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First mirror, 8" F/3.5-ish

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#176 chantepierre

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Posted 01 October 2020 - 05:27 AM

Hello,

You see the TDE because of the abrupt turn of the bands (too much "inside" when inside, too much "outside" when outside) ?

I'm not against starting again from a good sphere. I've been too aggressive at the initial parabolization and dug the center too much.

Since I'm fighting the edge, continuing to work TOT would be the way forward ?

Also, regarding the lumps you were talking about, can you be more precise ?

I'm taking pictures with a lot of noise, and it's hard to evaluate smoothness.


Edited by chantepierre, 01 October 2020 - 05:29 AM.


#177 luxo II

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Posted 01 October 2020 - 05:39 AM

I know what I'm seeing in your pics, they're quite clear. You have indeed dug quite a crater in the centre, and there's something ugly at about 80% radius as well. Then the edge literally falls off a cliff in the outer couple of mm. Plus the dog-biscuit problem which is also evident.

 

You could ignore the TDE and instead mask off the outermost 2-3mm of the mirror, but given the central crater and the other issues IMHO you're better off taking it back to a good sphere, and starting over and this time learn to do it without making a crater, or the TDE, and without the lumpy dog-biscuit surface. In doing so I'd also re-do the pitch lap as I described above, it will take an hour or two, compared to the time wasted to fix a lumpy surface.

 

To avoid creating a TDE I always removed some of the pitch around the edge to make the tool about 2cm smaller in diameter than the mirror being polished, or shaped it like flower petals so there was less surface contact near the edge.

 

At a guess you worked MOT to produce the crater. Working TOT may correct it but what you have right now is so bad it may be quicker to find grind it out, back to spherical and repolish with a LOT MORE TESTING to see what's going on and avert a catastrophe before you go too far. An alternative way of looking at this is that you're making changes far too aggresively between testing, and to solve that you could slow down the figuring by using rouge instead of cerium oxide.

 

While I appreciate your enthusiasm, what you are attempting is not trivial and even very experienced mirror-makers think very carefully before attempting one under f/4. You aren't one of them, yet, and there is no easy shortcut.


Edited by luxo II, 01 October 2020 - 06:26 AM.

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#178 chantepierre

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Posted 01 October 2020 - 05:55 AM

Thank you for the elaborate answer. I’m okay with the process taking the time that it will need and did not want to sound like I put your remarks in question, but help to see what you were talking of.

Masking isn’t interesting. If starting over from a sphere is the way then let it be and I’ll work in this direction. Despite the enthusiasm I’m humble in front of the glass.

Lucas

Edited by chantepierre, 01 October 2020 - 07:59 AM.


#179 luxo II

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Posted 01 October 2020 - 05:58 AM

Agreed I found it humbling too. It took me a year to make my first really good mirror but I went on to make several more. While I had made a couple of fast ones  my favourite though was an 8" f/7 and several decades later I still think f/7 is nirvana as it results in a scope with a negligibly small secondary mirror, and can utilise eyepieces spanning the whole range from about 4mm all the way to big 2" 50mm eyepieces - roughly 12: 1 from lowest to highest power - without the need for coma correctors, Barlows and other ugly contraptions. And best of all, relatively easy to achieve a very good figure. 


Edited by luxo II, 01 October 2020 - 06:05 AM.

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#180 chantepierre

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Posted 01 October 2020 - 08:06 AM

Regarding pitch hardness, and given that now ambient temps are around 18deg C, would it be beneficial to let go the Gugolz #64 for a more tender pitch ?

I’m open to do things differently even if I’d prefer to change only one variable at a time and observe.

#181 dogbiscuit

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Posted 01 October 2020 - 11:40 AM

For now I prefer to follow dogbiscuit’s advice regarding laps (even if I’m too symmetrical) because it’s quite straightforward and can be evaluated... with a nonstandard lap I couldn’t know if the lap or my strokes were at fault.

Your lap looks pretty good, but just one thing I don't like is the center of the lap is in a channel.

I like for the center of the lap to be on a facet but not at the center of the facet.

chantepierreLapCenter.jpg

 

 

I like that the you don't have many small partial facets at the edge.

So to get the channel grid a little more off center so the center of the lap is on a facet.

LapGrid-A.png

LapGrid-B.png

 

 

Working by hand I prefer a truly full size lap, not even a mm or so smaller because I get better edges that way.

By this I mean a "full size" should have a diameter equal to that of the mirror.

I'm not referring to "sub diameter" laps that are significantly smaller than the mirror.  That's a different animal and at times might be useful for you f/3.5.

 

I haven't made a f/3.5 parabola by hand.  I don't know how well suited a full size lap would work for that, although if I were doing it I would start with one and see how far I got before changing things. 

 

I think you have been doing pretty well making progress with your mirror.  The figuring difficulties are typical for first mirror.

the fast f/3.5 will be more difficult and maybe the full size lap won't get you all the way to the parabola.  I believe you should be able to get to even 70% correction before you see significant trouble related to fast f ratio, rather than difficulties related to just learning to make a smooth prolate figure. 

 

Cool temperatures and working a fast mirror are two reasons for softer pitch.

 

 

 

 



#182 chantepierre

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Posted 02 October 2020 - 12:13 AM

Thank you dogbiscuit for your comment. I also think I shouldn’t yet blame the hardware and go a bit further with the current lap, but I took time to think a bit.

Here’s the plan according to recent input from both of you :
- Go back to a sphere working TOT with the current tool and sufficiently diluted compound.
- Receive #55 pitch and synthetic plaster (current lap was plaster, that made me take too much precaution while working)
- Make a better stand that allows me to put enough pressure without rocking
- Restart working with a full-size #55 lap (one variable change, pitch hardness)
- See what’s different
- When necessary, switch to undersize lap
- See what’s different
- ??
- Mirror is finished - I move on in my life, observe with it after the cloud season and soon after buy another blank
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#183 ccaissie

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Posted 03 October 2020 - 08:32 PM

Re: the lap, you could begin trimming the channels one side only and move the relative position of the squares a bit.

 

Maybe polishing back towards a sphere would be best....certainly short strokes a la fix TDE would be in order and on the way back toward a sphere, you might find a place that shows a good outer zones and edge and go from there.  

 

Get the edge zones good and work deepening inward with a star lap...my 2 centavos.

 

A lot of regular work will tend to squeeze out any stig that can creep in on these babies.



#184 Pinbout

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Posted 03 October 2020 - 09:51 PM

 

Go back to a sphere working TOT with the current tool and sufficiently diluted compound.

i wouldn't

 

fix the tde,,, again

 

tot centered on .7 zone tangential strokes not very long.

 

no need to go backward, you'll wind up where you are again.



#185 chantepierre

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Posted 04 October 2020 - 09:30 AM

Okay folks,

Thanks a lot for all the advice — more than getting to the result I want to understand what I’m doing.

Since I can’t realistically drill through the glass by figuring, I poured two new laps : a 3“ round one and a 2x4 rectangular one. That makes three laps to take care of.

I’ll try actions on the mirror following what I feel I should do and see what it produces until I’m satisfied with my comprehension. Nothing I’ll do would wreck the mirror definitely (except breaking it in halves but I won’t go there) and nothing would be irreversible.

The club has plenty of scopes for me to use meanwhile.

Let’s experiment for a while instead of bruteforcing
Cheers !
Lucas

Edited by chantepierre, 04 October 2020 - 09:35 AM.


#186 PrestonE

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Posted 04 October 2020 - 10:12 AM

No idea what you think a Rectangular lap is going to be useful for???

 

I would tend to think that it will cause you All Kinds of Problems...

 

Best Regards,

 

Preston


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#187 chantepierre

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Posted 04 October 2020 - 10:18 AM

Hello Preston,

That’s certainly what it will do - but I need to try things to understand the relationship between local actions, figure as shown by ronchi tests, ups & downs relative to the best conic, ups & downs relative to the nearest sphere, and to the « real » figure of revolution of the mirror.

Right now I still have trouble reading ronchigrams and deciding based on that - and that feels frustrating because I struggle with that from the start.

Maybe finishing the bath and seeing the « real » shape of the mirror would help me.

— what I understand about the mirror at the moment :

The edge is turned : that means it focuses further than the rest of the mirror. That means when you focus, the edge contributes blur to the picture because it produces « another » image that would be focused by pulling on the eyepiece.

The center is too deep : this is more ambiguous, because if I polish more the middle zones and a lot more the edge, this « too deep » won’t be anymore because everything will have moved to a shorter ROC parabola

Is that alright ?

Would measuring the ROC of the turned edge with a Foucault test (because that’s what « finding the null » means if I understood correctly) be beneficial to see how far it focuses ?

Edited by chantepierre, 04 October 2020 - 10:28 AM.


#188 chantepierre

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Posted 04 October 2020 - 01:59 PM

Well, I did three five minutes sessions with the 3 inch round polisher, tangent strokes running TOT around the mirror.

So far it seems to help the edge : the really abrupt edge hook seen on precedent ronchis starts to « melt » and curve.

I vary amplitude and position at the edge to avoid boring a donut.

I will continue working with this small tool for a while and see if I can progressively move towards center : the ability to have a local action seems to help. It feels like painting with small touches : the difficulty, as in painting, might lie in producing a smooth gradient.

Compared to the big lap, the pitch hardness seems to be less of a problem. The smaller surface area with equal pressure helps conform faster. I did the nail test shown in Texereau, and as suspected, I boiled my pitch way too much : it’s hard as rock. Maybe this is a cause of trouble with the big lap.

Will continue to work and note observations for the next few days.

Good evening
Lucas

Edited by chantepierre, 04 October 2020 - 02:00 PM.


#189 PrestonE

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Posted 04 October 2020 - 02:03 PM

Hola Lucas,

 

I cannot remember if anyone has recommended Polism simulation software for understanding

just what the lap is doing to the surface of the mirror when figuring...

 

http://martin-cibuls...lator/index.htm

 

That way one has a fighting chance and is Not Experimenting Too Much...

 

I find Ronchi to be Ok for general guidance, but really like Foucault testing to provide

an easier understanding of exactly where one needs the work to occur...

 

Best Regards,

 

Preston


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#190 chantepierre

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Posted 04 October 2020 - 02:21 PM

Thank you Preston,

I did not try Polsim but will look into it and see if I can plan next strokes with it and « feel » the delta between its predictions and my results.

Regarding Ronchis, I am more than waiting to be able to use Foucault tests with Foucault Unmasked... but for that the edge needs to be fixed. I don’t trust my optimism enough to take couder mask manual measurements.

Have a good evening,
Lucas
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#191 chantepierre

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Posted 08 October 2020 - 11:57 AM

Hello !

The experiments are done. The small polisher was a powerful but dangerous tool. It introduced roughness. I smoothed things away with the big lap and concentrated on eliminating the central hole.

Sessions were short (2-3 turns of planned strokes, then ronchi) and although I lost a lot of correction in the process, surface is smoother and the central hole is almost gone.

 

With regard to the edge, I don’t understand : it seems that everything I try leaves it untouched. Hope the smoother pitch comes fast. I’ll try Danny’s TDE strokes too again. Now the path forward is adding correction slowly.

 

First time I tried to parabolize I dug myself a hole thinking it needed a LOT of correction and dug that central hole. Now I’ll work shorter sessions and should avoid this. Last sessions felt great because the outcome was quite as planned towards removing that hole. It’s still there but I’m going for it.

 

D3D20DD9-C282-4830-AAAA-B5E103FE4254.png
 

Good evening,

Lucas


Edited by chantepierre, 08 October 2020 - 11:57 AM.


#192 Pinbout

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Posted 08 October 2020 - 12:43 PM

Hello !

The experiments are done. The small polisher was a powerful but dangerous tool. It introduced roughness. I smoothed things away with the big lap and concentrated on eliminating the central hole.

Sessions were short (2-3 turns of planned strokes, then ronchi) and although I lost a lot of correction in the process, surface is smoother and the central hole is almost gone.

 

With regard to the edge, I don’t understand : it seems that everything I try leaves it untouched. Hope the smoother pitch comes fast. I’ll try Danny’s TDE strokes too again. Now the path forward is adding correction slowly.

 

First time I tried to parabolize I dug myself a hole thinking it needed a LOT of correction and dug that central hole. Now I’ll work shorter sessions and should avoid this. Last sessions felt great because the outcome was quite as planned towards removing that hole. It’s still there but I’m going for it.

 

attachicon.gifD3D20DD9-C282-4830-AAAA-B5E103FE4254.png
 

Good evening,

Lucas

with MoT, I use the edge of the tool to dig where I need more removal...scrubbing around the zones... kind of a zig zag... but I call it scrubbing.



#193 chantepierre

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Posted 08 October 2020 - 12:47 PM

Hey Danny,
How do you do this MOT ? Do you « lift » the mirror a bit to remove a bit of its weight ?

#194 Pinbout

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Posted 08 October 2020 - 01:11 PM

just like how I'm taking down this hill

 

https://www.youtube....r5c3NL9yR4&t=5s

 

same way but over the zones needed.

 

I need to fire the camera person on this vid... ha ha ha


Edited by Pinbout, 08 October 2020 - 01:13 PM.

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#195 chantepierre

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Posted 08 October 2020 - 01:49 PM

Thanks ! I should watch your whole channel one day it would have helped me.

My test setup is now permanently set up, that helps a great deal to have a short feedback loop. It feels like when I discovered hot reload in programming \o/ .
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#196 chantepierre

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Posted 15 October 2020 - 01:01 AM

Hello,

I think my pitch is dead - still waiting for the new one.

It was getting slower and slower to adapt, and now a whole night of pressing with weight on did not change the channels or anything. The edge was misplaced and did not even make a mark.

What would help soften it back ? Linseed oil or turpentine ? Is it worth it, as in, will it be as qualitative as before, or do I just wait for new pitch ?

Have a nice day !

#197 Pinbout

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Posted 15 October 2020 - 07:57 AM

 

What would help soften it back ?

i lightly brush olive oil on my pitch and wait for it to sink in over night.... lightly.


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#198 chantepierre

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Posted 15 October 2020 - 02:27 PM

i lightly brush olive oil on my pitch and wait for it to sink in over night.... lightly.


We mediterraneans know how to use olive oil lightly >:-) !

I poured a new synthetic plaster lap and collected all my pitch. Put it in a « bain-marie » to soften it without evaporating solvents too much and incorporated slowly 2 tablespoons of linseed oil.

We’ll see if that produces something, if not, the new pitch is still coming and I’ll have learnt something.

#199 Pinbout

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Posted 15 October 2020 - 04:18 PM

That may be too much. It only takes a little 


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#200 chantepierre

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Posted 16 October 2020 - 08:16 AM

I'll see if it was too much - but before summer I really over-boiled the pitch, it lost a lot of solvents and I got a big headache.

 

This morning it was easy to work with, nice and shiny, malleable under very hot water. I let the lap pressed TOT for the day.

If people read this thread before deciding on making a mirror, don't do the same mistakes : use synthetic plaster.

It made a world of difference to have a waterproof substrate to get the lap into contact with the mirror.




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