barkis

Not a brilliant example Damian. But here's a Ronchigram of a paraboloid. Photo by Samuel Borello from NE Howards Telescope Making Book. Not the best photo. you'll see Ron.

Centre oblate is no big worry Damian, it's easir to get rid of a hill, much more difficult to cure a depression careful you stay away from TDE.. Sub. diameter tools can change the surface very quickly, so work slowly, and test regularly. If it takes a long time, so be it, you have to become as one with the task, and if that sounds silly, it isn't Don't be in too big a hurry to judge the surface after a session, what you see shadow wise, will certainly change as time passes. Reserve final appraisals long after polishing sessions. Remember there's no rush. Are you using Rochi gratings, I think you said you were, those shadows indicate such. Perhaps a Foucault tester, although not suitable as a final tester, will give a better insight as to where you are with the surface as it passes through the light cone At the radius of curvature. The knife edge tests can reveal much useful info. Ron.

White Spirit was my friend at times of need. Moves pitch double quick, but can be messy.

I agree completely about that part of mirror creation. My wife used to get quite upset when ever I embarked on that phase. However careful I tried to be, I would inevitably carry pitch chippings into the house, where they would leap off me, and jump onto something prominent, like a duvet, or cushion cover, where they melted instantly, to blend with the material. Sometimes a Pillow case would be infected too. Divorce was almost certain more than once . I used to get a laugh when the late David Sinden, a close friend of John and myself, when he was describing the use of Jewelers Rouge when polishing. He told of carefully removing small amounts from a container, whilst standing in the corner of his workshop, knowing how contaminating the stuff could be. However, just when he thought he had managed to contain it successfully, he would turn around to find it covering almost everything in the vicinity, as though there had been an explosion of it . Right Damian, I think I've interfered enough in your thread, I'll withdraw now, and let you get on with the job in peace. Ill keep an eye on your progress though. Take Care matey. Ron

Damian, why don't you make individual pitch squares, and place them onto a grid drawn on the tool? A little melted pitch on the tool grid squares will stick em on easily, a hot air gun is very useful here too. I used the mirror as a mould for making a full sized lap. I made a collar that fitted around the mirror edge, protruding above the mirror depth sufficiently to accommodate the tool thickness. Grease proof paper was placed over the mirrors surface as a protective layer. I then poured dental cement into the mold. Dental Cement generates some heat whilst it cures, so I placed the mirror in a bath of cold water to minimise the heat transfer to the glass. When the lap tool was fully set, I removed it from the mirror, and cemented a disc of 18mm Plywood to the back of it. I lacquered the lap face with two or three coats to seal it, drew a grid for the pitch squares, not forgetting the channelings. Wallah!, A lap that after warm, and cold pressings, a bit of channel trimming, and job done. Wear on the facets were always uniform, as the surface created matched the curve of the Mirror very well. I made a few extra pitch squares to replace any that need replacing. Caution The lap I made by this method, was for a 14" mirror, yours is a much larger one, so I urge you to think carefully before you proceed, that is if the procedure tempts you. I also had made myself a polishing machine to do the work. All I had to do was keep the lap charged with the Ceriun Oxide squirter bottle . You've come a long way on this job, and the method I adopted may not appeal, and if there is doubt in your mind, I recommend great care, and perhaps even advise you against it. As I said, your Mirror is a great deal larger than the one I did. I would hate myself if anything went wrong with yours using that method. I'm sure your own instincts will guide you best towards the completion of the 22", You seem to have been be well equipped and prepared throughout this project, so I hope you forgive my being perhaps a bit presumptuous in recommending another method. There are many of us following this thread, and really wanting to see a very successful conclusion for you. Great Stuff indeed, you've almost got me fired up to do another one too. . Best Wishes. Ron.

I'm sure any difficulties can be surmounted with Johns guidance Damian. My largest mirror completed was a 14". A self blame disaster with a 16" terminated that effort. However, I always felt a huge satisfaction when a project was finished, be it a 4" or 14". I cannot see you failing at all, you have too much grit. And I think I deliberately intended that pun. Besides, you are a Yorkshireman, and the grit is inherrent .

I would hope that by the time the SGL11 Star Party comes around, this telescope will have been commissioned, and fully operational. I also hope you can find some means of attending said SP, with your creation, as a symbol of what can be achieved by the determined Amateur Telescope Maker. Plus, if the skies permit, there would be queues around the Lucksall Camp Site, of eager beavers waiting for a A look see at some far distant Star Island . Ron.

I got the Sagitta to 9.4mm for your target F of 3.7 Damian, but rarely does one get exactly the hoped for result. It's looking so good though, and I think fist light is on the minds of most of us, but still a ways to go yet, so we'll just have to be patient . Congrats on the fine job you've accomplished thus far though. Ron.

You've done a great job Damian, and you can be mighty proud of this. You are going to finish up with a superb Mirror, no doubt about that. Still a ways to go, but you're under full sail now, and land is in sight. I take my hat off to you, this is going to inspire more people to make their own too. :icon_salut:. Ron.

I made some sub diameter polishers from Dental Cement Damian. It was reasonably cheap from a local dental technicians workshop in the town, so I made a 16" full size polishing lap for a later project, a 16" f3.5. I never did finish that job. It was a thin mirror, and I made the mistake of grinding the back surface with 400 grit to tidy it up. Unfortunately, doing so relieved stresses in the glass that allowed it to badly distort the lovely sphere I had on it. It would have taken fine grinding and re polishing to get it back to a sphere, the position where I always liked to start the figuring process. It disheartened me too much to want to rectify it. It was stupid mistake to make really. Ron.

You're making good progress, and on track. That grinding tool is looking A1. Ron.

A huge well done to both of you Olly and Tom OD. You have both put a great deal of time into constructing this piece of work, and nobody should nit pick over the proportions of time devoted to it. It is simply a joint effort which has worked very very well. It is a superb result, and if it doesn't make APOD, then it can only be down to petty jealousy, or some other petulant reasons. You should both get that accolade, simply because you deserve it. Brilliant!!! . Ron.

Sure is a brave task you have set yourself Damian, and I wish you every success. The rough stuff is hard work, but once you have the finely ground curve generated, then comes the polishing and figuring stages. It is challenging, but exciting too. We're all rooting for you, and look forward to the commissioning of your creation. Best Wishes. Ron.

That will be one deep Sagitta Damien, your lump of glass will be a bit lighter .,

What specifications have you decided on for the final mirror Damian? Ron.

I developed a great interest in this thread, and from the point of view of the Cooperation and support each participant has provided throughout. It has been a very interesting read, and I would like to thank all of you. You are all a credit to SGL, and yourselves. It has been a pleasure following the project through to what will be without doubt a very successful conclusion. Well done all, and again, thank you very much for a great read, :icon_salut: :icon_salut: . Ron.

Post 171 says 1,886. You sneaked off elsewhere for three more .

As Mr Blessed would say, "YES!, ARE YOU DEEF" ?

LET'S GET ON WITH IT THEN..

I like big B.B. Especially when he was hosting 'Have I Got News For You' Funny guy.

A Medley of Mods too, we have our own Choir you see. Mostly all tone deaf at present, but gradually getting there, somewhere, anywhere. We are rehearsing Twinkle Twinkle Little Star at present . Ron.

You're wise to treat the rear of the disc first. I had partially figured a 12" mirror a number of years ago, and I wasn't happy with the condition of the back side. It had suffered a few abrasions during the grinding and polishing stages, and I foolishly decided to clean it up, just using 400 grit, which leaves a nice very fine ground surface. Of course the disc back wasn't perfectly flat, so it took a fair number of operations to get it uniformly fine ground. The shock came when I placed the mirror on the test stand, and found it was all over the place. It had not even the resemblance of a sphere, let alone a figure of revolution. I was disgusted with myself, as if I had applied the two pennyworth of grey matter I do possess,. I would have left the back alone. After all, it was a fairly thin disc of plate glass. The removal of glass that had occurred, was obviously sufficient to release some stress in the glass, resulting in the complete waste of time of all the figuring work I'd done previously. That disc is still in a cupboard in my workshop. I lost the will to recover it. Anyway, sorry to hear about your Car problems. Always a bad, and usually expensive time, and Christmas too makes it worse I guess. Ron.

I love to see people taking on this challenge, and you have embarked on a very big one here. You certainly have the right attitude, which is a big plus in itself It's difficult to gauge the thickness of the blank, about 40mm perhaps? Also, does it have a pre generated curve, which would be a big help in reducing the amount of coarse grit grinding? How fast are you making it? f3.5 - f4.5 perhaps,? I'm only guessing. My largest mirror was a 14" f5, but I did the grinding with a full size blank. I only used sub diameter laps during figuring stages. I wish you well, you will have a mighty big photon collector, and I will follow your progress with extreme interest. Best Wishes . Ron.

:D:D. Very amusing. I enjoyed reading that, although I believe you are more capable than you imply. Anyone who can relate a tale like that, is no duffer.I too dabbled in stepper motors years ago, but in the end I resorted to RS Components for a work of art they sold. I made a camera platform with it as the drive, for the 1999 Solar Eclipse, and I and a couple of friends motored all the way to Devon. Unfortunately, cloud dominated the event. Anyway, it was an adventure we all enjoyed. I actually dug it out of the dark cupboard it was locked in, just a couple of weeks ago. Connected it to my Power Tank, and it still works. Cheers . Ron:)