38 replies to this topic

[iTesla]

 

 

This is an 8 inch ƒ8 mirror with Foucault KE from the left. I don't know what the tests are telling me, but I think the Ronchi images are showing a possible TUE. FigureXP surface error analysis says I need to lower 65% and 100% zones and raise the 85% zone. I only have a full size tool. If I have to wear down the outer zones, should I design a wax paper star to modify the surface of the pitch lap?

 

Thanks for your advice.

[Pinbout]

Your outer zone is way under corrected - from the kink to the edge.

 MoT narrow W’s longer strokes  draw a 30% circle on the back of the mirror and don’t let that line go over the edge of the tool 

[duck]

the classic stroke for the up errors at the edge would be lap on top, wide W strokes (lots of sidethrow) with not much time with the lap in the center  (at low sidethrow).  I don't like the mirror on top long stroke with less sidethrow because the the large overhang tends to turn down the edge.

[Pinbout]

 

I don't like the mirror on top long stroke with less sidethrow because the the large overhang tends to turn down the edge.

its an f8, going too wide will overcorrect, he's acutally over in the 2nd & 3rd zone.

Edited by Pinbout, 27 January 2021 - 06:00 PM.

[dogbiscuit]

Your outer zone is way under corrected - from the kink to the edge.

 MoT narrow W’s longer strokes  draw a 30% circle on the back of the mirror and don’t let that line go over the edge of the tool 

Wouldn't a 30% circle result in a wide W,  large overhang to the side?

 

Did you mean 70% circle on the back of the mirror and don't let that circle go outside the edge of the tool?  That would result in a narrow W.

[Pinbout]

If you keep the w narrow the 30% circle is obviously not for the sides 

 

so front and back on the W have a 2.8” overhang and I would go 1.25” left and 1.25” right but I hate giving the left and right numbers 

 

one circle on the back keeps it simple.

Edited by Pinbout, 27 January 2021 - 08:54 PM.

[Pinbout]

Your too deep center flat edge 

 

A III 

 

[dogbiscuit]

If you keep the w narrow the 30% circle is obviously not for the sides 

 

so front and back on the W have a 2.8” overhang and I would go 1.25” left and 1.25” right but I hate giving the left and right numbers 

 

one circle on the back keeps it simple.

That makes sense. 

[DrewFamily]

All of the above. On the other hand, you do realize that by reading the scale on the left of the figure xp file that your surface error is miniscule, right? I'm sure you will be delighted with the high power views that optic will throw up as is. The figure on a slow mirror like that can pop out of whack with a few simple turns around the barrel. So as an alternate piece of figuring advice, there is no shame in calling that one done. Ship it! Jay in CT

[pjmulka]

You have undoubtedly spent a great deal of time on your mirror, if a jobs worth doing it’s worth doing right. Besides if you do end up messing it up think of how much you’ll learn fixing it!

[Pinbout]

 

On the other hand, you do realize that by reading the scale on the left of the figure xp file that your surface error is miniscule, right?

not according to the ronchigrams...

 

for the tue use C II to fix the edge.

[ccaissie]

A little TUE isn't bad.  The mirror edge/corner cools faster and contracts a bit diminishing the high zone.  But you can get it Right on!

 

Polishing slowly with a well-pressed lap, and some really slow polish (washed and settled cerium/rouge) TOT, blending W's as mentioned.  Since you can easily see the TUE, go slow and retest often.  My theory is that the edge of the lap is where the action is, especially as it reverses direction, so don't dwell over the R3.5" trough you have...better to let the lap edge frequent R2.5?

Attached Thumbnails

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[Pinbout]

 

A little TUE isn't bad.

yes it is

[iTesla]

First I'd like to say that a good Ronchi screen makes a difference in the quality of the image.

 

 

Next I want to thank everyone for their advice. I've done a lot of A III and 1/3 COC trying to get to a sphere. I've also made six rotations around the mirror using C II and testing after two laps for fear of turning the edge down. I think I could use 1 or 2 more rotations. This is where I'm at now. 

 

 

(Image above corrected from earlier posting)

 

 

Is the "Surface Error Analysis" depicting the contour of the mirror? Therefore, do I need to polish away or remove glass from the center and outer 1/2 inch of the edge? From what I have read, you should correct the edge first, then the center. So some C II on the edge and then strokes to lower the center resulting in a perfect paraboloid. 

 

KE from the left.

 

 

Edited by iTesla, 17 February 2021 - 07:18 PM.

[iTesla]

The last image has a lot of turbulence.

[Steve Dodds]

Why are you trying to get to a sphere???  Wouldn't it be a lot easier to start from where you are at.

[iTesla]

From what I've read, I thought a sphere is the preferred starting point. That you can get there after hours (my words) of using 1/3 COC. Then using the "classical" wide "W" parabolising stroke allegedly gets you very close to your goal.

 

I've reduced the end spikes with TOT and its center working the outermost 1/2 inch of the mirror. However, it reappears when I attempt to lower the center of the mirror. So I thought I'd try going the spherical route.

 

Regards,

Barry

[Steve Dodds]

No a sphere is just where you usually end up after polishing.  You don't need to TRY to get there, there is no point.  Just start from where you are at.

[Pinbout]

i'd fix the edge mot very short strokes, once the edge is fixed...

 

https://www.youtube....&index=4&t=163s

 

do figure 8's to smooth your lumpy surface.

 

https://www.youtube....jPcDoB&index=12

 

then do some w's since your undercorrected.

[iTesla]

Pinbout, don't I show a TUE if any and not a TDE?

In the Foucault panel, if the % zonal correction says zone 5 is -204%, that means that zone is undercorrected?

[Pinbout]

then you center the tool (on top) just inside the edge. hold the tool by the sides, with both hands, trying to help hold off the weight of the tool (a little) and do short tangent strokes... as you walk around.

 

don't worry about correction yet... but your center hill means your under corrected. but at your long focal length, it doesn't take much to correct.

Edited by Pinbout, 17 February 2021 - 04:53 PM.

[Pinbout]

Your pics looked labeled incorrectly 

 

both 3 line rochi look labeled backwards

 

the lower left 3 lines says inside but the lines bow inward

 

the Upper 2nd pic with 3 lines says outside and the lines bow out.

[iTesla]

Pinbout,

 

Thanks for catching that! I have much to learn. I edited the image and it should be correct showing a slight TUE.

[Pinbout]

Ok, just do some figure 8’s MoT to smooth stuff out then see how the edge looks 

[dogbiscuit]

KE from the left.

L1010886 copy.jpg

 

Except for the rough and irregular surface and how it affects the edge, the edge looks otherwise very good to me.

 

About the roughness... I don't know for sure how much of it is the real surface condition or caused by less than idea test conditions.

Maybe it is the surface has not been cleaned well. I see what looks like dried residue about 1/2" from the edge, mostly but not only on right side and lower left.  Also I see what I suspect could be signs of incomplete polish in this area.  Might just be dried stuff on the surface, but I recommend after the next work session don't let slurry dry even for a moment on the mirror surface, wash clean with soap so you don't have to look past any residue to see the surface condition.  And check closely for complete polish all the way to the edge all the way around.

 

 

There is some larger scale roughness, like a high area that looks like a column, same width as the central hill (1/3 diameter) extending all the way down to the edge 6 o'clock.   Maybe there is some air turbulence between mirror and tester, or the mirror was put on stand to dry and has not been given time to completely stabilize at test room temperature.  I pretty sure that column is from a temperature gradient  in the glass or air.

 

Other than those not so out of the ordinary things, the mirror looks pretty good.

Make sure polish is complete.

 

That raised 70% zone extends from about 55% to the 84% zone.

If it were my mirror that is what I would work on now.

I think that is easier to fix by making a good sphere. 

The f:8 sphere is close to a parabola is so it isn't like going out of your way.

I'm not sure if or how much of the apparent roughness is real.  If it's real then work on making the surface smoother.

 

 

The central bump is most likely the result of your A-III strokes all having the same maximum overhang on left and right sides of the W.   The edge of lap is under that black circle on the last stroke on far left and far right side of the W.  That is what creates the zone.  When you do MOT parabolizing strokes more than 1 trip around the barrel, change the width of the W just a little so the last stroke at sides of the W are a little (~ 1/4") closer to center. Next time around you could go 1/4" closer to center. Next time go back to the 30% circle.  The more overhang the more effective the stroke. So don't think you need to linger doing any extra strokes near center to get that hill down.  You just extend the side overhang of the W and the hill will get smaller. Avoid making a hole.

 

Another thing that helps to make a smoother center is to alter the stroke direction to more left right than forward back at the last one or two strokes at the far sides of the W.  This allows taking the edge of the lap closer to the center of the mirror.

See the reasoning for this recommendation in post 42 in the linked thread.

https://www.cloudyni...e#entry10462964