I've got an 8" collapsible skywatcher dob, and I love it. Having this scope has proven to me that I really enjoy observing, and that this is what I love about astronomy, as opposed to imaging, which doesn't really interest me....
So, I know that down the track I am going to want to build my own. This is something that will be a long-term project, so I want to start getting my head around it...I really don't have a clue as to how people start to plan such a job. Can anyone offer any advice to a complete novice? My initial thoughts are:
- I'm thinking 14" - 16" would be a good size. Can I realistically expect this to be as portable as the 8" skywatcher, given that the design of the 8" is pretty clunky and needs to be transported in two parts, base and OTA? The base alone is 70cm high...Would a custom built one also take up less space when stored?
- when building a scope, is it usual to get a primary mirror first, and then design around that?
- what is the typical overall cost for such a project?
- I have a friend who is a master with wood, and has an autocad(?) setup....are there plans/templates that people use?
Don't know if I can answer all your questions but here's what I would suggest:
Get a copy of The Dobsonian Telescope by David Kriege and Richard Berry. It explains the design principles you need to understand. The book takes you through how to build an Obsession style scope. This may not be what you ultimately want but the sections on building materials, how to make an effective truss, how to build connectors, how to align the optics etc are all relevant no matter what you decide to do.
Write some goals for your scope design, you mention portability and visual observation. Think about what you really want from the scope - one person set up? no tools? What size car do you have, when and where you will use it. Then look at the sites on the net and see if there are any designs that meet your objectives. Mel Bartels site and Albert Highe's are a good start for ideas.
A 14 to 16 inch portable scope is certainly possible I have my doubts as to it being as easy to set up as an 8 inch dob. I have a 12 inch home made goto dob. It is easy to transport although it takes up most of my old commodore wagon's luggage space. I have worked out that a 14 would take about the same space because I overbuilt the 12. A 16 would be more of a challenge (but still doable). If you can buy a thinner mirror this keeps the weight down and makes everything easier.
Cost is hard to guess at. It depends on the materials you use and the design you choose. I like Mel Bartels ultra light approach. He takes a minimalist design approach which helps minimise cost (small mirror box, longer trusses, single ring secondary cage). Have a look at his Tri-dob to see what I mean. Your biggest costs though are the mirror, eyepieces and focuser. The other costs are minimal by comparison.
If you have a friend who can cut the parts it makes the project much easier. Most people make their own plans. I am not aware of any available on the net but there may be. There are free design programs you can use so you can draw the plans and give them to your friend. I use eMachineShop that was suggested to me on this forum. It is easy to learn.
Thanks for taking the time to post such a great reply!
That's plenty of info there to get me started. I'll track down a copy of The Dobsonian Telescope as a first priority.
It's good to know where the main cost will be. I've just started towards building a good eyepiece collection, so I think the main cost for me will be mirror and focuser.
In terms of portability, I'm definitely after a one-person setup, and something that can fit in a good size sedan, 2001 Magna. I think there's a hatch through to the boot from the back seat so trusses could be transported there...
I suggest you start small and work up. Seeing you already have an 8" scope why don't you try a 10 or 12 " first. Everything is that much smaller and lighter ................ and cheaper. Also with these sized scopes you can conduct most if not all your observing seated.
As a general rule it seems to me that; telescope weight increases as the square of the aperture and the cost increases as the cube of the aperture.
All my scopes have be built around second hand mirrors so I have always acquired the mirror first. I have then designed the scope to the design criteria, around the weight/focal length of the mirror.
My experience is that home made scopes are NOT cheap. Marine ply, stainless steel fittings, high quality focuser, powered ground board, etc, etc.
Here are some images of my 10" F4.5 scope. I use this mainly for outreach tasks and for when I don't feel like setting up the 16". The scope and accessory case occupy a single seat in a car - suitably restrained of course.
The 1 metre steel rule is for scale.
I also have a 16" built on similar lines. I find a 16" scope more of a commitment. Sufficiently heavy to present elevated risk of back injury. I ended up buying a van for telescope transportation duties.
That scope looks great....I think I'd be very happy if I got halfway to that quality of woodwork...
I understand what you're saying about starting off a bit smaller...I think I might even be getting ahead of myself by considering what size primary mirror. After an enlightening conversation with Alex and Peter (thanks again! ) I think my first step will be to get a copy of The Dobsonian Telescope, have a good read and do a few sums regarding focal length, and think about what height scope would suit me best...
I'll also keep in mind your rule about weight and cost vs. aperture....
I happened to have the 16" set up tonight so I also set up the 10" to provide a height and bulk comparison. Both scopes are F4.5. There is a 1 metre steel rule in some of the photos to provide scale. At the zenith, 10" scope eyepiece height is about 1100mm, 16" is 1760mm. I can pick up and carry the 10", not so the 16".
At 1760mm eyepiece height 16" f/4.5 might just be possible for me...(I'm 180cm without shoes)
These photos really show the difference in size clearly...when you say you can pick up and carry the 10" but not the 16", is that fully assembled or packed?
There are a lot of designs out there, so I'm just trying to look around, see what I like and get my head around what the basics are...at the moment I'm basing my thoughts around a 16" f4.5....I think this would put the eyepiece at a reachable height at zenith...
My 17.5" is inspired by Albert Highe's design. I've taken his principles of design and further developed it to my needs, tools and capabilities.
Tnott helped me out a lot in the design development too. There are lots of little details that need as much attention as, say, a mirror cell. They are not difficult details. Really, nothing is really difficult, we are not building a rocket. Just details. Get these right too, and your machine will be sweet-as.
details, details...I imagine these will pop up as I go, and I may have a couple of questions as I get further into this...
tnott, I'm pretty slow sometimes.... I've just realised it's your scope I referred to in my last post here...
It's a beautiful design, and looks quite manageable for one person? I'm very interested in the CAD aspects...I'm lucky enough to have a friend who is a pattern-maker, and he's got a setup that would be able to do this part of the job very well.
In terms of materials, has there been much done with carbon fibre? I'm wondering if this would be good to use for the upper end....
I found the Tridob design very complex with lots of wooden bits to design and cut. No exotic materials like carbon fibre.
The big advantage of the design is the fold over bearings, which is the tallest part of a big collapsed truss dob. Not a problem for apertures up to about 14 inches usually
The crossed over wire spider design I used led to a see-saw pressure on the single upper ring, which meant that the secondary did not hold collimation too well. I had to fix this by reinforcing the top ring with square aluminium tubing, so it is OK now.
The UC design is inherently more rigid between secondary and primary, as well as having the fold-over bearings. It also had a lot less wooden components than the Tridob but a lot of steel to cut, bend and weld.
The Highe style 10 inch was much easier to plan and build and didn't need any welded steel components. It works well up to about 18 inches and the components can nest for transport.
What I really like about the tridob is way the the base ring, flex rocker and mirror box go together, and the low profile..... The design of the bits sounds tricky, especially the altitude bearings, but hopefully the cutting side of things will be something my friend can help me with...
As far as the upper end goes this is where I was wondering about the carbon fibre----but I'm still learning about what I need to consider when looking at upper end design. I also don't know how carbon fibre's rigidity-to-weight ratio compares to wood. My friend suggested we use it, when he thought it was a solid-tube design I was after, but then I started thinking about just the secondary/eyepiece cage....
I'm not sure what you mean by UC design...and I don't think I have the equipment or ability to cut, bend and weld steel...
Have you had any problems with the truss rigidity between secondary and primary?
A conglomeration of various ATM design aspects from the last decade and built by the same person who authored 'The Dobsonian Telescope'.
Very good rigidity between primary and secondary. Steel mirror cell, bolted to interlocking poles, bolted to steel pyramid spider, bolted to secondary holder. The wooden top ring, altitude bearings, rocker and ground board absorb vibrations. No problem with the secondary going out of collimation between horizon and zenith with this design.
One difficulty of this design is getting the fold-over bearings to meet exactly. I designed my 22" on CAD got most of the metal and wooden bits laser cut, then had to go to a metal shop to get bending, welding, tapping done etc. A few hundred bucks extra is not much for a 22 inch telescope, but for a smaller one.......
Both the UC and Tridob are balanced by carefully choosing the diameter of the altitude bearings. Too small and the scope is top heavy and visa versa. The resultant oversize bearings make he scope a little firmer to push in altitude.
The lack of a center pivot around which to shim in the Tridob means that you will also have a firmer motion in Azimuth for scopes bigger than about 16 inches. Need to keep it waxed too.
The Highe-style scope has smaller altitude bearings and ability to shim around the center pivot. Can also fine tune balance by moving the side bearings up and down the poles.
Peter (Hickny) put me on to a 12" GSO at Andrews (thanks again Peter) that needs recoating. The price was right, so I grabbed one (Andrews actually had three...) and took it off to Isaac Qin here in Sydney. As he explained, sometimes the GSO mirrors need refiguring before recoating, so it was a bit of a nervous wait until Isaac could remove the old coating and see what the situation was. Luck was with me, all that needs to be done is the recoating, so I'll now have a nice GSO mirror with Isaac's excellent coating and protective coat.
So, it's a 12" for me. Probably the best way to go for my first build, and I'm happy to follow the path of least resistance. It'll still be a few months before things can get going, but I've at least got a start on the project...
Great post Stephen - I bought some gear from Alex many months back to build a 12inch, which I STILL haven't got around in doing. Seeing the Master Woodwork from Ian is really inspiring as well.
The Book by Kriege and Berry I purchased from the advice from a few here (Thanks Alex) and it is very useful -Highly Recommended- I have written down some plans, obtained some marine ply (1.2 x 2.4 Mtr) that was going to be put on a Bon Fire (NO !!!!) so really I have no excuse now....
Buy the book by Kriege & Berry...you wont regret it..
) I think my first step will be to get a copy of The Dobsonian Telescope, have a good read and do a few sums regarding focal length, and think about what height scope would suit me best...
