saac

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

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

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.  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.

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

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

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

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

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

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

Ade, don't know if these may help (sorry for the focus). Roll pins are split along their length as seen in the second photograph, the hole is drilled undersized and the pin drifted in under under compression; it's held in place under an interference fit. 

Jim

Ade, sorry it's been a while since I'd logged on and I've just spotted your message. Firstly, thanks for your comments and good luck when you get around to doing your build; great fun and frustration in equal amounts   As jacksdad commented, the threaded connectors are attached to the vane using roll pins.  The hex connectors are first slotted - I used a hack saw, perhaps a Dremmel cutting disc would be quicker and neater. The connectors were then drilled to accept the pin which was then trimmed and peened once inserted in the vane.  Useful if you have a drill press to help keep everything square.  I sourced a selection of mixed size roll pins from Amazon, fairly easy to come by but if you have any trouble let me know, I have a fair few left over. 

I made a wee bit of progress on the upper OTA over the weekend.  I cut the tenon and mortise joints for the vertical supports (spider supports) together with the focuser board. These are now installed and will be glued up in due course. I also had a trial fitt of the spider itself and all looks good; I need to order some more fasteners and install the focuser and that should be upper OTA finished.  I'll post some photographs in due course but this is a busy week at work and at home so may take a wee while.

Jim  

Hi Damian, the secondary is 125mm on the MA and, together with holder, comes in at just over 1kg.  I've no real worry about the spider holding the static load of the combination; I'm just hoping that it will not flex overly making retaining collimation problematic.  I think as Andrew suggested, I'll have a better feel for how it will perform once installed and under tension.  I've never used this type of telescope before, would I be right in thinking that it would need to be collimated each time it assembled anyway.  How well do you find they hold alignment during use?

Jim

Thanks Damian, I'd love to say that it was the result of one take but it took a few goes to get it right.  This type of thing can be really frustrating but hugely satisfying at the same time. I have my fingers crossed that it will hold the secondary secure and stable enough; it is a large secondary.  I used 1 mm thick mild steel but in the back of my mind I'm open to the need to revisit it with a thicker gauge; I'll see how this one works first.  

Jim

A weekend full of fettling produced lots of scrap metal but thankfully one secondary spider vane.  I thought I would have time to finish off the upper OTA and have the spider installed but the good weather over the weekend saw other diversions.  Local bank holiday coming up next Monday so hope to make further progress then  

Jim

Some of you may remember the thread back in August of last year in which Andrew S very generously donated a 16 inch mirror together with mirror cell (a lot of other associated astro gear).

https://stargazerslounge.com/topic/275743-build-your-clubschool-a-telescope-16-f5/#comment-3021275 

Well it seems so long since I met Andrew in the Lake District in October but I've eventually found some quiet time in the school calendar and we are making progress on our build.  I hesitated for quite some time on the design, I wanted to do justice to Andrew's donation, and so I eventually settled on a circular mirror box design.  Our inspiration comes from  http://www.waiteresearch.com/ .

So here's what we have so far:

Mirror box is largely done and fitted with its mirror cell, now waiting to be glued together and sanded.  The photograph below shows a trial fitt with the mirror in its cell.  We also have the OTA assembled and waiting to be fitted up with the secondary.  I had a go at making the secondary over the weekend but my welding skills fell somewhat short against how I had remembered them  I now have a second spider design to be trialled this weekend. I'm keen now to get the kids involved before we spilt up for the summer holidays so the plan is to attach the mirror box to the OTA in school next week - we already have the truss tubes waiting to attach to the Moonlite sockets.  Our task is to experiment finding the correct length required for the truss tube before cutting them to size - this will be a good challenge for my S3 class and fits in well with work they have done to date.  The aim is to get everything ready for our return to school in Aug and then a proper shake down at a nearby dark sky site from October onward. 

I'll post updates as they happen, but please be patient - we are making a few mistakes as we go along    Andrew, once again, a massive thanks.

Jim

magnus I can well understand what you mean by the improvement on "setting up and the dreaded teardown" - you now no longer have to worry about that  Good luck with the remaining work - can't wait to see it installed.

Jim

Magnus I've been following your progress rather quietly here but now is the time to say  "Wow"   That is an enviable remote observatory, it reflects well on your hard work, congratulations.  Are there any significant hurdles to overcome before you are ready to position it and start commissioning?

Jim

Gina, Paul, thanks for the advice, appreciate it.  You know in the classroom I often remind pupils to confirm the output from the bench supply with a voltmeter and not assume it will kick out what the dial is set to. I should have taken my own advice.  I need to be more respectful with this stuff or I'm going to lose some expensive kit. At the end of the day  I think you are right in that I was just unlucky with this one, it wasn't meant to be my "forever" psu as Kirstie Allsop would say  The new one is humming gently in the background and the my little panel meter shows a steady at 13.8V.  I'm happy, and I've learnt something new about power supplies.

Jim

Paul, yep I don't think I'd embarrass myself now by taking it into Maplin, I should have done that in the first instance - I'm too impatient at times.  I may ask the technicians at school to take a quick look at it, if there is no easy fix then I'll chalk this one down to experience. I'm just so relieved that the mount wasn't damaged.  Overall what is your experience with this line of psu regarding reliability? I was happy to go for a like replacement given the comments from other SGL users (above) and I must admit, I do like the build quality of it.

Jim

10 hours ago, The storm fisherman said:

Hi Jim

i was just reading your thread contrary to popular myth you do not require a receipt as an ex Maplin manager a proof of purchase is all that is required  IE a bank statement and also i can confirm that there is a fault with regulator unless they have change the regulation circuit there is an internal variable resistor to adjust the voltage i had great success in turning one of these into a variable voltage psu by mounting a pot variable resistor out to the front panel.

Thanks for that, I must admit I've always had good a experience with Maplin, they do tend to be very reasonable.  I've been a bit silly though - I opened the faulty psu to have a look inside.  The variable resistor you described was  there (2 x) one I believe to control the regulated output the other not sure - part of the regulation circuit.  Could not see anything obvious - capacitors all looked good as was the external fuse.  On the back panel it says there is also a internal fuse but I couldn't spot it - maybe a thermal fuse on the transformer - something was wrapped in heat-shrink coming off the transformer. I may yet have a word with Maplin to see if there is anything they can do but I'd need to let them know I've opened it - I think that will the killer - ach well lessons learned.  

Jim

Picked up a new psu all is well, looking like clear skys tonight as well - one happy astronomer

Jim

Thanks Gina. There is a Maplin store in Dundee, I'll take a drive in tomorrow after work and pick up a replacement.  This time I'll make sure to do two things - measure the output voltage and keep the receipt!

Jim

Thanks Steve and Steve, good to have positive confirmation on the psu's normal output characteristics.  In one respect, it looks like I've been lucky that the mount hasn't suffered, I suspect it has been operating like that for some time.  Also ironic that the work I've been doing on the observatory was to install a control panel - few switches for the mount and ancillaries and a panel meter to monitor the supply to the mount.  I should have known better but that is the first time I have checked the supply output. That little panel meter could well have just saved my mount from being fried! It may be cloudy tonight but I'm happy

Jim

Yes you want to avoid any trip hazard, I only have a few metres to walk to mine but its always with trepidation as my eyes adjust to the dark - I've walked into the washing line a few times too many!    Looks like the rotovator is the way ahead with the grass cut before.  How are you planning to level out the path once the rotovator churns it over - would the tractor bucket smooth it over?

Jim

I've never used one of those things Gina but I would imagine that as you say it may well not like long grass.  My petrol lawnmower often stalls if I try to go over grass of any length.  Be careful with it.

If the rotovator doesn't work you could try spraying the grass with weedkiller then laying the fabric direct on the top once it has died backed.  It would of course mean you'd have to use a raised edge each side to act as a border. Maybe not ideal but it may save a lot of work. 

Jim