube with brazed joints. The three adjustable sub triangles sit on domed brass bolt heads with mating spherical sockets in the underside of each triangle.

Francis

[robbie c]

Hi Steve great project I'm looking forward to following the build and seeing the final result all the best and hope all go's well:icon_salut:

[swamp thing]

Steve, Triangles are 8 mm ali and the subframe 1 1/2" x 3/4" steel tube with brazed joints. The three adjustable sub triangles sit on domed brass bolt heads with mating spherical sockets in the underside of each triangle.

Francis

Looks great Francis. Thanks for sharing.

Hi Steve great project I'm looking forward to following the build and seeing the final result all the best and hope all go's well:icon_salut:

Cheers robbie. Looking forward to getting on with the build myself. Really enjoyed building my 10" Dob.

Regards Steve

[skybadger]

. The three adjustable sub triangles sit on domed brass bolt heads with mating spherical sockets in the underside of each triangle.

Francis

Francis - what holds the two things together though - just the weight of the mirror and gravity or some retaingin mechanism?

Cheers

Mike

[PeterW]

Great news, good to see you started on the upgrade! 20 f4 should improve the view... The Kreig design is not the lightest, but as you say it is tried and tested.

All the best

PEter

PS Frances, are you currently building a 22"?

[spaceboy]

Looking good Steve.

[swamp thing]

Great news, good to see you started on the upgrade! 20 f4 should improve the view... The Kreig design is not the lightest, but as you say it is tried and tested.

All the best

PEter

That's exactly the way I looked at it Peter. Maybe try something a bit more "out there" on another project.

Looking good Steve.

Thanks Nick

[michael.h.f.wilkinson]

Hi Steve,

My 20 inch Dob, Sir Isaac, was also built by a builder (Dennis Mien from Oldham) from the same book. He's a mighty fine scope. I did once have to speak quietly to a group of young guests who had started addressing him as 'Isaac.' Not the way to keep his collimation sweet...

Best of luck with the project and keep us posted.

Olly

I hadn't realized I had observed with a telescope with such a distinguished name and title. You should have introduced us properly

[swamp thing]

Haven't had much time on the build the last couple of days, I have only just managed the odd quarter hour here and there.

However I have managed to clean up the edges and drill the location, and mounting holes in the support triangles. Then drill and countersink the support triangles. Then join both together with zinc plated screws and nuts (couldn't locate any stainless fixings, although experience has taught me that stainless fixings are not always as clever as they sound, being extremely brittle).

I also managed to fix the Teflon mirror pads to the support triangles on the twenty seven support points. Not exactly rocket science. But a job out of the way nonetheless.

I have also cut the tailgate rungs and side bars from 1"x1" tube and 1.25"x0.25" solid bar respectively. These I will pre-drill and tap before welding it together. This is all in mild steel. Was going to post a pic of them but they are temporarily misplaced ( I'm a tidy so and so at times:))

Steve

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[Matthew]

Its coming along nicely Steve,Ive sourced a 20" mirror set and im planning to build a truss tube dob in the summer im hoping it will be done ready for the autumn.