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16 " Open Truss Dobsonian Build


saac
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The holder is indeed hollow as Andrew has intimated; the present configuration is unlikely to impede air circulation around the secondary to the extent that it induces any discernible impact in the mirror attaining thermal equilibrium beyond the usual wait period.  As for removing the secondary from the mount itself, I don't envisage any problem that an gas axe wouldn't overcome ;)  Joking aside, I suspect that my carpentry skills and ability to join two pieces of wood at right angles will introduce a far greater percentage uncertainty to the optical performance of the telescope than the mounting of the secondary.  That said, given that this is a school project then all comments are greatly received and will be useful to get the kids thinking about the design process and the balance of theory against practical constraints.

Jim

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Had some fun in the class today getting the pupils involved with the build.  We are very fortunate to have access to a thermal imaging camera on loan from a local company.  I've borrowed the camera before and it's been a great way to enrich the experience of the pupils with some industry standard equipment. In the past, the camera has been used to examine methods of controlling heat loss from buildings.  Anyway, I was describing why the thermal response of a telescope mirror can influence the optical performance and we thought  "well let's take a look with our own IR camera" :)   It was good fun playing around with the camera in general, things can look so different in IR and there were some surprising results. As for our secondary, what we found was that the mirror itself showed no temperature gradient across its major dimensions.  It also looked to be reaching equilibrium with its surroundings reasonably quickly - our lessons are 45 mins long so some more testing is needed.  I'll post some more pictures once we have downloaded them from the camera.  I also need to give some more thought to some meaningful tests with the IR camera before I hand it back; might be interesting to look at the primary for example to see how it cools :) 

 

Jim

 

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Edited by saac
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Well done Jim on expanding the telescope experiences to include other aspects of science, that are still telescope related.

It also takes me back to my school days to see "proper science benches"

Keep at it, you are obviously inspiring the next generation, well done and good on ya!!!

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7 hours ago, Uplooker said:

Keep at it, you are obviously inspiring the next generation, well done and good on ya!!!

Not just the next generation...

Both @Mrs Racey and I are enjoying this thread enormously 

:thumbsup:

Edited by Racey
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Interesting bit of research and good to see the pupils involved. A thermal camera is a novel idea.

I was ronchi testing a 14" mirror yesterday in preperation for a build will you be trying this with the students? This is also a good test for showing how thermals in the light path affect the performance of the optics.

I have yet to do a matched test image to see how it compares but the software is easy to use although having a micrometer stage is the hardest part to set up. The mirror thread I did may help explain better.

Damian

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Jim,

Depending on the sensitivity and the range of the camera you could observe the boundary layer of the primary, see how long it takes to get rid of.

 

21 hours ago, saac said:

... might be interesting to look at the primary for example to see how it cools :)

Jim

 

 

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On 2017-6-10 at 00:49, Uplooker said:

Well done Jim on expanding the telescope experiences to include other aspects of science, that are still telescope related.

It also takes me back to my school days to see "proper science benches"

Keep at it, you are obviously inspiring the next generation, well done and good on ya!!!

Thanks Ian. Re the benches - I have a love hate relationship with them. They are some 50 years old, original to the school, but they are great for setting out various experiments where you need a lot of space - like building a telescope :)

Re ronchi testing, that's a really good idea Damian. I hadn't really planned on doing any testing as such as it is a completely finished mirror that Andrew donated but there is possibility here to link into some content of our Higher Physics course on interference.  I have a Bath Interferometer which I made for a previous mirror project.  I've had a project on the back burner to upgrade it with a proper XY stage that I sourced about a year ago, so looks like another job for the holidays. We've used it in the classroom before and, despite its rudimentary stage, it has given some really pleasing interferograms.  I'd probably need to swap out the Grin lens to fully illuminate the larger mirror disc but it should be a good project to make further use of the big mirror.

 

Jim

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On 2017-6-10 at 09:48, Chriske said:

Jim,

Depending on the sensitivity and the range of the camera you could observe the boundary layer of the primary, see how long it takes to get rid of.

 

 

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

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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

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13 hours ago, brantuk said:

Looks fabulous - and there's gonna be a lot of weight in that mirror box so you'll be able to mount loads of accessories on the top tube. :)

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. :unsure: 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.:smile:

8 hours ago, Moonshane said:

Those clamps ooze quality. Build looking excellent and original too.

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

Edited by saac
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1 hour ago, saac said:

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. :unsure: 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.:smile:

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

Bespoke is what you get when you build your own. 

Nice to see another build under way and progressing well. 

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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

 

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7 hours ago, saac said:

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

Hahaha! Yep - I've done that too - had to totally rebuild a mirror box cos of one little slip - but you won't find any photos it no matter how hard you try. lol :grin:

Edited by brantuk
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15 hours ago, brantuk said:

Hahaha! Yep - I've done that too - had to totally rebuild a mirror box cos of one little slip - but you won't find any photos it no matter how hard you try. lol :grin:

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

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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]

 

 

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