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The secrets of a perfect sphere and black polish on a mirror?

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#1 macleod

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Posted 14 December 2018 - 04:17 PM

Apart from shear time and effort, what are the secrets to avoiding tde and getting that  perfect sphere and so called "black" polish on your glass? 

I,m 5hrs into polishing a 10inch f3.3  x 3/4in plate glass mirror, on a power turntable at 6rpm BY HAND , and man its an effort when the lap is in good contact - you feel all that effort working the glass. I,m doing plenty lap trimming/repair etc, warm pressing before each session, and working the glass for 1hr straight , and thats about all I can manage at one session. Do 1hr morning and afternoon, with lap maintenance and cleanup in between.

I have plenty of time, find it fun and rewarding to see those Cerox bubbles moving up and down the pitch channels, and feel the solid even drag of the lap working. My aim is to get as near to the "ultimate" sphere and polish that I can achieve before even thinking of parabolising - that will be my phase3 of this build. Any tips  on  achieving my aim will be greatly appreciated thank you. MACLEOD.


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#2 Lognic04

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Posted 14 December 2018 - 04:20 PM

All you really need to parabolize is a good, smooth surface and a good edge. I'm learning so far the lap is just as important as your strokes. Make sure it has very even channels, and an edge with no bevels.


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#3 Pierre Lemay

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Posted 14 December 2018 - 04:24 PM

Think less, polish more.


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#4 DAVIDG

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Posted 14 December 2018 - 05:08 PM

  To achieve a true black polish takes time. The rule is 1 hour of machine polishing on pitch for ever inch of aperture. For hand polishing that number is doubled to 2 hours per inch on pitch   Most amateur don't polish long enough. You can cut  that time in 1/2 again or more by using polishing pads. That is what we use at Delmarva mirror making class which allows us to a make 10" and 12" mirror from grinding to a finished mirror in 3 days. Once the mirror is fully polished we then switch to a pitch lap to figure it. 

   We use Cerium oxide to polish. The surface smoothness is not some much a function of the polishing compound but the condition of the lap and the technique used to polish. 

 

                  - Dave 


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#5 TOMDEY

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Posted 14 December 2018 - 05:35 PM

Apart from shear time and effort, what are the secrets to avoiding tde and getting that  perfect sphere and so called "black" polish on your glass? 

I,m 5hrs into polishing a 10inch f3.3  x 3/4in plate glass mirror, on a power turntable at 6rpm BY HAND , and man its an effort when the lap is in good contact - you feel all that effort working the glass. I,m doing plenty lap trimming/repair etc, warm pressing before each session, and working the glass for 1hr straight , and thats about all I can manage at one session. Do 1hr morning and afternoon, with lap maintenance and cleanup in between.

I have plenty of time, find it fun and rewarding to see those Cerox bubbles moving up and down the pitch channels, and feel the solid even drag of the lap working. My aim is to get as near to the "ultimate" sphere and polish that I can achieve before even thinking of parabolising - that will be my phase3 of this build. Any tips  on  achieving my aim will be greatly appreciated thank you. MACLEOD.

YES! That "drag" feeling is key... feels like dragging your toes thru a hot Sugar Daddy or wading thru the tar pits!  Tom

Attached Thumbnails

  • 057 Sugar Daddy.jpg
  • 058 tar pits.jpg

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#6 TOMDEY

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Posted 14 December 2018 - 05:42 PM

Ummm... take the above as metaphors... I don't actually do those things, on a regular basis.  Just imagine doing them, when I'm polishing glass.  Tom

 

And This one goes thru my mind, when I get the mirror stuck to the pitch... >>>

 

https://www.youtube....h?v=Y3LoQrw1RVM

 

Tom


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#7 Pinbout

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Posted 14 December 2018 - 09:17 PM

 

dragging your toes thru a hot Sugar Daddy or wading thru the tar pits!

That would be good for exfoliating the feet lol.gif



#8 Pinbout

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Posted 14 December 2018 - 09:19 PM

Firsthanddiscovery.com sell polishing pads also.



#9 SandyHouTex

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Posted 15 December 2018 - 10:26 AM

YES! That "drag" feeling is key... feels like dragging your toes thru a hot Sugar Daddy or wading thru the tar pits!  Tom

I miss those “Sugar Daddies”.  Then again it might be why I have type 2 diabetes.



#10 TOMDEY

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Posted 15 December 2018 - 11:36 AM

I miss those “Sugar Daddies”.  Then again it might be why I have type 2 diabetes.

Pardon me for chuckling!  Ummm... staying on-topic, speaking on "polishing it off", I fondly recall Massive sugar/salt binges, with impunity, as a kid. Like, sitting in the neighborhood movie theatre watching Sci-Fi movies or at a birthday party.



#11 Mike Spooner

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Posted 15 December 2018 - 03:39 PM

Ummm... take the above as metaphors... I don't actually do those things, on a regular basis.  Just imagine doing them, when I'm polishing glass.  Tom

I'm really trying not to think about the "irregular basis".shocked.gif

 

Mike

 

laugh.gifsmile.gif



#12 macleod

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Posted 15 December 2018 - 04:27 PM

thank you for the humour ! My question was aimed at a serious reply though. For that good polish should you decant the surface layer off a jar of cerox to get the ultrafine stuff, or change to decanted Rouge, or what. Is there a special technique ? A lot of members make mirrors and I would  have imagined  this topic would be of greater interest. MACLEOD.



#13 macleod

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Posted 15 December 2018 - 08:51 PM

CN has some very experienced and very helpful members , and appra-pro my original post, most advise a full polish / spherical before even testing or going to parabolization. Ive done 9 hrs so far pushing glass on a rotary tt at 7rpm. Seem to have a good polish by laser test. Doing COC 1/6th overlap strokes, a bit varied, but is this suitable approach to get a good sphere ?Also trying to avoid tde, ripple etc - this is advice I would like. Macleod



#14 Pierre Lemay

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Posted 15 December 2018 - 09:51 PM

At Delmarva, two years ago, I made two 8 inch f/5 mirrors. Well, actually, retouched a 1/4 wave under corrected parabola I had made 40 years before on one mirror. It's now perfect!

 

The other 8 inch I made from scratch. Rough ground the curve at home. By the time I got to Delmarva, it was ready for fine grinding. Did all the fine grinding by hand on a fixed grinding stand, then polished by hand, also on a fixed stand and did the laser test. I used polishing pads at first with about three hours of very aggressive polishing, then switched to about 6 hours of much slower pitch polishing by hand. All along I was doing the laser test and could see the glass getting "blacker", or so I thought frown.gif , as did others who looked at it. At some point I called it fully polished.

 

I then spent a few hours polishing in a very nice parabola using Dick Parker's DPAC tester as witness. Jail bar straight lines, just like the other mirror I had retouched. Brought the two mirrors to Normand Fullum for aluminizing. After a few days Norm calls to tell me my mirrors were ready but, pausing and with a hint of frustration in his voice he tells me: you could have finished polishing before parabolizing! The mirror I had made from scratch had a definite haze about 1/4 inch wide at the edge. Aluminizing takes no prisoners! This was the second time in my ATM life I had made that mistake. I can tell you this: IT WON'T HAPPEN AGAIN!

 

The concept of "black" polish is a cute term but I'm not sure what it means. The entire surface is either fully polished or its not. The laser test is a good test but when the polish is nearing the end, the laser test can be subject to interpretation and, especially, bias (as in: "I want it to be fully polished so the laser beam I see near the edge, that must be dust...).

 

If you can, use other tests to confirm the polish is complete like:

  • look at reflection from an incandescent or other bright light at a grazing angle and/or,
  • use one of these small eyepiece type microscopes to examine the surface under a bright light

And I would add this: Once you think you've polished fully, polish a few hours more, just in case. It's very embarrassing to pay good money for an aluminizing job only to have to strip it off to finish the polishing job for real and have to go through parabolizing all over again.

 

Been there, done that. Twice undecided.gif .


Edited by Pierre Lemay, 15 December 2018 - 10:04 PM.


#15 macleod

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Posted 16 December 2018 - 01:07 AM

Hi Pierre, I fully understand your frustration !! This is what I want to avoid - done a full 10hrs now actual glass pushing, irrespective of lap repair / channeling etc. Looks good, but will do another 10hrs, giving 20hrs polishing in total, and hopefully that will be it.  Then on to phase3 , the dreaded parabola on an f3 optic - that will test my sanity for sure. Thanks for reply, have a great family Christmas. Macleod



#16 DAVIDG

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Posted 16 December 2018 - 12:20 PM

" Black polish" is  a term that has been in the optical industry for years. If you have a truly optically polished surface and then look at an angle, it will look like a pool of black oil. Just no scatter at all. 

  The bottom is that it takes time to achieve a full optical polish to the very edge. As I said most ATM's don't polish long enough.  When  some says that they polished for say 8 hours, if you actually counted the time the surface was moving on the lap it would most likely be 25 to 50% less because you stop to add compound, watch TV for a minute, let the cat out the door, take a sip of a drink, rest for a minute, etc. At the Delmarva class I see this happen all the time, it is just hard to stand there and  polish non stop for any length of time.  So ATM's should polish for 25% to 50% longer when they think their surface is fully polished out to be sure it actually is.

 

               - Dave 


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#17 MitchAlsup

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Posted 16 December 2018 - 01:41 PM

" Black polish" is  a term that has been in the optical industry for years. If you have a truly optically polished surface and then look at an angle, it will look like a pool of black oil. Just no scatter at all. 

A coronagraph is essentially useless unless the optical elements have a black polish (i.e., no scatter).



#18 JohnH

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Posted 16 December 2018 - 01:56 PM

I took video of myself mounting to you five or six hours when I first started polishing out my big 12 inch mirror before I moved, and went over it to see just how much actual polishing time I did and to critique my own technique of course. I was usually pretty good about really going into it if I said it was 2 hours that most of the time was actually either pushing the lap around or adding more Polishing Compound to complete a wet.

#19 SandyHouTex

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Posted 16 December 2018 - 03:44 PM

  To achieve a true black polish takes time. The rule is 1 hour of machine polishing on pitch for ever inch of aperture. For hand polishing that number is doubled to 2 hours per inch on pitch   Most amateur don't polish long enough. You can cut  that time in 1/2 again or more by using polishing pads. That is what we use at Delmarva mirror making class which allows us to a make 10" and 12" mirror from grinding to a finished mirror in 3 days. Once the mirror is fully polished we then switch to a pitch lap to figure it. 

   We use Cerium oxide to polish. The surface smoothness is not some much a function of the polishing compound but the condition of the lap and the technique used to polish. 

 

                  - Dave 

20 hours seems like an awfully long time for a 10 inch.  And why would manual polishing be double a machine polish since I assume the strokes of the machine mimick a human?  I would also suspect that it depends on how small your last wets grit was.  I used 3 micron Aluminum oxide which is only .00011 inch.  I’ve done an hour with a pitch lap and pure cerium oxide, and I’m amazed at how quickly the polish is coming along, compared to a 16 inch I was working on 20 years ago. Back then I used the tan cerium oxide and a 12 inch sub-diameter lap, but things seem to be going a lot faster now.



#20 davidc135

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Posted 16 December 2018 - 04:42 PM

Hi Pierre, I fully understand your frustration !! This is what I want to avoid - done a full 10hrs now actual glass pushing, irrespective of lap repair / channeling etc. Looks good, but will do another 10hrs, giving 20hrs polishing in total, and hopefully that will be it.  Then on to phase3 , the dreaded parabola on an f3 optic - that will test my sanity for sure. Thanks for reply, have a great family Christmas. Macleod

I use a 25mm Kellner ep back to front to check for the last pits.  David



#21 mark cowan

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Posted 16 December 2018 - 08:27 PM

 

The concept of "black" polish is a cute term but I'm not sure what it means.

 

 

Hmm, well "black polish" is a real thing, and it's what I see on quartz after a couple days run time on the fixed quill polisher.  It's quite obvious comparing it to a similar mirror that appears fully polished out by laser test, but it's the result of going past that.  No easy explanation suggests itself.  I wouldn't try to do that by hand though.  :shrug:



#22 df_2112

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Posted 16 December 2018 - 09:12 PM

In my mind there is no real secret that hasn't been outlined already.  It is just the application of the time honored tenets of time and patience.

 

A good polish or black polish as you say starts way back at the rougher grits.  The larger the starting grit say 60 or 80 on a flat blank or 220 on a well regenerated blank induce subsurface cracks that can show up later during the finer grits or polishing.  If you are starting with the intention of going to that level each stage of grit needs to be run a good bit longer than what is the norm...if you are just sampling a mirror and using a loupe to check the surface then run until you see none of the larger pits then run several more full wets before proceeding (looking at my old log I have doubled the grit time on a few occasions)

 

When you do get to polishing there are those that will use specialized rouge or ultra fine cerium oxide but honestly I'm not sure those add anything more substantial at the amateur level than good old time time time.

 

Once you do get to the polished sphere point then it's just repetition.  The hardest thing is resisting the human nature side of things to change each wet.   Having used a machine for most of my work I can say the estimations here in time are not unreasonable...you can make it better with a LOT more time than would be considered typical.

 

i think the absolute hardest part aside from admitting that you are going to spend that extra time and risk of extra polishing sessions for what some may consider marginal gains.  I tend to be CDO with my mirror stuff and have always just had the tendency to push things beyond the limit (CDO is just like OCD...but alphabetic like it should be).  I suspect the OP has similar tendencies given the nature of the post..

 

The rest of the detail is in the technique....cold pressing, warm pressing, lap channelling and working environment temperature management.  You have to be very diligent..nearly neurotic about pressing, mircofaceting the lap and channelling the lap when it's needed and considering skipping the evening when the work room has cooled or warmed by 5 degrees..etc etc.  just keep everything as consistent as you can.. Once you get into that routine then you need to be militant and resist the urges to do 'just one or two more sessions before tending to the tool or faceting or mixing a new polishing slurry or just adding a few more minutes to each session etc...It is in those times when you are likely to go the wrong way, make a mistake and/or cause a problem and undo all of the hard 'extra' work you out in. 

 

Hope that all was helpful or at the very least coherent, no 'secrets' really.   In 30 years or more of mirror making It has never ceased to amaze me that amateurs just about anywhere can produce such wonderfully accurate objects with such rudimentary materials and relatively simple basic techniques....



#23 macleod

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Posted 17 December 2018 - 03:25 AM

Thank you all for the great advice ! As df2112 says, I am a little OCD about this mirror. I made an 8inch F8 in the 60,s  using only Texerrau as there was no Internet then. Always wanted to do another mirror - my  principal  scope has 10inch Zambuto with Quartz secondary and all the goodies, home built , so I do have a good scope. But  I want a "fast" OTA to do deep sky faint fuzzies to fit my existing base, and as I,m retired with plenty of time I thought I,d push the boundaries a bit with F3 10in 3/4 thick ala Mel Bartels.So reading the CN posts over the years the advice seems to be to get the best polish possible before parabolising, so thats what I,m attempting to do. Made a 12v electric turntable with a heavy duty gearmotor, and away I,ve gone.Did about 12hrs actual grind ( glass pushing time) down to 5micron Alox - had to go back to 25 to remove scratch then back down to 5. Now I,m at 11hrs polishing and the laser reflection has almost entirely gone allover surface, but I hate to think its not a sphere ! Have resisted the urge to look until I,m satisfied with polish.Maybe a few more hours and completely polish out before Christmas. Let the "fun" start in new year when I,m fresh ! Everyone have a great family Christmas , best wishes from NZ. MACLEOD.


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#24 sopticals

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Posted 17 December 2018 - 03:59 AM

Hi macleod,

 

I would be wanting to know what sort of figure at this point (after 11 hours of polishing). Could be that many hours of actual figuring could be required and that would take your polish to a higher level.

I've been working on a 33" f3.94 (my second 33") and after 20 hours (all by hand-no machinery) I had a good polish using a sub-diameter lap (17" [tried a 22" lap, but much too difficult to move smoothly]).

Since then been working another 75 hours on figuring (only wish I had checked out figure a little earlier than at 20 hours polishing as might have picked up on defects then). So at 95 hours I have a pretty

good polish-and I have a lot of work ahead yet.

 

Stephen.(45deg.S.)



#25 DAVIDG

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Posted 17 December 2018 - 10:37 AM

20 hours seems like an awfully long time for a 10 inch.  And why would manual polishing be double a machine polish since I assume the strokes of the machine mimick a human?  I would also suspect that it depends on how small your last wets grit was.  I used 3 micron Aluminum oxide which is only .00011 inch.  I’ve done an hour with a pitch lap and pure cerium oxide, and I’m amazed at how quickly the polish is coming along, compared to a 16 inch I was working on 20 years ago. Back then I used the tan cerium oxide and a 12 inch sub-diameter lap, but things seem to be going a lot faster now.

 Because a machine doesn't stop while a person does. Like I said try to polish non stop for even 30 minutes. Most people can't, they stop to rest,, they take a slip of drink, they glance up at the TV etc. So the actual polishing  time that the mirror is actually moving is 25% to 50 % less  when you manually polish vs the time you thought you polished.  Having been making optics and teaching classes for close to 40 years now,  I consistently see ATM's not fully polishing their optics. A  mirror will shine up pretty quickly with cerium oxide and looked polished but you haven't polished enough to get all the down past the fracture layer and down to a true optical polish. So ATM's  think  they have but when correctly tested for a full polish they haven't. So my advice is when your think you have fully polish your optics go another hour or so just to be sure. 

 

                - Dave 


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