barkis

HehHeh I understand Damien, Foucault shadows can be deceptive until you become completely adjusted to what they are indicating. Sometimes an apparent hill, looks like a crater, and vice versa. The shadows that define a very slow Newt. mirror, a six inch f 8 for example, can be very faint. some may be tempted to leave such a mirror spherical to avoid the difficulty of figuring it, however, that figure when created accurately, makes a huge difference to performance, everything else being equal of course. Anyway, I wouldn't dream of dictating how you should do this job, my knowledge of Ronchi testing is far from good enough to guide you away from it. I'm sure that like me, some of the other mirror making fanatics contributing to your thread, would like nothing better than to descend on your home with a need to help out. Whether or not we would be welcome is another matter of course HehHeh! Too far away though, so you're probably pleased about that. Besides, with John on your side, you are in good hands. Good Luck on the run in anyway matey, I think you have the will and determination to lick this big fella. Ron.

It's like staring into the faces of weird Aliens. I never made a mirror large enough to make use of the Ronchi gratings, although I did use a home made one to get a general picture a finished 12" f6 parabola. The Foucault/ Couder, and Everest test methods were mostly my bag. A general overall view of the shape and smoothness, and edge diffraction were always more apparent to me. The more modern methods seem to have relegated the old methods to the second division now though. Whichever one uses, matter little, as long as the goal is attained. Ron.

Many congratulations Raymond. That is a beautiful sight. It's a long time since I've a knife edge view of a perfectly smooth figure. There's something infectious about it that stirs the soul. All the work and effort, including the disappointments, and frustrations that can plague the mirror creator, disappear completely under the spell of this magic vision. I can't express it any better than that. I can only express a wish that Damian too reaches this pinnacle too, I'm certain he will. Ron.

Random is good, in fact it's desirable. as long as you resist the temptation to waltz the tool around, everything will be A1 with random. That was a dictate from a book, not my personal wording. What I do recommend, is that you think about what you are doing with the lap, and predict as best you can what result a particular action is going to produce. Once you start to get that right, you are going places, and in the right direction too . Ron.

Yes that is also a useful guide Damian. I used to do the small torch trick, starting close to the mirror and walking backward until moving the light up and down reverse direction when passed the ROC Point. Walking in the dark, and trying to keep the light on the mirror whilst in a stooped position gave me a hell of a backache at times. I always chided myself for making the test stand too low down, But it had to coincide with the height of my Foucault Tester table :grin:.

I used to spray the fine ground mirror with water after setting it up on the test cradle, and check the radius of curvature length, and thereby /2 it's focal length. A reasonable estimate could be gained, but also moving the knife edge back an forth to intercept the reflected light beam could give a reasonable indication of it's spherical condition. The illuminated mirror would darken alost instantly as the knife cut into the light source. Not a sure fire result, but any drastic event such as astigmatism might be apparent. It would be extremely unlikely, but not altogether impossible. A rare occurrence after fine grinding though.

Ah!, the Henry V. Shakespeare quote. Should do the trick .

I love that velvety feel of the very finely ground surface, soon to be completely polished and figured. You can foresee, and sense the beautiful and highly reflective and precision worked surface straining at the leash, ready to nestle down in the cell, and awaiting collection of the Billions of ancient photons that will soothe away all the left over anguish in your mind from the effort expended giving birth to this big baby. Now Damian, with that load of tripe out of the way, it's onward and upward in the real world me lad. This time, success is assured, but take nowt for granted, there is an abundance of work and thought left to negotiate. I have the utmost confidence in your ability to write the final chapter of this very interesting log book. Go to it matey, I'm going to watch from the Gallery as you take that longish, triumphant walk to victory. :icon_salut: . Ron.

Damian, for what it's worth, I used to add a tiny drop of Washing up liquid to my fine grind wets. It seemed to help in distribution and lubrication, and prevent locking up of the discs. Unlikely anyway with the smaller grinding tool, but it could happen. I've had a full sized tool get stuck fast on the mirror, because I didn't keep the tool moving, It was fatal to stop. Devils own job getting them apart too . You've been through that phase once though, so as I say, unlikely you will experience it this time around either, but I'm just making you aware of the danger. Ron.

Damian, I applaud your decision to return to the fine grinding to remedy the problem. It will speed up the correction required, and then your polishing and figuring can be accomplished now you are aware of the reasons why it all went wrong first time, you are now forearmed with the knowledge John has given you. Don't for one moment think yourself a failure, on the contrary, your upfront reports on this terrifically difficult project will have given others contemplating making a large objective, an excellent insight into the difficulties that await the unwary. You might feel a bit disappointed, it's only natural, you have worked long and hard to create a massive light bucket, and you have learned much, and you will now progress to a successful conclusion. It is still going to be no easy task, but you are better prepared now. The old saying still applies. Those who say they have never made mistakes, have never really made anything. All the guys will remain loyal to your efforts, and will you on to complete the job you set out to do. Remember, you are not in a race here, you are going to create something you will be immensely proud of, and when it's complete, and nestling in it's Dob. base, waiting to take you back through time, in a splendid way, the rest of us will want a look see too . Go for it buddy. Ron.

I have to concur with John, Rob. With one exception. In the ten years I have sailed across the universe as crew member on the good ship SGL, your post is the most pleasurable I have read, sorry, as yet, only partly read. I will return to it though, it deserves a complete study. It contains an eduction in itself, of this magnificent enigma we know as the universe. Very much more awaits discovery, and posts like this may be the inspiration to young potential scientists who read it, to go on in the future to help further man's knowledge of the vast cosmos in which we are but a microscopic resident of. That itself is quite extraordinary. Thanks for your very educational, and informative inputs to the forum. Ron.

I,ve just concluded that the confusion about your mirror size, is due to quaoar's 28" Saturn image, which had a big impression on the forum. A mobile phone capture through that huge mirror, must have focused my mind on the mirror's diameter of 28". That's a good excuse Heh? Ron.

So sorry for inflating your Mirror size Damian. I don't know why I thought it was 28", blimey, that's a lot larger than 22" obviously, but no less easy to fashion. I'm, happy you have decided you don't want the break from it, I made the assumption based on the feelings had at times, when the frustration started to grip like a Vise, and my patience was very short lived. But, like you, and I'm sure many others who have trodden this path, you learn to cope, and rationalise your thinking, All the clues are there, and when you decipher them, the remedy for different anomalies can usually be worked out, but can still be time consuming. However much it gets to you, never give up, and I know you won't. I hope the other contributors, and I know some are your friends, don't feel any animosity over my suggestion to rest up, It was purely a protective measure, as you seem such a friendly sort, you wouldn't have the heart call a halt. Now I know you are fine about it, we can all rally around and cheer you on to the chequered flag. I am certain John N. will see you on the right path when you see him, and he has a look at the surface. It will be easy peasy then, hopefully . Good luck anyway. Ron.

I think perhaps Damian might prefer a little respite from all of us for the time being. He is aware the job he has undertaken is a Mammoth one, and he's now at a crossroads. His visit to see John, should result in a plan of action that will hopefully re invigorate him, and he will carry on with a stronger purpose. Many post have been generated in this thread, some of them with the sole intent of aiding Damian in getting to grips with the problems. Let's face it guys, a 28" by hand is a Herculean task for anyone, let alone a first time aspirant. We are all attempting to help by proxy, but since the task is fundamentally a hands on operation, and the analyses of the progress being made, is the domain of the operator. From what I have read from all the experienced contributors, their advice has been quite valid, and under normal circumstances, would have brought about some forward progress. That doesn't appear to be the case though. So, with that in mind, I suggest a respite for Damian to allow him to regroup. Also I would like to say, there has to be no animosity developing in the thread, because will quickly propel it downhill. No aspersions to be cast to maintain our respect for Damian himself, who has been a very pleasant and cooperative young man, in his responses to all who have posted in his thread. He is a credit to SGL his forthright attitude, and friendliness towards everyone. I leave it open to Damian to respond of course, but If he would prefer the thread to be locked for a spell, until he has time to gather his thought, and reflect on what he gleans from John, then that can be done, and reopened when it suits him. This is a suggestion only, not a threat of any kind. For the most part, You have all built a good thread here, and it needs a successful conclusion, for Damian's sake, and for potential mirror makers too. Ron

When you get around to making a 1 metre diameter mirror Damian, I hope you build yourself a grind and polishing rig. It sure take a lot of the pain away . I hope your visit to JN. is a fruitful one. I'm sure he will quide you on the right approach to fixing the last anomaly. There's no way you can remove enough glass by polishing, sufficient to ruin it completely. Any thing you polish into it, can be undone by polishing. If the anomaly is too bad, and needs many hours of polishing, then reverting to fine grinding would be a quicker route, and that too would not take too much glass off. I've had to do just that myself on several occasions in the past. It takes more time, but less frustration. Best Wishes. Ron.

Try some W strokes Damien, traversing across the mirror, but staying within an inch or so from the edge, but varying it slightly. Keep rotating the 6" tool anticlockwise if you walk clockwise around your table. Take it slowly with these strokes, and just take care with the amplitude. Concentrate the stroke frequency in the central regions of the mirror, but don't dwell too long there, keep the tool moving. Do this for a few whets, then study any progress being made. Your aim is to reduce that raised centre without digging a hole. Good luck buddy, I hope this helps you, I really do. Ron.

A sense of humour, the antidote to the mirror makers frustrations. The joy of reading off those final, and successful figures Isn't too far off.

Good man. I, and everyone else gripped by your thread, would be very sad if you dug a hole, which is a beggar to recover from. I know you have gained some discipline through this labour of love, and I only wish to keep you on your toes. Not that you need it .

You be careful with that tiny lap young man. Small it may be, but it can appear to be benign at work, and might appear to be ineffective, but if you are not on your guard with it, it can quickly put a hole where there was once a hill. I jest not . Test often to give yourself a some braking time. Ron.

The apparently huge mountains and valleys the shadows reveal under a knife edge/ Ronchi, are really infinitely small. A simple test of how a mirrors surface can be affected by simply placing your thumb on the surface for a few seconds, will raise the temperature sufficiently so that under the knife edge, that area will appear as a huge hill, but as you watch, it will reduce in size, and eventually disappear. You might even witness the waves of heat as they dissipate into the air. Even when air currents are present around the test rig, they can be seen swirling around That is why testing, and in particular, final testing of the mirrors Parabaloid, should be in a stable temperature, and free from draughts. Some Mirror makers build a test tunnel in order to make that situation more achievable. @ Damian. I'm pleased you are still full of determination, and winning the TDE battle. It can be a long journey, but perseverance always wins. Ron.

The thread seems to have gone to sleep. Damian, are you taking a break to re group, or are you working away in silence for a while. Either way, I hope the project is still alive and well, and going well. It's a long journey, so rest periods won't harm anything. Best Wishes matey. Ron

I agree with RAC Damian. Foucault shadows are very revealing of the whole mirror surface. At the the centre of curvature, the Foucault tester will really show the edge condition. A good edge will show a dffraction ring all around the mirror edge. Slightly brighter on the lamp source side, but a turned edge will show none on the knife edge side at all. Of course the tester is not adequate for final quantitative measuring of a mirror that size. Ron.

"What's with the Negative Waves" To quote (Vernon L Pinkley), Donald Sutherland in the Movie 'The Dirty Dozen'. Damian needs inspiration from us, and hopefully we can give it.

You could polish a mirror for years before it got appreciably thinner to any degree. Actually, the School is still out on whether the glass molecules get removed at all, or just merely pushed around. Damian won't quit, I think it's quite obvious the man has grit and determination. If anyone sees a Pun there, it wasn't an intended one. He could take a break, which shouldn't shock anyone. After all, many makers of mirrors of this size would have used a grinding and polishing machine, It's damned hard work pushing glass around. We talk of microns, and could assume 'Oh, it can't be that difficult then'. WRONG. It's the getting them in the correct place that brings the difficulty, and the larger the mirror, the more difficult it becomes. It's largely why Newtonian, and compound telescopes are so expensive. There are cases of potential mirror makers not even getting started, but it's rarer to find many who don't finish what they start. It may seem a glamorous thing, but not a walk in the park either, and this one could be described as Herculean. He's a braver man than me, even if I was 50 years younger . Ron.

Let us try to keep this thread on topic folks. Anyone with their own projects running, make a thread of your own and don't post in this one. I'm sure Damian would prefer continuity, with no interruptions. Ron.