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The 22" mapstar mirror


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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

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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 :grin:  :grin:.

Ron.

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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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Hi Damian,

In that video of John Dobson he talked about the sound changing to squeaking when it became time to replenish the cerium polishing solution. I noticed he just spread a bit cerium by spoon on the surface and wet it. Then used his finger to spread it about. From what you have said you make a solution. So I guess you think that is better. Is it because of consistency of mix?

Derek

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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. :icon_salut:

Ron.

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Hi Damian,

In that video of John Dobson he talked about the sound changing to squeaking when it became time to replenish the cerium polishing solution. I noticed he just spread a bit cerium by spoon on the surface and wet it. Then used his finger to spread it about. From what you have said you make a solution. So I guess you think that is better. Is it because of consistency of mix?

Derek

I have been making a solution of cerium oxide up the day before using. I've been using in the proportions of half a teaspoon to about 300ml of water. It looks like milk to be fair but you can vary this to what you need. Thicker slurry works faster, thinner is more for polishing.

I'm working by hand so don't get to the squealing stage

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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.

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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.

Well explained Ron

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I would have thought that could only happen  if machine polishing rather fast or really soft pitch. I must admit the Luton polishing surprised me. He might have been pre polishing.

I've wondered if Damian used a straight chordal stroke with his lap say a couple of inches in from the side of the mirror at centre stroke without much overlap at the ends of the stroke it might sort his  problem out more rapidly.  A mixed stroke could be added to prevent the bump in the middle. This should keep the lap in good contact with the final curve.

Anyway while  nosing around on another type of problem I found Waite doing something similar to the above but with a small star lap. He mentions that it can also be used for a turned edge. Keeping a round lap further in should achieve the same thing. Part 2 of this video - on a small mirror but the same idea should work out. He calls them tangent strokes - :laugh: I recollect Tex....... calls them chordal.

John

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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.

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Nicely put Ron... And you are very right Damian has an invaluable mentor in John Nichol and we all watch and wait with baited breath for this monster mirror to come to fruition. :)

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All I'll add is that the 2nd part of the video can also be used to  reduce a turned down edge - with a sub diameter lap - that's the only reason I posted the link - plus some interesting factors about laps.

John

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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.

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

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Damian,

There is no rush to answer this, I know you are somewhat pushed for time. I'm just filling in mine waiting for gear to arrive for imaging. I'm in the middle of thought experiments. At times very boring others my brain hurts. :eek:

Just how much are you trying to remove, maybe a difficult question I know. It is a question possibly answered in time or microns. From the bit of the process that I have watched and seen on other videos, it seems that as mirror makers you try for a slice of a circle first then change this into a parabola by using a different stroke type. ( if all goes well). As everything I have seem so far suggests, it is more of an art with careful thought than an scientific exercise. I know that the bit removed will be small but by what amount? You told me that the solution you use is made up from the powder you buy. You also mentioned the grade was not as fine, sorry wrong wording there, but cheaper than that used by another mirror  maker. Would it make sense  in the last bit to use the finer cerium for the very last few laps, thus saving money?

Can't help wondering. Goodness........ so many questions!

Just keep going got to finish before Galloway you know........ :p

Derek

Edited by Physopto
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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.

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Damian,

There is no rush to answer this, I know you are somewhat pushed for time. I'm just filling in mine waiting for gear to arrive for imaging. I'm in the middle of thought experiments. At times very boring others my brain hurts. :eek:

Just how much are you trying to remove, maybe a difficult question I know. It is a question possibly answered in time or microns. From the bit of the process that I have watched and seen on other videos, it seems that as mirror makers you try for a slice of a circle first then change this into a parabola by using a different stroke type. ( if all goes well). As everything I have seem so far suggests, it is more of an art with careful thought than an scientific exercise. I know that the bit removed will be small but by what amount? You told me that the solution you use is made up from the powder you buy. You also mentioned the grade was not as fine, sorry wrong wording there, but cheaper than that used by another mirror  maker. Would it make sense  in the last bit to use the finer cerium for the very last few laps, thus saving money?

Can't help wondering. Goodness........ so many questions!

Just keep going got to finish before Galloway you know........ :p

Derek

Hi Derek

It is difficult to know but I think Nigel may have mentioned something in the thread a while ago which I'll have to check back on. You are quite right with the technique's and there is a lot of careful thinking that goes off on the next polishing/correcting session  

I totally agree that it may be better to finish with some finer cerium although the average particle size that is stated on the packet is 2micron  where as I think the stuff from Lapmaster is 1micron 

I'm sure Galloway will be the best gathering I've ever attended this year (wonder why that will be  :grin: )

Damian

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Hope you are right and Galloway is great Damian. Although I have to temper that as I'm going to Kielder first. I need the time to get some imaging in. S Shields where I live is diabolical for light pollution. I really am jealous of all the lads and lasses with darker skies and especially with observatories. Working towards it when I move, but Hey Ho.

Looked at quickly it does not seam a lot 2 to 1 micron but I suppose 100 % bigger., wow!

Whiskey maybe and of course our scintillating whit.............Ok then, just the whiskey! :drunken_smilie:

Mike and I are still trying to fathom out what we are going to charge for entry tickets to the big event. Pie and peas  cost money you know! :p

Derek

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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. :grin:

Ron.

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

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

Inside ROC

post-28847-0-28478500-1428179904_thumb.j

Outside ROC

post-28847-0-21230300-1428179888_thumb.j

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  :grin:

Damian

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

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

Inside ROC

attachicon.gifSDC13368.JPG

Outside ROC

attachicon.gifSDC13392.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  :grin:

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.

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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.

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

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

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  :undecided:

Damian

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

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

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  :undecided:

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?

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