16 " Open Truss Dobsonian Build

[saac]

Some photographs to show the setup for the Schlieren imaging above.

 

Jim

 

 

[Chriske]

To determine the distance between Primary and secondary cage I always use dummy poles. Thin square wooden poles, or whatever I have  at hand. I fix one set of poles to the secondary cage and the other set of poles to the primary cage. I fix them together with clamps. It's all very basic, don't even think collimating, see to it you can focus on the moon or a bright star, just find the correct distance. If it's not correct just loosen the clamps and slide a bit further. You need clamps, lots of them. If you don't have clamps just screw them together.

While removing clamps PROTECT YOUR MIRROR...!!!

[saac]

That sounds good Chriske, it's what we were really trying to do at school but we didn't have the means of securing the upper OTA well enough. So clamps it is

Jim

[saac]

I made some further progress today and managed to finish cutting out of one of the altitude bearings. I'll look to cut out the rest of the second bearing tomorrow and then get them trimmed and ready for gluing up.  I wonder if anybody could point me in the direction of a supplier for ptfe strips.  I found some sheets on Amazon but the delivery cost was more than the sheet itself!  

Jim

[mapstar]

47 minutes ago, saac said:

I made some further progress today and managed to finish cutting out of one of the altitude bearings. I'll look to cut out the rest of the second bearing tomorrow and then get them trimmed and ready for gluing up.  I wonder if anybody could point me in the direction of a supplier for ptfe strips.  I found some sheets on Amazon but the delivery cost was more than the sheet itself!  

Jim

Ptfe sheets? Or laminate/formica sheets? 

Ptfe/teflon is normally used for small pads that the formica/laminate on the bearings run on. Ptfe can normally be had quite cheaply on fleabay 

Edited August 1, 2017 by mapstar

[saac]

44 minutes ago, mapstar said:

Ptfe sheets? Or laminate/formica sheets? 

Ptfe/teflon is normally used for small pads that the formica/laminate on the bearings run on. Ptfe can normally be had quite cheaply 

It's the ptfe I'm trying to source Damian, I already have the formica. I may just bite the bullet and go with the supplier on Amazon but it seems a wee bit overkill for what I need.

 

Jim

Ptfe Sheet

Edited August 1, 2017 by saac

[brantuk]

These guys are usually at most astro trade shows and I've used them a couple of times via mail order. They're also very good with advice if you give them a call:

http://www.awrtech.co.uk/miscprod.htm#PTFE

 

Edited August 1, 2017 by brantuk

[saac]

38 minutes ago, brantuk said:

These guys are usually at most astro trade shows and I've used them a couple of times via mail order. They're also very good with advice if you give them a call:

http://www.awrtech.co.uk/miscprod.htm#PTFE

 

Thanks brantuk that looks useful. I'll give them a call tomorrow.  

Jim

 

[mapstar]

6 hours ago, saac said:

It's the ptfe I'm trying to source Damian, I already have the formica. I may just bite the bullet and go with the supplier on Amazon but it seems a wee bit overkill for what I need.

 

Jim

Ptfe Sheet

Just an example the same as what I have used in the past. Being 6mm thick you can drill and countersink it so you don't have to use etched stuff to glue on. 

http://m.ebay.ie/itm/360978773408?_mwBanner=1

If you want thinner material I have some 3mm sheet in the garage somewhere I could sort you out with. PM me if you are interested.

Damian 

[saac]

Thanks Damian that's another good source; the 6mm dim looks ideal as well.  

Jim

[saac]

Been a wee bit busy at work so progress on the Dob has been a bit slower than I would have hoped.  These things always seem to take longer than planned   Anyway, I finally have both bearings cut and the bearing supports mounted on the base board.  The photos below show a trial fit, the outer faces of each of the bearings have yet to be glued hence the clamps. At the moment, the mirror box is simply sitting on the bearings, but everything moves smoothly so far.  I hope to sort out an attachment arrangement for the mirror box to the bearing assembly next weekend together with finishing the ground board; I have some thrust bearings and a few router bits being delivered mid week hopefully   So far, no major snags; everything is moving along, just taking a wee bit longer than planned. We are hopefully still on track to have it ready for the return after our Oct mid-term.

Jim

Edited September 11, 2017 by saac

[saac]

I also picked up a nice SW 2" EP (28mm) from AstroBoot, I thought it would set off the scope nicely.  I'm also keeping my eye out on AstroBoot for a focuser, my own scope is missing its Moonlite now that the sky is getting dark. No real urgency, I still have my ED80 mounted so should keep me happy enough  

 

Jim

 

Edited September 11, 2017 by saac

[MikeCM]

Just picked up on your thread Jim. So this is the telescope Euan has been telling me about - Looking pretty impressive so far and look forward to seeing pictures of the final article.

Regards Mike

[Paul73]

That looks really impressive. I really like the mirror box(?) / Rocker assembly. Yes. Telescopes should be found!!!

How much the bottom section weigh (box,rocker,base and mirror)?

Paul

[saac]

Hi Mike, yes this is the monster. I had thought it would have been completed  before Euan left but I underestimated the time needed. All being well, it should be ready for the return from the Oct half term. I'm busy looking for a good dark site in the local area for a first light test - Kingsbarns near St Andrews looks promising, good car park site down by the beach.  I've got a group of 5th year students itching to get their hands on it so when we are ready to go you and Euan are more than welcome to join us. After that, the plan is to take a trip out to the Scottish dark sky observatory down in Dumfries and Galloway or maybe up to Skye.  I must admit, I'm getting quite exited now, I've never looked through a large aperture scope so I'm hoping for a big difference.  

 

Jim

[saac]

17 minutes ago, Paul73 said:

That looks really impressive. I really like the mirror box(?) / Rocker assembly. Yes. Telescopes should be found!!!

How much the bottom section weigh (box,rocker,base and mirror)?

Paul

Paul,

Thanks Paul. I'm trying t o emulate the Waite Research design as much as possible although I've had to make some changes due to use of hand tooling and my woodworking skills.  I must admit, I do like the look of the circular mirror box, it's my intention that it will also serve as the mirror storage box when off the telescope.  The lower OTA , with mirror and cell, is heavy; I haven't got around to weighing it properly yet, maybe do that at the weekend and have a look at the turning moments as well. I fitted the azimuth bearing yesterday and it rotates really smoothly in both azimuth and dec. There is something quite pleasing watching a big scope move around - simple things I know  

 

Jim

[saac]

2 hours ago, Beulah said:

Fabulous work. I've not seen the multi-laminated rings before as a primary mirror box; very original!

I have found out that Flexi ply is the best thing since sliced bread!

Thanks, Beulah. I spent some time, perhaps too long, looking at different designs before I settled on this. I found a few that used a circular box but the Waite Research design caught my eye.  It is a more expensive and time consuming way of doing it (as DIY) vice a rectangular box but it is supremely strong and aesthetically pleasing. I've used FlexiPly in the upper OTA, akin to a monocoque design where the main loads are carried by the ply. First time using FlexiPly and I must admit it's a fun material to use; I need to find something else to build with it  

Jim

Edited September 14, 2017 by saac

[saac]

Had a good run at the build over the autumn break. Bearings and mirror box are all glued up now; really messy job but everything is nice and secure now, no more clamps  Ground board is also complete and rather pleasingly the mirror box still moves smoothly in the alt bearings and rotates  in dec without snagging. The truss tube are still waiting to be sized and fixed, I had hoped that would have been completed but the weather over the last two weeks has been dreadful with permanent cloud cover. So sizing and fixing the tubes will hopeful be done in the coming week. What remains then is the final strip down, sanding, staining and varnish, flocking the upper OTA. There is also some minor tweaking to be done - I'm guessing this is never really finished.  The mirror cell has 3 upright posts that secure the mirror from sliding completely of the cell. To place the mirror in the cell when installed in the box one of the upright posts needs to be removed. Unfortunately, the screw securing the posts is not easily accessible when the cell is installed in the mirror box. I had though about drilling a hole through the mirror box to allow access to remove and secure the upright post but I think that would spoil the look of the box. I think what I'll do is removed one of the posts competently and install a modified "stop" fixed to the mirror box itself that can be removed from the top of the box. All of that probably doesn't make sense without a picture so I'll posts something later. Other than that, I think we are ready -  so truss tubes fitted then a first light session hopefully early November with pupils at school with a visit to a dark sky site to follow  

Jim

Edited October 22, 2017 by saac

[mapstar]

Looking good. 

These clouds are not good. One session out this winter so far.

Hopefully they will clear and you can get it all set up. Nearly there though 

[saac]

Cheers Damian, I'm really itching to get my eye to eyepiece on this. The weather really has been beyond frustrating but such is life I guess. Once I have the initial set up I'll be pestering you and the Dob Mob for some advice on fine tuning and operating tips   Straightaway I can see that the size of the thing is going to present its own challenges. 

Jim

[saac]

Hurrah - it flys   We had a clear night in Fife yesterday and I finally managed to set up a temporary jig so that I could measure and cut the truss tubes, wasn't easy but we got there. I took the scope out tonight and although the collimation is way out I was relieved that it could get the stars in focus. A couple of immediate observations:

1. Movement and balance - movement is nice and smooth, no jerking in either axis. Despite my concerns about the weight of the mirror box, the large moment arm means the scope is a little top heavy. It balanced perfectly with a 2lb lump hammer hanging from the box (hey it was easy to hand) so should be a relatively easy fix (i'll use something more refined than my lump hammer).

2.  Moonlite connectors - performing really well although the tri point knobs for tightening the tubes in their sockets are not suitable for cold dark Scottish nights. They are too small, fiddly and the pointed ends really hurt against freezing cold fingers. I can see these being replaced with some round rubberised knobs. 

3.  It's big !  I need to rotate the upper tube so that the focuser is in a more convenient position for viewing (easy fix) . Notwithstanding that, the scope at F5, places the eyepiece well above eye-line of your average Scottish astronomer!  Our diminutive height up here helps when running down haggis on the hills but not so good for tall telescopes  Some form of suitable step will need to accompany the scope. 

4.  Setup - connecting the upper tube to the truss tubes and then to the lower assembly was a bit tricky - as one tube was located in a socket another would pop out. Maybe it was because it was the first time but it was a wee bit frustrating, there is probably a routine that works best so I'll figure it out. Strike down was really simple - took about 10 minutes to have it all packed away and back in the house.

Ok so some fettling to do before I can take it into school. Apart from getting the scope stained and varnished, I need to sort out collimation and fashion some sort of shroud.  December is looking good for our first star party, a nice Christmas treat. I'ts been hard work, about a year in total since picking the mirror up from Andrew,  but it's a really great feeling getting to this stage, I'm quite excited about getting it into school its going to be a real talking point in class and it will certainly add something extra special to our courses. I'll be using the mirror on its own next week with the Higher Physics class as we look at refraction (Snell's law) - our trial at Schlieren photography (see earlier post) worked really well so I'm looking to incorporate it as an enhancement to our normal practical work. We are getting there Andrew, I now just need to work out how to mount that massive Van Slyke Inst focuser as well  

Jim 

 

 

 

 

 

 

 

Edited November 12, 2017 by saac

[andrew s]

If it's too tall for you wee Scots use the video camera! 

With the VS focused it will be very top heavy. Why not sell it and get a smaller one?

Well done a magnificent effort on your part.

Regards Andrew 

[saac]

On 12/11/2017 at 23:44, andrew s said:

If it's too tall for you wee Scots use the video camera! 

With the VS focused it will be very top heavy. Why not sell it and get a smaller one?

Well done a magnificent effort on your part.

Regards Andrew 

Yes the video camera of course, I was concentrating so much on getting the scope ready I forgot about that, thanks Andrew. While the kids will definitely want to get their eye to the eyepiece the video camera will be ideal for group sessions.  To be honest the height of the scope just adds to the drama, it's going to look really impressive in the classroom.  I took a fresh look at the VS focuser last night and it is a capable bit of kit but it is a bit heavy. I think I'm going to use it as a vertical stage for our DIY interferometer - it already has as good xy stage but lacks fine motion control in height adjustment (z axis). The VS will be more than up for the job.  As for the effort - well what can I say, I spectacularly underestimated just how much time this would soak up. It has however been wholly enjoyable and very satisfying; you should have seen my face when I got the upper OTA secured on the truss tubes and had it fully assembled for the first time. I was skipping around the garden like a 12 year year old   Photographs to follow of our first outing with it hopefully early December.

 

Jim

Edited November 14, 2017 by saac

[brantuk]

Looks fabulous Jim

Are you going to paint or stain it? Does the alt axis feel stable turning the scope at lower elevations or d'you reckon it might need a brace near the end?

[saac]

Thanks brantuk, it really has been an enjoyable project.  Yes, I'm looking to stain it then put a clear satin varnish on it. I'm hesitating at the moment trying to find a decent wood stain/dye - I'm too indecisive. The ones I've found in the usual high street DIY chains all seem to be non alcohol based - the dye doesn't seem to penetrate the wood very well. The one I'm trying out at the moment is Coloron Wood Dye  - a chestnut colour. I also need to flock the inside of the upper OTA and paint the inside of the mirror box black.

I think the alt axis is fairly stable at the moment but it will need a proper shake down to expose any problems. I think you are right about a brace though between the two bearings. I think it would certainly do no harm but I'm worried it may detract from the clean lines of the circular box and bearings. I'll see how it performs once we start using it properly then make a decision, but I wouldn't be surprised if I end up adding a bracing bar or two. There may be a slight wobble between the two ground boards at lower elevations - I think it bounces a little when its moved in lower elevations. I think it can be solved with perhaps a few more contact points between the two circular boards - I've used teflon furniture gliders. Either that or a heavy mass sitting on the ground board but I'll try to avoid that.  What I really need now is a good run of clear skys and decent weather to shake it down and find out how to use it.

Jim 

Edited November 14, 2017 by saac