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Figuring first mirror - 8" f/6

Mirror Making ATM Beginner
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#26 Knight Sky

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Posted 15 October 2021 - 03:55 AM

I was sent optical grade CeO2 (quite possibly Cerox 1650) by a kindhearted soul from another city. Since, they have a lab for optical grade instruments, that cerox, I doubt, is contaminated. Water used for washing and slurry making or one of the drying processes could be the reason then, if not the pitch lap.

 

Do you suggest moving forward from here or use one of your nearly dry techniques?

 

PS: Yeah, calling them pits is basically using bad terminology. grin.gif


Edited by Knight Sky, 15 October 2021 - 03:57 AM.


#27 dogbiscuit

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Posted 15 October 2021 - 04:39 AM

I was sent optical grade CeO2 (quite possibly Cerox 1650) by a kindhearted soul from another city. Since, they have a lab for optical grade instruments, that cerox, I doubt, is contaminated. Water used for washing and slurry making or one of the drying processes could be the reason then, if not the pitch lap.

 

Do you suggest moving forward from here or use one of your nearly dry techniques?

 

PS: Yeah, calling them pits is basically using bad terminology. grin.gif

Just considering all the possibilities I can think of.

Not likely trouble with the cerium oxide.  The type of sleeks I mentioned are related to rouge. I've used many slurry mixtures, thick, thin, wet and dry and haven't had sleeks like those caused by rouge.

 

 Sometimes what might appear to be a poorly polished surface is just that the surface wasn't cleaned well at the end of a work session.  Don't let the slurry dry on the surface at the end of a work session.  Keep it wet and wash all cerium and swarf off immediately.  If it dries even for a moment it won't be easy to get it clean.

I'd say continue and when you get that new grating maybe that will cure the problem of foggy Ronchigrams.


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#28 Knight Sky

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Posted 15 October 2021 - 04:59 AM

@dogbiscuit, as you mentioned these are sleeks. and indeed a million of them. grin.gif I was afraid of going dry in my polishing as I could hear screeching noises doing so. Perhaps there is a balance between how dry or how wet you go, and most likely I have missed that fine mark. Should I try to go a little on the dry side in next few sessions?

 

I'll carry on with the parabolizing process then.

 

Back of the mirror is scratched up quite bad.

 

IMG20211015143913


#29 dogbiscuit

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Posted 15 October 2021 - 05:59 AM

I wouldn't say you should polish while squeaking, but i have done it and made a beautiful smooth surface doing so.  Sometimes squeaking can occur because the lap is not pressed well or maybe a hard spot in the lap resists flowing and plays a tune, and might do so even when the lap is wet. Sometimes you can see the spot that is making the noise if working MOT as the shade of the spot will change as you change motion.  I seen and heard a spot about the size of a pencil eraser make a near perfect sine wave tone as I stroked.  Have to stop and fix those small spot noise makers.  I shave them off or dig them out.  Haven't had those with Gugolz pitch but had them infrequently when using burgundy pitch. With a good lap (channels and pressed) and working until dry the screeching lap can make a good surface.  Probably safer not to go too long in screeching territory.

 

You could try it to see if it removes or reduces your sleeks, but there is some chance it won't work and maybe will cause trouble with the surface smoothness or shape.

 

If you looked at that video link I posted earlier you might be able to see there is very little slurry on the exposed glass. A high resolution video would have shown the slurry layer was so thin that rainbow colors were showing. About 30 seconds or so in the stroke slows from resistance to motion and more force is needed to move the glass.  The stroke speed could have remained the same but there is a feel to keeping resistance high and smooth at the same time.  Sometimes speeding the stroke will keep smooth motion, sometimes slowing is better. The idea is to keep the smooth feel to the motion not necessarily steady speed.  There is a difference between grabbing (jerky is bad) and smoothly changing speed and force.  At about a minute in the the slurry dries completely on the exposed glass and force is needed to continue but it's beginning to lose the smooth feel.  Time to stop and add a mist of water, or a little more slurry, and try to get into the smooth resisitance zone and maintain as long as possible, or as long as necessary for the desired result.


Edited by dogbiscuit, 15 October 2021 - 06:15 AM.

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#30 dogbiscuit

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Posted 15 October 2021 - 06:13 AM

Some, maybe most figuring you wouldn't want to get into the very high and dry resistance. It might take too long to get into that state and would do more work than desired, or you might not be able to maintain that state for whole turns of the mirror and work not be even across all the worked diameters of a turn.

 

Sometime the work needed and the stroke to do it allow working farther into the dryer part of the wet. I've gotten some crazy smooth surfaces when that happens.

 

There are times. :)

 

Might be that could reduce your sleeks, but might take you a bit farther from a parabola while you did that.


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#31 Knight Sky

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Posted 15 October 2021 - 07:45 AM

Thanks dogbiscuit for excellent insights!

 

I am beginning to think, and this is still just a hypothesis, that these sleeks are caused by my lap being a little too hard. Lap is custom made with rosin + beeswax + varnish. What is creating these sleeks is the microfaceting then. I will try and work without microfaceting and that should confirm this hypothesis if the sleeks start to reduce. The only issue is that the contact might become slippery. Nonetheless I should give it a shot, maybe work slow to avoid pushing hard and avoid slips. Why I am saying this is the case is because whenever I have trimmed the lap, the edges have caused sleeks that have disappeared within 3-4 rounds around the barrel. And those sleeks were big and prominantly visible on Foucault, for these ones, they are hardly visible on Foucault.

 

I will update once I have a definitive reason to believe so.



#32 Knight Sky

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Posted 15 October 2021 - 11:35 AM

After 1 more round of .7-1. and .85 to 1. correction and a 3 stroke parabolizing 3/4D W to add correction to the center.

 

IMG 20211015 212048

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#33 Pinbout

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Posted 15 October 2021 - 12:51 PM

your kink is pushed out further and less, keep going


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#34 Knight Sky

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Posted 15 October 2021 - 01:05 PM

your kink is pushed out further and less, keep going

Pinbout, which zone(s) should I correct? With my little understanding I see that zones from .5 onwards have mild overcorrection, about 10-20% in my estimation except that 5% of the edge which needs to be corrected. Is my reading about the correction right?

 

You idea seems to make sense to me though, if I work on that last bit of kink, I think the overcorrection in .5+ zone should reduce as well. But I could as well be reading it all wrong. lol.gif


Edited by Knight Sky, 15 October 2021 - 01:05 PM.


#35 Pinbout

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Posted 15 October 2021 - 02:54 PM

 

Pinbout, which zone(s) should I correct?

put it on the stand and mark the kink with a sharpie while ronchi testing it,  then transfer that radius to the back of the mirror and draw a sharpie cirlce on the back. numbers? you don't need to get hung up on them yet... just get rid of the kink.


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#36 ccaissie

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Posted 16 October 2021 - 12:40 PM

Thanks dogbiscuit for excellent insights!

 

I am beginning to think, and this is still just a hypothesis, that these sleeks are caused by my lap being a little too hard. Lap is custom made with rosin + beeswax + varnish. What is creating these sleeks is the microfaceting then. I will try and work without microfaceting and that should confirm this hypothesis if the sleeks start to reduce. The only issue is that the contact might become slippery. Nonetheless I should give it a shot, maybe work slow to avoid pushing hard and avoid slips. Why I am saying this is the case is because whenever I have trimmed the lap, the edges have caused sleeks that have disappeared within 3-4 rounds around the barrel. And those sleeks were big and prominantly visible on Foucault, for these ones, they are hardly visible on Foucault.

 

I will update once I have a definitive reason to believe so.

Microfaceting, in my experience helps to keep the lap rom glazing...hard spots of Cerox, which can sleek the glass.  Scrub it well with a brass brush.  If you want to try working with a softer lap, raise the worktemp...either heat the room more, or slowly warm the mirror and tool in a bucket of warmer (like +10 degrees) water, press and go while there's warmth.  Might help.  I tend to use a fairly rich mixture in my slurry and don't get it too dry.  


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#37 Knight Sky

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Posted 18 October 2021 - 12:00 PM

@Pinbout and @dogbiscuit, there is an update but it's not related to having worked on the mirror. I took time off to fix my tester as it was becoming a little wobbly. I also changed the design a bit so that taking outside RoC pics became a little easier.

 

Now for the big update, and I apologise to you both that the numbers that I provided you initially were incorrect. frown.gif Mirror diameter is 200mm so about ~7.87" and RoC is at 91.5". Ronchi images still look fogged up (no pun intended).

 

Here is another image of the mirror taken just a few hours back.

 

IMG 20211018 211325


#38 dogbiscuit

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Posted 18 October 2021 - 12:16 PM

The shape looks pretty good.

Under-corrected. and outer zones lagging the center.

 

Why not take some Foucault pictures?

Try to get one with the null (crest of the doughnut) at about the 70% zone, and maybe one near center and one near the edge.

 

Maybe a picture of your tester too.

 

What sort of camera are you using?


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#39 Knight Sky

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Posted 18 October 2021 - 01:07 PM

I am using a phone camera with 4x magnification.

 

Isn't the surface a little zoney/jagged? I feel the surface could be smoothed a bit to see better results on Foucault? Images look that way, partially because I enhance them a bit (color correction).



#40 Knight Sky

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Posted 18 October 2021 - 01:09 PM

The shape looks pretty good.

Under-corrected. and outer zones lagging the center.

 

Why not take some Foucault pictures?

Try to get one with the null (crest of the doughnut) at about the 70% zone, and maybe one near center and one near the edge.

 

Maybe a picture of your tester too.

 

What sort of camera are you using?

You are right about under-correction. I can notice a kink at 55-60% zone, but it doesn't seem to be uniform across the surface.



#41 dogbiscuit

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Posted 18 October 2021 - 01:54 PM

Foucault shows some things better than Ronchi.

You don't need to take knife readings, don't need a mask, just see what it looks like with knife edge testing.

 

It could also show if your foggy images are from the grating or some other source.

 

There is a kink but it really isn't a big deal.  It is under corrected and less corrected in outer zone than the center.


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#42 Knight Sky

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Posted 18 October 2021 - 03:17 PM

Here's a Foucault image at about 70% zone nulled.

 

Surface looks uneven but doesn't seem to be out of control to my untrained eyes. Maybe smoothing will remove this irregularity on the surface? 

 

IMG 20211019 012738

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#43 ccaissie

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Posted 18 October 2021 - 06:13 PM

Nice pic, nice diffraction edge.  Nice work.  It is lacking smoothness.  A softer lap...maybe warmer environment, washed and settled cerium and some overall smoothing strokes can reduce that ripple.

 

As far as correction, can't tell.  The right side of the focogram suggests the center looks deep, then there's a flattish area from 50-80%, then drops off steeply.  If you spend some time and watch the approach of the shadows under knife edge you'll begin to get a sense of the slope changes across the zones.  

 

I'm a big fan of the Everest pinstick method of Foucault.   It allows you to view the whole mirror when measuring the zones and you can study the shadow movement which tells all.

 

https://www.cloudyni...stcc-tape-test/


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#44 dogbiscuit

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Posted 18 October 2021 - 08:05 PM

That looks pretty good.

there is slight roughness. Some of that is from trying to work various zones.

W strokes can smooth the curve, and the mirror being under corrected you could use big Ws that would add some correction. You are close to a good parabola and could get there in just a few short sessions.

 

The lap should be made just right and press well before working.  If the lap is good and you stroke smooth the surface can be made smooth as you work toward the parabola. Being so close you don't have much work time left.

 

Think first, stroke later.  Press while you think. Don't want to make a mistake now.   :)

 

Is that still the same as seen in the Ronchi in post 37?

Correction looks to be more in the Foucault than in the Ronchi but that really has a lot to do with the exposure settings.  But to be safe verify the grating is really 133 lines per inch, that's 133 black lines per inch.  Some sources (a source) for gratings counts 1 black and 1 white as 2 lines. 

 

Post a picture of your lap.


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#45 davidc135

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Posted 19 October 2021 - 03:15 AM

Coming along well. Great edge.

 

It seems to me that outside the 71% zone there is much less correction than there is inside. There should be the same amount. In fact, the outer 3/4ins looks spherical whilst the centre (may) be a little over-corrected.

The outer zone makes it easy to measure the overall knife edge travel and get a handle on the remaining error. I'd guess still a bit over 1/4 wave.

You want a stroke that corrects the outer zone whilst not adding to the centre correction and that smooths and blends but doesn't turn the edge.

Edit- I'd try a MoT, medium wide W with long strokes for a short while and then check. But I'd try Dogbiscuit's approach first.

 

David

 

PS An F/6 8'' sphere suffers from .83 waves SA which is corrected by r^2/2.R or 2.08mm ke travel if source/ke are moving. Even a ruler and fine pencil lines can resolve .5mm and probably .25mm or 1/10 wave.


Edited by davidc135, 19 October 2021 - 04:18 AM.

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#46 dogbiscuit

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Posted 19 October 2021 - 03:45 AM

That looks pretty good.

there is slight roughness. Some of that is from trying to work various zones.

W strokes can smooth the curve, and the mirror being under corrected you could use big Ws that would add some correction.

To be clear,  The big W strokes I would use would be as seen in the video link in post # 8.

TOT, 1 trip around the mirror and test.


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#47 Knight Sky

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Posted 19 October 2021 - 04:41 AM

Thanks everyone for encouraging words!

 

Nice pic, nice diffraction edge.  Nice work.  It is lacking smoothness.  A softer lap...maybe warmer environment, washed and settled cerium and some overall smoothing strokes can reduce that ripple.

 

As far as correction, can't tell.  The right side of the focogram suggests the center looks deep, then there's a flattish area from 50-80%, then drops off steeply.  If you spend some time and watch the approach of the shadows under knife edge you'll begin to get a sense of the slope changes across the zones.  

 

I'm a big fan of the Everest pinstick method of Foucault.   It allows you to view the whole mirror when measuring the zones and you can study the shadow movement which tells all.

 

https://www.cloudyni...stcc-tape-test/

 

Yeah, I think I may have to work with slightly warmer lap. It has started to cool down slightly here. I do have the couder mask printed but I was thinking of using Everest pins, wasn't too sure that I would be able to discern small little changes. But now that you have given the push, I might just go ahead with it as I am not too keen on carving the Couder. Just that instead of notches I will try to have protruding radial markers. Thanks for that suggestion! waytogo.gif

 

That looks pretty good.

there is slight roughness. Some of that is from trying to work various zones.

W strokes can smooth the curve, and the mirror being under corrected you could use big Ws that would add some correction. You are close to a good parabola and could get there in just a few short sessions.

 

The lap should be made just right and press well before working.  If the lap is good and you stroke smooth the surface can be made smooth as you work toward the parabola. Being so close you don't have much work time left.

 

Think first, stroke later.  Press while you think. Don't want to make a mistake now.   smile.gif

 

Is that still the same as seen in the Ronchi in post 37?

Correction looks to be more in the Foucault than in the Ronchi but that really has a lot to do with the exposure settings.  But to be safe verify the grating is really 133 lines per inch, that's 133 black lines per inch.  Some sources (a source) for gratings counts 1 black and 1 white as 2 lines. 

 

Post a picture of your lap.

Nothing has changed between #37 and now on the mirror. Ronchi grating is 133 lpi. Should I go ahead with parabolization W stroke? Or should I first correct z4-z5 and spread the correction evenly on z2/z3?

 

Coming along well. Great edge.

 

It seems to me that outside the 71% zone there is much less correction than there is inside. There should be the same amount. In fact, the outer 3/4ins looks spherical whilst the centre (may) be a little over-corrected.

The outer zone makes it easy to measure the overall knife edge travel and get a handle on the remaining error. I'd guess still a bit over 1/4 wave.

You want a stroke that corrects the outer zone whilst not adding to the centre correction and that smooths and blends but doesn't turn the edge.

So, everything that Dogbiscuit says except I'd try a medium wide W with long strokes (not much difference) for a short while and then check.

 

David

 

PS An F/6 8'' sphere suffers from .83 waves SA which is corrected by r^2/2.R or 2.08mm ke travel if source/ke are moving. Even a ruler and fine pencil lines can resolve .5mm and probably .25mm or 1/10 wave.

Thanks David! I thought for some reason ke travel from z1 to z5 would be about ~1.65mm. Seems like I miscalculated. If that's the case then my rough measurements on ke are pretty much falling in line. Except that, as everybody mentioned, some zones are under-corrected.

 

To be clear,  The big W strokes I would use would be as seen in the video link in post # 8.

TOT, 1 trip around the mirror and test.

So, big 4-stroke W with ToT. Should this be 3/4D or even longer? I find your video to be quite eloquent (well this word might be misplaced but for me it's poetry on mirror...maybe I don't make sense at all grin.gif ), the issue is I am not so confident about smooth turn and, apriori, in my mind, I imagine it to screw up no matter what. But I shall give it a shot. laugh.gif Should I soften the lap a little for this session? Say by a warming it up a little (5-10 degrees or so)?

 

IMG20211019125536
Album: 20191019_tester_lap
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#48 Knight Sky

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Posted 19 October 2021 - 04:46 AM

Please do not make fun of me for how cheapo this tester is. It was built with all the junk that I could find at home - bobbins, printer shaft, shoe polish boxes and what not. lol.gif lol.gif The only slightly expensive item there is the dial gauge.


Edited by Knight Sky, 19 October 2021 - 04:46 AM.


#49 dogbiscuit

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Posted 19 October 2021 - 05:53 AM

Wait before working.

 

Looking at your lap... It needs to be trimmed.

I am marking some things on your image of the lap to show how to improve it when you trim.

Not sure about the temperature. The roughness, which isn't all that bad, is almost certainly from working with the lap that needs trimming.

 

Don't trim until I post the marked up picture.

There is a small section of channel that is about the right depth and width.  I'll mark that area with a green circle. The idea will be that when trimming you try to make all the channels as much like that all over the lap.

 

You could be only a few short (~5 minute) sessions from a very good parabola.

 

As for heating the lap and mirror, I don't do that but I work in a temperature controlled space.

I think for one session you should continue without warming the lap and mirror.  After trimming the lap rinse the pitch chips off and apply slurry and press. 

 

How much have you been pressing until now?  How much work have you done since the last time you trimmed the lap?  I can get a pretty good idea how hard the pitch is from that.  For now I will say that you have a pretty good surface so the pitch is pretty good.  With the lap trimmed it should work better.

 

Picture coming... not to long from now.


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#50 dogbiscuit

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Posted 19 October 2021 - 05:56 AM

Your tester look nice.

And very practical... you can polish your shoes while you test.  :-)


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