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8" mirror making

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#1 mihaiG

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Posted 26 November 2019 - 03:48 PM

Hello everyone,

I've been watching for a long time the "Cloudy Nights".A good part of my ATM knowledges comes from this forum.I reach a point where I am stuck,so this is why I am posting now.
   Few months before I decided to start grinding my own mirrors,being unhappy with the quality of the usual commercial ones.I start with an 8"/7.5,a bit unusual f,but I  want a planetary tool.
I follow the Mel Bartels path(by hand): rough grinding using a subdiameter ring tool(4"diameter),grit 80 and 180 till reach 5/6 focal lenght,fine grinding using a plaster and tiles tool,half TOT half MOT,grit 320,25,15,9,5 and 3 microns;I checked carefully the focal lenght using a feeler gauge after every grit.I did always the "pencil test" before changing grits.I verified also the grit pits before changing the grit size.
  Materials are  good quality,also the mirror blank(1",the back was also verified to be flat).1/2" of newspapers was alway placed bellow blank to avoid astigmatism.
Strokes:mostly 1/3 Center-Over-Center;after10 strokes,rotate tool step to the right,mirror to the left and repeat.
Started polishing with a slightly oversize lap(9")to avoid turned down edges.MOT always,1/3 center-over-center first hour.Second hour,trying to lowered the central hill wich seems to be present on the mirror,I shortened the stroke.
My mirror seems to be in a bad shape,away from spherical;I am doing something wrong for sure,but cant figure what.
In the Ronchi pictures,upper line is after 1h polish,the bellow one after 2h polish.80 LPI
Kindly asking for some guidance:)

 

      Mihai(aka Mike)

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  • Mirror8.png

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#2 dan_h

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Posted 26 November 2019 - 05:00 PM

Welcome to CN!

 

You are not stuck. You need to keep going.

 

Two hours is not enough polishing to finish an 8" mirror.  You need to continue polishing until the mirror is completely transparent all the way to the edge. It is easy to get a flash polish but close examination will show you are not done.  There is a danger of not polishing enough if you start testing before the polish is complete.  What would you do if you tested and found you had an excellent figure? 

 

I am not familiar with the use of an over-sized lap. My intuition tells me that the outside of the lap will not get worked as much as the centre so it will remain high.  You also cannot press the lap to good contact with the mirror. Both these conditions will guarantee that the mirror will plow into the lap and a turned edge will almost certainly result.  This is already showing in your Ronchi pics. 

 

To my way of thinking.  Use a full sized lap to complete the polishing.  Standard procedure is 1/3 to 1/4 stroke,  centre over centre.  Press well to obtain good contact before every work session.  Avoid short work periods. The lap never warms up and good contact is not established. Avoid the temptation to test until you have a full polish. You don't want to start worrying about the figure until your polish is complete. 

 

Again:  You are not stuck. You need to keep going.

 

The fun is not yet started.  

 

dan


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#3 Pinbout

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Posted 26 November 2019 - 08:11 PM

You need about 1 hour per inch for polishing

 

you have a hill in the center so polish more MoT for an hour

 

but get back to tot to make sure the edge gets a good polish.

 

keep an eye on your channels and keep them open

 

https://youtu.be/T03b-5LyWGk

 

how your channels close tells you hows your mirror is getting polished

 

And while MoT watch the cerox and how it flows 

 

https://youtu.be/erF2jO34pgg

 

And when you need to open up the channels, if you use a soldering iron, keep the tip out of the puddle so it won’t smoke

 

https://youtu.be/_81Mi3FfQo0


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#4 mihaiG

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Posted 27 November 2019 - 01:56 PM

Thank you,guys,for the guidance....I  change to a full sized lap and use the 1/3 stroke....very nice trick to use a soldering iron to keep the channels open....I will keep the post updated.


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#5 mihaiG

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Posted 28 November 2019 - 01:28 PM

I followed the Dan advice to  change to  a full size lap,with nice channels and micro-channels.The strokes were short,10 over the center then tool  to the right 10-15 degrees,mirror to the left same amount and repeat,MOT,1h...The result is way better,the center hill is reduced as one can see in the picture...Tde still present.

I will concentrate to  finish the center,then I will take care about the  tde.

In my case,what should be the recomended stroke for removing the tde?

 

              Thank you

 

                   Mihai

Attached Thumbnails

  • Mirror8.png

Edited by mihaiG, 28 November 2019 - 01:46 PM.

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#6 Pinbout

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Posted 28 November 2019 - 04:12 PM

To take down your hill - 2x’s around

 

https://youtu.be/Vr5c3NL9yR4

 

then test

 

to fix the edge

 

https://youtu.be/Q7a7s-BwRPk



#7 dan_h

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Posted 28 November 2019 - 05:05 PM

Not only is the central hill disappearing but the edge is beginning to blend as well. The hook at the end of the bands is not a sharp and the affected area is much broader. You are moving in the right direction.  It will continue to improve as you work towards full polish. 

 

dan


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#8 mihaiG

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Posted 01 December 2019 - 04:04 AM

It seems that I got a kind of spheroid,parabola or hyperbola...I use MOT,1/3 and 2x’s around to get the central hill down,,

I think I should use the Foucault to identify what kind of surface I create.Or should I go back to a sphere?In this case,what are the strokes to safely go back to a sphere?

 

   Thank you

 

               Mihai

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#9 Pinbout

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Posted 01 December 2019 - 05:01 PM

Short narrow ovals, tot, will lower correction. 

 

You look more like an f5 right now 

 

but you can back into proper correction with above stroke

 

check edge with ke 


Edited by Pinbout, 01 December 2019 - 05:02 PM.

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#10 mihaiG

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Posted 07 December 2019 - 01:53 PM

I am fighting my way toward a sphere,after some oval strokes, 1/3 Coc MOT/TOT and W's 2/3D and 3/4 D same TOT/MOT.I was also using a lot of short and fast MOT strokes,as Danny recomended,for reducing TDE;I also tried short W's method(Texereau);It seems that I cant go further on reducing TDE using those strokes.Some ideeas about how to continue to get the sphere?Pictures are outside both,80 lpi.Thanks!

 

 

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  • Outside.png


#11 Pinbout

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Posted 07 December 2019 - 07:47 PM

;It seems that I cant go further on reducing TDE using those strokes.

you have to look at your lap and see where the channels are closing first... probably the center since your edge is turned...

 

keep the channels open... evenly... and watch how they close...

 

https://www.youtube....03b-5LyWGk&t=4s

 

 

keep the channels open

 

https://www.youtube....81Mi3FfQo0&t=4s


Edited by Pinbout, 07 December 2019 - 07:50 PM.


#12 mihaiG

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Posted 08 December 2019 - 03:49 PM

Some progress has been made today...checked again the lap tool,opening channels to 1/4",hot press for micro-channels and cold press before starting to work,as Danny advised.Use a series of Mot Coc 1/5,1/4/,1/3,alternate with large  W's(3/4) to controll the center...

At the end follow Sam Brown's method(Tot,accented pressure ,using edge of the lap as a tool) for correcting the Tde...it seems reduced a bit.

Next step would be to continue strokes for correcting Tde,meanwhile trying to controll the center and avoid zones.

Both pictures outside,80 lpi

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  • Out.png

Edited by mihaiG, 08 December 2019 - 03:50 PM.


#13 dave brock

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Posted 08 December 2019 - 04:48 PM

I wouldn't worry too much about zones or the centre while fixing the edge. Don't combine strokes in a session. Do one then test so you know exactly the effect of each stroke type.
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#14 Pinbout

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Posted 08 December 2019 - 05:49 PM

 

Use a series of Mot Coc 1/5,1/4/,1/3,alternate with large  W's(3/4) to controll the center...

 if your outside RoC in the images above you have an extreme tde.

 

do this and only this for an hour

https://www.youtube....h?v=Q7a7s-BwRPk

 

always paying attention to the channels and how they are closing...

 

I'm barely breaking the edge of the lap with the edge of the mirror.

 

I like my polisher's edge cut straight down, if its chamfered its basically a subdia tool... don't do that. grin.gif


Edited by Pinbout, 08 December 2019 - 05:51 PM.


#15 mihaiG

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Posted 09 December 2019 - 07:41 AM

After 1h polish Mot/Coc short and fast strokes...outside,80 lpi

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#16 Pinbout

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Posted 09 December 2019 - 09:17 AM

Keep going ...but

 

Are you watching your channels and how they close?



#17 mihaiG

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Posted 10 December 2019 - 04:33 PM

Hi Danny...yes,I am watching the channels...always open them with the soldering iron before starting the polish session(ofc warm/cold pressing before).Today I got another 1h30m of fast strokes...not much improvement...I think i will make another pitch lap...I broked some edges during fast strokes for correcting TDE.

  Maybe I have to consider to get back to fine grinding 9 microns or 5 microns to repair the TDE.

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#18 Mike Spooner

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Posted 10 December 2019 - 06:11 PM

Okay, my take is completely against convention so print it out and start a fire with it if needed.wink.gif

You don't have a turned edge - instead you have a short radius in the middle. It's what pitch laps do with MOT and we become afraid to work the edge which means we never work the radius out to the edge without a ton of time and pressing. Some folks use small laps to work the edge and this may be prudent on larger mirrors. For small mirrors I just like to use a full, straight stroke  - yes 8" stroke for an 8" mirror. No W or anything as I just want to get this edge on the same page (or radius) with rest of the mirror. Basically I'm trying to blend to a new curve that is slightly longer than the majority of the mirror is at currently. I use soft pitch and warm press and go straight to polishing. When I final figure an 8" mirror I use an 11" lap (approximately) and warm pressing is about the only option, at least for me. Small mirrors I do MOT even with machine work but that's not relevant here so forgive my ramblings. 

The long, quick strokes should help with ring zones from pitch channels (but watch for roughness and adjust accordingly). I don't obsess about the pitch channels - well maybe a bit more with large mirrors due the amount of work needed if I screw it up bad enough. Trust me, size matters.lol.gif

 

So I know I've basically insulted at least half of Tex's procedures (his methods can work wonderfully trust me) but you have spent hours and I was there at one time and finally threw caution to the wind in frustration and tried some off the wall stuff that worked for me. As amateurs we often don't work in temperature and humidity controlled environments that helps pitch behave in a civil manner. 

 

Again, most of the optics work I do today is by machine (with its own set of frustrations) so as they say, YMMV.

Hopefully none of the good opticans here hurt themselves laughing at my methods.

 

Mike Spooner (started as an ATM, got lucky and made a few decent optics ... Then got old and tired.)tongue2.gif


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#19 mihaiG

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Posted 11 December 2019 - 02:59 PM

Mike,thanks for your advice....I gave it a try today,just to see whats happening...here  the result,after 3rounds,8" strokes,MOT.

Does it worth to continue?

 

 

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  • 8inchStroke.png


#20 Mike Spooner

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Posted 11 December 2019 - 07:01 PM

To me it looks like it's moving in the right direction. Maybe shorten the stroke to 6" or 7" for the next session to keep the center from digging deeper at the end of the travel if that is in fact happening. Think about this - it looks like you have put a slightly longer radius on the mirror so as this new curve spreads over the surface then the short radius from before is concentrated in the center and looks like a hole. But it is pretty easy to dig a central hole so test often and adjust if that gets worse. It has seemed to me that transitions like near the edge or central zones look sharper as they get narrower - this can make you think they're getting worse even though they're improving. If the zone is getting narrower then progress is being made. 

As far as what I see in the outside ROC photo, I like that the transition seems very sharp. To me this indicates the new radius is marching right out toward the edge. 

Just for reference, how long did the last session take? 

 

I also want to emphasize that if this doesn't seem to be working it's no big deal to abandon for another method. As I've stated I now use machines and there are many folks here that have many more hours hand polishing experience than I have (at least in recent years). I don't post too often about technique as I don't want to add confusion and I know everyone's goal here is to help you get a nice finished mirror with a minimum of stress but edge problems sometimes take much more time than seems reasonable so I wanted to offer a possible differing tactic and perspective. I personally believe the concepts of rolling the edge vs shortening the central radius is well worth understanding. The same could be said for understanding slope vs height in reference to an optical surface. I got confused earlier this year in interpreting a surface I was working on and made it wildly hyperbolic before I came to my senses. I felt pretty stupid by the time I figured it out. I lived through it, fixed it and life goes on so its all good!

 

Mike Spooner


Edited by Mike Spooner, 11 December 2019 - 07:05 PM.


#21 dogbiscuit

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Posted 11 December 2019 - 08:47 PM

Lap troubles

MihaiG8Lap.png

 

First... the lap is not pressed well.  Looking at the picture of your lap, I have marked with a red ellipse a few mm inside the edge of the lap.  Notice that outside the ellipse the light shade coloring of slurry on the lap indicates there is a thicker layer of slurry between the lap and mirror than in the darker shaded areas inside the red ellipse.  This layer of slurry is thicker outside the ellipse because there is more separation between pitch and mirror surface,  that is poor contact.

 

For now the poor pressing has limited the effective diameter of the lap.  When you get that fixed here are a couple of other concerns.

On the near side it can be seen that the lap edge is about 3/8" inside the edge of the mirror.  I can't see the far side so well but it appears the lap is at least 1/3" less than the diameter of the mirror.  My opinion is width of TDE is related to the slightly undersized lap.  Aggravating this is the missing partial tiles around the edge of the lap which also effectively reduce the diameter of the lap.

 

There may be other lap concerns but those are what jumps out at me from your picture.  I can't tell everything from that one picture. Your results in the Ronchigrams suggest to me the pitch may be very soft, but might work ok if you get those problems I've mentioned fixed.

 

My opinion is a little different than most others here. I see no significant improvement of the edge.  I think whatever stroke you use you will not see much improvement until you get the lap right.


Edited by dogbiscuit, 11 December 2019 - 09:01 PM.


#22 mihaiG

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Posted 12 December 2019 - 04:44 PM

@Mike Spooner  Thanks for encouraging words:)...Today ,after I got a new lap,I continued to work,3 sessionx2 minutes,MOT,COC, 6",4",2"  sessions.It seems the radius of the new curve is expanding.The first polishing session,the one creating the deep hole, was about 10 minutes.

  The Ronchi picture is outer ROC,80 lpi.

  It is not a big deal for me if this method fails,maybe.I am here to learn,willing to learn.Most important is to get  good knowledge about creating  slopes on optical surfaces and the way different strokes work.And I am very happy that such professional  people from CN are correcting my errors:) .I am not in a hurry to get the mirror ready,I need  first to get a good understanding  about what I am doing,

 

@dogbiscuit  You were perfectly right,the lap was 2-3 mm shorten in diameter than the mirror;also I got the point about poor edge contact.I molded a new lap,think now is better.If you notice something wrong with the new one,please tell me to correct.Channels are not fully open,the picture was taken after polishing sessions.

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  • Lap.jpg
  • Lap1.jpg

Edited by mihaiG, 12 December 2019 - 04:52 PM.


#23 Pinbout

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Posted 12 December 2019 - 04:53 PM

you can get rid of the hole and fix the edge in one shot now that your pretty much spherical and not oblate

 

https://www.youtube....ZAqvzU_GA&t=42s


Edited by Pinbout, 12 December 2019 - 04:53 PM.

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#24 Mike Spooner

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Posted 12 December 2019 - 05:21 PM

Lap looks better. May need to trim the grooves a bit. Since you don't mind experimenting, try a 30-40 minute polishing spell with only the longer (approx 6-7") strokes. Then post an inside/outside test. We're looking for edge improvement. 

 

Remember, anything polished in can be polished out. Easy for me to say since you are the one doing the work.wink.gif

But I have been there; about the time I think I've figured it out, I get another portion of humble pie. Well fed in fact!

 

Mike



#25 Pinbout

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Posted 12 December 2019 - 05:33 PM

you can use a soldering iron to open the channels... just keep the tip out of the puddle of wet pitch, otherwise you'll smoke the room up. but keep the tip out of the puddle and you'll have very nice wide channels

 

https://www.youtube....h?v=_81Mi3FfQo0

 

 

you want to watch the cerox and how its drawn out of the channels...

 

https://www.youtube....rF2jO34pgg&t=9s

 

 

every so often get a brush and brush the cerox up out of the channels...




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