406mm F4.5 String Truss Dob Project

stringscope (Ian) · #1 03-06-2006, 12:11 AM

Well the 16" mirror arrived today all in one piece, many thanks Lester

.

I am still considering some basic design criteria:

Nested design, ie the UTA nests inside the mirror box for storage and transport?

UTA load paths. The "obsession" type UTA's have the struts offset 45 degrees from the truss attachment points. It would seem to me it is preferable to have the struts lined up with the truss attach points. I am planning to use glued timber struts as opposed to bolted aluminium tube and glued 1.8mm ply as opposed to Kydex. On this basis I think I should be able to have straight load paths from the truss attach points through the UTA?? Also due to this more rigid type of construction I might be able to go down one size in plywood thickness with the consiquential weight reduction??

2X80mm boundary layer fans blowing across the face of the mirror. appropriate vents on the opposite side of the mirror box.

UTA ID 450mm OD 530mm and Altitude bearings 600mm diameter (teflon/ebony star).

Thats about as far as I have got to date, any thoughts???

Cheers,

spudrick · #2 03-06-2006, 12:42 PM

Hey Ian i can`t help with suggestions as far as design and operation but it will be fantastic to watch this project through to final scope. My longer term goal is to have a 16".
Keep us up to date with progress reports. Do you have a timeline for this?

SMR (Steve) · #3 22-06-2006, 11:43 AM

I agree with you about the UTA/truss configuration. I've never been able to figure why Kreige chose the offset-by-45 degrees arrangement. Yes, your approach will allow you to use thinner ply for the rings.

Try to make your UTA lining as light as possible. Every gram of weight up top moves your center of gravity up, which means bigger trunnions, taller rocker box, etc, etc, etc ...

Steve.

Starkler (Geoff) · #4 22-06-2006, 12:37 PM

Quote:

Originally Posted by stringscope

2X80mm boundary layer fans blowing across the face of the mirror. appropriate vents on the opposite side of the mirror box.

You would be much better off with the fans close to the mirror edge sucking air and blowing it out of the mirror box. This keeps the airflow laminar and will ensure that the mixed tempertaure air exits the box cleanly.

To have fans blowing will create a turbulent mixture of cool and warm air in the mirror box which wont help views at the eyepiece.

I think the ideal arrangement might be to have a couple of small low powered fans to "dig up" the boundary layer, and then a number of more powerful fans to suck that air out of the mirror box.

AstroJunk (Jonathan) · #5 22-06-2006, 02:11 PM

Quote:

Originally Posted by SMR

I agree with you about the UTA/truss configuration. I've never been able to figure why Kreige chose the offset-by-45 degrees arrangement. Yes, your approach will allow you to use thinner ply for the rings.

If I understand what your talking about correctly, it's probably so the focusser can be mounted square on rather than at the jaunty angle of the smaller dobs.

Also, one doesn't have to counter sink the the truss fixing bolts with this arrangement.

http://www.sdmtelescopes.com.au/SDM006.html

stringscope (Ian) · #6 25-08-2006, 04:37 PM

Well I can finally make a real start on this project

.

I have decided on some basic design criteria to assist with minimising weight/complexity:

Single ring UTA similar to my 8" only larger 440mm ID/500mm OD => 1.2 degree unvignetted.
HC-2 2" focuser (200g)
Rigel QuikFinder (much lighter than Telrad)
Protostar 3.1" secondary and 4 vane spider.

4 sided string truss similar to my 8" but with screw as opposed to spring tensioning.

"standard" mirror box design ID 510mm to allow the UTA to "nest" inside the MB for storage/transport.

18 point mirror cell with sling. Front access for collimation similar to my 8".

"Standard" rocker and ground board.

Altitude and azimuth bearings both about 600mm diameter. Teflon on Ebony Star.

MB probably about 400mm high.

I am planning to cut plywood for the UTA this weekend

.

Cheers,

xstream (John) · #7 25-08-2006, 04:48 PM

Good luck with it Ian.
Looking forward to your progress reports.

stringscope (Ian) · #8 25-08-2006, 05:00 PM

Thanks John, I have just sent you a PM.

Cheers,

stringscope (Ian) · #9 11-10-2006, 09:45 PM

After being distracted by some heavy work commitments and sidetracking to make a couple of observing chairs I have finally cut some plywood for this scope. The single ring UTA will comprise 2 rings of 18mm plywood glued together 440mm ID and 520mm OD.

3.1" secondary and 4 vane secondary support with integral dew heater are on order from Protostar.

Focuser shelf is a piece of 100mmX50mmX4mm Aluminium angle and I will be using a light weight KineOptics HC-2 focuser.

A couple of pics for what they are worth.

Cheers,

spudrick · #10 11-10-2006, 10:02 PM

Good to see some progress Ian. I have just ordered the Berry/Kriege Book on how to Build Dobsonian.. so i just might follow in your steps, well sort of, I want to make a Classic 16" Obsession style Dobsonian.

stringscope (Ian) · #11 03-03-2007, 11:34 PM

Hi All,

I have been off line for a while but I am back now.

I have finally managed to get back to this project. I have been distracted by a whole bunch on non-astro matters and what asto time I have had has been devoted to putting together an 8" refractor - see other thread -. After some thought I have decided to redesign the UTA to allow for a 4 sided truss and still have the eyepiece axis horizontal. This evening I cut all the parts, trial fitted same plus a gross error check with the secondary mount. All glued up now. Tomorrow I will fabricate the secondary mount attachment brackets from 40mm aluminium angle stock.

Cheers,

iceman (Mike) · #12 04-03-2007, 05:35 AM

Welcome back Ian! I love your handy-work.

h0ughy (David) · #13 04-03-2007, 08:03 AM

looks like a lot of TLC is going into that work. Looks great, will it be ready for SPSP?

Lester · #14 04-03-2007, 09:25 AM

Hi Ian,

I have been wondering how you were getting on with this project.

Looking good.

All the best.

Orion · #15 04-03-2007, 09:28 AM

Ian, what type of plywood are you using?
By the way nice project!

stringscope (Ian) · #16 04-03-2007, 11:19 AM

Thanks Mike, its great to be back.

stringscope (Ian) · #17 04-03-2007, 11:21 AM

Hi Houghy,

The plan is to have it ready for SPSP. However, due to a family issue there is only a 50% chance I will be able to attend this year

Cheers,

stringscope (Ian) · #18 04-03-2007, 11:25 AM

Hi Lester,

Yes I am back into it now. The first delay was a delay in the delivery of the secondary mount and mirror. The second delay was my distraction with the 8" refractor.

Cheers,

stringscope (Ian) · #19 04-03-2007, 11:27 AM

Hi Ed,

I am just using Hoop Pine plywood from a local building supply place.

Cheers,

stringscope (Ian) · #20 06-03-2007, 10:57 PM

UTA sheeted with 1.8mm ply to provide additional stiffness.
HC-2 Focuser fitted.
Secondary mount fitted.

UTA truss mount blocks currently being fabricated from 40mmX12mm Mountain Ash glued into 3 laminations.

Total weight excluding finders = 2.7kg which is about the same as the UTA for a 317mm Obsession.