barkis

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.

Imitate the action of the tiger, stiffen the sinews, summon up the blood,

Richard

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

Hi all,

I have over the last few days done quite a bit more work on the mirror centre but things are not really working the way I want them to. As the days go on the images seem to be getting more confusing to read. With the amount of edge work then the centre should have had a huge hill so was going to take a lot of work to rectify and I have done quite a bit already.

I have a trip loosely planned to see John this week so the mirror is on hold until he takes a look at it and gives me his expert opinion 

Damian

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.

I hope I get it finished as it'll be a public stoning like the one in life of Brian!

We see no Messiers he's a very naughty boy!

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.

John has advised the same Ron so I shall be careful.

I did half an hour to start with and then tested and then went on to do another hour and test.

I shall do the same tomorrow and fingers crossed if it starts to get near I shall get back to the 10" lap to finalise the smoothing

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  .

Today was an hour and a half with the smaller lap after cold pressing for an hour before starting.

I worked just on the raised area quite vigorously and it's the only time I've ever felt a discernable temperature difference to the glass.

The hill is going to take a lot more work and using the small lap is like using a dinky toy compared to every other one I've used.

After cooling off for a period of time I retested but it's obvious that it is gonna be a lot more work to smooth it down.

More tomorrow and hopefully a good update to post

Damian

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.

Hi Damian,

If you are pressing the lap now, does it not take on the shape of the hill in the middle? Or does that not matter as it is relatively small?

Derek

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.

Hi all,

Apologies for not posting anything for a few day's and thanks for all the comments and help 

 Ron, I hope you didn't take offence to the joke comment about the polished edge. The lap is 10" in diameter and after a discussion with John I am now moving in a different direction and will update the thread later on with the result's hopefully more positive we shall only see?

I have also ordered more Cerium as unfortunately I am again running low

Damian

Certainly not me lad, I don't offend easily anyway.

I've taken time off myself for a little while, trying to come to terms with a very persistent cough I've inherited from a recent chest infection.

It has wearied me somewhat, and still no end in sight. Maybe it's waiting until your mirror is figured and ready for the Vacuum  :grin:.

(Now that is meant to be a lighthearted jest    ).

I sincerely hope you are on the last lap. (Whoops!, a pun now too.)  .

Good Luck, you deserve some.

Ron.

It might be working Damian but not in the way you expect - trouble with laps. Say you use a chordal stroke  with the edge of the lap just running just past the centre of the mirror. Stroke length so that the max overlap at each end is say 1in.  This would scarcely work the edge at all. If things are working efficiently it should produce a hill in the middle that runs out gently to the cord the lap ran on and then rise up towards the edge leaving your turned down edge. Extend the stroke so that the max overhang is about 1 1/2 and eventually the edge should start cleaning up. You need to work up to that. There's a need for a sense of humour and determination to meet the challenges.

Ronchi isn't something I am very familiar with but I'd guess you need to adjust to see 5 lines with one over the centre of the mirror. That way you may see that you are making progress as outlined above. In fact you can probably look at the lines and guess how things will turn out if you continue. The shape of the lines will relate to errors but wont measure them unfortunately.

Small laps can make big changes as some have mentioned but yours has a pretty large area really. It isn't going to work a 22 in mirror as quickly as say a 15in.  In your case you can get round the problems by doing some centre over centre from time to time with no overlap at all to try and keep the curve that is being worked out spherical. That can be aided by mixing in some mild w's again with no overlap. Eventually when the edge has gone or staring to go that is the sort of stroke that will be needed but this time  with some overlap. Only testing can indicate when you should do this. You might find 1/2hr work takes longer to clean up or a few mins. The time spent correcting can be balance to suit.

I'd guess you already know that it's very important to spin the lap a bit regularly and alter the amount as randomly as possible. Every stroke wont hurt.

If there isn't any efficiency I would wonder about the polishing mix being too thick and in any case add weight to the lap.

However you do it - try it for sufficient time to see what it's doing, clean up and test. At this stage I don't think you need worry about letting things cool. That comes when you are worrying about wavelength sized adjustments. Here you need to lower a substantial area of glass. Waite's video might well be correcting 1/4 wave or less over correction - that really does need doing slowly and carefully. The problem with people like him is that he has done so much of it that he knows how much work will be needed to do what ever he is trying to do - us poor sods have to learn. If the work doesn't do what we want at least we know how long it took to make the change. If there is no change and it should be possible to detect it - something some where is very decidedly wrong.  Some people make notes.

John

-

Some very sound advice in your post John. I hope when Damian surfaces again, he has some positive news. Goodness knows, he deserves it.

Ron.

That made me chuckle Ron, and you said David Sinden was good at joke's   

Definitely polished out, especially after I think nearly 30 hours extra now after the initial polish. I wish it was that simple 

The image's make it look like you could see it with the naked eye it seem's that big!

I worked today with the lap centre about 2" from the edge and then about 6" from the edge. Doesn't seem to have worked 

Damian

What size is the polisher you are using, and how are you working it on the mirror,

When you said 2" and 6", is  the centre of the lap traversing that variation in vertical strokes across the centre of the mirror.

or are they  chordal?

I can imagine how messy the Rouge is Ron as everything else about mirror making seems to be messy 

I have done another 2 hours today mainly edge work but very little change to be fair

Inside ROC

SDC13368.JPG

Outside ROC

SDC13392.JPG

I did 10mins and then cold pressed for the same repeating in about 1 hour sessions. I have roughed up the centre and created a little mound in there.

When I finished today I cleaned and re-bevelled the lap.  This turned edge is nearly as stubborn as what I am 

Damian

Forgive this question Damian, it might seem a little impudent.

Have you looked at the edge to make sure that it is polished all the way?

Just a thought that crossed my mind. I'm sure it will be, just being nosey is all.

Ron.

Jewelers Rouge is  slow acting as a polishing agent. The big drawback with it, is the mess due to staining it makes.

A mutual friend of John Nichol and I, the late David Sinden, amused me with his tales of rouge.

He would relate that, standing in a corner of his Optical Shop, carefully opening a container to extract a small measure which he would then place in a squeezie bottle

to make up a mixture. On turning around, he would be confronted by an area behind him covered in Red staining. 

He was exaggerating of course, merely trying to emphasise how badly  the stuff stains anything it contacts..

David was good at telling jokes. Much better than I can even write them.

Ron.