barkis

My heartiest congratulations Damian, on the completion of your mirror.

You must be relived to reach the end of the journey at last.

I'm pleased you decided to buy a flat for it, I think you have done enough battling with the 22".

It's of no detriment buying the Secondary, making one may have been in your mind, but flats are not

an easy task either, there is flat, and then there is flat, not easy to make at all.

I think the telescopes first light will be the moment all the hard work you have devoted to this project will

overwhelm your emotions, when those distant photons hit your retina, and flood your mind

with extreme satisfaction. Almost as good as you know what, perhaps better even.

We have accompanied you most of the way on this labour of love, and we all very much look 

to the report of the christening of this instrument, and the writing of the final page.

Well done young man, you are a credit to yourself, and this forum.

:icon_salut: .

Ron. 

It's been an exciting, and pleasurable experience tracking you through your Mammoth task Damian.

I so wished I could have spent some time in your presence to encourage, and perhaps offer some on site advice, but

I know deep down this mirror making job is an extremely personal journey.

You've shown a great deal of patience, and may I say restraint too, as there must have been times when the pressure Cooker valve

got very near blowing off. However, I'm sure you accepted Johns experience had to be tapped into, and there is nothing wrong with that at all,

in fact I would call it a  common sense move.

Congratulations on your fortitude and stickability my friend, you certainly have plenty of it, and in due course,

I look forward to your experiences as you gaze out into the magical universe, and drink in the beauty of those magnificent

Deep Sky gems that only a Big Eye like that 22" will deliver.

Smaller apertures are never to be mocked of course, and I would never do so, but there is no substitute for 

huge light collectors, and I hope one day I get a looksee through the Mapstar Monster.

Well done    .

Ron.

One must never denigrate the use of machines in the  making  of mirrors, even the smaller diameter ones, although it is  a lot less arduous

getting down to fast numbers.

If one has the ability to construct  one, then do so.

 Anyone who may think it's a cheating method would be quite wrong. 

A machine simply saves time, and prevents weariness. The skills of the people  making

objectives, are still the prime requirement, and the quality of the end product is down to them, not a machine, which is simply another

tool in the process. A damned good one too  .

Ron.

If I were to even consider making a fast mirror 25", or 30",  the blank would need to have a pre-generated curve  .Far too much glass to hog out from 

a plane surface  .

Ron.

Nice to know you are getting to grips with it Damian.

Proceed with caution here on in, work a little, and test often.

Don't let anything nasty creep up on you, as you are likely on 

path to  "Eureka".

Ron.

I just hope nothing I said hurt your feelings in anyway Damian, that piece was a generalisation, and not targeted at anyone at all.

I was reflecting    (No Pun), on my own experiences over the years, and my own lack of so many fundamental preparations that resulted in

many problems, and the need to retrace a lot of work that could have been avoided with some forethought.

I would encourage anybody to take it up though, It has it's many rewards, and not least the utter satisfaction of a job well done.

It's never easy, if it was, there would be a shortage of the necessary materials  .

Anyone contemplating making their own Newtonian mirror, and especially if it's a first time  project,

I recommend doing lot's of homework before you start.

There are so many variables that come into play during the task, it pays to be fore armed with certain knowledge.

The degree of difficulty is much  more than directly proportional to the size of the disc.

In the case of very large diameters, the full size lap becomes a liability, and therefore smaller tools have to be employed.

These can be very difficult to use without introducing  some serious anomalies that are difficult to correct.

Without extreme care, they are positively dangerous, and in not too long a time at that.

Pitch consistency, allied to temperature changes can also cause havoc. 

Turned Edge is the Bane of mirror makers, but the best cure is not to let it happen in the first place.

Read about what the causes are, and do your best to avoid them, many do, because they educated themselves as to why it can happen.

It isn't easy to have a place of work where a constant temperature can be maintained.

Working with the mirror face up also enhances the risk of scratches, small particles floating about and landing on the 

mirror face is all takes, so cleanliness is another must.

Not intended to dissuade anyone from having a go at this, but be aware it takes much consideration, and a determination to succeed.

A first time creator, would be advised to take on a smaller disc, about 200mm diameter is  good size for a beginner. No more than a 250mm if

you are brave. Those sizes will permit you to work with a full sized lap for polishing and figuring stages,

and although it make some cringe, allow working the mirror on top of the lap.

It is easily controllable,  and with small alterations to the lap facets, parabolising can be accomplished too.

You can make two full sized laps, the second one can be  a repair tool if it becomes necessary

It is a very rewarding thing to complete precise optical objective, I hope amateur mirror making stays healthy,

If you have the mettle to do this, you will likely become a very good observer too.

Ron.

LOL! I  made a model boat that took 16 years and cost me about £20 

That confirms my Fortitude and Stickability theory. If you have it,

You'll never ever give up.

Fortitude and Stickability, the bywords of Newt. mirror makers.

One has to have it, or failure will result almost for sure.

You have both attributes Damian, so success is probably just  around the corner.

This was a monumental task you set for yourself, and that realisation dawned on you as time

went on I'm sure, but you have battled on, and all credit to you for that.

I hope to see a nice Xmas gift for you, in the form of the  completed Mirror, and I think that will be so.

I had the pleasure of viewing through a 20" f4.7  last week, and mind blowing just doesn't cover it,

so I hope one day to put my eye to the Mapstar Scope  Focuser one day, and get the awesome feeling 

of being back in time again.

Like all in this thread, We're willing you to get this done, but in your own good time.

Ron.

I dismantled my large garage workbench today,  and clearing out the junk from the good stuff,  (not much of that I might add),

a lot of paint tins etc. the contents of which  are still usable.

I also found, tucked away at the back, my 16" lap I made from Dental Cement years ago.

The pitch squares have suffered a bit, but the memories flooded back. I got the mirror spherical, but made a foolish, and cardinal error,

I made an effort to clean up the back of the mirror with 400 grit, and when I re tested to see if the surface had changed, I was horrified at the 

funny shapes on there. The mirror blank was only a thin plate glass one, so the clean up had obviously released stresses in the disc.

I abandoned it altogether at that point, I gave myself a telling off for being stupid, then parked the lap in the bench cupboard, and forgot

about it.. 

I will give some thought to making another mirror though, I do have a  15" blank tucked away somewhere.

Ron.

Mel Bartels is highly respected, and has given much to mirror making.

That's a very useful link to lots of good Info, and it's free too.

Got to be good.

Damian, first let me say that your PM on another subject is ongoing, and a solution is fairly imminent.

Your mirror surface seems to be reducing in the centre and intermediate zones, but left behind in the marginal ones.

You are wise to cease work until you have discussed with John.

Personally, I would make myself a full size lap, and warm press it with a pliable mesh, to manufacture hundreds of small facets within the large ones.

The purpose being, to make it easier to polish with the mirror on top.

I know it sounds crazy, but I used that technique on a 14" with success.

Mix a little detergent in the Cerium polishing mix, it makes a good lubricant. You get a few bubbles, but what the heck,

it will break the monotony a little.

This is just a remedial suggestion. The strokes should be an average of 2" overlap, and no more.

You will need to press several times to repair the small facets, as they will disappear quickly.

You could also place a plywood  disc over the back of the mirror, it would need an edge to fit around the mirror wall to keep it in place.

A couple of knobs screwed glued to the on the back of the disc to make it easier to push it back and forth.

The pitch should be fairly hard, and stay with centre over centre strokes  4" Max, ie  an average of 2" overlap each side. No need for accuracy, but stay away from too long a stroke. and work slowly, 

This should start to bring your edge to heel, but you might finish up with an Oblate spheroid, but that's easy to cure.

Now, I'll qualify this by repeating, it's what I would do, and have done.  Others may tell you different.  

A large disc like yours is no pushover on top a like sized lap, but multi faceted pith square will reduce some of the friction using the polishing agent with a little washing up liquid.

This is simply a means to an end.

Anyway, good luck, it's a difficult stage you're at, and you will need all of your resolve to get by it.

Ron.

PS, How is your Foucault tester coming along.  I'm sure some  images of the shadows will give a very clear picture of the surface.

Those Ronchi lines, although useful, just make me dizzy  .

Damian, we do appreciate you taking time out to keep your audience happy, and we appreciate it.

None of us would complain if you curtailed some of your precious time keeping by keeping us informed.

A summary at the end of a weeks work or more, would more than satisfy us I'm sure.

I know you love to keep us in touch with what is, or isn't happening, but we aren't going anywhere matey,

and we'll hang on every word you write when you do get back to these pages.

Time is very important to you, but you mustn't rush just to make some deadline for our benefit, it doesn't  work like that  .

Ron.

Keep your attention on the  mirror Damian, the  clothes for it were never going to be as demanding to create

as the Optical masterpiece you are striving for, much work to that end is still ongoing.

Doesn't harm to take breaks and do the other things that need doing of course,

but the main focus has to be on the mirror.

What about the secondary by the way, are you buying a ready made, or making one,

and have you made progress on the  Foucault tester you were going to make?

Might help if we didn't ask so many questions hey?    .

Ron.

I drew the pattern of the grid onto the lap, coated with Beeswax, which allowed the pattern to stay visible, then

played a light flame over the wax to position and set in the pitch squares.

One recommendation I got from one ATM book, was to mix a little beeswax into the pitch before making the lap.

I tried it once, but found that the lap often skidded when polishing, so I quit that,

Ron.

I did the same thing Raymond.

I used Dental cement, which  is a superb medium for a lap,and quite inflexible when set.

It isn't expensive, in fact when I explained to the Dental technician where I bought it , what I was going to use it for, 

he gave me a large bag full of it FOC  It made a 2" thick base for a 15" lap, and a couple of sub diameter ones too.

It does get rather hot when it's curing, and as I used the ground mirror as the mould, I used a tissue and baco foil lining over the mirror

to prevent too much heat transference. It was a heart in mouth wait, but no disaster happened .

 The pitch squares were individually made to stick on later, so not a lot of pressing 

to get a good match was needed, and little or no channel pruning  out was necessary either.

Ron.

There are many variations of the Foucault tester, but you  rarely find one as basic as the one pictured here

from N E Howards Telescope Making Book.

Of course it is crude, but the Barr Scale was used to measure the longitudinal 

readings quite accurately. How to make it also appears in the Book.

I think a Micrometer, or DTI  Gauge are preferable though  .

The gap between the light source and knife edge is a bit too wide,

but of course the Image is purely to show the simplicity of the device.

Ron.                                                                                                                                

Looks a decent setup Raymond, I too used a DTI gauge, although there are simpler methods, I preferred the Gauge.

I used to use a very tiny, and hopefully star like pin hole, but I surrendered to a slit source for better illumination.

I found that fresnel fringes could be a nuisance with too small a light source.

Ron.

I do believe you are cooking with gas now Damian, that flippng glass is starting to listen to you at last  . 

 Keep your thinking cap on now.

Ron.

Taking pictures of the various shadow patterns isn't usually difficult Damian.

Positioning your camera as close to the knife edge as you can when you are intercepting the focus of a particular zone on the mirror.

You saw Raymonds lovely parabaloid Image. That would be at 50% of the total longitudinal knife edge movement. which 

images the 71% zone of the mirror, if my memory serves me right.

I almost went to find the book, but If I'm wrong, someone will correct it for me.

Ron.

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.

I don't think having the mirror not spinning will effect it too much, just more work walking around. I don't try and make every stroke perfect as there are so many strokes it all averages out and a bit of random may be a good thing.

It sure is nice having a spinning table.

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.

John mentioned the reflection test with fine misted mirror but I never tried it.

I instead went for 3 hours of polishing then test.

I can't remember where I read it but you can also shine a light source at the mirror whilst moving it side to side and the reflection should reverse around the focal point? Think I got that right but I do stand correction.

I must invest some time in making a proper foucault tester as I will then have two test methods to go on.

I will eventually call on Johns good nature and expertise to test it on his auto collimation bench to make sure it is figured correctly.

Damian

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:.