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8 inch, F/6 - Correction and Figuring

ATM mirror making
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#1 Astrokhan

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Posted 14 September 2020 - 02:21 PM

So I have completed polishing my current project of 8 inch, F/6 and will be starting correction and figuring soon. I had great help on my preview project of 6 inch, F/8 by dogbiscuite, pinbout and many other experts on CN (link here) and I am expecting the same great help again!

 

I did the Ronchi testing today and found it is oblate (looks like I always polish out oblate and wonder why!). Am I good to go ahead with parabolizing?

 

Inside CoC

IMG_4235_cr.png

 

Outside CoC

IMG_4237_cr.png


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#2 chantepierre

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Posted 14 September 2020 - 02:27 PM

Impatient to follow your thread, your previous mirror was very instructive. Cheers.
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#3 dogbiscuit

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Posted 14 September 2020 - 03:13 PM

Looks pretty good.

Short strokes near center makes oblate.  Not unusual.

Parabolizing methods are the same from oblate or sphere.

If polish is complete you have a very good figure for beginning parabolizing.


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#4 dogbiscuit

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Posted 14 September 2020 - 04:13 PM

Same strokes that got you a good parabola on the 6" will work pretty well for the 8, but you have farther to go to get to the parabola.  Maybe 3 or 4 times as far.  It will take longer to get to the parabola.

 

Or you could get there more quickly with using more of the longer stroke and/or wider Ws than were used doing the 6" f/8.

 

I'd recommend starting parabolizing just like with the 6" and see how far toward the parabola you get in the same time you worked first sessions of the 6".  Of course you will reach a sphere first and if it's a good sphere and edge continue parabolizing.

 

The 8.25" will be a little more difficult because the Foucault shadows will be deeper quite as easy to see how smooth the figure is from center to edge all at once.  Not much more difficult though.

 

It will be fun watching your progress.


Edited by dogbiscuit, 14 September 2020 - 04:28 PM.

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#5 Astrokhan

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Posted 14 September 2020 - 10:18 PM

Thank you smile.gif

Impatient to follow your thread, your previous mirror was very instructive. Cheers.

 


Edited by Astrokhan, 14 September 2020 - 10:19 PM.


#6 Astrokhan

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Posted 14 September 2020 - 10:22 PM

Thanks dogbiscuit :) I will start parabolizing as suggested and post the progress soon.

 

Same strokes that got you a good parabola on the 6" will work pretty well for the 8, but you have farther to go to get to the parabola.  Maybe 3 or 4 times as far.  It will take longer to get to the parabola.

 

Or you could get there more quickly with using more of the longer stroke and/or wider Ws than were used doing the 6" f/8.

 

I'd recommend starting parabolizing just like with the 6" and see how far toward the parabola you get in the same time you worked first sessions of the 6".  Of course you will reach a sphere first and if it's a good sphere and edge continue parabolizing.

 

The 8.25" will be a little more difficult because the Foucault shadows will be deeper quite as easy to see how smooth the figure is from center to edge all at once.  Not much more difficult though.

 

It will be fun watching your progress.



#7 Jeff B

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Posted 14 September 2020 - 11:48 PM

So I have completed polishing my current project of 8 inch, F/6 and will be starting correction and figuring soon. I had great help on my preview project of 6 inch, F/8 by dogbiscuite, pinbout and many other experts on CN (link here) and I am expecting the same great help again!

 

I did the Ronchi testing today and found it is oblate (looks like I always polish out oblate and wonder why!). Am I good to go ahead with parabolizing?

 

Inside CoC

attachicon.gifIMG_4235_cr.png

 

Outside CoC

attachicon.gifIMG_4237_cr.png

A very nice edge.

 

I really enjoyed your first thread and I'm looking forward to this one too.

 

Jeff


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#8 Astrokhan

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Posted 15 September 2020 - 12:49 AM

A very nice edge.

 

I really enjoyed your first thread and I'm looking forward to this one too.

 

Jeff

Thank you Jeff :)



#9 dogbiscuit

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Posted 15 September 2020 - 08:07 AM

Foucault will show the edge and surface smoothness better.



#10 Jeff B

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Posted 15 September 2020 - 08:26 AM

Foucault will show the edge and surface smoothness better.

Indeed. 

 

Curious, and want to learn, just how far off is he right now from a parabola?

 

Here are 80 LPI Ronchis of an 8" F7.5 that's "fully corrected".

 

As my images are the "flip" of his, Astrokahn then has to polish down to a smooth sphere, then, "through to the other side" for a parabola.  Is that the process?

 

Jeff

Attached Thumbnails

  • Fagin, F7.5 Inside ROC.jpg
  • Fagin, F7.5 outside ROC.jpg

Edited by Jeff B, 15 September 2020 - 09:42 AM.

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#11 dogbiscuit

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Posted 15 September 2020 - 11:25 AM

Curious, and want to learn, just how far off is he right now from a parabola?

 

I haven't had time to think about that yet.  lol.gif  I'll work on that in a little bit.


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#12 dogbiscuit

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Posted 15 September 2020 - 12:13 PM

Astrokhan,

When you did your 6" I had suggested a couple of relatively long strokes and then a large W parabolizing stroke.

post 84 here in thread for the 6"

https://www.cloudyni...-testing/page-3

 

The reason I chose those strokes to parabolize was that the basic parabolizing stroke in texereau's book (and most other sources) usually corrects the central area from from center out to about the 60% zone but adds very little correction from there to the edge.  The books show two other parabolizing strokes, one to add more correction to the outer zone with more strokes in the center of the W, and another that has more strokes in the sides of the W for adding more correction to the mirror's center.

The three strokes I suggested accomplished something similar to doing the parabolizing stroke to add more in the outer zones.  I think it might be simpler to do the two  relatively long narrow W strokes that both have same spacing of strokes to get some correction in the outer zones plus the standard parabolizing W to correct the center areas.

 

If you watch the figuring threads, you will see that most people will have no trouble parabolizing from center to about the 60 or 70 % zone, then have difficulty getting correction into the outer zones.  They work a while trying to get correction in the outer zones and end up overcorrecting the central zones. 

 

So a little bit of the long narrow Ws can help you keep the correction in the outer zones near the same as the central zones.  Try to keep correction about the same from center to edge.

Don't do too much of either the long narrow strokes or the standard parabolizing stroke.

Astrokhan3Parab.png

You can do a little of the long narrow strokes to get some correction started in the outer zones.

Then the large parabolizing stroke can be used until the central correction starts getting to be more than the outer zones.

Then do a little more of the narrow strokes to even

And remember that last almost sideways stroke at the sides of the W is that way for a reason.

 

 

 


Edited by dogbiscuit, 15 September 2020 - 12:15 PM.

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#13 dogbiscuit

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Posted 15 September 2020 - 05:30 PM

Indeed. 

 

Curious, and want to learn, just how far off is he right now from a parabola?

 

Here are 80 LPI Ronchis of an 8" F7.5 that's "fully corrected".

 

As my images are the "flip" of his, Astrokahn then has to polish down to a smooth sphere, then, "through to the other side" for a parabola.  Is that the process?

 

Jeff

I think Astrokhan is using a 100 lpi grating.

Comparing Ronchi images with simulation in Ronchi for Windows it looks like the conic is about + 0.3

So approximately 30% farther from the parabola than a sphere.

 

I ran +.3 cc through Foucault analysis program and that shows pv wavefront error:

a little better than .25 wave oblate from sphere

about .9 wave oblate from parbola

From sphere to parabola is about .7 wave.

 

Yes, through the sphere to the parabola.


Edited by dogbiscuit, 15 September 2020 - 05:35 PM.


#14 Jeff B

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Posted 15 September 2020 - 06:34 PM

Thanks.  So if you look at it though another mirror so that it's reversed, does it become a parabola?  lol.gif



#15 davidc135

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Posted 15 September 2020 - 11:04 PM

If it's not being too picky I get .43 waves wf for a 8'' F/7.5 sphere. ( 22.55x8/(7.5^3) ).  David



#16 dogbiscuit

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Posted 15 September 2020 - 11:32 PM

Jeff B's is f/7.5

 

Astrokhan's is f/6, and I assumed he would have a bevel as wide as for his 6". That brought the diameter down to 7.92" and staying with 96" ROC came to f/6.16.

Then I rounded a little bit.  lol.gif


Edited by dogbiscuit, 16 September 2020 - 05:42 AM.


#17 davidc135

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Posted 16 September 2020 - 06:46 AM

Yes, my bad.  David



#18 dogbiscuit

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Posted 16 September 2020 - 07:10 AM

Glad you posted the formula.  It's a bit quicker than what I did. smile.gif



#19 Astrokhan

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Posted 16 September 2020 - 01:05 PM

And remember that last almost sideways stroke at the sides of the W is that way for a reason.

Yes dogbiscuit, I remember that :)



#20 Astrokhan

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Posted 16 September 2020 - 01:30 PM

Couldn't start the correction yet. Will be doing something tonight. Currently cold pressing

 

IMG20200916235359-01.jpeg



#21 Astrokhan

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Posted 16 September 2020 - 04:51 PM

So here are the results after 1 set of 3 strokes suggested by dogbiscuit in #12. Lines are moving to straight. Slight hill remaining in the center which should go with further 1 or 2 sets. Need to change the grating as printed lines have been deteriorated over time and not showing clear edges. Took first KE image as well. please see following images.

 

Inside RoC

IMG_4260cr.png

 

Outside RoC

IMG_4261cr.png

 

 



#22 Astrokhan

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Posted 16 September 2020 - 04:53 PM

First KE image showing central hill

 

 

IMG_4268cr.png

 


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#23 dogbiscuit

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Posted 16 September 2020 - 06:44 PM

Ok... There is a slight turn up in the outer zone and a slight hill. 

Would like to see the diffraction ring all around. That should get better if you get the high outer zone down closer to a sphere

 

You could work on that first with the long narrow stroke. Only one turn of this, too much can roll the outer zone down.

OneFourthWxThreeFourths.png

 

More of the post 12 strokes could work to add correction, but you know I like to mix things up.  So don't do that.  smile.gif

Let's try some more strokes that were used in your 6" mirror work.

These could take the hill down, and add a little correction.  They vary in length and width in ways that should (hopefully) produce a smooth relatively kink free curve.

 

 

The short stroke wide W will work mostly to take the hill down.

AstrokhanThirdx5sixths.png

 

the next two strokes are to smooth kinks out between edge and center zones.

AstrokhanHalfx4fiths.png

TwoThirdsXThreeFourthsW-1.png

 

All four strokes in order 1 turn each.  That is enough for one session. test and see if you have a better sphere and edge.  It might get you a little bit prolate.

 

Don't do too much of these things.  Too much of anything, even too much of a good thing can be a bad.

If you go slow stroke choices can be modified before the curve gets far off course.

It would be good to see the sphere first and get a good diffraction ring before continuing to the parabola.


Edited by dogbiscuit, 16 September 2020 - 06:46 PM.

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#24 Astrokhan

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Posted 17 September 2020 - 12:09 PM

dogbiscuit, I did suggested strokes as given in #23 and here are results. I don't see much changes in ronchi. However in KE, I see a donut kind of raised zone spread between the central hill and edge. Let me know what could be the reason and next steps. Also measured the RoC distance which is 99 Inch.

 

Inside of RoC

 

IMG_4274cr.png

 

Outside of RoC 

IMG_4278cr.png



#25 Astrokhan

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Posted 17 September 2020 - 12:10 PM

Here is the KE

 

IMG_4290cr.png




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