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Making a first mirror: 10" f6

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#1 Chris Adamczyk

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Posted 19 November 2019 - 03:39 PM

Hi,

I've never posted on anything before, I'm polished out on my mirror, in the attached I believe I have a rolled edge, but I'm wondering if I'm close enough to start figuring, or weather I should shot for a total sphere first.

 

Chris

Attached Files


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#2 siriusandthepup

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Posted 19 November 2019 - 04:00 PM

That is clearly a fairly severe case.

 

You might need to go back to fine grinding to get close to a sphere before repolishing.

 

Hang in there - these things are normal learning events on a first attempt, nothing to worry about. waytogo.gif



#3 dogbiscuit

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Posted 19 November 2019 - 04:12 PM

It would help to know more.

 

Don't know degree of turn or whether up or down without knowing

Grating density?

Inside or outside ROC?

 

Tell us how you are working.

Lap diameter?

Hand work or Machine?

Strokes you have been using recently that brought on the current shape?

 

Although I don't know exactly,  it doesn't look too bad. Shouldn't take long to get it right.

If you have completed the polish, then you are figuring, even if it is to get a better sphere before parabolizing.

You are now working on the shape of the surface.


Edited by dogbiscuit, 19 November 2019 - 04:15 PM.


#4 Chris Adamczyk

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Posted 20 November 2019 - 08:45 AM

Thank you for the response, The image is inside the ROC using 85 lines per inch.  I was using a full size lap center over center 1/3 strokes MOT the mirror was prolate with a turned edge and would not improve. I then made a 6" lap and star shaped the edges. I am using a W stroke rather than center over center to spread the work out, each time I complete a session (12 trips) and test I definitely notice improvement. I was testing outside in open air. I need to create a space inside to avoid the uncontrolled air currents. 



#5 dogbiscuit

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Posted 20 November 2019 - 11:10 AM

Most likely cause of prolate with tde from 1/3 COC strokes MOT with the full size lap is the lap not being  pressed well resulting in lap contact in central area but little or no contact in outer zone.  This causes the lap to be effectively a sub diameter lap.

 

I think the rolled or turned edge will be easier to fix with a full size lap. 

Pictures of your lap might help verify this or show another problem.

A good lap is a key factor in making a good mirror.  It is very important to get the lap into good condition and maintain it in good condition.  Everything about the lap is important.

 

How thick is the mirror blank?

Plate glass?  Clear back so you can see the lap well through the glass when working MOT?

 

Describe how you work.  Are you working around a "barrel", a work stand that you can walk all the way around as you work?


Edited by dogbiscuit, 20 November 2019 - 11:26 AM.


#6 Chris Adamczyk

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Posted 20 November 2019 - 04:00 PM

My mirror is 1" plate glass, my lap is poured on a solid thick concrete base, epoxy coated 2X's. I feel like I have good contact, when I press usually for a hour or so, I place a plastic mesh from a orange produce bag between the 2 surfaces to micro facet the surface. I can see that I have good contact through the mirror. It started raining here in SoCal yesterday so I'll be improvising and inside set up so maybe my pictures will not be so affected by air movement.Attached File  1in Mirror on lap.pdf   126.11KB   95 downloads



#7 dogbiscuit

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Posted 20 November 2019 - 05:47 PM

Lap looks fairly good as far as what I can see of it.

It would be nice to have  a picture from above and looking straight down with the mirror centered.

 

I assume the blue tape is used as a reference for mirror rotation increments.

My advice for more than one reason is remove the tape.  One reason being heat transfer from the hands and then radiation from the disk as you work will be different in areas covered by the tape and areas of bare glass.  Also I think 8 increments of mirror rotation is not enough and you certainly don't want to do 8 precisely aligned every time.  There are 8 radials but if each is worked you are really only working along 4 diameters. 

 

Better to have more increments, I like something near 12 but not exactly.  This way you won't work the same diameter on any two increments as you turn the mirror.

 

Working MOT the step size around the lap is not so important so your 12 marked positions is ok.  In time you should develop a habit and not need to rely on markings to make nearly the same size steps going around.  If you work TOT nearly equal steps sizes is more important going around the mirror, but still you should develop a habit stepping nearly the same size steps going around.  Same with turning the mirror in your hands when working MOT. Develop a habit of doing it very nearly the same every time.  Developing the habit allows you to get in a rhythm and that leads to smoother work.

 

It is ok to mark the mirror with so you know when a full turn has been made.  The last step before returning to the start position is the end of a turn.  When you get to the start position that is the beginning of the next turn.

 

 

What is the weight of the concrete tool?

What type of pitch are you using?

 

An hour is not necessarily an adaquate pressing.  Depends on a number of factors.

Just my opinion, micro facets are not always a good thing.  They can speed polishing but for figuring I don't see consistent results from micro facets.  Not saying (yet) that you should quit.

 

Ronchi image shows a pretty good sphere but being inside ROC the outer zone is turned up.

 

Looking pretty good really.

Press as normal, then get a picture through the mirror looking straight down, mirror centered after about 5 or 10 minutes of stroking.  After this 5 or 10 minutes of work don't remove the mirror from the lap or add water to the lap before taking the picture.  I hope you can bear to do without the tape on the back of the mirror.  :-)

 

You thought the edge was turned down, but it is up.  So maybe the full size lap was working better than you thought.  Or maybe it wasn't working good.  You have done some work with the 6" star.  I don't know what it looked like with the full size lap.  If  you want to continue with the 6" lap  hold on for someone else to advise you.  I wouldn't work a 10" f:6 with a sub diameter lap, so I don't have much experience doing that.  There are people on CN who have done this.


Edited by dogbiscuit, 20 November 2019 - 09:03 PM.


#8 tomb18

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Posted 20 November 2019 - 10:26 PM

Hi I made a 10" f/6.6 a number of years ago. I used a full sized lap, no micro-facets and it turned out better than 1/10th wave on the wave front.  I would just use the full size lap and try to get to a sphere before parabolizing.  That's what I did.  It took 40 sessions of figuring to get to the final figure.  It can be useful to use the faucalt test of zones to see the shape when plotted.

Tom


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#9 Chris Adamczyk

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Posted 21 November 2019 - 08:42 AM

Guy's thank you, I'll go back to the full size lap with a longer press. The blue tap was to keep track of overhang only, I have a mark on the tap at the approximate overhang that I very roughly match. I'll get a picture tonight after 5-10 minutes of work.



#10 dogbiscuit

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Posted 21 November 2019 - 01:23 PM

Guy's thank you, I'll go back to the full size lap with a longer press. The blue tap was to keep track of overhang only, I have a mark on the tap at the approximate overhang that I very roughly match. I'll get a picture tonight after 5-10 minutes of work.

Ok... Drawing zones on the back of the mirror can be useful sometimes.  I suggest using less tape or try a grease pen.  I don't know for sure the tape as you have it would cause trouble, I just suspect that it might.   

I think it can be good to be able to see the lap under the mirror while working.

 

Also I don't know that 1 hour of pressing is not enough.  I depends on the hardness of the pitch and how long the mirror has been separated from the lap, and some other things.  I suspect insufficient pressing from your description of the results with the full size lap.  The 1/3 COC strokes should have made something near a sphere with a raised outer zone, similar to what you have in the posted Ronchigram.  But that is after working with the 6" star, so I am not sure what your full size lap produced with the 1/3 COC strokes.



#11 dave brock

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Posted 21 November 2019 - 02:29 PM

In the pic it looks to me like you don't have contact in the outer area unless there wasn't enough polishing compound when the pic was taken.

#12 DAVIDG

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Posted 21 November 2019 - 02:56 PM

 Put the mirror on the bottom and the lap on top. Do a slight W stroke with no more then 1/2" over hang. Test after 10 minutes. You should see the turned edge start to get smaller.  A turned edge  as a  longer radius then the rest of the figure. You need to "push" the figure out the edge. Been making optical for 35+ years and this always works for me.

 

                       - Dave 


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#13 dave brock

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Posted 21 November 2019 - 03:12 PM

Dave, it's a turned UP edge according to the first pic.

#14 dogbiscuit

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Posted 21 November 2019 - 03:17 PM

In the pic it looks to me like you don't have contact in the outer area unless there wasn't enough polishing compound when the pic was taken.

The darker outer area of the lap could be because of poor contact, but it could be that all or most stroking had been done with 1/3 COC strokes and difference of embedded polishing agent is because the central area has done more work, being in contact with the mirror's surface at all times and outer area seeing less work because it is not in contact during overhang.

 

This is why I asked for the picture after 5 or 10 minutes of work.  This will sometimes show poor contact areas as a lighter shade because more slurry will fill the poor contact area, but be squeezed out of good contact areas.



#15 Chris Adamczyk

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Posted 21 November 2019 - 03:26 PM

The first Ronchi Picture was inside the ROC showing lines hocked out at the ends, isn't that a turned down edge?. The picture MOT had not no polishing compound. I need to reconfigure my cave so I can test inside. (Dave, I'm from Seddon originally)

 

Chris



#16 dogbiscuit

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Posted 21 November 2019 - 03:39 PM

The first Ronchi Picture was inside the ROC showing lines hocked out at the ends, isn't that a turned down edge?. The picture MOT had not no polishing compound. I need to reconfigure my cave so I can test inside. (Dave, I'm from Seddon originally)

 

Chris

The inside ROC Ronchigram shows a turn up.  Turn down would show a hook inward at the edge.

 

That you had no slurry on the lap can be seen and what is seen is the cerium that is embedded in the pitch.  The darker outer zone shows less embedded cerium.  that could be because of poor contact in the outer zone, but the lap surface looks fairly fresh (fairly dark even in the lighter central area).  The central area will get more work because it is in contact for all parts of all strokes.  The outer zone is not always in contact with the lap so it is natural that less cerium will embed there. After more use more cerium embeds and after a few hours you won't see much difference... if contact is good, and you don't renew the surface such as by (brass) wire brushing.



#17 DAVIDG

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Posted 21 November 2019 - 03:41 PM

 Your edge looks like this picture.  It is  a common mistake that ATM's  make that they use too long of a stroke  with the mirror on and the lap doesn't go all the way out to the edge of the mirror. The result is the edge doesn't get the same amount of wear and  curve does not have the same radius out to the edge.  So it has a longer radius.

   By putting the mirror on the bottom and lap on top and using a slight W stroke with no more than 1/2" over hang and  you'll  "push" the curve out  to the edge.

 

              - Dave 

turnededge.jpg



#18 dogbiscuit

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Posted 21 November 2019 - 04:04 PM

Some have refered to "first" Ronchi

Maybe there is a Ronchi that I am not seeing

This is the only one that I am seeing and if it is taken inside ROC, that is closer to mirror than ROC, I'm saying the hook outward at the edge indicates a turn up.
CARonchi.jpg

 

 


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#19 dave brock

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Posted 21 November 2019 - 04:23 PM

+1

#20 Pinbout

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Posted 21 November 2019 - 07:00 PM

Some have refered to "first" Ronchi

Maybe there is a Ronchi that I am not seeing

This is the only one that I am seeing and if it is taken inside ROC, that is closer to mirror than ROC, I'm saying the hook outward at the edge indicates a turn up.
attachicon.gif CARonchi.jpg

You beat me to it. 

 

Inside roc - that is tue 

 

with tot, supporting both sides of the tool, centered over tue, very short light strokes as you walk around.

 

2x’s around and test.

 

careful as you go, or you will have a tde. 

 

Always test the edge with ke. 

 

Tue will show a dark shadow on ke side

 

tde shows shadow opposite ke side,

 

you want that bright diffraction ring all the way round.

 

if one side is less bright that’s an unbalanced edge.


Edited by Pinbout, 21 November 2019 - 07:05 PM.

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#21 dave brock

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Posted 21 November 2019 - 08:47 PM

Tue will show a dark shadow on ke side

tde shows shadow opposite ke side,


Other way around Danny. I know you know that.
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#22 Mike Spooner

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Posted 21 November 2019 - 10:39 PM

Inside it's turned up edge. It's just me but if it's polished out I'd just go to a wide w parabolizing stroke and monitor carefully. I think Tex has examples. The long, smooth w should smooth up the kinks and an up edge is an advantage in my book, especially for first time MOT efforts. But consider that I do most of my figuring now by machine so others here may have better advice for hand work. Just saying what I would do.

 

Mike Spooner 



#23 Chris Adamczyk

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Posted 21 November 2019 - 10:57 PM

OK, this is great! I've been bashing about in the dark. I'll spend the weekend re configuring my work area to support testing in doors. At this stage I want to be able to test as I go. Looking at the Stellafane Ronchi Table "Curves into Center" Inside ROC the edge is under corrected, this supports the drawing by Dave above being a TDE?

 

Thank you

Chris



#24 dogbiscuit

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Posted 21 November 2019 - 11:11 PM

OK, this is great! I've been bashing about in the dark. I'll spend the weekend re configuring my work area to support testing in doors. At this stage I want to be able to test as I go. Looking at the Stellafane Ronchi Table "Curves into Center" Inside ROC the edge is under corrected, this supports the drawing by Dave above being a TDE?

 

Thank you

Chris

No. 

 

 

:-)

 

 relative to a sphere you have something like this.

 

\___________/

 

I would recommend just working with something other than 1/3 COC.

What you have is much to be expected coming out of polishing using 1/3 COC strokes with a full size lap.

 

A variety of Ws  1/4  1/3 1/2  2/3 might give you a fairly smooth curve and might lean a little toward prolate and the parabola.

except for the turn up you sphere is pretty good.  The turn up should not take much if any special attention. The larger W will tend to remove that tue.



#25 dogbiscuit

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Posted 22 November 2019 - 12:12 AM

How are you planning to test the mirror?

Ronchi testing with the 85 lpi grating is not very sensitve.

Get within +- 5% even correction and good edge it will be a very good mirror 

If you have even correction within 10% or 12% and good edge you will have a pretty good mirror.

If you are off 15% or 20% but smooth correction it will be "good enough".

My  judgment there. 

 

I would be near impossible for you to tell the difference between 100% correction and 80% over or under corrected.

Ronchi can show you the smoothness of correction.

Foucault testing can give you percentages of correction and a reasonable idea of wavefront error.


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