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First mirror, 8" F/3.5-ish

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#51 Pinbout

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Posted 03 May 2020 - 09:23 AM

i would do MoT and scrub it against the edge of the tool, from inside the kink to the edge

 

you edge does look turned but get rid of the kink first

 

you do have a lot more correction in the outer zones...

 

just take pics of 3 and 4 lines outside...


Edited by Pinbout, 03 May 2020 - 09:23 AM.

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#52 chantepierre

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Posted 03 May 2020 - 09:31 AM

Hello Pinbout,

In the meantime I decided to try and made it worse, 10 minutes of 5/6 zigzags MOT.

 

Here's the current state :

 

xjdAeoE.jpg

By scrubbing, do you mean doing small Ws turned 90° clockwise ?

I'll make less pics to concentrate on 3/4 lines after those ones.

 

Edit : for now it's pressing, I'll be able to do two more short sessions tonight depending on where things should go now.

 

Thank you,

chantepierre


Edited by chantepierre, 03 May 2020 - 09:42 AM.


#53 Pinbout

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Posted 03 May 2020 - 10:20 AM

 

By scrubbing, do you mean doing small Ws turned 90° clockwise ?

draw a new sharpie circle

 

EWYGG7TWoAQjbfb.jpg

 

oops wrong pic ha ha ha

 

8in mirror f35.JPG


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#54 chantepierre

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Posted 03 May 2020 - 11:27 AM

Two turns, with the sharpie line, one thumb of each side of the new sharpie circle :

 

ph3Iw4c.jpg

 

It seems it does something right but does it fast : shall I continue doing this by offsetting the new circle after each test and test often ?



#55 chantepierre

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Posted 03 May 2020 - 11:53 AM

Two more turns : it seems that every time I'm doing this scrubbing, I create a new circle on the image that's slightly different than the previous. Would that be too much pressure or not enough contact ?

 

After two turns, overall contact becomes bad. Currently pressing with a water jug for 30 minutes, will leave MOT without weight overnight.

 

1GShGUQ.jpg

 

Subsidiary question : I have in addition to cerium oxide 200g of Opaline which seems to be super-fine cerium oxide according to the manufacturer : http://www.abralis.c...-polissage.html

The french vendor I bought the kit from, Mirro-Sphère http://www.mirro-sphere.com/ told me to use it when I'm roughly 1/3ds of the way through parabolization but to keep cerium too. Should I mix some to my cerium oxide now or is it too early ?

 

For now I followed your advice on the picture and added hand sanitizer to the cerium oxide solution.


Edited by chantepierre, 03 May 2020 - 12:06 PM.


#56 Pinbout

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Posted 03 May 2020 - 12:10 PM

If you don’t feel confident using the edge of the tool, Then do the middle stroke MoT 

 

draw a 2” circle on the back of your mirror, that’s your don’t go over the the edge of the tool line.

 

456DC983-FF2B-4FDF-9386-5114C89C3F4C.jpeg


Edited by Pinbout, 03 May 2020 - 12:13 PM.


#57 chantepierre

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Posted 03 May 2020 - 02:27 PM

Thank you, I’ll go this way from now on. I did five minutes and am unable to see a change, so I plan 15 consecutive minutes of this stroke tomorrow and will post pictures.

If at some point it would be interesting to go back to a sphere and start over, I’m not against it either.

#58 chantepierre

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Posted 03 May 2020 - 03:27 PM

Well I found some courage tonight. Here are the pictures of the mentioned 5 minute session :

 

QOiRjej.jpg

 

And here is the session that followed : 5 minutes, press for 3 minutes with weight, 5 minutes, press for 3 minutes with weight, 4 minutes.

 

AAF7P51.jpg

 

I have to admit that I'm a bit lost in linking the manual work with the observation of the pictures. Maybe this was a bit too much for a single day.

 

Anyway, thanks a lot for all that guidance. I'll come back to it tomorrow with fresh eyes and hands :-)



#59 chantepierre

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Posted 04 May 2020 - 02:58 PM

Hello everyone,

Here are pics of outside ROC after a 12 minutes session MOT with the second stroke of the picture Pinbout posted yesterday.

Contact was excellent after leaving the mirror for 22 hours on top. It seems to have had a lot of effect whereas yesterday it was minimal.

 

Jr6jl2l.jpg

 

From what I'm able to deduce, I should continue with those strokes for a while before doing something else, maybe 3x10 minutes, but what do I know... It also seems to introduce something weird at the edge.

Good evening,

chantepierre


Edited by chantepierre, 04 May 2020 - 03:00 PM.


#60 Pinbout

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Posted 04 May 2020 - 04:55 PM

Do you get a diffraction ring with ke



#61 chantepierre

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Posted 05 May 2020 - 01:34 AM

Hey !

I took the best pic I could with current Foucault setup, it’s still not good, but there does not seem to be a ring around the mirror.

Maybe it’s the centering and tilt issues that prevent it from appearing ?

U8nj6GZ.jpg

 

In a way it looks coherent with the Ronchis to me... two clearly separate zones 



#62 dogbiscuit

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Posted 05 May 2020 - 03:39 AM

I've seen a few lap pictures lately showing laps with an "extra channel" as is highlighted by the red line in the image below.

Is there some website that is showing this sort of thing?

 

chantepierreLap1.jpg

 



#63 dogbiscuit

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Posted 05 May 2020 - 03:54 AM

Notice the very small row of channels between red arrows at lower left of the lap image in the previous post.

 

If the "extra channel" had not been cut the lap could have looked like the edited image below.

All pitch facets would be nearly the same size.

The very small partial facets at the edge between red arrows at bottom left of the image in the previous post could be larger and more useful as seen between red arrows in the image below.

 

chantepierreLap1a.jpg


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#64 chantepierre

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Posted 05 May 2020 - 04:27 AM

Hello dogbiscuit, thank you for your answer. It came from my haste of pressing channels while the pitch was still hot, I’ve made them too fast. I can remelt and repour this lap tonight if that would help !

#65 chantepierre

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Posted 05 May 2020 - 04:43 AM

Also, if that would be easier to maintain, I can press a smaller number of squares. I’ve seen many laps with only 2 or 3 divisions for 8 inch mirrors online.
Sorry for the double post, I can not edit on mobile.

#66 Pinbout

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Posted 05 May 2020 - 05:37 AM

Hey !

I took the best pic I could with current Foucault setup, it’s still not good, but there does not seem to be a ring around the mirror.

Maybe it’s the centering and tilt issues that prevent it from appearing ?

U8nj6GZ.jpg

 

In a way it looks coherent with the Ronchis to me... two clearly separate zones 

So if there is a ring, the next question is, is the brightness balanced on both sides. Can’t tell from the pic. 



#67 Pinbout

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Posted 05 May 2020 - 05:37 AM

Hello dogbiscuit, thank you for your answer. It came from my haste of pressing channels while the pitch was still hot, I’ve made them too fast. I can remelt and repour this lap tonight if that would help !

Your lap looks great, aside from the extra channel



#68 dogbiscuit

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Posted 05 May 2020 - 05:39 AM

Hello dogbiscuit, thank you for your answer. It came from my haste of pressing channels while the pitch was still hot, I’ve made them too fast. I can remelt and repour this lap tonight if that would help !

 

Don't do anything yet.  Mostly I was just asking Because I had seen 3 laps with much the same "extra channel"  I was thinking it was an unlikely coincidence and maybe it was something shown or recommended somewhere on the web.

I don't know if that extra channel and very small partial facets at the edge are causing you any trouble.  I'm somewhat obsessive about pitch laps.  Of course I think anything less than a perfect lap might cause trouble. 

 

See what Pinbout thinks. 

 

If you were to decide to do something about it, or if you ever have a need to repair a damaged lap such as a severely chipped area and need to fill in missing pitch...

What I would do is  scrape cerium oxide off the walls and floor of the two channels marked with red line and arrows. Then melt a small amount of pitch (from same batch) and fill those channels to very slightly above the top of the facets. With a razor blade, shave the top of the filler pitch to level with facets.

Cut another channel in the right place. Trim to reopen the crossing channels where melted pitch flowed into them as the unwanted channel was filled.

Press the lap.

 

I think the number of squares you have (except the extras) is good for a f/3.5.  Larger squares might not flow as well as you figure close to the parabola.  Overall I think you have done a good job making the lap.


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#69 Pinbout

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Posted 05 May 2020 - 06:27 AM

this is what i see in your mirror

 

on the left, the overall curve needs to flow like what i sketched on the left

 

at 1 oclock i circled a sharp change in RoC, it needs to flow smoother.

 

at 2 oclock all the way out before the edge you need to scrub, MoT, on the edge of the tool to add correction on the small ring

 

another overall corrective stroke is a figure 8, it could help blend some of that ish.

 

at f3.5, with mot, you'd do a figure 8 motion, not going over your 2" dont cross circle, you could draw a 3" circle to be safe.

 

Jr6jl2l.jpg


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#70 chantepierre

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Posted 05 May 2020 - 06:56 AM

Thank you both for your answers.

I'll try figure 8s on a 2" circle / do not cross 3" tonight.

 

It seemed pretty effective when I used them to smooth a sphere, I guess I should work 5 minutes per session max.



#71 Pinbout

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Posted 05 May 2020 - 07:07 AM

Thank you both for your answers.

I'll try figure 8s on a 2" circle / do not cross 3" tonight.

 

It seemed pretty effective when I used them to smooth a sphere, I guess I should work 5 minutes per session max.

also to add more correction in the middle, to help with the sharp turn in RoC, you could do some of the 3rd pic previously posted from Sam Brown's book.

 

in the end, to after blending everything you have right now, you may end up overcorrected, but that's no problem... its just an additional step to lower it.


Edited by Pinbout, 05 May 2020 - 07:10 AM.


#72 chantepierre

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Posted 05 May 2020 - 12:23 PM

Thanks for the advice. I get only a few minutes of work per press, maybe the gugolz 64 does not handle very well the now constant 25deg C ambient temperature.
I’m able to arrange midnight work sessions if that could help, I’m a bit clueless on how sensible this is.

#73 chantepierre

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Posted 06 May 2020 - 01:49 AM

Hello !
Despite the short working times, I did a few seasons of edge parabolizing (3 turns), center parabolizing (2 turns), figure 8s (1 turn), test and repeat.
I see blending coming in and with those very short sessions there’s not much harm to be done. It also finally “clicked” why which stroke does what, it has to do with the part of the mirror passing on the edge of the tool.

I hope to come back when Ronchis are smooth and near target ! Hopefully at this time testing conditions will be good enough to take real measurements.

Have a good day

#74 Pinbout

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Posted 06 May 2020 - 09:50 AM

 

It also finally “clicked” why which stroke does what, it has to do with the part of the mirror passing on the edge of the tool.

lol.gif waytogo.gif



#75 chantepierre

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Posted 10 May 2020 - 10:50 AM

Hello,

 

Weather is more cold now and I can work again. It seems that this week's sessions did not have any effect. I rechannelled the lap today, scratched it with a xacto knife, and did a new bottle of polishing compound and things finally started to move. I manage to add curvature to the central bands that stopped being worked in the past week.

 

My plan is now to get to the point central bands are curved enough to meet the edge and produce nice curves, while being attentive to not kink the edge region, then measure exactly where am I with a couder mask. I tried to do this today, but being able to control tilt of the mirror is crucial and my current stand doesn't have it. I'll fix that.

 

Still, my new fixed tester with three-axis movement, way heavier than the precedent, is a huge help. The polish / rinse / test / polish cycles are quicker.

 

Work started to be at full speed again, but I'll take the necessary time.

Have a nice week,

chantepierre


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