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

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

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

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

Neat design and well executed, well done. Jim

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

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

Well, have to report a bit of a disappointing trial in setting the truss tube lengths. We were trying to do a visual confirmation of the focal point but from the school grounds it was quite difficult to find a suitable distant target to aim at. We eventually found a distant TV transmitter mast that we could see on the horizon but we couldn't reliably align the upper OTA to the mirror to do a trial focus. I need to have a rethink, most likely now over the summer holidays as we finish the term this week. On a more positive note, we had success in using the mirror to do some Schlieren imaging; a good bit of fun (told the pupils it showed them evaporating) and it throws up some really interesting possibilities for future work. The setup is quite straightforward; an LED acting as a point source is placed at twice the focal length and then photographed behind a knife edge (razor blade). On the very quick trial we did, there was evidence that it was resolving the boundary layer around some of our test objects. There may well be a possibility of visualising the cooling of a mirror as previously suggested. Next update on the telescope in a few weeks - holiday season is well and truly upon us - days to do In the meantime, if anybody has suggestions for sizing the truss tube to the correct focal length I'd be glad to hear. I know the mirror focal length so is it a simple matter of cutting the poles to suit - allowing for inner radius of OTA to focuser? Grateful for any advice, I'm nervous about trimming too much off the tubes and leaving them short Jim [youtube]

Way back in my student days we had a visit to Weir Pumps in Glasgow. Our lecturer was taking us through the various machining stages of a hydraulic valve block - it was really a work of art with lots of convoluted internal passages. We noticed a partly complete valve block set to one side and when we asked about it we were told that it was rejected due to machining faults. The valve blocks, even in a reject state had an intrinsic value of tens of thousands of pounds due to the lengthy and complex machining that had been undertaking. It still sticks in my mind today that every process done on a component increases it's value due to the time and effort already invested in it. It's really coming home to me now on this build. Jim

Thanks Brian; I'm getting good at not photographing the bits where the chisel slipped or the joints don't quite mate Jim

Made a fair bit of progress over the weekend; amazing weather so nice to be working outside in the garden Mirror box rings are now screwed down and secure, good to be able to move it without the rings slipping all over the place. The rings are held in place with wood screws through a section of three rings with two three ring sections themselves screwed together. I have some M12 ss bolts on order which I'll pas through the whole box once they arrive but it is secure enough now to continue working. I've also finished the lower tube ring, this will allow the tubes to be connected to the upper OTA separate from the mirror box in turn attaching to the mirror box as a complete unit. The idea being I guess to prevent the mirror being damaged by a pole slipping when getting attached. All of the truss tube sockets are also attached, rather than screw the socket blocks into the rings I tapped the delrin blocks (M5) and secured them to the rings with cap screws. I think it gives a neater finish, should be more secure and allows for a little adjustment if needed. It's funny but I guess in most project builds there are notional milestone moments. I've been itching to get the truss tube connectors attached for some time so I'm happy now; I couldn't resist a trial fit of the tubes once I got the ball socket inserts installed. It's starting to look like a telescope now. The tube inserts (ball sockets) drifted into the tubes quite easily and, while nice and secure, I'll go back later and screw in a self tapping screw through the tube wall. The next stage is to size the truss tubes so all being well I'll have mirror box and mirror together with upper OTA and the tubes into school this week. I'm really looking forward to this as I should be able to get a good number of the classes involved in setting the tube length. I'm still working out the detail but the idea of the cradle arrangement in the last photograph is that it will hold the upper OTA allowing it to slide up and down on say four of the truss tubes to allow us find the ideal focus/length. The next stage is probably the most challenging - the bearings. I've had some really good ideas from starman345, threw up some things that I hadn't really considered; some design features that may be possible or not but worthy of thinking about. I'm not going to rush this stage but with the summer (school) holidays almost upon us now I'll have time to work on it. I must admit, I'm really happy now to have this amount of work done, it's a good confidence boost. Thanks for all the words of wisdom and encouragement folks Jim

I must admit brantuk I was surprised at the weight. With the the mirror installed it really is at the limit of a one man lift. I wouldn't like to carry it any great distance so I think in normal operation the mirror will be stored separately and fitted in situ. Either that or some sort of detachable wheelbarrow arrangement. Having said that, the mirror box will be lightened a little by cutting sections from the front so that it can rotate on the bearings; a bit of a shame because I think that will spoil the aesthetics a bit. I'm not entirely sure at the moment how I'm going to do this though, something else to work out I'll also cut some circular holes in the base of the mirror box to lighten it there (improve air flow around the mirror as well). At the end of the day though you are right, there will be a lot of weight in the mirror box - I hope I can get it to balance ok and move smoothly. I like the idea of being able to mount lots of accessorises on the top tube - once I get confident I can swap out the Moonlite focuser for the big Van Slyke crayford , it has a robofocus attached. Moonshane, thanks. Moonlite really do have an eye for attention to detail and quality, good products allround. I like the look of the clamps and being made of delrin they can be tapped to take a screw thread for attachment to the tube rings. I'm glad I held onto them and pleased to see them being used at last. I'm just hoping it doesn't take too long to trim each tube socket or that I don't take too much off ! Re the design, it's not mine but rather it's based on the telescopes by Gordon Waite of Waite Research http://www.waiteresearch.com/ . I was drawn by the simplicity of the circular mirror box. Overall it's not the easiest thing to make - I'm not looking forward to doing the bearings, but I think it will be worth it. I wanted something special to do justice to Andrew's mirror. Jim

Well the upper OTA is effectively finished now, just needs final sanding, staining, inner flocking installed and then gluing up. Next job is to join all the rings of the mirror box together, install the truss tube connectors and mate up to the upper OTA. I'll need to put together a jig to hold and align the rings before I bolt them together; another job for the weekend. I'm hoping to get things ready so that we can size up the tubes next week in school - we have 2 weeks to go before before we spilt for the holidays. No pressure then. Andrew included in the package a 3'' Van Slyke crayford focuser. It's a marvellous piece of engineering if not a little intimidating, but it will certainly offer a heavy lifting capacity for future developments. At the moment, and to get things up and running, I've borrowed my own Moonlite focuser from my 8 inch scope. The Moonlite is positively tiny next to the big 3 " focuser but I thought it would be a lot easier to fit to the upper OTA. I'm really pleased with the Moonlite truss tube connectors and would recommend them to anyone thinking of a similar project. I bought them about 10 years ago having intended to use them on an earlier project that was postponed. They are really well machined (made from Delrin) and should give a neat finish to the scope. One job remains however; the connector sockets are a little oversized for the inner diameter of my truss tubes. I tried turning them down on my drill press at home (see scuff marks on right socket) but it takes too long. I'm going to see if I can blag some time on the lathe at school, it would save a lot of time and effort. So the next task is to get the mirror box ready for next week so we can have it back in school to do trial focus and size the tubes. So far everything has been going well, only a few hiccups along the way. The bearings is where things will get interesting! Jim

Thanks for the suggestion Chriske, I'll need to take a good look at the camera, maybe touch base with the experts that loaned it but I'm not sure it has the resolution to pick out the boundary layer. Maybe if there was a sufficient temperature differential, I'll need to play around with it. Good idea though. In fact it has prompted a thought on scherliern photography as a possible demonstration with the mirror. If I remember correctly there was a recent thread demonstrating a setup using an installed mirror. Scherlien produces some lovely effects that should be quite eye catching for a classroom demonstration. Jim