How hard is it to build a telescope?

[SimonfromSussex]

Hi John,

You probably wont notice a massive difference between 16" and 18" however you said right at the start of the thread that you wanted to get it right first time and invest in something that will last so I would go 18" if you can afford it.

I very nearly went for 20" with my scope but I had the same thinking. If I had gone 20" would I always be thinking I wish I had gone 22"? My 22" is as practically big as I can go so will now hopefully last me for many many years! 

I think this might apply with the mirror as well. If you go for the explore scientific, will you be thinking in a few years time, I wonder what a better mirror looks like? 

I purchased the cell and secondary spider from astrosystems; http://astrosystems.biz they took a little while to post it (9 weeks) but I had to wait for the mirror anyway and the cost was very very reasonable and not much more than if I had made them myself probably.

This way I avoided the complicated bits and just went straight to the basic woodworking. I was very lucky to have Rusty's help (who is a master) with my scope but if you are any good at woodworking, I think the build is very possible.

There is a huge amount of satisfaction looking through a scope and thinking, I made this!

Best of luck what ever you decide.

[callump]

After reading the Kriege and Berry chapter on mirror cells, I believe that I could make one using the dimensions and bolt all the metal pieces to some plywood, which could then be marked to have a few holes cut in to allow air flow and fans to cool. Although all this talk of inches, especially for bolts and drill holes is giving me a headache!

So, the options would be:

1. An 18" F4 John Nichol mirror, focal length of 1828mm, with Moonlight focuser, telrad and truss poles c.£3k total build and a lot of work and learning.

2. Or a 16" F/4.5 mirror would knock £600 off the price in mirror alone.

3. Or an Explore Scientific 16" dobsonian  f/4.5, £1.6k and ready to go, give or take the odd 'fix' on arrival.

Hmmmm. Decisions! Still not sold that there would be much advantage to an 18" over 16", so may settle for 16"?

John

There is a general consensus that the K&B mirror cell layout is not the optimal. It would be better to download PLOP and feed in your actual mirror spec.

It might not show a great deal of difference, but is worth doing...

If you are bolting mirror cell elements to ply, you'd probably need a couple of layers of 18mm. And then the weight would go up. You need to worry a bit about weight, as if it becomes too heavy you won't be so keen to get it out...

/callump

p.s. why not push on to a 20"?

[gooseholla]

20" would never fit in or out of my house or shed.

My neighbour is onboard, I'm going to talk to a couple of my woodworking mates about them routing out the rings I need. I also hear rumours my neighbour has a welder, so may be able to build the cell out of metal.

[Moonshane]

you can make a basic scope very very easily. it's the fancy shapes and cutouts etc that take a lot more time. as does the finishing etc. if you have someone very experienced and a workshop this helps a lot. make no mistake that it's a lot of work to get it right but well worth the effort.

[gooseholla]

I have no doubt that it takes many hours. I have already told my neighbour the mirror takes about 3 months to come, so from ordering we have 3 months to make the calculations, cuts, and assemble, and then all the time in the world to get the trusses right and finish everything off neatly. Unlike when I made my last mount in a hurry, I have a scope to observe with, so no real rush to get it out under the stars - and if it slips towards summer (which I doubt but you never know), might as well let the build carry on until Autumn.

I have thought about it and the cost of buying a router, learning how to use it, etc. is going to be more than giving a woodworker a few quid to route out the secondary rings, altitude bearings, and finish off everything nicely for assembly. They'd probably do an excellent job in a couple of hours. So I shall talk to him on Sunday and say what I would like and all he will have to do is follow the markings on the wood.

You guys have been great help so far, and I am slowly taking inspiration from various big dobs on here and the net, piecing together exactly what I want and thinking about ways to do it. I saw a nifty way to make a spider that was so simple and I never thought of it! The next stage it to finish reading K+Bs book, working out some dimensions and figuring out what bits to get. Then I suppose bite the bullet and buy a mirror... I guess the good thing is, I am unlikely to lose money on a big dob if I ever need to offload it - assume it is built well!

John

[gooseholla]

There is a general consensus that the K&B mirror cell layout is not the optimal. It would be better to download PLOP and feed in your actual mirror spec.

It might not show a great deal of difference, but is worth doing...

If you are bolting mirror cell elements to ply, you'd probably need a couple of layers of 18mm. And then the weight would go up. You need to worry a bit about weight, as if it becomes too heavy you won't be so keen to get it out...

/callump

p.s. why not push on to a 20"?

Hi, I used PLOP and the measurements were near enough the same, give or take a fraction of an inch.

Now, what I don't know still is, the three bars are obviously centred at 120 degree intervals (360/3), but how far out along the mirror do they go? i.e. If an 18", are they 6" out from the centre, 10", 12" etc.?  PLOP gave a nice diagram, but no measurements for the positioning of each group of triangles, unless I am missing something?

Thanks

John

[callump]

If you look at the Part Dimensions graphic plot, it shows the relative position of the part at the top right of the screen.

If you are looking at an 18 point cell, scroll down to see the second part - the bar that holds the triangles. 

(sorry not able to see an easy way to paste a screenshot...).

Callum

[gooseholla]

Hi,

With the help of a couple of people (thank you) I believe I now understand PLOP and where the parts will go.

Looked at how pole blocks are made last night and clear on a way which meets my wood working ability to be able to anchor them at the lower points. Still looking at the top part to the secondary.

John

[Joseki]

Have you seen the Moonlite truss connectors?

http://www.focuser.com/cgi-bin/dman.cgi?page=category&plugin=dstore.cgi&category=7

John

[gooseholla]

Have you seen the Moonlite truss connectors?

http://www.focuser.com/cgi-bin/dman.cgi?page=category&plugin=dstore.cgi&category=7

John

thank you, an interesting product to consider.

Also, just so I am sure, can someone confirm that 1" x 1x tubular steel is box section? i.e. not solid? (for a mirror cell)

Thanks

John

[callump]

Yes - for the mirror cell 'tail-gate' you want box section.

Callum

[gooseholla]

Thank you. I have priced up the metal to be about £16 for the tailgate and bars for the triangles sent. I think I shall order the lengths, and mock up the primary mirror cell. A lot more than the free sheet of plywood I have lying about, but I am convinced of the benefits weight wise and thermal wise.

Oh one question, how thick should the walls of the box section be?

Thanks

John

[callump]

I'd have thought K&B would have the wall thickness.

Does not want to be too thin, as it needs to take a thread for the collimation adjusters.

Callum

[gooseholla]

All it said was "heavy wall". The only stuff I could find at 1" x 1" was 3mm walls. However, if that doesn't work I have a cunning plan to weld two small squares where the collimation bolts will go of slightly thicker metal and tap through that as well.

[mapstar]

You could weld on some nuts depending upon the skill of the welder a lot easier. Just need a tap running through them afterwards in case of distortion.

Have you looked for 16gauge thinner than 3mm?

[gooseholla]

I see they do 1 - 3mm thickness in 25x25 box section.

Nuts is an interesting idea... if it gets too much for the welder, I'll gorilla glue them on!!

John

 

[gooseholla]

The first part of the build is complete!

A few pence M8 x 60mm bolt is recessed into a 20p door knob. One collimation bolt made!



 

[gooseholla]

The Build Begins - 24/10/2014

So today I committed to the build. I ordered the metal 25x25mm box section for the frame and the two support bars for it which is 30mm x 6mm steel bar. I also have the 25mm x 6mm steel bar coming for the three connecting bars for the triangles. I am yet to order the material to build the triangles, though I did get a pack of 24 furniture pads. I also got some primer and matt black paint in spray cans to protect it all.

Three collimation bolts were made by drilling a hole into three wooden drawer knobs which would accept the hex head of an M8 set screw 60mm long (see above post for pics). These were Araldited into place and are rock solid. They were then covered in exterior varnish to give a nice look of expensive wood.

The telescope will be an 18" F4 truss tube dobsonian.  The budget for the build is £3k after costing 90% of things up. This involves provision of a bit extra for any unforeseen problems. The Moonlite focuser at £229 and the mirror at £2400 are the main chunks of money. I don't plan to have to buy any tools (neighbour + mates should cover everything), and will only need one sheet of plywood as I have a lot left over from the last build (if everything is cut right!). I plan to keep things simple, so that it is functional and easy to set up, yet buildable with my skills.

[SimonfromSussex]

Good luck Gooseholla! Look forward to watching your progress.

[gooseholla]

Thank you. So all the material for the primary mirror cell are on their way. I now have to try and work out what to do about the secondary. In the grand scheme of things, I want to save money, but a part of me knows spending c. £100 to buy one, in the long run, will work out simpler and make a more enjoyable experience I believe.

[Astro Imp]

Good luck with your project.

I will be following your build with interest, not because I will ever attempt anything like this but because I love to see the skill you guys exhibit.

[gooseholla]

Hi,

Just throwing ideas out incase I make a secondary. Thinking a few rings of plywood, one will have the vanes (thin bits of steel) bolted to it. The others will have a metal washer on for the collimation bolts to ride against. Then a 45 degree angle of wood, and maybe a sheet of metal bolted onto it for the mirror to attach to (or even just the wood)? Would this be strong enough to hold a probable 4" secondary mirror?

Below is a mock up of what I mean by the plywood ring.

Regards

John

 

[Sfarndell]

John - plywood cutouts work well. I did that in my 16" Dob (90mm MA secondary) and it works really well. I used silicon to glue the mirror directly on to the wood and have had no issues. (I also epoxied a safety wire to the mirror which is independently connected to the spider in case of failure!)

Here's the link to a photo before I neatened it up and painted it. http://stargazerslounge.com/uploads/monthly_06_2013/post-1508-0-70225500-1372244669_thumb.jpg

I made a wire spider similar to Mel Bartels' method. The wires are lightweight and very strong, but are thin enough that the diffraction effect is so minimal, I don't notice it even on bright objects. 

Scott

[gooseholla]

Thank you that is most interesting. I was planning on using steel rulers to make my spider vanes. However, the ones I saw were a few mm thick. I mocked up a couple of rings yesterday to see how the secondary would work out. Definitely would need to drill them out with a drill press, as hand drilling caused a bit of a wonk. But in principle it should work. As I told my dad though, I either spend £5 on this and potentially save £70+, or I've then spent £75 in total if it doesn't work, so worth having a go even If I don't use it.

Mirror Cell

Well the metal came for the mirror cell today. The metal is quite heavy! I have cut the side rungs to length today and I am going to cut the middle rungs tomorrow and tidy it all up for mocking up in a jig. My next door neighbour is in the middle of getting his drill press set up for the holes. Just waiting for the aluminium to come for the triangles. Hopefully within the next week I shall have a lovely, hopefully square, primary mirror cell ready to build up.

Also, I drew up the plans for my plywood to get cut. There is a good place down the road apparently that specialises in wood and cut it a lot better than B + Q, according to my neighbour who uses them a lot. So I shall be going down at some point to see them to see what they can do to get most of the bits cut nicely so I basically then just have to bolt it all together.
 

I talked to my woodworking friend yesterday and he has agreed to rout me some rings for the secondary and the altitude bearings as and when he gets the time within the next two months. Everything is moving along nicely in the preparation stages, just waiting on the money so that I can order my mirror. I have been in email contact with John NIchol who has given me some sizes and weight of the mirror. I don't think it will be overly heavy this build, maybe 50kg (obviously I won't have to lift all of that, as wheel will be involved.). My current mount and scope are about 20kg each, but the OTA is such a hassle to lug out and lift onto the mount. Looking forward to being able to wheel stuff about and set it up in stages.

John

 

[gooseholla]

Mirror Cell - 2

So today I finished cutting all the metal for the framework of the mirror cell. It isn't too bad, only a couple of mm out from square - not bad for hand cutting! My mate is going to sort it all out when he welds it.

Here is a picture of what I have. Everything needs filing to make the edges round and not sharp and so that they are roughly the same length.

The cell is made up from 25mm x 25mm box section steel, with walls of 2.5mm cut into 3 x500mm lengths. The side pieces are 30mm wide by 6mm thick flats of steel cut into 390mm lengths. The bars for the triangles are made from 5mm thick 25mm wide steel at 140mm lengths. This was going from the nearest conversion of inches that the Kriege and Berry book suggested. The cost of the metal was £7.70 but I had to get it posted, so was £9 postage, so it pays to find a place close you can collect it from.

Tomorrow's job will be to start making a frame to square it up in for welding, and then marking where to drill the holes for when my neighbour has his drill press ready. I am then going to think about the spider, as I am not going to order wood until after I have ordered the mirror, or it will be sitting around taking up space over the next few weeks untouched. When the frame is welded, I am going to install a couple of fans, and the hardware for a sling and mirror posts. It will be primed and painted a matt black colour. The aluminium I want to leave aluminium colour if possible to give a nice contrast to the open tailgate.  

As this is also a learning experience for me, I am not ashamed to admit I don't know everything and that I make mistakes. Here is something I would do differently a second time: No matter how thin the metal sounds, it is too much for a hacksaw! Yes, the job will be done, but it will kill and needs tidying up. Get it pre cut!

John