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Figuring first mirror - 8" f/6

Mirror Making ATM Beginner
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#1 Knight Sky

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Posted 13 October 2021 - 04:44 PM

I am working on my first mirror and I feel like I am close yet not quite there. At this point I decided to get some help so that I do not mess up anything too horribly to go back to sphere. RoC is at about 92.7". Ronchi screen is 133 lpi. Three attached images are taken at -4mm, -2mm and 6mm (-0.157, -0.079 and 0.237 inches) respectively.

 

I see a correction needs to be made at .7 and .9 I have had two sessions on .9 with a 4 segment Z 1/3D stroke length and 1/3D overhang and 1/4 overhang respectively, which seemed to have improved the .9 zone just that tiny bit, but in doing so I seem to have lost a little bit of correction in the center. Surprisingly though the center matches pretty decently with MB's ronchi matching web applet. Mirror also has a minor TUE, not sure how significant it is though. Any suggestions on how to go from here without messing much? Am I even going in the right direction?

Attached Thumbnails

  • IMG_20211014_012130.jpg
  • IMG_20211014_012228.jpg
  • IMG_20211014_012313.jpg

Edited by Knight Sky, 14 October 2021 - 12:57 AM.

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#2 Pinbout

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Posted 13 October 2021 - 04:54 PM

The top image is inside or outside RoC?

 

no one really measure the distant from RoC on ronchis


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#3 Knight Sky

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Posted 13 October 2021 - 05:00 PM

Two are inside RoC at 4mm and 2mm. Outside RoC one is at 6mm.



#4 Knight Sky

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Posted 13 October 2021 - 05:03 PM

I am just trying to see if I can get Ronchis as close to what MB's web applet suggests. I can then switch to Foucault and work with the readings.


Edited by Knight Sky, 13 October 2021 - 05:03 PM.


#5 Pinbout

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Posted 13 October 2021 - 05:09 PM

Two are inside RoC at 4mm and 2mm. Outside RoC one is at 6mm.

I have to guess the bottom one is outside… that’s really annoying. 
no one cares how much from RoC the images are. 
 

the outer zones need more work. The ronchi is flat.

 

while working one side of the mirror, use the edge of the tool and short zigzag from inside the kink out to the edge and back to inside the kink as you walk around the stand, keeping all movement fluid, almost like no stops and starts, no jerky movements. It’s like a dance…

when I saw Dick Parker do some parabolizing stfokes at Delmarva it was like wow, that’s how you keep it smooth.

Swayze does the same thing with the edge of the tool on local zones… fluid motion.

 

5mins and test 


Edited by Pinbout, 13 October 2021 - 05:13 PM.

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#6 Knight Sky

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Posted 13 October 2021 - 05:33 PM

Thanks Pinbout! And sorry about those details. Yes indeed, first two are inside RoC and the last one is outside.

 

Not quite sure though I understand the stroke you mentioned, specifically the non-stop motion and moving around the stand part. Do you have a video/image/sketch you can point me to? Also, what radius I start this stroke from and what length should each stroke be? This seems like a complex stroke for a beginner like me. smile.gif



#7 Pinbout

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Posted 13 October 2021 - 05:45 PM

I’d put the mirror on the stand and use a sharpie to mark where the kink is and the draw that circle on the back of the mirror with the sharpie.

 

and you can do it MoT so you can see the kink line you drew

 

this isn’t as fluid as I’d like, wife walking backwards trying to capture the stroke…smh

 

https://youtu.be/Vr5c3NL9yR4

 

also mark a starting point. 


Edited by Pinbout, 13 October 2021 - 06:09 PM.

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#8 dogbiscuit

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Posted 13 October 2021 - 09:19 PM

Best to ask now... is it completely polished?

 

 

I judge the kinks to be at .6 and .8 radius.

See that alot with the classic parabolizing stroke as described by Texereau and some other books.  Central ~ 60% corrects faster 60% to 100% zones.

 

 

That sort of shape, I like to do this. Works fast.

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


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#9 Knight Sky

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Posted 14 October 2021 - 12:55 AM

Hmmm, dilemma of choosing which path to take confused1.gif  confused1.gif Both are fairly new to me so it's going to be a challenge.

 

@Pinbout, that video makes it fairly clear the approach you suggested. like-button.jpg

 

@dogbiscuit, yes the mirror is fully polished. Wouldn't working ToT be risky and potentially create a TDE? Also, would it flatten the center?

 

Thanks gentlemen for your kindhearted suggestions! I will update once I execute one of these two approaches.


Edited by Knight Sky, 14 October 2021 - 04:01 AM.


#10 davidc135

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Posted 14 October 2021 - 02:11 AM

In Mel Bartels Ronchi matching site which you mention; of the five zones that the viewer can alter I changed zone 4 to 80% correction and the outermost zone 5 to 60% and got close to your pattern. With 0.28 pv wf error. What looks like tue may just be part of the very under-corrected outer zone.

Is there an MoT solution, say long coc to correct the outer zones combined with a W wide enough to maintain correction in the mirror's centre?

 

David


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#11 Knight Sky

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Posted 14 October 2021 - 03:22 AM

Thanks Dave, I had not realized that I could do that on that web applet! I will check Ronchi results and fiddle around with that feature on MBs.

 

For correcting outer zones, as far as my noob understanding goes, will work if you have oversized laps. For me I have same size lap as the mirror. Furthermore, you might be right about the TUE, I am not so sure though.



#12 dogbiscuit

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Posted 14 October 2021 - 03:54 AM

Hmmm, dilemma of choosing which path to take confused1.gif  confused1.gif Both are fairly new to me so it's going to be a challenge.

 

@Pinbout, that video makes it fairly clear the approach you suggested. like-button.jpg

 

@dogbiscuit, yes the mirror is fully polished. Wouldn't working ToT be risky and potentially create a TUE? Also, would it flatten the center?

 

Thanks gentlemen for your kindhearted suggestions! I will update once I execute one of these two approaches.

 

Within the limits of what can be seen in Ronchi your edge looks perfect. I think Foucault shows the edge better.  I'm pretty sure Foucault test would show a very well balanced diffraction ring around your mirror.

 

You do have under correction in the outer zones so you could view that as a turned up zone, but the edge is near, maybe is perfect.  For me when speaking of turned up or down edge, that is really the last mm or two at the edge.  Much wider than that it becomes a zone.

 

"Wouldn't working ToT be risky and potentially create a TUE?"

I'm pretty sure you made a typo and meant to say TDE.

Your edge is so good almost anything you do could make it worse.  smile.gif

The stroke shown works fine for me.  The edge on the mirror in the video is also near perfect but my lap is always maintained in perfect condition, channels uniform and well pressed, and work done at controlled temperature.

The TOT large W shown rolls correction into the outer zone. There is a knack to sizing the W to get just the right gradient of correction across the outer zone, and I would do only 1 turn around the mirror with the stroke shown and see what and how much happens and adjust if another round is needed. Wouldn't take much for an 8" f:6. Might be 1 turn would do it. Doesn't seem there are many mirror makers using this method. Maybe it's not really as easy as I find it to be for correcting outer zones.  smile.gif

 

The video link was as much to show stroking method as the particular stoke itself.

 

Go with Pinbout's suggestion.

Try my method on some future mirror or if something bad happens and you start from a sphere again on this one.  When I use this method it is 1 or 2 turns of MOT classic parabolizing stroke and an equal amount of TOT parabolizing stroke. Then adjust the ratio of MOT to TOT and maybe size of the Ws depending on test results.


Edited by dogbiscuit, 14 October 2021 - 04:12 AM.

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#13 dogbiscuit

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Posted 14 October 2021 - 05:36 AM

Post 194 is a description of the effect of the MOT and TOT parabolizing strokes in the method I mentioned above.

 

https://www.cloudyni...-6-in-f8/page-7

 

It's likely better you stick to more common methods for now.

It is difficult to describe things well enough in words and little chance a beginner do the strokes things exactly as I would and results could be wildly different.  Try this when you feel you have time for experimenting and not so worried about making a mess.

 

David mentioned a more common way of correcting what you have.

 

Always doing things a little different  lol.gif  I would modify to something like this to work on the specific undercorrected zones.

 

1 turn of the mirror with each of these...

Parabolizing length strokes but W only about 1.5" to 1.75" wide to add correction to the outer zone, the kink at about the 80% zone to the edge. About 5 forward and back strokes across the W and only 1 W at each rotation increment of the mirror.

 

Parabolizing lengthe strokes but width of the W about 3.25" to 3.5" wide to work from the edge to the kink near the 60% zone.  About 7 strokes across the W and only 1 W at each rotation increment of the mirror.

 

Then the normal full width parabolizing stroke. This would be done just as you have done before.  You curve is smooth all the way to center in the central zone, no dimple, no button. So what you were doing is very good there.

 

All together this works the outer kink to edge 3 times, the inner kink to the outer kink 2 times, and central zone to the inner kink 1 time. Putting more work where things are going slower.

 

Not saying you should do this, just giving an example of ways to think about getting work in the right places.  There are many ways.


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#14 Knight Sky

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Posted 14 October 2021 - 03:55 PM

Thanks dogbiscuit for alternatives and insights!

 

I was extremely tempted by your suggestion and almost went ahead with it, but changed my mind at the last moment and went ahead with Pinbout's suggestion. As you said, I might need a little more confidence to execute that complex a stroke, which looks very pleasing to the eye.

 

I did 3 sessions from .7-1.0 and 2 sessions from .85-1.0. Zones seem to have got a fair bit of correction but center has lost correction. Should I just execute a center deepening stroke at this point? Also how do I blend all the corrections so that the surface smoothens out once corrections reach to a satisfactory level?

 

Ronchi image, outside RoC.

 

IMG 20211015 013641
 
Ronchi, inside
 
IMG 20211015 013812

 


Edited by Knight Sky, 14 October 2021 - 03:57 PM.

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#15 dogbiscuit

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Posted 14 October 2021 - 04:16 PM

Why are the Ronchi images so foggy?

Looks like there is Vaseline on the grating.


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#16 dogbiscuit

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Posted 14 October 2021 - 04:55 PM

Ignoring the Vaseline :)

Looks to me like you have dulled the kinks into one broad kink.

Still the same basic shape with correction lagging in outer zones.

Overall correction looks like about 40%.


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#17 Knight Sky

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Posted 14 October 2021 - 04:56 PM

It might be a combination of mirror not having a smooth surface and the grating, being low quality, has some serious scratches.



#18 Pinbout

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Posted 14 October 2021 - 05:19 PM

I’d do more of those outer zone things, cause as dog said it’s still lagging. Worry bout the center after you get those zone flowing… the center should fall in line pretty well.


Edited by Pinbout, 14 October 2021 - 05:20 PM.

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#19 dogbiscuit

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Posted 14 October 2021 - 05:30 PM

Why do you say the mirror might have a rough surface? Something you can see?

What can be seen of the Ronchi bands looks smooth, but maybe there are pits?

Does the grating cover the light source?

Maybe focus could be better.


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#20 Pinbout

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Posted 14 October 2021 - 07:14 PM

Where did you get the ronchi? How clear is the film?


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#21 Knight Sky

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Posted 15 October 2021 - 01:19 AM

Why do you say the mirror might have a rough surface? Something you can see?

What can be seen of the Ronchi bands looks smooth, but maybe there are pits?

Does the grating cover the light source?

Maybe focus could be better.

Yes, the surface has been like this for quite some time...and I am not quite sure what might have caused such pits. On the surface they don't seem to be visible to the naked eye (except when pointde to at direct sunlight). My guess is that my pitch lap may be contaminated which is causing these pits? I don't however feel/hear that the surface is getting scratched up. I am genuinely perplexed myself. I have redone the lap twice to remedy this but to no avail. frown.gif I have gone back to fine grinding once but those were quite visible scratches. Are these scratches showstoppers?? bawling.gif bawling.gif

 

Where did you get the ronchi? How clear is the film?

Film is pretty substandard quality. I got the Ronchi screens printed on OHP slide.

 

 

I am from a small town/city in central India, you might have guessed that based on how sluggish I am in responding to you gentlemen laugh.gif. It has been very tough to procure materials here.

 

I am new to this arena and I am not sure how you feel about mirror making, but to me it seems like it's somewhat like being addicted to coke, once you have a taste of it you want more. Perhaps that's only true about newbies like me. Not quite sure if this one would see the light of the night sky or not.

 

I will get back to correcting z4, z5 as suggested by Pinbout.

 

KE (cutting from left) right before the parabolization attempt. I later noticed that I am slightly outside RoC in this image.

 

IMG 20211015 130311

 


Edited by Knight Sky, 15 October 2021 - 02:40 AM.


#22 chantepierre

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Posted 15 October 2021 - 01:42 AM

PM me, I'll send you a great ronchi screen.
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#23 dogbiscuit

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Posted 15 October 2021 - 02:29 AM

Not a show stopper.

Perfect is nice but you have to work and do the best you can with materials that are available to you. In almost all cases that will make a usable telescope that will make you happy.

I looked through some scopes with mirror not completely polished and other troubles and been surprised how well some of them worked.  You appear to be getting reasonable good shapes on the mirror and reason to believe you will be able to get a good parabola.  So if polish is not complete or there are many pits that is somewhat like having a dirty mirror except reflective coating in the pits scatters more light.  I have a 12.5" that should have been recoated 25 years ago.  it's really filthy but the parabola is very good and it still gives very good views.  If I couldn't clean it and the condition was permanent (since I haven't recoated it yet maybe it is permanent lol.gif ) I'd still be happy to have have that telescope.

 

It can be worth while to look at your grit under a microscope to compare grit gain sizes with labeling.

I once had some mis-labeled aluminum oxide grit, 30 micron labeled as 5 and 5 labeled as 30.

I thought that was the best 30 micron finish I ever had! After 15 micron it was obvious the surface was not as fine as before so I looked with a microscope and caught the mis-labeling. I transfer the grit to smaller containers convenient for working.  Thought it may have been my mistake, but checked the larger volume packaging from the well known reputable vendor and found that was mis-labeled.  Goes to show mistakes happen.  I was suspicious when I got such a fine surface with what I thought was 30 micron and was sure something was mislabeled when 15 micron made a less fine surface so caught the error quickly and it didn't really cause much trouble.  A newby probably would not have noticed, and it could cause much trouble as they returned to grinding and finished again with 30 grit mis-labeld as 5. That could turn into a long loop of trouble.

 

While you have the microscope out check the rouge or cerium oxide too.  Rouge I think you would know is rouge by it's color, but it's possible aluminum oxide could be mis-labeled as cerium oxide.

Or it could be the rouge or cerium is contaminated with grit.

 

I would have probably asked anyway, but the foggy look of you Ronchi image compelled me to ask if you polish was complete.  I thought slime on the grating more likely, or maybe a dirty camera lens. But incomplete polish might cause something like that.


Edited by dogbiscuit, 15 October 2021 - 02:36 AM.

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#24 Knight Sky

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Posted 15 October 2021 - 03:33 AM

Not a show stopper.

Perfect is nice but you have to work and do the best you can with materials that are available to you. In almost all cases that will make a usable telescope that will make you happy.

I looked through some scopes with mirror not completely polished and other troubles and been surprised how well some of them worked.  You appear to be getting reasonable good shapes on the mirror and reason to believe you will be able to get a good parabola.  So if polish is not complete or there are many pits that is somewhat like having a dirty mirror except reflective coating in the pits scatters more light.  I have a 12.5" that should have been recoated 25 years ago.  it's really filthy but the parabola is very good and it still gives very good views.  If I couldn't clean it and the condition was permanent (since I haven't recoated it yet maybe it is permanent lol.gif ) I'd still be happy to have have that telescope.

 

It can be worth while to look at your grit under a microscope to compare grit gain sizes with labeling.

I once had some mis-labeled aluminum oxide grit, 30 micron labeled as 5 and 5 labeled as 30.

I thought that was the best 30 micron finish I ever had! After 15 micron it was obvious the surface was not as fine as before so I looked with a microscope and caught the mis-labeling. I transfer the grit to smaller containers convenient for working.  Thought it may have been my mistake, but checked the larger volume packaging from the well known reputable vendor and found that was mis-labeled.  Goes to show mistakes happen.  I was suspicious when I got such a fine surface with what I thought was 30 micron and was sure something was mislabeled when 15 micron made a less fine surface so caught the error quickly and it didn't really cause much trouble.  A newby probably would not have noticed, and it could cause much trouble as they returned to grinding and finished again with 30 grit mis-labeld as 5. That could turn into a long loop of trouble.

 

While you have the microscope out check the rouge or cerium oxide too.  Rouge I think you would know is rouge by it's color, but it's possible aluminum oxide could be mis-labeled as cerium oxide.

Or it could be the rouge or cerium is contaminated with grit.

 

I would have probably asked anyway, but the foggy look of you Ronchi image compelled me to ask if you polish was complete.  I thought slime on the grating more likely, or maybe a dirty camera lens. But incomplete polish might cause something like that.

What I am noticing is that these are not constant pits. If that were the case that would have implied unfinished polishing. These are pits that appear at different parts at different times. So, most likely it's either contaminated pitch lap, CeO2 or something wrong in the process that I am using while polishing - I am using normal filtered water and wash the mirror using very light fingers. It could also be one of the felt cloth that I think could be the culprit. I will chuck that out for drying/wiping the mirror.

 

I did face a spurious grit issue early on. Some local vendor sold me a grit as #80 which was way coarser than that, I had to grind out quite a bit of glass because of that.

 

In any case, do you think I should proceed with the parabolization process?



#25 dogbiscuit

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Posted 15 October 2021 - 03:33 AM

I think it's possible what you are calling pits that you see only in sunlight could be sleeks.

There is a form of sleeks that occur when using rouge for polishing.

Many short (mostly about 1/4" to 3/8" or so) very fine scratches that show only when lit properly and occur when using rouge for polishing.

I had these sort of sleeks when making my first mirror.  Difficult to see, had to have a bright flashlight lighting at just the right angle (didn't try sunlight) and they became obvious, thousands and thousands of them.  Looking in the common atm books of the day I found a recommendation to work a wet until nearly dry.  Did that for a short work session  and they were gone.  After that I used less slurry, less rouge and less water.  After some experimentation I found a good quantity of slurry to apply and how dry I could work and get good polish without making sleeks. Finished that mirror and maybe 1 or 2 more then changed to cerium oxide.  lol.gif 


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