mapstar

Hi Raymond I will have a go at the mask, foucault tester and XP software and I fully agree that it will teach more about how the figure of the mirror changes than just one test alone. What you are saying about not needing a camera is right but I really put this thread up to show people what they will see if they have a go at making their own. There isn't that much info from beginners to look at on the Web especially when you are working with basic tooling and making a large fast mirror. Yes I have not got things right and it may take me a long time to get there but it is all part of the learning process and others hopefully will read the thread and not fall into the same pitfalls. Onto your Scope pic a bit of a different take with square truss tubes? Any reason for that? Easier to work with perhaps? Damian

Hi all, John Nichol is hoping to hold another one day seminar if there is sufficient interest. The previous two editions have been limited to mirror making, this time it will include aspects of telescope making. This is because some people would like to make their own telescope without making their own optics. As a result the day will be about 50% mirror making and 50% telescope making. Details of the day are... Date: 22nd November 2015 Time: 10am til 4pm. Location: BIllingham, Teesside. (Easy access from the A19) The day will feature a talk from SGL's Mapstar (Damian). More details to follow as the timetable is firmed up. This is not a profit making event, But There will be a charge for this seminar, sufficient only to cover costs of the facility, and materials etc. If you would like to register an interest in attending the day please send me a PM Damian.

Oh and it does seem that money does not carry the same gravitas as malt in the astronomy world! Or am I just mixing with the wrong types he he

Many thanks Pete I intend to create a new thread as I did with the last build to detail the build process. I have to that end been taking pictures and trying to fit in the details that would be beneficial to others. At present the mirror thread absorbs a lot of my SGL time. I will do it soon, as the last one I did was well received and interested people hopefully enough for some to have a go themselves as was the purpose of detailing the making of a large aperture mirror. I must admit it has been a lot longer than what I thought it would take. Damian

Many thanks for posting Steve and writing about your experiences.The word epic is a good one and saga may be more in keeping with my efforts? Good to know there are others willing to help out there even if it does end up costing a little malt to get there. Cheers Damian

Apologies Ron I didn't answer about the foucault testing. The problems I've been having is I like to take images and it's been difficult to do it with the set up I previously had. I have worked it out that the camera needs to be a distance back from the knife edge to get it to work as advised by RAC and an article on the Web I found. I have taken some images and it seems that the mirror has the classic doughnut of a parabola. I shall try to post some images. Damian

Thanks for the above post and the update on my PM. The Ronchi lines are difficult to read sometimes and working out your next move takes some head scratching? I have an 18" lap although this would need a good press and then a lot of work to get it up to speed. A very detailed post with a lot of great input and maybe John will say the same although I have kept a constant dialogue and until this week everything was going OK It may be some remedial work in the outer zone to bring things back but we shall have to see? I have to admit a full size lap and mirror on top fills me with dread! Fingers crossed. Damian

Afternoon all, I promised an update to the thread this weekend and here is the progress I've been making. Since the last update I've done roughly five hours on the figuring with a tight V through the middle then a W stroke across the whole of the mirror with a 1" overhang to try to smooth out the zone that I seem to have in the outer 3.5"-4" Here's how the mirror looks at offsets through the Ronchi from inside ROC to outside travelling a total of 17.5mm. Each step is 2.5mm I'm not 100% happy with how the surface is changing as it doesn't seem to be following the matched Ronchi images that I have. To this end I have suspended any further work until after Tuesday when I am taking the mirror up for John to cast his eye over. My initial thoughts are that the outer edge's will require work to catch the middle up and blend the zone in. I've not been idling away though and I've spent the majority of the day in the workshop (o.k. Garage!) progressing the rest of the build for the upper tube assembly (UTA) Hopefully Wednesday a plan will be formulated and I can continue. Thanks for reading Damian

Hi all, Just to update on where my thoughts are at present. I have only managed another 4 hours on the mirror due to work this is quite a snails pace. With time ticking by I am going to have to move a little quicker as I really want to be getting back to doing some viewing and hunting down those faint targets this winter instead of staring at a pile of bits! Things are coming along and I will make a decent up date to the thread this weekend when a little bit more time is at hand. Damian

Hi Ron The mirror is quite rightly my main concern although building the other bits is a way of spurring stuff on. I have modified my tester to make it a slitless tester with enough board at the back to mount the camera. I've tried this a couple of times but I think it needs to be on something more stable (mass wise) to hold the image stable. The secondary will be from John so I have no problem there. As for the questions I really don't mind as it shows people are interested Damian

It is and it's a bit of a shock when you see how much of a monster they become.Shame to admit that I enjoy the wood work far more than the glass bit although I'm sure when the mirror is done it will be provide the enjoyment far surpassing if I'd bought it in.

I will be using the matched Ronchi test mainly but will sort the figure XP out to run in conjunction as two sets of testing should be better. Have to sort out my test rig.

Hi all, I've been making steady progress with the mirror, although I have spent more time on the wood work and actually getting out to view lately so been getting up late and then work so progress is slow. Here's how things stand at present the new lap is working nice and feels great to push across the glass. Inside ROC Outside ROC I have a zone to blend in around 50% radius so have had an hour or so blending. I may have to star the lap (cut sections out of the edges as Raymond says above) to give the curves a smoother transition. I am going to have another couple of hours and see how things progress all the while checking against my test images to make sure I'm not going too far. Damian

Not yet Raymond I will see how it goes as I'm just working in the centre section as this is still under corrected with a slight hill. I'm also working on a set up for foucault testing so I can image with the camera I have. Did some initial testing and found the camera needs to be quite far back from the knife edge.

That's some chunk of glass Paul and I have to admit mine has looked to me almost like that at times Although I will admit that it is like buying a new TV. At first they look immense then seem to shrink (I don't mean by grinding and polishing bits off). I made the new lap from a mix of Gugolz 55 and 64 with a little boiled linseed oil in to soften it slightly. The new lap pressed overnight Sunday and then I cut the facets in it and then pressed again over night with approx 5kg on it. Today when I arrived home from work it was time to heat up the lap and do a warm press with some cerium on the mirror to stop it sticking. Here's the lap contact after just one warm press. The lap is face up the mirror face down on top with the mirror back wet just to give a better view. After a couple of presses it looked to be in good contact.To make sure I put the mirror on the polishing table and left it for half an hour with 5kg on, occasionally turning it and making sure it didn't stick while keeping it wet. I then made a start again with the polishing the 5kg remaining on top. I also added a threaded insert just so I could screw a bolt in just to be on the safe side so I could stop the weight accidentally going anywhere. I had half an hour using various strokes (narrow V through the middle to about 80% dia, W strokes varying from 60% up to about 90% dia) Will update with the test images when the mirror has stood to cool. Damian

Yeah it's a while since we chatted and hopefully will do a lot more in November. Fingers crossed everything will go OK from now on and constantly checking and pressing will ensure that. Back to it this aft once I've finished work. Probably more on the wood work too. The lap was pressing overnight and today so by the time I get home it should have formed to the mirror fully and be ready for the channels cutting. Once that's done I will cover it in cerium and do the mirror face down test to check contact and make a start when it's good. Damian

Hiya Derek.I know John uses both dental plaster and also ply laps and doesn't have any problems so both work. The ply is baltic birch and very dense. 2" thick over a 10" disc would be very difficult to flex. I have cast the lap using the method I used last time which was to put the lap face up and put a dam of foil around it and then just pour the pitch in. I have waited for it to cool to warm before putting it face down on the mirror to press with weight. It has grease proof paper between and gap nearly formed to the shape already. By tomorrow it should be ready for use. Damian

Dental plaster is something I've thought about trying but with the ply at hand it's just easier for now. What did you use to stick the squares to the plaster with? I think I read in tex that bees wax or something similar was used? I have some more work planned today as I've said already, but probably not much on the mirror. Damian

Here's the progress over the last few days, I took a check of the lap today after I finished work, and after a test press with the mirror face down on the lap I could see the contact was not that good. The lap is down to about 3mm in places on the edge as the ply wasn't the best and I can see it was slightly warped. After several attempts with warm pressing with the mirror face down and the back wet I decided not to continue using it and make a new one. It just wasn't pressing out in the middle. So I headed to the garage which turned out to be 4 hours as I got into making a new lap base this time out of quality baltic birch. Whilst the new 10" lap was gluing (4x 1/2" thick disc's) I continued with the upper tube assembly build which will hit SGL later. Here are the images I took this afternoon before I started Inside ROC Outside ROC It is a fair way off at present but getting nearer by the day. I should have the new lap completed by tomorrow evening so will resume work probably monday. It will also give me time to look into making a set up where I can get foucault images working with my camera although I am still going down the matched Ronchi route I would like to see how the other methods work too. Here's a peek at the upper tube assembly ply as I made a start More soon Damian

Evening Mirror watchers Over the last few days I've made another couple of hours progress and things are looking smoother. More work tomorrow and I will update with some ronchi images of how the mirror stands. The 10" lap I'm using is beginning to get a little thin with all the pressing so I shall make a new lap too. Thinking the temperatures were going to be a little warmer for this time of year I had picked up some gugolz 64 pitch from John to make a new lap. The temperatures don't look to be materialising so I shall have to test the hardness before use and soften it if need be. I've also made a good start on the shell build which will have to progress a little quicker to catch up with the mirror. Damian

Guys at present I use a micrometer stage with 25mm of travel. I posted this a while ago it is smooth and works well so I will utilise this in any future development of a tester.

There is a multitude of Foucault testers all over the net and as Ron says they don't need to be complicated. Thanks for the suggestions guys I'm going to work on what I have at present for now but will build something to make further tests.I have not made any progress today as I have been concentrating on getting the other bits that I need together. I'm really hoping to get back on with it tomorrow though Damian

Thanks for posting that Raymond. I will have a good look through it.

Thanks. I'll try my best to keep up with adding updates

I hope so Ron, slowly slowly does it. I have generated the images I need and hopefully I can match them to those.Need to suss out the pesky Foucault test though Damian