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20" F 4.6 Analysis

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#1 Stargazer

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Posted 14 February 2020 - 05:29 PM

Hi folks,

 

 First of I need to correct something, my mirror is a 20" F 4.6 not 4.5 as I stated before, my error and it needed to be corrected.

 

 Been working it today for the first time in awhile, did a Foucault test and input the readings, the graphs below show where it currently stands...

Ricks_20 inch_Surface.gif

and 

Ricks_20 inch_M-L_Plot.gif

 

Now I am rusty at these graphs so have questions...

 

Does this mean outer zones are focusing long so I should center 10" lap over the 60-70% area some

more or do I have it wrong?

 

What would be the easiest way to tackle this graph?

 

I have a 13",10" and a 6" lap to choose from, so the question is,what lap would you use, where

would you use it, how long a tangent stroke and how long before you would test?

 

Thanks

 



#2 Stargazer

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Posted 15 February 2020 - 05:48 PM

Hello All,

 

So I did what I had planned, 15 minute with 10" lap centered over 70% zone, 15 minutes worth (3 five minute sessions)

6" tangent stroke with table turning 3 rpm, and 5 lbs weight on top of lap because lap is really light,

here is where things stood after that...

Ricks_20 inch_Surface2.gif

 

OMG what have I done was all I could think,

after studying the results the plan was, 7 minutes centered over the 60% zone 6 " strokes with the

added weight still on top of lap, then 5 minutes over the 50% zone with the weight still and 5-1/2" strokes and then 2 minutes over the 35 % zone with no added weight, retested and here is the

results.

Ricks_20 inch_Surface3.gif

 

now to fix the center and see how that changes things. looking much better.


Edited by Stargazer, 15 February 2020 - 05:50 PM.


#3 dave brock

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Posted 15 February 2020 - 06:55 PM

My guess is that in your second test where things went crazy, the zone 5 measurment was read or

imput incorrectly. I don't see how one zone, which was at the point where you centered the lap, could

get so much correction compared to those on either side of it. On top of that, relatively little polishing was done to

cause that and to fix it.


Edited by dave brock, 15 February 2020 - 06:57 PM.

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#4 Stargazer

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Posted 15 February 2020 - 08:30 PM

Hi All,

Thanks for the input Dave,

something went wrong somewhere for sure, I will have to pay better attention in class.

 

I just finished testing again after one minute of 1/2" off center strokes with 10" lap and

1" overhang to lower center.here are the results...

Ricks_20 inch_Surface4.gif

 

things have lowered some, need to figure me a stroke that will lower the center and leave

the rest the same, at least it is heading in the right direction this time.



#5 dogbiscuit

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Posted 15 February 2020 - 09:57 PM

The zone 5 reading of .710 is clearly an error, probably just the 7 should have been a 1.

In the future check numbers for plausibility before figuring.  Measure again if in doubt.


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#6 Stargazer

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Posted 15 February 2020 - 10:29 PM

I rechecked my numbers dogbiscuit and zone 5 in that test should have read .071, hung a zero on the end

somehow, will have to pay more attention, thanks.

 

Just finished testing again a minute,10" lap of coc slightly longer strokes 1-1/2 overhang then a minute of

1/2 off center same length, not much change in results, going to try an edge to edge W one trip around and see

if I can smooth things down.



#7 dave brock

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Posted 15 February 2020 - 10:49 PM

Just finished testing again a minute,10" lap of coc slightly longer strokes 1-1/2 overhang then a minute of

1/2 off center same length, not much change in results,

Not surprising to see not much change with only two minutes of polishing.


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#8 Stargazer

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Posted 15 February 2020 - 11:08 PM

True enough Dave,

still learning how much work remove what amount and do not wish to over shoot or mess

things up this time so slow and steady and keep track of what is going on.

Just finished a walk around so to speak, W stroke from side to side.will test soon and see what changed.

six minutes of work.



#9 dogbiscuit

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Posted 15 February 2020 - 11:49 PM

To get the center down you will do better with the 6" lap.

Typically the center of the mirror out to about the diameter of the lap, maybe just a little less will remain spherical.  You might shorten the ROC in that area but  it will be near no correction out to near the diameter of the lap.

So with your 10" lap it would be expected that you would not have much correction out to near the 5" mirror radius.  That isn't what your measurements show but maybe you have worked with the smaller lap.

 

I think the best online reference for machine figuring is here

http://www.astrosurf...achines_eng.htm

Two figuring methods are described, Zambuto's being one of them.

 

Note that figuring session are made of several steps of a few minutes each with changing offset (stroke distance from mirror center) by something like 1/4" or 1/2", usually while maintaining an overhang (portion of mirror diameter overhanging edge of mirror at the end of the stroke).

Zambuto's method corrects from outer zone first and working inward.  As correction increases and correction is being added farther from the edge, overhang can become a negative number, meaning the lap's edge at the end of the stroke is some distance inward of the mirror's edge.

 

How many readings are you taking in your Foucault testing?  You might take another set of readings to verify the last set.  I'm not so sure the correction would have increased so much in the outer zone with a few minutes of work with 1" overhang.

 

If your readings are correct I think you will need to use the larger lap and work back toward a sphere to reduce correction near the edge.  The overcorrection is likely to morph into a tde as you work that correction out and you would need to continue work to make the edge good.

 

When you learn to get the outer 2 zones good, the rest is easy... relatively speaking.  lol.gif

 

It's not likely you would be able to get those outer zone percentages to go down with the smaller laps.  Maybe someone else knows how to do that without making an ugly edge.

 

 

Look at that link above.


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#10 dogbiscuit

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Posted 16 February 2020 - 12:00 AM

Another useful reference, an example of Zambuto figuring a 12.5"

http://www.astrosurf...to_12,5_Eng.pdf


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#11 Stargazer

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Posted 16 February 2020 - 08:40 PM

Okay, 

 I hit the central area today with the 6" lap, coc, offset, w strokes and long strokes out to the edge,

here are the latest results...

Ricks_20 inch_Surface8.gif

 

if I am looking at this correctly, the center needs to come down some more but what will

that do to the edge,zones 5-8 look good other then not corrected enough the difference

between those four zones and actual is only .005.

will lowering the center more turn the outer zones downward or raise them up more?

 

thinking it will make them longer and require more work to be done out there as well but 

I do think the center should be lowered more first with the 6" lap.

 

 



#12 dogbiscuit

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Posted 16 February 2020 - 11:35 PM

Yes the center could come down some more, but I think first a little work to get zone 4 correction percentage down would be good.  Some of that over correction can be moved to zone 5 which is lagging and some into zone 3 which is lagging.  It would take some luck for it to move only to 5 and 3.  If you got just enough work to wear the peak down it could turn out that way.   Main thing is getting zone 4 more like zones around either side of it.

 

 

The 142% correction at zone 4 is actually the correction between zone 3 and zone 4. Between 4.981" radius and 6.188" radius is over corrected.
Over correction between zones is shaped like an n.  Or you could look at it as 3 and 4 having this shape /\
Wear the peak down to make 3 and 4 look like this_ _ 
The peak is between Er3 and and Er4 and that is boundary between zone 3 and 4 at about 5.5" mirror radius (outer mask radius of 3 and inner mask radius of 4).

 

 

Small lap (I think your smallest is 6") worked with a short ~ 2" stroke tangent over that radius.
Turntable slow.  You have mentioned using 3 rpm before that would be ok.  If you can do 2 rpm that would probably be better.
doing 3 or 4 turns of the mirror I would work about 1 1/2 to 2 minutes, depending on the table speed.   with the table but count the turns of the mirror and continue work to complete the last turn.

 

 

No W, just straight forward and back short tangent stroke to the 5.5" radius. Or you not stroke at all, fixing the position of the lap at the 5.5" radius and let the mirror turn under the lap. After a couple of turns move the lap about 1/4" inward (5.25" radius) and do another couple of turns.

I know this is very little work for one session, but if you do too much you might get the th 3 to 4 correction down but make a big mess somewhere else.  We don't know how fast this might work. Might work fast enough to do too much or so slow no significant change will be made.  Hope for just right.

Could do some of that and then do some work to take the center down some more.

 

 

For any of this to work you must have good knife readings.  Averaging a number of readings helps to get more reliable results.  I think XP allows averaging input of 4 sets of readings.
A Ronchi image might show the kink at the 5.5" radius.

Describe the machine and speed range.


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#13 Stargazer

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Posted 17 February 2020 - 09:57 AM

Hi All,

 First I will explain my working situation.

My turntable speeds can be set to less then 1,2,3 and 4 rpm, by less I mean by 5 -10 seconds depending 

on which pulley being used,so my 3 rpm is really done in two minutes and fifty seconds.

All the working of the lap is done by hand after completing a mesh press then a cold press.

Glass is Pyrex so most of my testing so far is done with the hour of placing on the test stand.

 

I am not looking for a perfect F 4.6 focal length but am hoping to shorten it more, be it ever so little at this

stage, but either way, where it ends up when finished will be fine.

 

Dogbisciut, I did a short session last night of coc with offset very short (1/4) strokes then very narrow W over center

followed up with off center long strokes out over the edge in a hopes to smooth things some,left on stand to cool

over night and here are the results.

Ricks_20 inch_Surface9.gif

 

If I am looking at this right, 3 and 5 need work first but how?

that is the question.

 

 

 



#14 dogbiscuit

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Posted 17 February 2020 - 11:48 AM

What you have been doing seems to be getting you there.

 

After a little more work to get the center down some maybe it would be good to use the larger lap for a short time to smooth things out a little.


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#15 Stargazer

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Posted 17 February 2020 - 12:11 PM

Dogbiscuit, I must have read your mind,

 

Just finished a session of exactly that, did a short session over center with 6" lap 1/4 strokes again with

no long extended stroke this time, pressed the 10" lap and did one trip around side to side W and back

with 1" overhang at sides and 1-1/2 over center, glass is cleaned and on test stand,will test in an hour 

and post results.



#16 dogbiscuit

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Posted 17 February 2020 - 12:31 PM

Maybe I read your mind.  smile.gif


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#17 Stargazer

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Posted 17 February 2020 - 01:59 PM

Okay All,

 Test results are in

 

Ricks_20 inch_Surface10.gif

 

This mirror project has never been so close, starting to get nervous thinking what to do next,

I am thinking another smoothing session but which lap, the 10 again or the 13, and am I even

right with my logic, hmmm

What would the experts (I am definitely not one) do now? 

 



#18 Pinbout

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Posted 17 February 2020 - 04:04 PM

 

What would the experts (I am definitely not one) do now?

look at the ronchi to corroborate ke testing

 

I compare the ronchi's with jim burrow's diffract program images


Edited by Pinbout, 17 February 2020 - 04:04 PM.

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#19 dogbiscuit

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Posted 17 February 2020 - 04:46 PM

look at the ronchi to corroborate ke testing

 

I compare the ronchi's with jim burrow's diffract program images

Seems like a good thing to do now.


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#20 dogbiscuit

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Posted 17 February 2020 - 05:06 PM

I'd choose the 13" for smoothing.  If not used it for a while it would need extra pressing.

 

Go easy.   Notice that the vertical scale of the graph makes the maximum error height appear about the same.

In the last two tests the central hill goes up to about half scale, but actual height has been cut by 1/3.


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#21 Pinbout

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Posted 17 February 2020 - 05:17 PM

Having a 80x400 pcx laying around from optics-sigma to do a Ross null test also helps judge ke testing results  grin.gif


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#22 Stargazer

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Posted 17 February 2020 - 08:33 PM

thinking I knew what I was doing I decided to go with the 6" out over the 70% zone for one minute

worth of short 2" tangent stroke to fix that hump, here be the results...

Ricks_20 inch_Surface11.gif

 

Did not turn out as planned so have been pressing the 13" lap dry for an hour then pressed 

again with the mesh, scraped surface with wire brush and now cold pressing on the glass.

plan is for one trip around smoothing W stroke, clean and test again.

 

Danny and All, not moving my testing setup for a Ronchi till this mirror reads better then 1/4 wave

front. Mr Foucault has been nice to me so sticking with it.



#23 Stargazer

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Posted 17 February 2020 - 10:23 PM

Hi All,

 like I said, did one trip around smoothing W stroke with the 13" lap side to side and back again,

one trip around 5 stops with 1-1/2 over hang,and the results are in...

 

Ricks_20 inch_Surface12.gif

 

Looking good but more to go still.

back to work tomorrow so it gives you all time to advise me how to get this past the 1/4 wave

up to at least 1/8th I hope.

another question, I have some zirconium oxide polishing compound, suppose to be slower

then the cerium, maybe I should switch to it now??? or just stick with what I am using?

also have red rouge but I bought it 6 years ago and do not have a ball mall to roll it so best

not use that for fear of scratches or sleeks eh.

 

Let me know what you think.

thanks

 



#24 dogbiscuit

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Posted 17 February 2020 - 10:29 PM

I say stick with the cerium oxide.


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#25 dogbiscuit

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Posted 17 February 2020 - 10:34 PM

Keep going like you've been doing.

My advice would probably mess you up.


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