The 22" mapstar mirror

[barkis]

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.

[mapstar]

hi damian, after all the work you have done and time spent and got it so close, would it cost much for john to finish it off for you, overwise you could go to far and have to junk your blank. im realy not knocking you mate you realy have tried your best and most people woulld of given up by now. you should of become a doctor, you have loads of patience

mike

Cheers Mike.

John has been helping me through out this and is prepared to share his time and experience to get the mirror to first light. It's not really a matter of cost but I guess everyone has come to that conclusion about my thread.

I'm sure things are going the right direction but just need an experienced eye to look it over and put my mind at ease.

If the time comes when I need John to put me back on track so to say then the offer is already there.

I will know soon

Damian

[RLWP]

Will you be taking your laps as well as the mirror?

Richard

[mapstar]

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.

I can't think I'd ever attempt something that big but a grinding rig would be the first acquisition.

John is a busy man and to give his time to such a troublesome student is telling of his commitment to keeping amateur mirror making in this country alive.

It will get there and I've never given up on a project yet.

Chin up as they say more time will sort it

☺

[mapstar]

Will you be taking your laps as well as the mirror?

Richard

Not sure Richard maybe if John requests it

[estwing]

got to give it to you mate keep on keeping on..

[faulksy]

nice one damian, good luck. love reading this thread.

[Demon Barber]

Damian,why not give your  mirror a rest an do a bit of woodwork,that will be as good as a rest,i`m sure,where are you up  to in the frame work?

Joking aside you have to get it ready in time for Galloway,Derek and me have printed and sold loads of tickets (all first in line  ) and they are not refundable       ,

Keep at it mate any help just ask ,

Regards

Mike

[Soupy]

Never mind a doctor you need to be a saint... Perseverance, patience and resilience all in one. You will get it done buddy, the dob monster will rise at your hands

Edited June 13, 2015 by Soupy

[mapstar]

Thanks guys for the support and suggestions

Damian

[Physopto]

Even one suggestion of giving up is a no starter!

You have not struck me as "the giving up sort", so in a nut shell DON'T GIVE UP you aren't that man. We all get bad days when things don't go right. But after a while we see the light. It always happens. I've been there many times over the years, things always get better, you've just got to push through it. Easy to say I know.........

Been it, done it, seen it, and all that bull.

John will incentivise you during you're visit with him. Working on your own for so long on such a big project must be daunting. But you can do it, just keep repeating that to your self.

All the best.

Derek

[RLWP]

Bung them in the car, that way he can advise on the mirror and your tools and techniques as well. After all, it isn't the mirror you need advice on, it's the making of the mirror

Richard

[Mark62]

I'm sorry to hear things are not going how u want then to. Keep at it I'm sure u will sort it out and master the mirror.

We're all rooting for u Damian chin up [emoji106]

[Ajohn]

hi damian, after all the work you have done and time spent and got it so close, would it cost much for john to finish it off for you, overwise you could go to far and have to junk your blank. im realy not knocking you mate you realy have tried your best and most people woulld of given up by now. you should of become a doctor, you have loads of patience

mike

Blanks are never junked unless they are dropped etc and broken.  Seems I am rude because I pointed out that Damian has spent more than enough time to finish this mirror. Comment made because I am annoyed that he is having such a bad experience. It could put others off having a go.

Anyway as my comments are not appreciated and making another wont do any harm there comes a stage in mirror making where things have gone wrong and the only and quickest answer is to back step - even when forming the parabola.  In that case it's working it back to a sphere again or at least part way. Turned edges can mean going back to fine grinding as there may be too much glass to remove with a lap. Other  defects can mean the same thing even a hole in the middle as that means a huge area of glass has to be lowered just like with a seriously turned edge. Actually going back is beneficial as people then know that what they did at the next stage didn't work out and can hopefully react accordingly.

Personally I think Damian has 2 problems. Pitch hardness and lap size. The lap is about the right size for forming a parabola on a mirror of this size. Good for lowering the centre region and working down towards the edge. From the description it's also pretty light if made from wood. If I remember correctly John advocates concrete - I'd seal it with diluted water proof pva or diluted exterior grade wood glue - same stuff. Pressing down on a lap to make it work like a heavier one can't be a good idea as it will be impossible to apply an even pressure. Personally I feel it's best to work up toward  elbow height maybe above, easier to just push and muscles will work to their best advantage.  I have this aquarium I have used standing in end  with a board on it - not a good idea really. I've never measured it. It doesn't move around up to 10in with a bucket of water in it.

Pitch hardness is a toughy. I've tried the Texereau way - small piece in the mouth, let it warm up and see if you can chew it and still had sleeks at times. I'd guess this has something to do with him using pine rather than coal tar pitch and also him having very high skill levels. He does show coating the lap with beeswax - I couldn't find any at the time that happened. Another more recent view is pressing hard with the thumb nail. If an impression forms in under say 1/2 min or some level approaching that forget it as it's too soft. Pitch hardness will alter the amount of drag the lap has.

Some people pre polish - charge the lap with one of the abrasives rather than rouge. Might correct things more quickly but I have no idea if the lap is any good for anything else afterwards. Too small a lap and it will probably just cause problems more quickly.

For the next one I want to make a machine. Main reason is that I want to put it on a shelf in my workshop when it's running - bit difficult to walk round a shelf. No stroke arm as I can do that for the brief time it's needed compared with the time to do the rest. RPM has been tricky to sort out but from looking around I'm settling on about 15rpm, no more that 20. That's based on pro machines - usually intended for smaller work. Adding a stroke arm makes things more tricky even to the point of  deciding on a sensible rate that wont produce patterns in the work. The Waite's video's on youtube are a good source of info on how to use a fixed post machine.   Could add a manual stroke arm anyway easily.

Maybe Damian might realise that any comments I make are largely down to concern about the experience he is having and nothing else. As I see it there is a lot of junk about on the web concerning making precision mirrors and people can easily be miss lead and I feel that is where Texereau comes in. Little has changed really other than the materials used. It's a pity he didn't do another on bigger faster mirrors because that does need some changes. He does give sufficient info to work out how much glass needs to be removed to figure if I remember correctly - if that gets up to 0.001in it will be a lot of work. I don't think Damian is any where near that though and probably in real terms due to the size of a lap trying to figure a sphere. Something up to 15,16in should be a lot better but will still take a while to polish out to the edge and too much overhang will probably turn the edge. The idea of the sphere forming is all down to random motion as a sphere is the only shape that can cope with that and remain in contact.

 Anyway Damian probably has muscles like Popeye now. Don't give up. You've learnt a lot all ready. I just feel some one should have posted the above some time ago. Some will have been there. I have been lucky.

John

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Edited June 14, 2015 by Ajohn

[Astrobits]

I too have had times when nothing appeared to be happening to my mirror. At those points I have decided to change something significantly and see what happens.

The last images show a central hill. I would probably make up a very small lap, about 1/3rd the dia of the hill, certainly no more than 1/2, and work entirely within the central area and see if that made a difference. I wouldn't worry about creating zones there as you can easily switch back to a larger lap once some progress has been made. Don't forget that the central area will be obscured by the flat when in use so it doesn't really matter what happens under that flat. ( Cassegrains have a rather severe central hole ). I have used laps as small as 2" on a 20" mirror and on occasions even just my thumb.

I also use a heavier tool base making mine with plaster and sealing with varnish.

As stated previously, the biggest problem with figuring large fast mirrors is that significantly different amounts of glass need to be removed from adjacent areas of the surface and that cannot be done with large tools. So a bit of practise with small laps now will give you a feel for them when you actually get to the figuring.

Here's the 200" Palomar mirror being worked with a relatively small lap:

Probably equivalent to a 2" lap on your 22" mirror.

Nigel

[sloz1664]

200" Palomar mirror   :eek:      

The guy in the white shirt looks minute against that leviathan of a mirror.

Damian take a large print of this and put it in a prominent position in your "lap shop" and when you feel you're up against it - look at the photo and .... draw breath.

Steve

Edited June 14, 2015 by sloz1664

[Ajohn]

There is a difference when you are doing that Nigel - you can see from the testing you do where glass needs removing as per the Palomar mirror. There is also the brave way - forget the sphere and go for the parabola. Trouble is when surfaces start going up and down  probably even with slopes in different directions it can be very difficult to see what needs to be done.  One problem Damian has is that he can't actually measure how much glass needs to be removed. He could try taking measurements and sticking them into some software and setting the conic to zero.

I would say exactly the same as before - if a small lap is used or just one area of the mirror is worked because of the size of the lap do spend some time blending for each session. The blending in this case will still lower the hill. Same with the earlier work further out from the centre - do it often enough and no hill.

As to back stepping - just thought that it had to mentioned and would be surprised if others were not thinking the same thing. It's much nicer to have some measurements to base that decision on though - if they can be taken well enough and that can be tough.

John

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[mapstar]

I too have had times when nothing appeared to be happening to my mirror. At those points I have decided to change something significantly and see what happens.

The last images show a central hill. I would probably make up a very small lap, about 1/3rd the dia of the hill, certainly no more than 1/2, and work entirely within the central area and see if that made a difference. I wouldn't worry about creating zones there as you can easily switch back to a larger lap once some progress has been made. Don't forget that the central area will be obscured by the flat when in use so it doesn't really matter what happens under that flat. ( Cassegrains have a rather severe central hole ). I have used laps as small as 2" on a 20" mirror and on occasions even just my thumb.

I also use a heavier tool base making mine with plaster and sealing with varnish.

As stated previously, the biggest problem with figuring large fast mirrors is that significantly different amounts of glass need to be removed from adjacent areas of the surface and that cannot be done with large tools. So a bit of practise with small laps now will give you a feel for them when you actually get to the figuring.

Here's the 200" Palomar mirror being worked with a relatively small lap:

Probably equivalent to a 2" lap on your 22" mirror.

Nigel

Hello Nigel

Thanks for the post and words of wisdom. I have been working with the 6" lap which I made up a week or so ago and have been working solely on the centre with it.

I know I did a lot of edge work and that the centre would take a lot of work to smooth down but with the glass area being smaller I didn't think it would take as long to make an impression.

My ronchi images are confusing in the sense that inside focus it just looks like a hole? 

[mapstar]

It is always good to take lots of photo's for comparison and here's two Foucault type of when I started and how it stands at present

The Ronchi is a different beast altogether 

Damian

[Physopto]

Well it looks like it is reducing !

Derek

[barkis]

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

[mapstar]

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

Afternoon all,

Thank you for the post Ron and the steadying hand on the thread., also for the compliment.

I think it would be against my best feelings to lock the thread as such. It wouldn't be what I set out to do in the first place which was to give a feel to everyone of just how much effort (in some cases) goes into creating a mirror by hand. I have to admit that I never have thought small about most projects I take on hence why I went for the size I did and why I have taken so long. 

I have had quite a journey up to now you will probably all agree, an experience that many have had, but unfortunately not documented in the public domain to quite the same extent with live discussion and progress. 

At times, as Ron fully appreciates in his post, it is quite a challenge to keep thoughts and patience together over what has become many months of staring at a piece of glass wondering just what to do next. It is slow progress and I should imagine that you feel frustration as much as I do (I know someone has commented to that end)

I didn't realise at the start just how much interest the thread would generate from those that I know and from far afield. One thing I will say is I have enjoyed it, and appreciate all who take time to post, but do agree that if you are reading this from new it is an awful lot of pages to trawl through to pick out the relevant information. So I would like to ask everyone (and I have sent out a couple of pm's to the matter) that we should keep the information relevant (with a little banter)

In the future the thread I am told can be edited to shorten it so as to condense the info which I think people would appreciate.

I will reach the end as I have never given up on a project yet and will carry on sharing my progress whether it is fruitless or moving me to first light.

After all that seriousness I do have one little bit to add about Ron's post that has enlightened me as to why I'm having problems with the 22" I'm making. It's actually a 28" now   (Ron you must get those typing fingers sorted  ) 

Thanks to all again up to now for the support on this thread and for making SGL a great forum 

Damian

[Soupy]

Damian well said and you have all our support I imagine!

Back to the above images. The ronchi image at the bottom doesn't seem to be showing much TDE is it gone or am I reading it wrong?

[barkis]

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.

[Stu]

Damian,

As you know I have been (mostly) silently cheering you on from the sidelines, having little to add other than moral support.

To pick up your point, I think it takes plenty of courage to document and live the experience publicly, warts and all. It would be far easier to have made your mistakes in private so I applaud you for putting this all out there for everyone to benefit from.

Keep on keeping on!!

Cheers,

Stu