mapstar

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!

Thanks Jeremy, It's a lot of work which isn't really portrayed through posts, 100+ hour's (still counting) is a huge investment of anyone's life as everyone can appreciate.The thing about it is though it's a mirror for the rest of my life so worth every second I'm sure with determination I'll change your mind ha ha

Thanks James, I can only say have a go you'll not regret it, it's difficult and to be honest at present I still have the hardest part ahead which is figuring but that's another post Damian

Cheers Stu The pencil test during fine grinding is a good indicator of full contact and a sphere. To accurately measure the surface when I was at John's he checked it for me with a spherometer which is on his website. This one The measurement should be the same all over the mirror if you've got things right (i.e. perfect sphere). The only way would be to constantly check with one but as an amateur I don't have one so the sagitta depth is a rough guide and the mirror will end up around that figure. If it was fitting in an already made scope then it needs a lot more precision so I would've bought one. The spherometer works on four point's with the three outside fixed and the middle with micrometer measures the depth in between these. So subtracting the depth of a flat surface off the measurement you can calculate the radius of curvature through a bit of maths (which I'm not going to go into) but this is done through a computer program and spews out the focal length (it's magic!) It's quite strange to see a mirror without a coating in the flesh. As with most things pictures just don't do them justice. Damian

Onto the polishing Now the mirror was at the stage of polishing I required some more supplies. I trawled the internet for Cerium Oxide and came up with some from a well known auction site. The pitch for polishing was a different matter as it seems to be quite difficult to find a supplier that has some? A visit to John though sorted that problem and 2kg of Gugolz #55 pitch was winging it's way home with me so I could make the polishing lap. I made up a base for the lap from four 12mm plywood disc's of 17.5" diameter that I happened to have lying about. All glued together I sealed them with a coat of fibreglass resin. Now actually melting and pouring pitch is a horrible business and not at all nice. I melted the pitch in a pan that I would dispose of later. The pitch is then heated slowly making sure not to overheat it as boiling drives off the volatile's which give the pitch it's unique consistency. Gordon Waite does quite a good video on this and a video writes a thousand better word's than I could so I'll just go through the basics. The mirror is covered in tinfoil and the melted pitch is poured on top in the middle and slowly spreads out in an even layer. Then with this cooling the pitch lap base is pressed down onto it. The results are a perfect lap or in my case an absolute disaster! I had not quite got the blank level and the pitch started it's way towards one edge and I waited a little too long to press the base onto it as it hardly stuck! the lap wasn't exactly what I was hoping for and ended up a proper messy affair with pitch everywhere. I then let it cool and chipped it all off to redo and hopefully get it right. The second attempt was a success but as I've already said pitch is horrible stuff and just get's everywhere so glad I was in the garage doing all this. After cooling I cut facets in with a tenon saw finding out the blade doesn't clog if there is running water on it whilst cutting. I'm sure if there was a missus Mapstar living with me then she would be none too pleased with the use of the kitchen sink for this phase of the lap making! I then covered the mirror with grease proof paper and pressed the lap over night on top of the mirror to form a decent contact. Here's a shot of my rather messy lap cold pressing with oxide before polishing operations began The polishing compund comes in a couple of forms, Cerium Oxide the other being Regipol. I initially started out with Regipol optical polishing powder like this After suspending it in filtered water (I used a brita water filter) I did my first half hour of polishing and below show's the lap wasn't making contact in the middle so more pressing with more weight The lap eventually made full contact after warming it in water and pressing several time's. The bevel around the outside and facet channels were re-cut several times during this as they close up when the lap forms to the surface After 12 hours the surface looked to be polishing but the centre was still hazy which I'm not sure can be seen in this shot? I had been polishing without any weight on the back of the lap so added 10Kg and continued. The lap become's quite a chore to push across the surface and I would polish for about an hour then press the lap overnight and carry on. November by this stage was at an end. When you first see your mirror polished it is a genuine WOW. I can count it as one of the most satisfying things I've ever done in life. It makes the hard work of the rest of the journey disappear. These two shot show the curvature of the surface with the fluorescent tube reflection. I inspected the surface throughout using a laser to test on the surface to make sure the micro pit's had all gone. I had picked up a couple of scratches near the edge but these would stay. It had taken me nearly 22 hours to polish which probably would've been a lot quicker with weight on from the start so up to now well over 100 hours I had in this. It now looked like a mirror so onto the testing Damian

Cheers Ron and Rick for the encouragement and kind comments. Hopefully I have got all the details right and the thread make's sense in the way everything progresses. To quote what a very wise person said "it's a 22" sub F4 mirror and a beast compared to the average mirror so must be treated with the respect it deserve's" I'll get there in the end and maybe inspire others along the way. Damian

After a short break I returned to the mirror. Next stage was to work up through the different grades of grit. Just on a safety note the glass is always worked wet as inhaling glass dust can result in Silicosis, a very serious lung disease, and to this end in the initial stages I also wore a dust mask. For those that have never really looked into the grinding process, when the rough curve is generated the 80 grit Silicone carbide chips away the surface creating a lot of damage. After this is complete the use of ever finer grits then repairs the damage and brings the surface back to a polish. The process would involve several grades of grit to achieve a polish. Each and every change would be followed by lot's of cleaning of the work area and tile tool 80 grit silicone carbide 220 grit silicone carbide 400 grit silicone carbide 600 grit aluminium oxide 1200 Cerium oxide Grit quality can be quite variable so the particle size is only an average of what you get dependant on the supplier Here's a picture of the surface after the curve generation with 80grit The large chips or pits can clearly be seen without any magnification on the surface. Some of the 80 grit I recycled during the rough grind. Saved on quite a bit of grit usage Next was the 220 grit and although a lot more difficult to spot, the pits are still visible naked eye although I used an up turned eye piece over the entire surface to spot larger one's which had to be ground out. A combination of strokes and stroke length was used from straight through the centre to W strokes and also chordal all the time pencil testing to maintain a sphere. The grinding action gets a lot quieter at this stage Here's a couple of pic's of the 220 grind during and on completion The surface is still very opaque and has no shine to it at any angle. Next onto the 400 grit. At this stage the blank or mirror start's to take on a shine from a low angle which is quite a revelation As before I used several random strokes and rotated the blank every time a trip around was complete in order to prevent any astigmatism occurring. Next was the 600 grit aluminium oxide which is used due to it being a much softer and less aggressive abrasive. Here's a pic of some and has the consistency not much different to talc. I acquired this from John as the quality for this size is a bit hit and miss from other sources. Here's the surface after the 600 Al Ox has been completed. The angle at which the shine was showing steadily increased By this stage though I had been backwards and forwards through the grit's as it's so easy to pick a scratch up. But finally finished without scratching it The surface really spurs you on at this point with it beginning to look like a mirror. I checked yet again with the up turned eye piece throughout and finally thought the tile tool was redundant, so onto the next stage which is polishing It was by now early October so coming up a year since myself and Rich had received the blanks. During the process we have regularly exchanged experience, encouragement and tips to progress. Also constant contact with John along the way all of which has been invaluable. I can highly recommend this approach. Damian

Back to it and on with the thread. So as previously described I'd now got good contact with the tile tool and all was going well. I forgot to mention that throughout the mirror making process I was regularly rotating the blank on the grinding table to prevent any astigmatism. This is done entirely at random sometimes an Inch sometimes a 1/4 turn but always random. Having stated earlier that I'd ground the rough Sagitta to a depth of 12mm with the theory that the tile tool should shallow this off again as the spherical surface is formed I measured the sagitta depth again. It was at 11.9mm So onto a bit of maths which I'd not explained in my earlier post about calculating the focal length and sagitta. I'm no maths teacher so bear with me The focal ratio was to be F3.9 I'm a metric sort of chap so Focal length (FL) = dia of mirror x focal ratio FL = 560mm x 3.9 FL = 2184mm Next was Sagitta (S)= Radius of mirror squared (r2) / 2 X Radius of curvature (R which is 2 x focal length) S = r2 / 2 x R So S= 280 squared / 2 x (2 x 2184) S= 78400 / 2 x 4368 S = 78400 / 8736 S = 8.97mm This meant a lot of edge grinding to bring it back down to the desired sagitta and also a lot of bevelling! To do this the centre of the tool needs to be moved further out towards the edge as this is the part that does most of the work So off I went with the plan with chordal strokes interspersed with W stokes across the Mirror blank to try to maintain good contact The overhang of the tool increases the weight on the edge thus grinding faster After what seemed like an age (4 hours) I had removed 0.4mm of the edge and was down to 11.5mm (2.5mm to go!) so more bevelling and edge grinding to follow It was at this point that I reached probably the lowest part of my mirror making experience so far. My in experience showed and whilst grinding the edge I pushed for too much overhang and wobbled the tile tool. With a sickening clink I shelled (chipped) the side of the blank to a depth of about 2.5mm. It was a totally gutting experience to remove the tool and see the damage I left the blank for about three days to think before I had the heart to return to it. Several discussions later with John, Rich and others and I increased the bevel again to 5mm which went part way to moving the damage out. As others said if it was a finished mirror and you chipped it like that would you stop using it. Milk spilt time to mop up and get on with it. I did finally grind down the edge using a weight on the inside of the tool (near the centre of the blank) to balance it, but settled on F3.8 as the sagitta ended up at 9.2mm when I finally bedded the tool back in with 220 grade grit . Much harder task when it come's to figuring at the later stage. I also renewed the tile's on the tool due to them becoming that thin that the're was no facets left between them New tile's glued a top old one's Here's another shot of the chip and what the surface looks like after the 80 grit The 220 grit grind's much smoother and now the rough work is done it was all about repairing in finer and finer detail (working through the grit grades) all the damage to the surface that had occurred. here's a shot of the pencil test with 220 grit worked surface Bedding in tiles The blank was now just under 38mm thick and about 18kg. I'd spent 79 hours on the project to this point. Total grit usage 2 Kg of 60 grit 14 Kg of 80 grit and 2 Kg of 220 grit lots of water! Not everything always goes the way you want it and more lessons learnt. By the end of all this it was Mid June 2014 so time had flown and I had to leave the project for another one which was desperate for finishing. I resumed again in September hopefully with more up's than downs this time Damian

Such a shame Ron after all that work, I have been at this stage as my next post will reveal

Hi there, the blanks way back then cost £480 but have since gone up to about the £550 area? In financial terms the blank is only one part, you should also figure in the amount of grinding supplies below Grinding table Various grades of carborundum grit 60, 80, 220, 400 Aluminium oxide abrasive 600 grit Cement Tiles Cerium oxide Pitch Plywood for laps and transport box Test equipment Postage Mirror coating And last but not least the time you have to put in to make it happen.

Cheers Ian I've missed the Galloway gathering too could've done with getting up there!

On to the next stage of the grind The rough curve had been generated and a quick measure on the edge showed the blank to be down to just over 41mm. This curve is non spherical so we have to make a tile tool to bring this to a sphere. The tile tool was to be 18" in diameter (about 3/4 diameter of mirror). It consisted of a cement base with mosaic tiles glued on the front face for the grinding. Sound a little strange at first, for a smaller mirror you would cast the tool the same size but this would be too unwieldy at this size for me to use so 3/4 diameter. I made a former to cast the cement disc which would fit on top of the mirror blank. Here's a few picture's of the progress from me making the former to casting the cement backing I'd put packaging tape around the inside to help the cement release better. The former sit's on the greaseproof covered blank and when the cement is poured it forms to the rough curve that has been generated on the blank by the sub diameter tool (11" barbelll weight) After scree-ding the excess off and giving the former a few taps with a block of wood to release any trapped air bubbles I left it to set. I used 15kg of cement for this. After I'd separated it from the former I sealed it with a coat of fibre glass resin The mosaic tiles had arrived so it was the job of attaching them with yet more fibre glass resin. The convex surface of the tiles can be seen clearly Once they were well and truly stuck it was out with the angle grinder and diamond disc to trim them up and put a nice edge bevel on them. So onto the grinding with the tile tool, the theory being to work two surfaces together one a rough convex surface and the other a concave surface of the same radius to eventually produce a spherical surface. My understanding of the mosaic tile's is that they create facets (gaps) between them which carry the grit across the surface each tile being a cutting surface to grind the glass away (Apologies if this an amateurish way of putting it!) First of all the surfaces need to mate together and the first few wet's (carborundum and water) show on the tile tool how well these are mated together So the contact was not very good to start with. As things progressed it steadily got better. I used a variety of strokes at this point ranging from pushing the tool across the blank in a straight line through the centre to a W stroke across the blank in order to get the tool and blank to contact fully. So finally all in contact so onto what is the pencil test. A grid is drawn on the surface of the blank to see if even contact was being maintained. A trip then around the table and I inspected the lines to see if they were all grinding off at the same rate. I did this several times until I knew that I'd good contact between the two by all the lines grinding off equally (took 5 hours). At this stage I must've been around this table a thousand times between the rough and tile tool grind. More later but I was quite happy to this point Damian

Thanks again for all the encouragement guys. I'm not sure about the awe bit as the how mad must some people be? Damian

Thanks for the encouragement guys. Yes I hand chamfered the edge's and developed what's commonly known as Mirror bevellers elbow! Hard work

Yeah blank is how your mind ends up doing the hogging. If I ever do another I'll get it pre-generated. P.s. the blank lost weight a lot faster than I did

Cheers Guys I made a start on the front face by yet again flattening it making sure it was the same thickness all round. It was better doing this side as the ridges were no where near as pronounced as the other side. This time I started with 1Kg of 60 grit moving onto 80 grit to finish with. As can be seen it was a messy business!! This took 5 hours in total, the 60 grit cutting a lot faster. I then renewed the bevel on both sides which was another hour Once the front was flat, I started the hogging out progress after preparing the 10Kg 11" barbell weight by grinding the middle off to leave the ring around the outer edge I'd worked out that the sagitta for F3.9 was going to end up at roughly 9mm so a lot of glass to remove. I used a centre through stroke testing with a straight edge as I went. I needed to take the sagitta deeper as the barbell would create a non spherical curve. A tile tool is used then to bring it to a spherical curve and would grind the edge down (in theory) so lots more grinding later and another 1kg of 60 grit, 9 kg of 80 grit 12 gallons of water and I was at the depth I required. Here's a few photo's of it progressing. A total of 41 hours later I had a depth of 12mm. Just out of interest I weighed it at 18.7Kg. Wish dieting was as easy! Just to say hogging has to be one of the most boring things in the world!!!!!!!!!!!!! I re-bevelled several time's during this process So next I was ready for the tile tool Damian