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Parabolization of a 10" f/4 using star laps, etc.

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#26 Lognic04

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Posted 11 November 2018 - 04:58 PM

What's the point of deforming the tool? Surely it is just easier to do it the normal way - it took me 2 or 3 parabolizing sessions to get down to an f3 parabola!



#27 stargazer193857

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Posted 11 November 2018 - 08:11 PM

The more I think about it, the more I realize mirror makers have probably experimented with many strokes on their turn wheels, sometimes just for fun, and looked at many Rochi diagrams to verify theories.

#28 danmdak

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Posted 11 November 2018 - 10:21 PM

I ran this in polsim, the polishing simulator program.  It seems to show that a straight stroke just 25mm off center and 175mm long does the job.  

 

I'm sure the W helps blend the action, a feature that you can't plug in with polsim.

 

Pretty sure that a 6 sub lap will do what you want.    

Well...the frustration continues. Going 1" off center with a 7" stroke didn't do anything to reduce the edge. I then tried a 6" stroke, 1" off center. Still have a reluctant edge. Suggestions? Perhaps I should try a 5" sub lap? Perhaps just do a full size lap with a star pattern? Throw it at a concrete wall maybe???!!!

 

Somehow this topic of mine got really hijacked, but I do appreciate all the help I am getting.



#29 PeteDCard81

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Posted 11 November 2018 - 11:38 PM

Try a 5-inch pitch lap. You will likely need one anyway.



#30 stargazer193857

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Posted 12 November 2018 - 12:28 AM

I apologize for my tendency to miss or forget the big picture and go off the intended topic.

The ideal stroke probably depends on the current figure, and is only good for a while. Now that you have made changes, I bet a Rochi picture is needed.

Please tell us what changed from before to after, and how many strokes and how much weight you did. Also picture of lap.

Bad as my memory is, you may have already posted most of that.

#31 ccaissie

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Posted 12 November 2018 - 07:14 AM

Yes.  Your descriptions of progress are good, but the usual format of getting advice is to post test images or actual foucault measurements, or screenshots of your FigureXP or Sixtests results.

 

You've tried a number of things, so you have a lot of hands-on experience.  It's way too soon to fling it.  Short focus mirrors require extra effort.  You're in it.


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#32 dogbiscuit

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Posted 12 November 2018 - 11:46 AM

Well...the frustration continues. Going 1" off center with a 7" stroke didn't do anything to reduce the edge. I then tried a 6" stroke, 1" off center. Still have a reluctant edge. Suggestions? Perhaps I should try a 5" sub lap? Perhaps just do a full size lap with a star pattern? Throw it at a concrete wall maybe???!!!

Why always 1" off center?

Why not go 2 1/2", 2 1/4", 2" off center?



#33 Pinbout

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Posted 12 November 2018 - 12:09 PM

 

Well...the frustration continues

if you want help, post a ronchigram inside and outside 3 lines



#34 danmdak

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Posted 12 November 2018 - 08:55 PM

Why always 1" off center?

Why not go 2 1/2", 2 1/4", 2" off center?

Because I have next to zero experience using sub diameter tools and there is no or limited data online or in books that I have run across (Kestners article in ATM magazine is about all I have) and thus I'm clueless here other than the advice others have posted for me!!! So.........do strokes farther off center with a subdiameter lap reduce the edges of a mirror?


Edited by danmdak, 12 November 2018 - 09:01 PM.


#35 danmdak

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Posted 12 November 2018 - 09:03 PM

if you want help, post a ronchigram inside and outside 3 lines

No idea how to get a ronchigram picture. I'm a dinosaur with most computer stuff!

The curves when using a ronchi screen are nicely curved (no zoning or edge problems (yet)). The problem I am having is the outer part of the mirror (70% to outer edge) lagging way behind the center to 70% part of the mirror. This has been a problem on all f/4 mirrors I have made. Eventually through brute force, I guess, I wind up with a usable figure. Just want this 10" f/4, probably my last mirror, to go faster than the usual 9 months to a year of figuring I usually have!



#36 dogbiscuit

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Posted 12 November 2018 - 09:58 PM

Because I have next to zero experience using sub diameter tools and there is no or limited data online or in books that I have run across (Kestners article in ATM magazine is about all I have) and thus I'm clueless here other than the advice others have posted for me!!! So.........do strokes farther off center with a subdiameter lap reduce the edges of a mirror?

I think you are working by hand.

Using sub diameter laps, I think you would do well to try to emulate machine work.

Here is the best (my opinion) single source on how to use a grinding/polishing machine, and describes two methods of parabolizing about as complete as can reasonably be expected.

 

http://www.astrosurf...achines_eng.htm

 

Note the stroke descriptors of "offset" and "overhang".  A stroke length is defined by offset and overhang.  For a particular offset, and chosen overhang, there is only one stroke length.

So think offset and overhang, and that determines the stroke length.

 

To emulate a machine, you could use straight back and forth strokes, at the chosen offset, with the chosen overhang, while stepping around the mirror.

 

On a machine rather than W strokes varying offset, the offset is incrementally change frequently.  So an offset would be used for several turns around the mirror, and then the offset would be stepped in one direction or the other. The chosen overhang would be maintained. This stepping of the offset would be made in  maybe about 1/4" increments.

 

Generally zero offset, dead center over center is not used on a machine.

Overhangs can be positive with the edge of the lap extending beyond the mirrors edge at the end of the stroke, can be zero with the edge of the lap at the edge of the mirror at the end of the stroke, or can be negative with the edge of the lap some distance inside the mirror's edge at the end of the stroke.

 

So working through various laps, stepping through various offsets and overhangs, you control where correction goes.

 

Stepping through various offsets is the machine method of doing W strokes, or conversely Ws are the manual stroking way of emulating incrementally stepping through offsets.

Read that linked article.  The answer to your question is in there.

 

I think this will help you understand how sub-diameter laps are used in figuring. 

Adapt the concepts to hand work.  Be a machine.  It's not so ba ba ba ba ba bad.  lol.gif


Edited by dogbiscuit, 12 November 2018 - 10:14 PM.

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#37 danmdak

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Posted 12 November 2018 - 10:42 PM

I think you are working by hand.

Using sub diameter laps, I think you would do well to try to emulate machine work.

Here is the best (my opinion) single source on how to use a grinding/polishing machine, and describes two methods of parabolizing about as complete as can reasonably be expected.

 

http://www.astrosurf...achines_eng.htm

 

Note the stroke descriptors of "offset" and "overhang".  A stroke length is defined by offset and overhang.  For a particular offset, and chosen overhang, there is only one stroke length.

So think offset and overhang, and that determines the stroke length.

 

To emulate a machine, you could use straight back and forth strokes, at the chosen offset, with the chosen overhang, while stepping around the mirror.

 

On a machine rather than W strokes varying offset, the offset is incrementally change frequently.  So an offset would be used for several turns around the mirror, and then the offset would be stepped in one direction or the other. The chosen overhang would be maintained. This stepping of the offset would be made in  maybe about 1/4" increments.

 

Generally zero offset, dead center over center is not used on a machine.

Overhangs can be positive with the edge of the lap extending beyond the mirrors edge at the end of the stroke, can be zero with the edge of the lap at the edge of the mirror at the end of the stroke, or can be negative with the edge of the lap some distance inside the mirror's edge at the end of the stroke.

 

So working through various laps, stepping through various offsets and overhangs, you control where correction goes.

 

Stepping through various offsets is the machine method of doing W strokes, or conversely Ws are the manual stroking way of emulating incrementally stepping through offsets.

Read that linked article.  The answer to your question is in there.

 

I think this will help you understand how sub-diameter laps are used in figuring. 

Adapt the concepts to hand work.  Be a machine.  It's not so ba ba ba ba ba bad.  lol.gif

FANTASTIC!!!! THANK YOU!!!!



#38 stargazer193857

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Posted 13 November 2018 - 11:15 AM

No matter what method you use to parabolize, I don't think any strokes converge to and stay at parabolic. Optical testing or forehand knowledge is needed so you know when to stop or when to change the stroke or the polish. Same is true of my deformed too plan. Power to the people whose optical tests show them very well where they are at.

Some good measurements to calculate removal rate and specific conditions would be useful for people whose optical tests are not as good for the complex stuff. Know how much glass comes off with each stroke would be nice. It depends on the pitch hardness, grit size, grit type, grit concentration, and how well pressed into the pitch, which varies for each person. So I don't think there is a specific answer.

Edited by stargazer193857, 13 November 2018 - 11:21 AM.


#39 Pinbout

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Posted 13 November 2018 - 12:40 PM

 

No idea how to get a ronchigram picture.

hold up the cell phone to the ronchi, zoom in on the image, and tap the screen.

 

then post the pic

https://www.youtube....h?v=SJWsqTOnF5o

 

 

without seeing where your mirror is. no one can tell you what to do.

 

you may not be seeing correctly if you never did this before.

 

take some time to learn how to take a pic and post it.

 

it doesn't have to be perfect.

 

if you can't take a pic, sketch it, take a pic of it and post the sketch.


Edited by Pinbout, 13 November 2018 - 12:40 PM.


#40 hamishbarker

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Posted 13 November 2018 - 08:18 PM

Thank you for the pictures. I agree the third one removes less glass. Tempting. Finer polish could be started with. But I don't know how I would approach it. My plan likely works best on the first one.


I was just this morning making a spreadsheet to produce precisely these curves. For my 22"f4.6 the depth of correction for the third method is about 1/3 of either of the first two, and the volume of glass to remove is 1/4.

So it's easy to see the attraction! But indeed how to work the bit at the edge is the question.

In my case i have about 20mm width of TDE which can reduce some of that required edge work, but i need to measure precisely how far it is down. In the meantime I'm working back to sphere to reduce the tde as much as possible, and have a consistent basis to work from.

If i did method 1 or 2, correction is about 10 waves. But maybe slow and reliable is better than a quick but chancy method.

#41 stargazer193857

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Posted 13 November 2018 - 10:49 PM

I have a creative newbie idea:

From a sphere that is well polished, make a triangular sublap 30% as tall as the mirror radious. Add pitch. Press in polish. Spin the mirror on a turn table. Put the triangle base on the edge, with the peak pointed at the center of the mirror. Star test often so you don't over correct.



Inside the 70% radious, use a sub diameter lap that is smaller than 70% of diameter. Maybe 40%. Do a star pattern with the edge stopping at 70% radious.


Then use a full size lap that's thin enough to push, and polish out the lumps and bumps.


The key is to creap up on this and measure how much it is changing each time. Measuring star size on spherical aberration star test might be an easy way to quantify.

#42 stargazer193857

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Posted 13 November 2018 - 10:52 PM

Other way is a full radius triangle that addresses picture A. A foot powered turn table might be nice.

And I would make the full size lap before figuring with the sub laps.


No matter how you do this, the glass your strokes take will approach faster in some areas than others. You just have to get close, and then fine polish the jumps with the full size.


As for a turned down edge, I have bad news: you have to take down the whole mirror to match now. But depends how turned down it is. Maybe not an issue yet.

Edited by stargazer193857, 13 November 2018 - 11:07 PM.


#43 dave brock

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Posted 14 November 2018 - 02:02 AM

As for a turned down edge, I have bad news: you have to take down the whole mirror to match now. But depends how turned down it is. Maybe not an issue yet.


The only time someone would have to take down the whole mirror to match a tde is if the focal length must be some exact measurement.
Have you made a mirror?

#44 totvos

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Posted 14 November 2018 - 08:51 AM

I have a creative newbie idea:

From a sphere that is well polished, make a triangular sublap 30% as tall as the mirror radious. Add pitch. Press in polish. Spin the mirror on a turn table. Put the triangle base on the edge, with the peak pointed at the center of the mirror. Star test often so you don't over correct.



Inside the 70% radious, use a sub diameter lap that is smaller than 70% of diameter. Maybe 40%. Do a star pattern with the edge stopping at 70% radious.


Then use a full size lap that's thin enough to push, and polish out the lumps and bumps.


The key is to creap up on this and measure how much it is changing each time. Measuring star size on spherical aberration star test might be an easy way to quantify.

???

 

Have you made a mirror?




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