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Figuring a 6-in f/8

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#51 dogbiscuit

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Posted 31 August 2020 - 07:43 AM

Be sure channels and pressing are good.

This is to reduce correction up and make the edge better.

Ws starting small and getting bigger, but not too big.

 

Stroke smooth with downward pressure.

 

Cycle through the A strokes (top left, top right, bottom left, bottom right) 3 times 1 turn each.

kcoles51-a.png

 

Then do the b strokes 1 turn each.

kcoles51-b.png

 

Don't do anything else, I want to see the effect.  Hope you haven't already done something, but not to big a deal if you have.

 

Stop and test, tell me how long it takes to do all those strokes.

 

 


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#52 kcoles

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Posted 31 August 2020 - 09:05 AM

I did make a Couder mask after the plans in Berry's book. I just calculated the radii, drew them on a file folder with a compass, and cut carefully with a mat knife. That said, pinbout's video showing the shortcuts in a cad program is amazing. I just did three zones, which makes clear the overcorrection:

 

Zone  Readings (inches)

1     .504  .507  .495

2     .566  .581  .574

3     .621  .623  .636

 

If you look at the range it is about triple what I get from Berry's formula for a parabola, so clearly overcorrected.



#53 dogbiscuit

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Posted 31 August 2020 - 09:12 AM

Tell me the dimensions of the mask.

Inner and outer radius for each zone.



#54 kcoles

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Posted 31 August 2020 - 10:38 AM

Used the Stellafane calculator (back in mid-July) to get:

 

Zone  %   center   edge

                   0.0

1   40.8  1.21 in

                   1.72 in

2   70.7  2.10 in

                   2.42 in

3   91.3  2.71 in

                   2.97 in

 

Of course I don't claim I cut it out to that accuracy; these are just the numbers I started with. But made it as close as I could.


Edited by kcoles, 31 August 2020 - 10:39 AM.


#55 Pinbout

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Posted 31 August 2020 - 01:06 PM

here's right on the money with a 100 lpi

 

6f8.JPG

 

here's a quarter over.

 

6f8 qrtr.JPG

 

 


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#56 dogbiscuit

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Posted 31 August 2020 - 01:17 PM

Used the Stellafane calculator (back in mid-July) to get:

 

Zone  %   center   edge

                   0.0

1   40.8  1.21 in

                   1.72 in

2   70.7  2.10 in

                   2.42 in

3   91.3  2.71 in

                   2.97 in

 

Of course I don't claim I cut it out to that accuracy; these are just the numbers I started with. But made it as close as I could.

Are you sure your grating is 100 lpi?



#57 dogbiscuit

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Posted 31 August 2020 - 01:25 PM

WillmannBell counts white space between lines as a line.  If you got that gating from them, their 100 lpi grating is what most people would call a 50 lpi grating.

 

Numbers indicate about -4 conic.   For your mirror that is still less than a wave on the wavefront.  Now that your lap is working right it won't take long to reduce correction.


Edited by dogbiscuit, 31 August 2020 - 01:25 PM.


#58 Pinbout

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Posted 31 August 2020 - 02:09 PM

WillmannBell counts white space between lines as a line.  If you got that gating from them, their 100 lpi grating is what most people would call a 50 lpi grating.

 

Numbers indicate about -4 conic.   For your mirror that is still less than a wave on the wavefront.  Now that your lap is working right it won't take long to reduce correction.

for a quarter wave overcorrection I used -1.9, If his edge was better I'd call that around a quarter wave over...

 

if you had the scope built, I'd star test it, just for the halibut. 


Edited by Pinbout, 31 August 2020 - 02:10 PM.


#59 kcoles

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Posted 31 August 2020 - 02:16 PM

It is a Willmann-Bell "100" grating. So that is 50 for everyone else? Would explain a lot! 50 gives a match on the bbastrodesigns Ronchi matching page that makes more sense. I did get the next level up grating ("133" I think?) that I could swap in if I disassemble the tester.

 

I've cleaned the channels and am pressing now. Will go start figuring shortly. Thanks.

 

Edited to add: I don't have a tube for this mirror, so no straightforward way to star test at present.


Edited by kcoles, 31 August 2020 - 02:21 PM.


#60 Pinbout

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Posted 31 August 2020 - 02:30 PM

here I get .6875 waves over with 50lpi lol.gif

 

6f8 half.JPG



#61 kcoles

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Posted 31 August 2020 - 02:36 PM

I used a loupe and ruler to look at the grating in the tester, and sure enough, it is 2 dark lines per mm: 50 per inch.



#62 Pinbout

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Posted 31 August 2020 - 02:40 PM

I used a loupe and ruler to look at the grating in the tester, and sure enough, it is 2 dark lines per mm: 50 per inch.

that's what I used on the last image



#63 kcoles

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Posted 31 August 2020 - 07:54 PM

Trimmed the channels, pressed for an hour. Did one set of the strokes as suggested, about 45 minutes of figuring. There are CeOx smudges, as (chastened by the two scratches I made yesterday) all I did was rinse in clean water and then distilled water.

 

Looks like correction is a little less, but the tde has changed also? I will be away tomorrow, so if you aren't tired of giving advice (and I wouldn't blame all of you if you are) you can take your time, and I'll check back Weds. the 2nd. Thanks for the great and abundant help.

 

f8_eighth_test.jpg



#64 dogbiscuit

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Posted 01 September 2020 - 06:55 AM

The secret sauce of making a fine mirror is a good pitch lap.  Don't tell anyone, it's a secret. 

 

Uniform channels and pressing

 

Post some pictures of the lap. 

 

The same strokes as in post 51 again.  you can do the entire thing either 1 or 2 times.  There is nothing in the series that should cause bad things in the figure or edge.

 

Even though it is twice as bad as it looked 50 lpi vs 100, it is not far off, not so bad.


Edited by dogbiscuit, 01 September 2020 - 07:42 AM.


#65 dogbiscuit

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Posted 01 September 2020 - 07:39 AM

The correction is being removed from the center outward.  the "crest of the doughnut" between the less corrected central area and more corrected outer area will move toward the edge and look like a tde.  You can see that your diffraction ring is actually better so the edge is good on the outside of that overcorrection in the 3/4 wide outer zone.  As the crest is pushed closer to the edge it will become more like, will be a tde.  The crest and tde will be pushed off the edge in a little while and you will have a nice edge and figure very near a sphere.


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#66 ed_turco

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Posted 01 September 2020 - 10:15 AM

With this thread going onto three pages puzzles me.  Six inch f/8 mirrors were said to be easiest to figure.  I could have figured any number of 6" mirrors in the duration of time getting to this point in the thread. 

 

Talk about two many teachers.  Please note I'm not offering advice here.   :)


Edited by ed_turco, 01 September 2020 - 10:20 AM.


#67 dogbiscuit

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Posted 01 September 2020 - 11:15 AM

It's only been two weeks.  Some threads go on for many months and hundreds of pages. There aren't too many teachers. Why not offer advice?

 

Takes some work sessions and time to get all aspects of the process, lap, strokes, pressure, number of lines on Ronchi grating under control.  Then have to remove the ugly stuff.

 

6" is easy to figure to 1/8 wave to just ehhh quality,  I think he's had that already.  But if the goal is a very smooth surface and smooth correction better than 1/8 wave with and excellent edge it gets a bit more difficult and usually for a first time mirror maker it takes three or four weeks to learn to get every thing right.

 

kcoles is doing fine, learning to do it right.  He will be finished soon with a high quality mirror.



#68 kcoles

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Posted 01 September 2020 - 05:32 PM

If this thread gives the impression of slow progress, the credit for that belongs primarily to me. If any of you were standing next to me in the room (at a safe distance I'm sure) you would have noted that I had way too little pressure on the mirror (for several rounds), that I wouldn't learn to read the knife edge or get good photos of it until I got in a really dark room, and what a well-maintained lap looks like. And number of lines on the Ronchi screen! blush.gif Having to guess what I'm doing wrong is not the fastest way for any of you to tutor a new mirror maker. But I am grateful beyond expressing that so much professional-level advice is available here simply for the asking. And older threads here on CN inspired me to tackle this challenge after 14 years - I'm thinking of astrokhan's thread not too long ago, which I was fascinated to read. Maybe this thread will help someone else. Thanks to all of you, both those participating and those just watching - I can feel the encouragement and it helps a lot. (over 1100 views and a "hot topic" even though it must be the oldest question in this business.)

 

I play several musical instruments and years ago gave lessons on a couple of them. There too one finds many paths to success and each teacher had one or more to share. I guess you pick one and try it.


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#69 Pinbout

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Posted 01 September 2020 - 05:57 PM

With this thread going onto three pages puzzles me.  Six inch f/8 mirrors were said to be easiest to figure.  I could have figured any number of 6" mirrors in the duration of time getting to this point in the thread. 

 

Talk about two many teachers.  Please note I'm not offering advice here.   smile.gif

Yeah after having how many years of experience 

 

but not on your very first mirror without a mentor in the same room.

 

dont compare apples to cabbage 


Edited by Pinbout, 01 September 2020 - 05:58 PM.

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#70 Pinbout

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Posted 01 September 2020 - 06:02 PM

Trimmed the channels, pressed for an hour. Did one set of the strokes as suggested, about 45 minutes of figuring. There are CeOx smudges, as (chastened by the two scratches I made yesterday) all I did was rinse in clean water and then distilled water.

 

Looks like correction is a little less, but the tde has changed also? I will be away tomorrow, so if you aren't tired of giving advice (and I wouldn't blame all of you if you are) you can take your time, and I'll check back Weds. the 2nd. Thanks for the great and abundant help.

 

attachicon.giff8_eighth_test.jpg

Swayze would have you use accent pressure with the edge of the tool and scrub that zone with slight thumb pressure on top of the tool. Workin on the right side of the mirror, walking right, tool rotates left.



#71 dogbiscuit

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Posted 01 September 2020 - 06:41 PM

If this thread gives the impression of slow progress, the credit for that belongs primarily to me. If any of you were standing next to me in the room (at a safe distance I'm sure) you would have noted that I had way too little pressure on the mirror (for several rounds), that I wouldn't learn to read the knife edge or get good photos of it until I got in a really dark room, and what a well-maintained lap looks like. And number of lines on the Ronchi screen! blush.gif Having to guess what I'm doing wrong is not the fastest way for any of you to tutor a new mirror maker. But I am grateful beyond expressing that so much professional-level advice is available here simply for the asking. And older threads here on CN inspired me to tackle this challenge after 14 years - I'm thinking of astrokhan's thread not too long ago, which I was fascinated to read. Maybe this thread will help someone else. Thanks to all of you, both those participating and those just watching - I can feel the encouragement and it helps a lot. (over 1100 views and a "hot topic" even though it must be the oldest question in this business.)

 

I play several musical instruments and years ago gave lessons on a couple of them. There too one finds many paths to success and each teacher had one or more to share. I guess you pick one and try it.

Nothing unusual about it taking some time and missteps to learn that some aspects of making an optical surface are more critical than first imagined.  There is plenty of room for less than ideal practices if a mediocre result will be acceptable, but if real quality is desired it requires getting a lot of details right.

 

I think you have learned faster than most with distance mentoring.

 

Now if you wanted to see a slow learner, you could try to teach me to play a musical instrument.  Hopeless !



#72 kcoles

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Posted 01 September 2020 - 06:45 PM

Actually, Steve Swayze worked briefly on this mirror at Delmarva 2006 after one of the rounds of testing, demonstrating for me a stroke like that I believe. At this time it had the raised zone about halfway out that is in my first set of photos. If it hadn't been so many years ago I might have remembered how to do it. But things happened so fast at Delmarva, and many of the terms were new to me so it was a challenge to make proper notes and drawings of the advice and strokes there, coming from several different experts.

 

I'm sure Steve's karma, and all of yours, is still with this mirror. Now I will get back to figuring - thank you.



#73 Pinbout

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Posted 01 September 2020 - 07:18 PM

You could just like taking down this hill, but in your zone 4x’s around and check, even with MoT 

 

https://youtu.be/Vr5c3NL9yR4



#74 dogbiscuit

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Posted 03 September 2020 - 08:40 AM

Getting a grasp of how pressure changes during stroking will help with understanding how to figure.

 

Increasing overhang reduces contact area between top and bottom disks.

With steady down force applied to the top disk increased overhang increases pressure per square inch because of the smaller area of contact.  The  edge of the bottom disk under the overhang becomes a fulcrum bearing more of the downward force than areas of contact away farther from the fulcrum.  Increasing overhang concentrates more and more of the total downward force toward the edge of the bottom disk. There is less contact area between the disks so psi increases and with increasing overhang the local pressure between the lap's edge and the mirror increases exponentially until with 1/2 D overhang (not stroke length) all of the pressure is between the edge of the lap and the mirror's center, and psi is extremely high.

 

OverhangPressureDist-b.jpg

 

I've suggested you apply about 3.5 to 7 pounds of down force by hand when stroking. I really meant to say 3.5 to 5 pounds to be applied by hand but 7 will work ok for the not too much overhang strokes you've been doing. This is in addition to about 2 pounds mirror weight (my estimation, not critical to be right on).   I was shooting for between .2 and .25 psi total pressure and for a 6" mirror that would be between 5.5 and 7 pounds .

 

To refresh memory the strokes in post 51, the A chart shows small stroke patterns that will work toward less correction, done long enough would get to an oblate spheroid.  The strokes in the B chart are to lessen the tendency to go oblate, maybe A and B done long enough would still be slightly oblate but not as oblate as only the A strokes, or might be very near a sphere.  Of course as it gets near a sphere The ratio of small A strokes and medium size B strokes, can be adjusted to hold it near a sphere for making a good edge. The some strokes to parabolize.

 

 



#75 kcoles

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Posted 03 September 2020 - 07:20 PM

Well, it is interesting, the figure is clearly changing. Here is how it looks after yesterday's session. I cleaned the channels, pressed for 90 minutes. It was time to trim the edge (actually, the mirror knocked a few chips off). Did the new AAAB cycle and then a look on the tester. Pressed and did another AAAB cycle and then took these photos. I found it tricky to judge how far in to move the knife edge but maybe these will give you an idea. Seems to my untutored eyes like the middle is getting closer to no correction and the outer part is getting narrower, as predicted.

 

I'll do a photo of the lap in the morning, but I open the channels (where needed) after almost every cycle.

 

f8_ninth_test.jpg




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