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Polishing 6 inch f/11

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#1 mazdak

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Posted 20 April 2021 - 08:45 AM

Capture_00002.jpg

Hello friends.

I am in the polishing stage of a six inch mirror. today after several hours of polishing decided to ronchi test the mirror and the result as not good. 

plz tell me what should I do? return to 250 grit makes me really sad.   


Edited by mazdak, 20 April 2021 - 08:47 AM.


#2 photomagica

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Posted 20 April 2021 - 10:15 AM

Wow! That is really strange. Try a Foucault test as well to get a better idea of what is going on with the surface. The Ronchi pattern makes me suspicious that something could be wrong with the test.

Bill


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#3 dan_h

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Posted 20 April 2021 - 10:33 AM

Finish polishing, then test.  It doesn't take much error to look really wonky at f11.  Returning to 250 grit is not at all necessary. 

 

The central portion polishes first and shows a reasonable Ronchi pattern.  This will grow to include more and more of the outer zones until it is complete.  

 

Finish polishing, then test.  You can't correct an unpolished mirror. 

 

dan


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#4 Pinbout

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Posted 20 April 2021 - 10:41 AM

 

today after several hours of polishing decided to ronchi test

never do that...

 

 

Finish polishing, then test.  You can't correct an unpolished mirror.

exactly


Edited by Pinbout, 20 April 2021 - 10:42 AM.

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#5 TOMDEY

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Posted 20 April 2021 - 10:50 AM

Hi mazdak!

 

I did a nice 6-inch F/12.5 about sixty years ago! One of my first and it was a learning pleasure and great performer.

 

You'll get a superb figure pretty soon. The good news is that a 6-inch F/11 will be a piece of cake to get right. Perfection is nearly spherical, so forget the Ronchi and just use a common traditional knife edge.

 

1) First, work the polishing simply and gently until you get a complete polish --- with no testing at all. You will need to get that rhythm where the action feels viscous and smooth drag and just hang in there, entirely by hand, for a few hours. Then give it an extra hour just "for good measure". Don't overthink it and lay off the obsessive testing.

 

2) Next, polish very conservatively center over center with slow deliberate, shorter stokes c/c, maybe some W, and more rotations than before. About half an hour. This will reliably drive the figure toward spherical. No testing yet - don't cheat.

 

3) Rinse the mirror off and let it stabilize in your testing environment for a couple of hours.

 

4) Now take the knife to it. It should be very close to spherical with no edge problems.

 

5) Do the figuring in the traditional manner to coax it to a perfect sphere --- smooth, bland, boringly wonderful. Nearly all of your time will be working other mirrors, as the six-incher comes to testing stability. At this stage nearly all of your time will be waiting for meaningful stability. Figuring runs will be very short --- just a few minutes each.

 

6) Now that you have a perfect sphere --- parabolize it in very short runs. To take a 6-inch F/10 from a perfect sphere to a perfect paraboloid could well be just a minute or two on the lap. It's very easy to overshoot, so don't get too aggressive or anxious.

 

That's really all there is to it! Assuming that you glass substrate is superior (e.g. fine annealed Pyrex) things should go quickly. The wait-times can be doing other mirrors or reading a book.

 

I watched the professionals doing this kind of hand-figuring work at B&L back in the 1960s. German artisans from the Old Country. A good optician would knock off around half a dozen superb mirrors in an 8-hour shift, without even breaking a sweat. I only did my hobby stuff, so could spend days or even weeks on a mirror.    Tom


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#6 mazdak

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Posted 20 April 2021 - 10:53 AM

Thank you All

I took some other pics with a better ronchi screen . the one that i used first had some problems in bandings. 

 

Attached Thumbnails

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#7 mazdak

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Posted 20 April 2021 - 11:59 AM

One thing that i forgot to mention is that the mirror would be spherical .

#8 dan chaffee

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Posted 20 April 2021 - 12:07 PM

If this is your first mirror, I recommend just doing what it takes to

get a perfect sphere.  At that aperture and focal ratio, it will be a

very good performer without parabolizing.   Your ronchigrams are

exactly what I'd expect for  a mirror that is not fully polished, as

the edge polishes out last.


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#9 mazdak

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Posted 20 April 2021 - 12:14 PM

Thank you for very helpful comments.



#10 Pinbout

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Posted 20 April 2021 - 02:24 PM

Thank you All

I took some other pics with a better ronchi screen . the one that i used first had some problems in bandings. 

are you fully polished out?

 

also if you'd like insightful interpreting of ronchigrams...after your fully polished out... no laser on the surface....

 

label the pictures, which is inside and outside RoC... it helps. your first picture isnt even a sphere... if and only if its inside RoC. and that would mean your edge is up... probably cause your not finished polishing.


Edited by Pinbout, 20 April 2021 - 02:25 PM.

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#11 RalphMeisterTigerMan

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Posted 20 April 2021 - 11:24 PM

That is a great project. Back in the late 1980's, possibly 1989, there was a great article of a super-slow reflector in Sky and Telescope. A Japanese amateur built a 6" F/14 reflector using a mirror which he ground and polished himself. He was able to use a one-half-inch diagonal!

Now if this isn't a "Planet Killer" I would like to know what is.

 

A picture of the author and his "pride and joy" were also included. That was a very long tube mounted on a GEM and it looked impressive! I believe once you are finished you will have a reflector which would give "Apo-like" images.

 

Good luck and thanks for the nostalgia "episode"!

 

Clear skies and keep looking up!

RalphMeisterTigerMan


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#12 mazdak

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Posted 21 April 2021 - 12:49 AM

when you love planets observing and telescope making first project 

 

are you fully polished out?

 

also if you'd like insightful interpreting of ronchigrams...after your fully polished out... no laser on the surface....

 

label the pictures, which is inside and outside RoC... it helps. your first picture isnt even a sphere... if and only if its inside RoC. and that would mean your edge is up... probably cause your not finished polishing.

No the mirror not fully polished out.    the edge in laser test shows bright dot.



#13 dan chaffee

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Posted 21 April 2021 - 02:13 AM

There actually is a good reason to test a mirror on the bench before polished

completely and that is to see if there is gross astigmatism.  Years ago I spent

 an afternoon helping Jeff Baldwin polish a 40 mirror ...by hand....and he was

very anxious to see if he had ground in astigmatism. We did a test on a light

reflected off a ball bearing in the shop and saw gross astigmatism.   That saved

him a lot of wasted polishing time. I had the same experience when I attempted a

5 inch mirror of ridiculous thinness...and was summarily defeated. 

 

To keep this in perspective though, one should factor just what they are looking for

in such early polishing and if that is not understood, it's better to just  get on with 

the polishing.


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#14 Steve Dodds

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Posted 21 April 2021 - 09:49 AM

when you love planets observing and telescope making first project 

 

No the mirror not fully polished out.    the edge in laser test shows bright dot.

There is nothing there that is not polishoutable.  But if you don't have much time invested in polishing you might want to go back to 9u, might be quicker.


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#15 ccaissie

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Posted 23 April 2021 - 07:02 AM

If the edge is slow to polish, TOT.    Most edge issues are from incomplete fine grinding.  Avoiding work on the edge because you think it's turned down will just make things worse.  Needs more work.   The Sam Brown books have picture illustrations on working the edge...I don't have the actual page copies, but they are posted here frequently.

 

I use a 6" f/10.6.  During one iteration I just left it a very good sphere, and enjoyed every view.  Later I parabolized it and really couldn't tell any improvement.  If it finishes as a smooth sphere, you'll be very happy....


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#16 dogbiscuit

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Posted 23 April 2021 - 08:18 AM

Are you working by hand, or on a machine?

Is the polishing lap the same diameter as the mirror?

What sort of stroke are you using?

These things matter.

Ronchi looks as though the lap might be smaller.  It could be the lap is as large as the mirror diameter but not pressed well leaving the outer area of the lap not in contact with the mirror, making the lap effectively smaller than the mirror.

Maybe the stroke is such as to work the center and not the edge.

It is normal for polish to progress slower toward the edge.  TOT will work the edge a little faster, but there is more risk of causing astigmatism.   I recommend MOT to reduce the chances of astigmatism. The extra work is not that much on a 6".  Learn about methods to avoid astigmatism on another mirror or maybe after full polish learn it in figuring part of this mirror.

Assuming the lap is not super soft or some extreme stroke is used, something so extreme it is not likely you would be using, you don't have a ground in tde. I'm pretty sure I can see in the Ronchi that there is some polish extending all the way to the edge.  If the edge shows some polish, it can be fully polished.

It could still be possible that some grit stage of grinding was not sufficient to completely grind all previous grit pits all the way to the edge. That could require going back to grinding. But if your polish looks good in the center it is likely you can get a good polish all the way to the edge.  It just takes longer to get polishing completed all the way to the edge.

Most likely your strange Ronchi can be explained by lap size, or lap pressing and trimming, or stroke, or some combination of these.

 

Your figure now is not so far out of line for the polishing stage.  Check those things I mentioned and just put some more polishing hours in until polish is complete from center to edge.  Then you can work on making the figure spherical.


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#17 mazdak

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Posted 24 April 2021 - 01:25 AM

Are you working by hand, or on a machine?

Is the polishing lap the same diameter as the mirror?

What sort of stroke are you using?

These things matter.

Ronchi looks as though the lap might be smaller.  It could be the lap is as large as the mirror diameter but not pressed well leaving the outer area of the lap not in contact with the mirror, making the lap effectively smaller than the mirror.

Maybe the stroke is such as to work the center and not the edge.

It is normal for polish to progress slower toward the edge.  TOT will work the edge a little faster, but there is more risk of causing astigmatism.   I recommend MOT to reduce the chances of astigmatism. The extra work is not that much on a 6".  Learn about methods to avoid astigmatism on another mirror or maybe after full polish learn it in figuring part of this mirror.

Assuming the lap is not super soft or some extreme stroke is used, something so extreme it is not likely you would be using, you don't have a ground in tde. I'm pretty sure I can see in the Ronchi that there is some polish extending all the way to the edge.  If the edge shows some polish, it can be fully polished.

It could still be possible that some grit stage of grinding was not sufficient to completely grind all previous grit pits all the way to the edge. That could require going back to grinding. But if your polish looks good in the center it is likely you can get a good polish all the way to the edge.  It just takes longer to get polishing completed all the way to the edge.

Most likely your strange Ronchi can be explained by lap size, or lap pressing and trimming, or stroke, or some combination of these.

 

Your figure now is not so far out of line for the polishing stage.  Check those things I mentioned and just put some more polishing hours in until polish is complete from center to edge.  Then you can work on making the figure spherical.

Thank you

I am polishing By hand and the lap diameter is nearly the same size of mirror. maybe a little (about 10mm ) smaller.

The stroke is normal(1/3) .



#18 dogbiscuit

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Posted 24 April 2021 - 05:23 AM

That 10mm smaller diameter lap makes a big difference, tending to make a 5mm wide turned down edge. Try to make it 0mm difference. A lap diameter slightly less than the mirror diameter tends to make a turned down edge. It is easier to make a good edge if the lap is at least the same diameter as the mirror.

 

If the tool is the same diameter as the mirror, let the lap spread out to equal the diameter of the mirror. You can let it spread outward as you continue polishing. Don't bevel the edge of the lap.

When it has spread beyond the edge of the tool trim it vertically.

 

A good well pressed and trimmed lap is one of the most important aspects of mirror making.  Making the lap good and keeping it in good condition will show in your figuring results.

 

With 1/3 center over center strokes I think your Ronchi image is post 1 indicates the lap was not pressed into full contact all the way to the edge of the lap as you worked.  I estimate it was pressed only out to about the 70% or 75% of the radius of the lap.  So the lap would behave as a smaller lap, about 100mm or so diameter.  The Ronchi images in post 6 show some improvement and I guess the area of lap contact has increased from additional work and pressing.

 

When the lap is the same diameter as the mirror and pressed into full contact you will see less difference and gentler transition between the mirror's center and edge zones. And there will be at least some increase in polishing speed at the mirror's edge.

 

I'll say it again...

A good well pressed and trimmed lap is one of the most important aspects of mirror making.  Making the lap good and keeping it in good condition will show in your figuring results.


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#19 Pinbout

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Posted 24 April 2021 - 04:14 PM

Thank you

I am polishing By hand and the lap diameter is nearly the same size of mirror. maybe a little (about 10mm ) smaller.

The stroke is normal(1/3) .

How much time have you polished for? are you alternating tot/MoT ? 

usually, at Delmarva Mirror Making Seminar, we’d say it takes an hour per inch before you even think about using a laser to see if your polished out.

 

and did you do a reflection test with a bare bulb 

 

https://youtu.be/mWOGACcR6H0
 

if you havent passed the test, don’t even bother looking at the ronchi.

 

and at DMM, we found as Dog stated, even a slightly smaller lap can cause tde. So everyone started to trim the laps straight down instead of chamfering them 


Edited by Pinbout, 24 April 2021 - 04:15 PM.

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#20 mazdak

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Posted 25 April 2021 - 03:00 AM

That 10mm smaller diameter lap makes a big difference, tending to make a 5mm wide turned down edge. Try to make it 0mm difference. A lap diameter slightly less than the mirror diameter tends to make a turned down edge. It is easier to make a good edge if the lap is at least the same diameter as the mirror.

 

If the tool is the same diameter as the mirror, let the lap spread out to equal the diameter of the mirror. You can let it spread outward as you continue polishing. Don't bevel the edge of the lap.

When it has spread beyond the edge of the tool trim it vertically.

 

A good well pressed and trimmed lap is one of the most important aspects of mirror making.  Making the lap good and keeping it in good condition will show in your figuring results.

 

With 1/3 center over center strokes I think your Ronchi image is post 1 indicates the lap was not pressed into full contact all the way to the edge of the lap as you worked.  I estimate it was pressed only out to about the 70% or 75% of the radius of the lap.  So the lap would behave as a smaller lap, about 100mm or so diameter.  The Ronchi images in post 6 show some improvement and I guess the area of lap contact has increased from additional work and pressing.

 

When the lap is the same diameter as the mirror and pressed into full contact you will see less difference and gentler transition between the mirror's center and edge zones. And there will be at least some increase in polishing speed at the mirror's edge.

 

I'll say it again...

A good well pressed and trimmed lap is one of the most important aspects of mirror making.  Making the lap good and keeping it in good condition will show in your figuring results.

Thank you again for very helpful information

you are right. I had bad contact between mirror and lap and guess using too much polishing agent (in m case red Rouge) decreased bad contact. images in post #1 and #6 are in the same time. just in #1 I used a Ronchi screen with bad banding.



#21 mazdak

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Posted 25 April 2021 - 03:02 AM

How much time have you polished for? are you alternating tot/MoT ? 

usually, at Delmarva Mirror Making Seminar, we’d say it takes an hour per inch before you even think about using a laser to see if your polished out.

 

and did you do a reflection test with a bare bulb 

 

https://youtu.be/mWOGACcR6H0
 

if you havent passed the test, don’t even bother looking at the ronchi.

 

and at DMM, we found as Dog stated, even a slightly smaller lap can cause tde. So everyone started to trim the laps straight down instead of chamfering them 

Thank you Pinout

Nearly  15 hours with red Rouge.

Yes I am alternating TOT/MOT.



#22 Pinbout

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Posted 25 April 2021 - 08:56 AM

Do you decant the red rouge? 


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#23 mazdak

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Posted 25 April 2021 - 09:15 AM

Do you decant the red rouge? 

No I shake the bottle and ...



#24 MKV

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Posted 25 April 2021 - 10:06 AM

No I shake the bottle and ...

I think that's not right. You need to let all the heavier particles settle down and decant (remove) only the top layer with smallest and lightest particles and use that suspension for a final finish. 



#25 mazdak

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Posted 23 May 2021 - 12:07 PM

Hello My friends

I think my mirror finally is fully polished out.

here are some ronchi image.

Does the mirror suffer from TDE?

Attached Thumbnails

  • Capture_00003.png

Edited by mazdak, 23 May 2021 - 12:49 PM.

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