mapstar

Hi all, Thank you all for the help and advice about the smaller lap . I know this method work's for some so I will as Rick says 'try it and see how things go'. Out of interest on this I did do some calculation's on the actual difference in the lap thickness between the edge and middle making a 10" lap. The result is just over 1.8mm between the side and the middle thickness of the lap. I should imagine that this could be the difference between one side of a lap to the other if anyone's lap does not sit entirely flat whilst making? How many use a spirit level whilst setting up and on the back of the lap when casting? I shall see how it perform's over the next few day's and revert to a different method mentioned above if it does not behave. Damian

I hope I can finish it off to everyone's expectations so no pressure! I've got one of those imaging balaclava's like kenny has to stop hair loss he he More this weekend as I've been on with other things most of this week. I have picked myself up from the lap incident and optimism is coming back again Onwards and upwards and thanks for the continued support it's much appreciated. I'm sure messing about with all those imaging cable's would drive me nuts, I do enough of that at work :-D

Ha ha cheers Derek I get to spend lot's of time (and money) on it as there isn't a missus Mapstar so the foil would be very safe I think I may do one in my next decade on this earth (it's gonna take me ten years to get over this one!!!!) Cheers Pompey Monkey I'll be carrying on this weekend so look out for update's Damian

Thanks Nick glad your enjoying The notes and I'm sure one day you'll get to see the results. Cellotape is a good suggestion thanks. Unfortunately the pitch being hot make's the glue on the tape melt and let go. I used Gaffer tape in one of my much earlier pours and learn't my lesson there. White spirit is definitely on the shopping list Ron

Unfortunately I'd run out James so used normal width stuff over lapping it on the mirror. Unfortunately The pitch ran underneath during my bit of a disaster

Thanks for dropping by Derek and I agree it's very tedious at time's and enough to test the patience of a Saint.(although being an imager patience shouldn't be in short supply) The pitch is poured onto aluminium foil on top of the mirror, so come's off the mirror quite easily unless like mine it goes under the seam in the foil and sticks to the glass itself. I think the stuff just doesn't like me although the feeling is mutual Damian

Lovely story there Ron and I can quite sympathise with the pitch and rouge get's everywhere analogy. I'm finding it does very quickly. John has mentioned David quite a few times and recounts very fond memories of him. Please don't think you are interfering as it's a pleasure to read the experience and interest in the subject people have. Feel free to pop a comment in as I'll always welcome them Many thanks Damian P.s. hope you do make another mirror, although I won't be blamed for any divorce proceeding's

Hi Ron, Many thanks for the kind word's and encouragement all of which is gratefully accepted especially when at a low point. I've managed to cast the lap and although not pretty I'm sure it will work. The lap is 10" in diameter I have read of the method you describe before and it is also in Texereau's book or something very similar as I recall. I have tried a couple of methods up to now and thanks for the suggestion on the way you managed to make your's as there is always more than one way to complete the same job some much easier than others. Casting lap's is one of my least favourite bit's of mirror making and I'm always apprehensive when it come's to doing one. As far as this bit is concerned I'm not on my own as Rich hate's it too.Finger's crossed I can complete this and show it can be done but a few more knocks to take along the way Damian

The cold light of day has revealed the turps has dissolved all the pitch into an oil like sludge on the mirror. After going through several sheets of kitchen roll it's had a final clean and wash with detergent I've inspected the surface and I seem to have got away with no damage The lap isn't pretty but it will work although I will try to clean it up a little more. Damian

It may even drive me to drinking whiskey!

Cheers Ian I hope your right and I still have some teeth left! Damian

I think it probably would Daniel except there's a slight hitch in that. I can't get the mirror in my fridge :-( The turps seem's to be doing its bit. I'll leave it to soak and then give it a clean tomorrow.

Cheers soupy. My second attempt at a lap was a complete disaster with the pitch actually getting onto the mirror I have managed to get the lap and most of the foil off and now it is soaking with turps sub's to dissolve the pitch. Another set back which if it means going back again due to scratches will be a massive blow :-(

Had a go at casting the new lap and as always it turned out to be a disaster. I hate pitch!!!!!

I may do that Nigel. I didn't do it with the last lap though Damian

Just a quick update today Whilst I've been on with this I decided that it would need a transport box so this is another thing to add to the list of bit's. So this is what I knocked up with a bit of scrounged packing foam The mirror I also wrapped in two soft towel's. More packing foam on top and then screw the lid down. I will also be adding something between the mirror and the towel when I finally get it finished. I also started on a smaller lap made from 2" of glued up ply. I've also sealed it with two coats of fibre glass resin Should be pouring the pitch tomorrow and then when it's cooled channelling the facets Damian

Quite correct Peter and I've only ever known of one earth quake in these parts. I'm sure the blu tac is there just in case of this ;-)

No worries it was as safe as houses ;-)

It has been difficult and I have thought about the giving up so many time's. It's been a big help doing it along side my good friend Rich (crashtestdummy) as we have both kept each other motivated along the journey when things are not exactly going right. The saying "don't struggle in silence" come's to mind

Onto the first testing To test the mirror I was going to use a combination of Ronchi screen and the knife edge Foucault test. Firstly I had to build the testing rig. I managed to pick up a single axis linear stage with 25mm of travel and went about fitting it to a camera tripod. Next up was a piece of ply to hold the LED light source and Ronchi screen. Here's the results So with that complete I made up a test board to mount the mirror on via a sling suspended between 2 bolts. I was now ready for testing. I was going to perform the test's with a moving light source so here is the test rig labelled up The test's are carried out at the Radius Of Curvature (ROC for short) or twice the focal length. There is a lot of info on the net about both these tests and others that are used but I shall stick to just these two as they are probably the easiest for the amateur to use and set up. Firstly the Ronchi test and I took these inside the ROC Then onto the Foucault test Interpreting these image's was my untrained eye, so I exchanged a few email's with John and a verdict was given which I can't fully remember all the details. As can be seen the centre is raised (think it was actually mount Everest in mirror terms) and there were other problems with it too mainly the edge. So John advised a stroke through the centre with lot's of weight on. So back home and I tried this but again I had problems. I was in the garage so however I warmed the lap it would cool off and basically school boy error No.2. Looking at the surface I might as well have used a brillo pad to polish as when I took the lap off to inspect it there were more scratches than my parent's record collection. The pitch was obviously too cold and the lap just skidded about. You can feel when the lap is working right as it sort of suck's onto the mirror and the polishing action is smooth and quite a resistance to push I should have known. A return trip to what I thought was the redundant tile tool and another trip through the fine grinding (400, 600) pencil testing along the way and then back to polishing. I moved all the table into the kitchen where I could work at a constant temp and have the lap warm. After cleaning and pressing I tried again with the lap I had to polish but struggled to polish out the centre. The lap had become too thin at the edge's and was never going to form to the surface. To cut to the chase I made a new lap below and redid all the the polishing operation having purchased more Cerium oxide. The new lap was a world away from the old one with the lessons I'd already learnt And the table in the kitchen with the old lap pressing The lap at the correct temperature worked beautifully and in no time at all I was polished again Again now I could return to testing, but not nice retracing your steps and doing it all again but I should at least be getting goods at it! Just realised the back door put's into perspective the size of the mirror the table will just fit through the door jamb. Damian

I'm not sure that I'll be attending that Ron but will try to make it if I can. It's nice to put face's to name's on here and thank people for the encouragement they give. I have some parts of the scope build already complete along the same line's as the 24" I built for a friend. Fingers crossed I plan to have everything done by September so 6 months away but I know that is a small amount of time to get it finished. I've quite a few star parties planned and really want to get out there and use it and as you say show what can be achieved with maybe the odd problem here and there to overcome. I still have to finish the mirror though which I know is going to be the hardest part as it's something that I've never done before Damian

Hi Ron Thank you again and right that just over 9.4mm is the sagitta for F3.7. I ended up at around F3.8 so roughly 9.2mm. I was aiming originally for F3.9 so a fraction under 9mm but as I said I took the rough ground sagitta to 12mm and because I didn't want to grind much more off I ended up slightly shorter focal length. Yes there is now the hard part of figuring which is a proper challenge Damian

Think that would send me over the edge Soupy!