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Figuring first mirror - 8" f/6

Mirror Making ATM Beginner
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#51 Knight Sky

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Posted 19 October 2021 - 06:56 AM

Wait before working.

 

Looking at your lap... It needs to be trimmed.

I am marking some things on your image of the lap to show how to improve it when you trim.

Not sure about the temperature. The roughness, which isn't all that bad, is almost certainly from working with the lap that needs trimming.

 

Don't trim until I post the marked up picture.

There is a small section of channel that is about the right depth and width.  I'll mark that area with a green circle. The idea will be that when trimming you try to make all the channels as much like that all over the lap.

 

You could be only a few short (~5 minute) sessions from a very good parabola.

 

As for heating the lap and mirror, I don't do that but I work in a temperature controlled space.

I think for one session you should continue without warming the lap and mirror.  After trimming the lap rinse the pitch chips off and apply slurry and press. 

 

How much have you been pressing until now?  How much work have you done since the last time you trimmed the lap?  I can get a pretty good idea how hard the pitch is from that.  For now I will say that you have a pretty good surface so the pitch is pretty good.  With the lap trimmed it should work better.

 

Picture coming... not to long from now.

I have had issues with the trimming and rechanneling of the lap, so I try to avoid it. I reckon that my pich is a little on the hard side, and hence the problems that I see due to trimming and/or rechanneling. Temperatures have gone down a bit so perhaps what was a good smooth lap a few weeks back is not as smooth.

 

Here's a photo of what happened on the mirror the last to last time I trimmed the lap. This was on 2nd Oct. I have done trimming and rechanneling once more after that. Not quite sure what I did to avoid these sleeks.

 

IMG 20211019 171206

 

Your tester look nice.

And very practical... you can polish your shoes while you test.  :-)

Hahaha. I quit wearing leather and hence won't need it for polishing unless I buy synthetic...I am too lazy so I just wear sneakers with suit. cool.gif cool.gif



#52 dogbiscuit

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Posted 19 October 2021 - 07:04 AM

You have done good not beveling the edge of the lap. That is why you aren't having trouble with tde. Reducing the diameter of the lap to less than the mirror diameter is a major cause of tde. Some of your lap defects also can cause edge troubles.  Getting you channels opened up, press well and when you work the edge might get even better than it is.

 

 

I have marked some, but not all of the problem areas.

Three red ellipses show some of the worst examples of closed channels.

All except a few short segments of the channels have narrowed too much.

Those sort of things cause bad thing on the surface.

I'm surprised you mirror is not a real mess.

I think you pitch hardness is ok.  If it was too hard the mirror would be much rougher and the curve would have ugly zones.

 

KnightSkyLap.png

 

The red lines don't show how wide to make the channels, they show which side of the channel to trim to open it.

choose the side that will keep as much pitch as possible on the small pitch facets and removes some of the pitch of the fat facets, or that moves a channel toward equal spacing of the channels.  I didn't mark all of them, use your judgement to improve the channel grid within the limits of the suggested channel width.

 

The channels are trimmed to about the depth and width of the channels in the green ellipses. Make them like that all over the lap.  Yes I understand the channels were probably all the same size when you started and the outer ones have not closed as quickly.  When you trim try to make channels uniform over all the lap, and evenly spaced.

I know you can't get it perfect in one trimming.  When you trim plan how you trim so it gets better.

And I understand the lap was in better condition for much of your work and maybe only a little while like in the picture.  Don't work at all with the channels in the condition as in the picture.

 

You will probably finish this mirror without trouble before you get everything perfect with the lap.These are details that can make things about keeping the lap in good condition and action more predictable.

 

Trim channel more frequently.  Less trimming will be needed to remove less pitch from channel walls. It is easier to correct channel creep.  

Less trimming more frequently.

My old saw...  The mirror maker's secret sauce is the lap.

 

 


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#53 Knight Sky

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Posted 19 October 2021 - 07:12 AM

@dogbiscuit, here's a proper top-down photo of the lap.

 

IMG 20211019 173745


#54 Knight Sky

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Posted 19 October 2021 - 11:55 AM

Redone lap, @Pinbout style, just that I have done a terrible job grin.gif

 

IMG 20211019 221750


#55 dogbiscuit

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Posted 19 October 2021 - 04:23 PM

now press well, extra time before the next work session.


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#56 Pinbout

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Posted 19 October 2021 - 04:36 PM

I would take sometime to build the scope and star test it.  Even if it’s not “finished” you’ll learn some stuff about star testing while comparing with your ronchi to the star test. 


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#57 Jeff B

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Posted Yesterday, 10:03 AM

I would take sometime to build the scope and star test it.  Even if it’s not “finished” you’ll learn some stuff about star testing while comparing with your ronchi to the star test. 

Always good to have two "points of view".


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#58 Knight Sky

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Posted Yesterday, 03:46 PM

Seems like the last session has not gone my way. I had two sessions with ToT W (3/4D) suggested by dogbiscuit and one session of smoothening short Z (1/4D). Mirror now has TDE, I am not too sure about the figure and the surface seems worse than before. Seems like I was not able to control ToT W properly and may have overshot multiple times and hence the TDE.

 

Here's a 65 lpi outside Ronchi.

 

IMG 20211021 014917

Edited by Knight Sky, Yesterday, 04:12 PM.


#59 Knight Sky

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Posted Yesterday, 03:55 PM

With Ronchi matching, figure appears not to be too bad. To my naked eyes it seems however that the bottom part of the edges seem to have got tiny bit more correction than the top part. Could just be an optical illusion though.



#60 Steve Dodds

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Posted Yesterday, 04:06 PM

That turned edge is nothing to worry about, a few passes with a beveling stone and it will be gone.


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#61 Pinbout

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Posted Yesterday, 05:05 PM

 

To my naked eyes it seems however that the bottom part of the edges seem to have got tiny bit more correction than the top part.

probably a tillt error in the grating to the mirror surface

 

can't use ronhi to read tde... use a ke to look at the edge, bright ring around the disc

 

 

observing planets with an uncoated primary always gives pleasing results.


Edited by Pinbout, Yesterday, 05:07 PM.

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#62 dogbiscuit

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Posted Yesterday, 07:38 PM

I don't understand why it is so much trouble to take a Foucault image.


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#63 dogbiscuit

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Posted Yesterday, 08:31 PM

Seems like the last session has not gone my way. I had two sessions with ToT W (3/4D) suggested by dogbiscuit and one session of smoothening short Z (1/4D). Mirror now has TDE, I am not too sure about the figure and the surface seems worse than before. Seems like I was not able to control ToT W properly and may have overshot multiple times and hence the TDE.

 

Here's a 65 lpi outside Ronchi.

 

When you say "two sessions with ToT W" do you mean you did 1 trip around the mirror and tested? Then another session of the same and tested. Then the smoothing stroke? Was there roughness seen by testing  after the TOT W's that caused you to do the short 1/4 Z?

Was that Z MOT or TOT.  I'm not familiar with that stroke.  I guess it is stroking a Z left to right, or else it would be an N.  If so how is that different in effect from an N stroke.  Please describe that stroke.  

 

The 65 lpi Ronchi looks good, would like to see what it looks like with the 133 lpi grating. The 65 lpi grating is not as sensitive as the 133 lpi.  65 is ok but it is difficult to compare smoothness of the curve to the previous Ronchi with the 133 lpi.  I think you have improved the curve but I can't be sure it is not only that the latest test was not as sensitive.

 

Foucault will show the edge better than Ronchi.


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