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help with ronchi 8in f2.6 pyrex mirror

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#1 macleod

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Posted 16 July 2019 - 01:01 AM

Making an 8in f2.6 Pyrex  mirror , and at the pitch/lap polish stage, and looking at  Ronchi I had a "perfect" sphere showing after 2hours polishing. However I needed to improve polish, so have done another 2hrs TOT and MOT. Have warm pressed lap ( excellent contact ) and been doing 1/3 COC strokes as per sphere. To my horror Ronchi is telling me I,m heading for some other figure of revolution ! I have done nothing different from my first effort so am bewildered. I have done a 10in glass at f3.4 with good results, but this is weird .

My Ronchi photos are "burned out in center" ,too hi an iso, also very bright LED. In fact the bands are straight, even edged, and visually vastly better - except for turned edge!! At the moment best I can do. Outside ROC:-

 

 

 

 

 

post-233319-0-75967900-1563256640_thumb.

 

Inside ROC:-

 

post-233319-0-54498000-1563256778_thumb.

 

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#2 macleod

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Posted 16 July 2019 - 01:27 AM

Oh I forgot to say my polishing is hand pushing glass on a 6rpm turntable. My ain question is why I ended up with this figure, after having a near perfect sphere yesterday.The only difference was my warm pressing - my stroke length COC 1/3 is the same. I do have an excellent polish now, so maybe Parabolise now ? Thank you, Macleod.



#3 Mark Harry

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Posted 16 July 2019 - 03:47 AM

Oblate?



#4 macleod

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Posted 16 July 2019 - 03:57 AM

If its oblate, how do I correct to a sphere please Mark ? Macleod



#5 Stargazer

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Posted 16 July 2019 - 04:48 AM

 

My Ronchi photos are "burned out in center" ,too hi an iso, also very bright LED. 

 

Parchment paper in between light source and screen will fix that.



#6 Pinbout

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Posted 16 July 2019 - 06:14 AM

Your on the wrong side of the conic 0 and your edge is lagging the center...so even further away from your desired conic.

 

Mot narrow w’s longer than what you’ve been doing since your edge is laggy, once it catches up short length a little.


Edited by Pinbout, 16 July 2019 - 06:17 AM.


#7 MKV

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Posted 16 July 2019 - 06:26 AM

Parchment paper in between light source and screen will fix that.

A piece of milk jug or acetone, alcohol, etc. similar bottle makes an excellent diffuser. Also, Scotch-type "invisible" tape, one or two layers, very easy to mount too.


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#8 Lognic04

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Posted 16 July 2019 - 07:09 AM

Are you finished polishing now? Imo it may be fine to parabolize. You'll polish all that out in the super deep figure!
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#9 macleod

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Posted 16 July 2019 - 04:27 PM

Hi Logan, I did have a "perfect" ronchi sphere, but not polished out.Did 2 more hours 1 TOT and 1 MOT. I think it was the MOT that ruined it ! My aim was to do more TOT as I new the edge would lag, but with the heavy MOT I overdid the work. So I,ll get back to a sphere, or a more respectable figure before moving on. How,s your big mirror going ? Best wishes, Macleod.

PS:- the pyrex blank I,ve got is 8in x 2in thick - quite a slab.



#10 Pinbout

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Posted 16 July 2019 - 04:44 PM

if you want to fix just the tue

 

tot center the lap right where the ronchi turns and do short chordal strokes, supporting the over hang of the tool so it doesn't add to the weight on the outside...

 

if Mot did that, you couldn't have had good contact.


Edited by Pinbout, 16 July 2019 - 04:46 PM.


#11 dave brock

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Posted 16 July 2019 - 05:00 PM

Your on the wrong side of the conic 0 and your edge is lagging the center...so even further away from your desired conic.

 

Mot narrow w’s longer than what you’ve been doing since your edge is laggy, once it catches up short length a little.

 

 

if you want to fix just the tue

 

tot center the lap right where the ronchi turns and do short chordal strokes, supporting the over hang of the tool so it doesn't add to the weight on the outside...

 

if Mot did that, you couldn't have had good contact.

Top photo says outside Roc, bottom photo says inside.

The edge is not lagging or turned up. 



#12 Pinbout

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Posted 16 July 2019 - 07:00 PM

Top photo says outside Roc, bottom photo says inside.

The edge is not lagging or turned up. 

thanks for the clarifying

 

and he needs to label it better

 

its amazing how people need help and can't label ish so its well understood

 

#Dobetter for free lessons

 

so your turned down your edge...

 

Using the edge of the tool

 

Starting from just inside the turn

 

slight thumb pressure on outside edge of tool

and scrub from the hook out to the edge and back

 

walk around right, turn tool left

 

2x's around and test

 

if its going in the right direction keep going.


Edited by Pinbout, 16 July 2019 - 07:02 PM.


#13 macleod

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Posted 16 July 2019 - 10:20 PM

Danny, could it be the "the Queens English V USA English ? Heck I even use punctuation ! It comes from my 1940 education ! Anyway thank you so much for help, I,ve almost got edge correct, just a little more But not sure I,ve helped the " Conic" ! Best wishes, Macleod



#14 Pinbout

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Posted 16 July 2019 - 10:48 PM

It’s labels next to the pic the large pics 

 

what’s up the small pics ?  Smh 

 

its not just you, other people post more horrible pics then ask, what do you think and what should I do.... really smh 


Edited by Pinbout, 16 July 2019 - 11:06 PM.


#15 Pinbout

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Posted 16 July 2019 - 11:11 PM

When reading  ronchis always look at outside RoC to see what you have

 

the center bulges out -  hill

the center bows in - hole

 

the edge flares out - tde (possibly) 

edge flares in - tue 

 

the outside looks how it is.


Edited by Pinbout, 16 July 2019 - 11:12 PM.


#16 macleod

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Posted 17 July 2019 - 02:16 AM

Danny, thank you for your great help with Ronchi. I,m still getting to grips with it , read thru Mel Bartels  Ronchi pages a few times. Unfortunately my abstract geometry visualisation is not too good ! Maybe age ? Thank you anyway for your help, your very patient. Macleod



#17 totvos

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Posted 17 July 2019 - 07:21 AM

Danny, thank you for your great help with Ronchi. I,m still getting to grips with it , read thru Mel Bartels  Ronchi pages a few times. Unfortunately my abstract geometry visualisation is not too good ! Maybe age ? Thank you anyway for your help, your very patient. Macleod

I would suggest using an interactive tool, like Mel's online tool or RonWin to play with theoretical results and get a feel for the impact of under- and over-correction, and what your targets are.



#18 Pinbout

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Posted 17 July 2019 - 04:31 PM

are you going to parabolize this sphere?



#19 macleod

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Posted 17 July 2019 - 05:56 PM

Hi Danny , I did as you advised and here is where I,m at. To me it ooks good and ready to parabolize ? OUTSIDE ROC:-               post-233319-0-63568200-1563404077_thumb.       

 

 

 

 

 

                                                                                                                                                                                                                                          

                                                                                                       post-233319-0-63568200-1563404077_thumb.

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#20 macleod

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Posted 17 July 2019 - 06:30 PM

On second thoughts still TDE !! Macleod



#21 Lognic04

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Posted 17 July 2019 - 06:48 PM

Apart from your TDE i recommend you use subdiameter tooling to deepen, much better than full size parabolizing! :)



#22 Pinbout

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Posted 17 July 2019 - 07:02 PM

Apart from your TDE i recommend you use subdiameter tooling to deepen, much better than full size parabolizing! smile.gif

is that why you have stig on your 16... uhm.



#23 Pinbout

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Posted 17 July 2019 - 07:03 PM

On second thoughts still TDE !! Macleod

use a ke to judge the edge

 

you can see the bend out and back in the the whisp around the edge...

 

so definitely use ke to look at the edge b4 you do anything.



#24 Pinbout

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Posted 17 July 2019 - 07:10 PM

but, tot, coc, short fast strokes after you properly warm pressed and let cool.

 

fast strokes to keep the pitch from conforming to the tde.

 

what I see could take a couple of hours and be a workout....

 

so be prepared.

 

https://www.youtube....A&index=65&t=0s



#25 Pinbout

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Posted 17 July 2019 - 07:13 PM

along with fixing the tde, you have to pay attention to the channels... keeping the channels the same-same across the face.

 

where the channels close 1st, that's where the glass is getting more work, so keep the channels same-same while working  a mirror.




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