barkis

Thank you Ron for the very thoughtful post, written with sympathy, experience and dignity from a true Gent. The Jaffa's will get me to the end

I have returned to it today after a session yesterday morning for 2 hour's. I will be working solely on the edge today to see what results I can come up with so I shall feedback later with some image's 

Damian

Please be very careful with the small lap, they can change an area in double quick time.

Goes without say my friend, but keep a wary eye on what is happening by very frequent tests after allowing cooling periods.

The Video AJohn suppled is a great guide to working the edge with the small tool, but I still urge caution.

A 22" disc is far from an 8" one, when it comes to remedial work.

Sorry if I seem to be Clucking like a mother hen, but I dread the thought of you having to return to fine grinding.

I'm sure it won't happen though.

That beefed up table will make a difference too. Good job that.

Hi. John. That video is very  informative, and that TDE correction method is indeed a great way to correct it. Short centre over centre with a hard lap.

An Oblate centre is the usual penalty for such an action, as the gentleman in the demonstration explained. That fortunately, is easily corrected, again adequately explained.

Unfortunately, the mirror being worked in the Demo, looks like a 150mm dia disc to me, worked with a full size lap.

Damian's disc is 22", and a full sized lap is going to be a beast to work on a mirror of those dimensions.

As much as I personally dislike sub diameter corrective laps, they are necessary on Damians project.

Many instances of TDE are a legacy from the fine grinding process, and in severe cases, reverting to fine grinding is the only sensible way to reverse it.

Damian is persevering in correcting the  anomaly  using his sub diameter tool, and it does appear to be responding. It's a condition mirror makers dread,

and either one gives up, and decide to mask it off, or prepare himself for the difficult job of Eradicatiing  it, which is Damian's goal I'm sure.

I think Damian is very pleased with all the attention his project has attracted, and all of us watching, and sometimes adding our owns tips and suggestions

to the mix, maybe getting a little tiresome for him at times. He is probably too much of a gentleman to say as much, and in fact I might be wrong in suggesting such a thing.

However, at such a critical time in his progress, he might wish to enter the last phase of the job with less comment from us, and just let him alone for a while.

He knows what is required, and if he needs qualified help and advice, he has John Nichol to turn to, and in better hands he couldn't be.

I hope no one is offended by what I've said, and indeed, if Damian himself doesn't agree, he is perfectly at liberty to say so, and I won't mind one bit.

Ron.

Edit] My apologies. The mirror in the Vid. is  a 200mm  Diameter Disc.

I've read that pitch that is too soft can cause a turned edge. Maybe Ron has too. I can't figure out why other than the lap tends to follow it so wont completely remove it. Damian can guess how I test pitch for hardness but I would describe the results as pretty stiff chewing gum, doesn't shatter. If this is the reason turned edges can be hard to get rid off the only solution will be to make sure the lap spends plenty if time inwards from the turned edge so that it can keep  the required shape - also why I reckon a larger star lap might fix it. On the other hand that might not be the reason some reckon that soft pitch turns edges.

I saw the mirror making man from Luton at work in his industrial unit.  His polishing machines ran at something like 60 rpm, he offered me one to take away but the things were way too big for me at the time. The drive was interesting though.  He polished till they squeaked and then added more rouge, about maybe 1 1/2 teaspoons for a 10in mirror. Consistency about that of very runny salad cream - plenty of colour. This would last for some time, longer than I was there. Personally I use it a little runnier and spread it out over the lap area with a finger and then get to work. Same when pressing over night other than a little work to spread the rouge. His test for scratches and sleeks was a little single AA torch all most  horizontal across the mirror. Sleeks look like broad highly polished scratches.

John

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The reason is usually because the central areas of the lap tends to collapse due to the constant friction. There is no respite in the middle.

The softer pitch unsupported on the mirror due to the sagging, causes the lap to plough into the edge zones, resulting in  turning the edge over, and can get pretty bad if not recognised soon enough

The mirror also turns Oblate, because the central areas are  not being worked by the lap, due to the same sagging condition.

Hard pitched laps are an insurance against TDE, but can result sleeks on the mirror. Not as bad as scratches though, and can be polished out.

Lots of pressing is also mandatory to maintain good contact all over.

Ron.

Small update

After an hours polishing the table is working lovely and the drainage system is a revelation.

May be psychological but it feels that little bit different

Everything that feels good, is good, so stay in that mindset  Damian, it's worth hours of work.

Ron.

Cheers Ron

There isn't the other half to worry about as I live alone except when my daughters come to visit. So all clear

I hope too you are right on the mirror behaving itself now we shall see

I have this morning added a drain hole and length of tubing so that's all sorted

Mentioning recutting the channels I have also done that too, cleaned the lap and inspected it before a bit of warming and now it's pressing ready for a couple of hours work.

Results I shall post later as I have work this aft which will give the mirror chance to settle

More bits for the scope build also arrived this am which was a surprise ;-)

Damian

Onward and Upward!, as they say.  Not that I know who they are   .

Ron.

Hi Ron

I have been working in the kitchen as you quite rightly point out. The reason being that during the winter the pitch was just too cold to work right in the garage. I've just stayed in the kitchen ever since and there is also much less chance of contamination as my garage is quite a mess and needs a tidy out

Damian

Cheers mate, I thought it was an indoor environment. My other thought was, if you're married, then you have a very understanding wife. 

Mind wouldn't let me near the kitchen. It was bad enough I transported pitch chippings into the house on my person.

Trimming the channels after periods of polishing with a trim knife, the little bits flew all over the place,  in my hair usually, where some little bit would drop off during the night, and embed itself into

the pillowcase. Then the Flak flew I can tell you   .

The garage can be a cold environment in which to work, but a hard cold lap was a plus in preventing a turned edge, but contact conformity was also difficult, so pressings were frequent through necessity.

One had to guard against sleeks too.

Good Luck in your forthcoming sessions, I think the mirror is about ready to do your bidding  .

Ron.

Just where are you doing the work Damian? You seem to be in the house somewhere.

The Kitchen maybe? Either there, or you have a very posh Garage/Workshop  .

Apologies if you've explained this elsewhere in your thread.

Ron.

Cheers Ron I'll hold you to the Jaffa's

Damian

For Sure matey  .

Good luck on the run in Damian.

I'll be back to join in the celebrations, and I may send you a Pack of Jaffa Cakes, SGL's 

usual celebratory snack   .

Best of Luck Matey.

Ron.

Thank you Ron, a very encouraging comment and thank fully he got it finished using the ultimate star test. Bet he was as happy as I'll be once I get my shaving mirror complete    

You could also have a few wets if you could wrestle the lap away from me   although I think after 5 mins we'd be having a cup of tea and you'd be reminiscing about your own mirror making exploits and acquaintances. 

I've done three hours today and maybe a little change to the edge 

Inside ROC

SDC13299.JPG

Outside ROC

SDC13289.JPG

Not much to report and the battle continue's maybe tomorrow. A well deserved beer now and some relaxing to think of strategies for tomorrow

Keep watching

Damian

You're certainly approaching the tape Damian. Carry on the excellent work, and I'll recede from the workplace and let you

get to Nirvana.

I'll be back to join in the celebrations. I'll send you a pack of Jaffa Cakes, SGL's usual celebratory snack   .

Best of luck on the Final Approach Matey.

Ron.

Yeah, but he got paid well for the privilege!

John

I don't think King George III would miss the £4,000 he granted for the telescope John.

Cheap for the 40 foot monster I would say.

Consider the discoveries made with it. I don't think 

there would be much for William and his Sister Caroline,  (Who helped him) to pocket after paying for the materials.

It was the largest scope in the World for 50 years. Believed to have helped in the discovery of Saturn's sixth and seventh Moons, Enceladus, and Mimas.

Anyway, I think we have hogged Damians thread too much as it is.

He must be getting fed up with interruptions  :icon_mrgreen:. 

Ron.

Try to imagine William Herschel figuring that 49" Speculum metal mirror of his, way back when  Damian.

I think he used a star as his light source, and lugged the mirror in and out of the huge truss to make corrections to the 

surface as required. Some Job Heh?

Carry on yor good work sir, I wish I wasn't so far away from Barnsley, I'd love to pop in to watch progress, and maybe even get a 

few wets in. You've got me yearning to get back on the grind again you bad lad      .

Ron.

I would love a big Dob, a 36" f3.5 will do nicely thank you.

Housed in a weatherproof Obs, an a simple Alt Az  Swing through Fork Mount to gain access to the Zenith.

Not too high to climb to the focuser, and no awkward eyepiece positions to annoy me.

Just wait for the Cosmos to roll by me nightly through the years,  and pour the starlight down to the big collecting eye, and back up to mine.

Nice thought.

Sorry Damian, I digress in your thread.

Ron.

I sold my Texereau book. All I have now are NE Howards Handbook for Telescope Making,

and Albert Ingalls  Volumes 1, 2, and 3. Old, but crammed with useful stuff. 

Modern methods have long overtaken them, but one can still home produce good optics from them.

Ron.

PS.

I'm sure Damian won't need to attempt the following  .

A little humerous drawing from Ingalls book 1.

I have been doing the wire brush scrub but not quite often enough by the sound's of it. Great tip and thanks for that Raymond. I will test the mirror later after a cool down and see if things have changed for the better. 

Can I also add that is one lovely Flickr page you have 

Damian

Eeeek!!  Wire Brush??  Tell me you are kidding please  .

Imagine the myriad scratches a piece of wire could do stuck in the pitch.

Ron. 

This post isn't to try and offer any advice Damian.

I just want to make the point about the incredible attention you are giving this project.

Not just your almost running commentary on the work, but to highlight the extreme patience you posses,

both in applying yourself to the task in hand, and your willingness to pop on SGL every day to reply to everyone's posts,

without an Iota of impatience reflected in your replies.

I know each one of us is willing you success, and it will happen.

The only equation left is being resolved by you, and that is.. Patience x Time + Thought  = Eureka!!!

We'll all share in your elation when that moment arrives.

Keep pushing those glass molecules up the hill to the edge  .

Ron.  

I always used micro faceting on my laps, whether it was a 12" lap, or a 6".

The theory that it is the edges of the lap squares that facilitated the polishing action,

means that multi. facets theoretically would speed up the polishing, and also

the figuring process.  Of course we know that speed is not the factor that is important.

Good contact, and at a speed that prevents micro ripple is important.

Keep the Faith Damian, you've already got the winning post in sight,

you just have to give the mirror a quiet talking to  .

Ron. 

Damian, I'm sure you were under no illusions about the task you embarked upon with this large objective, and John Nichol would certainly have 

highlighted the difficulties you would encounter. Frustration will always be your companion when you are attempting the figuring process,

and when it gets too bad, you should take a break. A 22" f3.7 is a very difficult adversary for anyone to tackle.

All of us following your progress are pretty much mesmerised by what you've achieved so far, and certainly willing you passed the winning post.

You have the discipline to complete the job, but don't be averse to leaving it to one side for a spell, and get back to it when you feel like it.

Have a read of Mel Bartel's Parabolising pages might inspire , they are quite interesting.

Best Wishes.

Ron.

                                                                     http://www.bbastrodesigns.com/JoyOfMirrorMaking/Parabolizing.html

Have you been using  1/3rd centre over centre strokes Damian? 5 minute periods with cold pressing regularly?

You can shorten to 1/4 strokes if it is reluctant to cooperate.

I assume your lap is fairly hard, which is good?

That is a good way to fix TDE, but you might inherit an Oblate surface in the  centre, which

is not unusual. That is easy to remedy though.

Not trying to teach Grannie to suck eggs matey, and you are winning the battle

with that huge lump of glass. You're a braver man the I  .

Ron.

You'll get there alright, you have achieved enough already to show you have the will to 

get past the winning post.  I look forward to that day  .

Ron. 

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.