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First mirror, 8" F/3.5-ish

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#26 chantepierre

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Posted 16 April 2020 - 11:14 AM

Thank you pinbout and dogbiscuit for your answers.
I redid the lap, which can be seen here :

https://i.ibb.co/QJW...-F4-C43-E05.png

Dogbiscuit, I’ll study your answer in a detailed fashion tonight and apply those behaviors. Thanks a lot for that much insight.

#27 Pinbout

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Posted 16 April 2020 - 01:05 PM

why are you getting those big chunks knocked out... that's not good, everything else looks great...


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#28 chantepierre

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Posted 17 April 2020 - 01:14 AM

Hello pinbout, it happens when I trim the channels with a razor blade. I’ll take fresh pitch fragments and heat them to complete these squares.
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#29 chantepierre

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Posted 18 April 2020 - 09:20 AM

Dogbiscuit, Pinbout, David, your advices did show a real gap in quality.

The microfaceted lap changed everything in regard of suction, I’m now able to stroke slow and steady instead of a janky motion. It seemed to help with cerox distribution too.

Polish progresses fast and Ronchi bands are more and more straight since I applied those narrow W strokes and figure 8’s.

I’ll take time to totally finish the hunt for pits, get a rather flat surface at the knife edge, build the XY stage accurately for the next parts and come back here.

Fortunately I have spare linear motion assemblies scavenged from old 2D & 3D printers so I should manage to build an average KE tester.

Have a nice day,
chantepierre
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#30 chantepierre

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Posted 20 April 2020 - 10:05 AM

Hello,
I took time to polish a bit more and smoothen things out. Also built a tester (finally let linear thingies in their box at work and used wood and rods) which for now has Y movement with 0,00428mm/step, and I now realize that I need a better edge and a better pinhole, and X and Z adjustment would be nice too. Or at least X and tilt.

My setup is : mobile phone has a 0,3mm pinhole on Agfa film (very clean) and a lesser 0,1mm (not really clean) aluminum pinhole on top of it on its camera flash.
The knife edge is a cutter blade taped to the phone camera at half diameter, so light source moves with the knife.

Now that fine movement is okay-ish, I have been able to measure ROC a bit better : approx. 145,5cm, so F/3.63. I’m okay with that.

I’m also able to see that the bad contour of my aluminum pinhole is a problem and that the knife edge is also not good enough. I can dress it as shown in Texereau or use an Agfa film (black on transparent) straight edge that I used for my previous tests.

Last, but not least, I’m able to travel through ROC back and forth with the tester... or by leaning a bit more forwards or backwards, so I need a harder floor or any measurement will be truly awful.

In those conditions, here’s a list of pictures showing before / at / after ROC, but centering and measuring is not stable enough because of the floor to give any numerical value.

https://imgur.com/a/HqOdR6Y

I have access to a concrete floor basement in which temperature is stable enough and will use this for future tests.

But right now, how should I proceed ?

Thank you for being companions of this exciting ride,
chantepierre

#31 Pinbout

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Posted 20 April 2020 - 10:35 AM

You need to get better taking ke pics cause those pics don’t say anything.

 

take some ronchis - they’re easier 


Edited by Pinbout, 20 April 2020 - 10:35 AM.


#32 chantepierre

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Posted 20 April 2020 - 10:45 AM

Thanks for the feedback. I’ll move to the basement and come back with better pics when hardware and method are sorted.

 I will first try to bolt the tester to a concrete or marble slab then see if more action is needed.


Edited by chantepierre, 20 April 2020 - 11:18 AM.


#33 Pinbout

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Posted 20 April 2020 - 11:54 AM

Thanks for the feedback. I’ll move to the basement and come back with better pics when hardware and method are sorted.

 I will first try to bolt the tester to a concrete or marble slab then see if more action is needed.

Try this when posting

https://youtu.be/SJWsqTOnF5o


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#34 chantepierre

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Posted 20 April 2020 - 05:29 PM

Yeah, sorry for the recent links - I do not have a computer at home since lockdown, only at my office.. CN’s complete post editor is unpractical on mobile devices. I’ll post from the office.
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#35 chantepierre

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Posted 29 April 2020 - 12:34 PM

Hello everyone,

I've been away a bit, with work and getting diverted by building a better tester, waiting for parts, building an adjustable chair while waiting for one more part, etc.

 

I decided to polish more while waiting for those parts. I rechannelled the lap, fixed the holes, microfaceted it, etc, and worked 2 hours more TOT. It seems that some pits do not go away. I cannot see them but when scanning the entire mirror with the laser, there is sometimes a "blip" near the edge that's almost impossible to find again. Planning two more hours.

 

Here's a handheld ronchi of the current situation. I circled in red one of the many notches that are baked in the grating and that I also see on my generic F/9 3" and 4.5" spherical mirrors. It's 85LPI.

There are a lot of thermal currents on the pic on the bottom right, but it was mostly smooth.

 

mawk38R.jpg

 

Shall I continue 1/4W, 1/3W, 1/2W as I've been doing for the last 4 sessions ?

 

have a nice day,

chantepierre


Edited by chantepierre, 29 April 2020 - 12:41 PM.

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#36 dogbiscuit

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Posted 29 April 2020 - 01:12 PM

I prefer looking for pits with a flashlight and magnifier (looking backwards through a 25 or 32 mm eyepiece).

A laser just shows too many dust particles for me to be sure if polish is complete.

 

If you see the same defect on  all mirrors I think you are safe believing it is a flaw in the grating.

 

Within the limits of what I can see with the thermal distortions, It looks like a reasonably good sphere and edge.

A nulled Foucault would show edge quality better than Ronchi.



#37 chantepierre

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Posted 29 April 2020 - 04:39 PM

Thank you for your answer : I’ll try to get a stable KE null pic with enough resolution tomorrow.

I inspected the whole mirror with a 32mm plossl and a flashlight, and discovered that the areas where I was able to see the laser had cerium stains. I also discovered two small but deep pits that won’t polish out at the near edge. They’re both smaller than 1/10mm in diameter but deep. I must have them since the start.

Good evening
Chantepierre

#38 Pinbout

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Posted 29 April 2020 - 06:40 PM

Hello everyone,

I've been away a bit, with work and getting diverted by building a better tester, waiting for parts, building an adjustable chair while waiting for one more part, etc.

 

I decided to polish more while waiting for those parts. I rechannelled the lap, fixed the holes, microfaceted it, etc, and worked 2 hours more TOT. It seems that some pits do not go away. I cannot see them but when scanning the entire mirror with the laser, there is sometimes a "blip" near the edge that's almost impossible to find again. Planning two more hours.

 

Here's a handheld ronchi of the current situation. I circled in red one of the many notches that are baked in the grating and that I also see on my generic F/9 3" and 4.5" spherical mirrors. It's 85LPI.

There are a lot of thermal currents on the pic on the bottom right, but it was mostly smooth.

 

mawk38R.jpg

 

Shall I continue 1/4W, 1/3W, 1/2W as I've been doing for the last 4 sessions ?

 

have a nice day,

chantepierre

why is your dot on the right side and then on the left side... 

 

the reflection test is used to see polish.

 

https://www.youtube....h?v=kjpkflDCBEI

 

I use my handheld microscope at 25um.

 

 

pits wouldn't show up in the Ronchi like that.

 

you have a pretty nice sphere.


Edited by Pinbout, 29 April 2020 - 06:45 PM.


#39 chantepierre

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Posted 30 April 2020 - 03:44 AM

Hello Pinbout, hello dogbiscuit,

The "dots" are dust grains on my camera.. changes position every time.

Here are, I hope, better pictures than yesterday :

 

U0DEDcG.jpg

 

On the pits topic, I do not see "gray" at all by the reflection method but confirmed the two deep pits. They will stay with me, it's all right. I won't sell this mirror.

Foucault is still hard to do with my first iteration tester. Ronchi grating is the same but rotated, printed on film, so it will always have a bit of irregularities.

 

have a nice day,

chantepierre


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#40 Pinbout

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Posted 30 April 2020 - 09:44 PM

 

Foucault is still hard to do with my first iteration tester. Ronchi grating is the same but rotated, printed on film, so it will always have a bit of irregularities.

you have a slight oblate sphere so ke will not show much. good to check edge... even ring all around the outside.

 

 

start parabolizing


Edited by Pinbout, 30 April 2020 - 09:45 PM.


#41 chantepierre

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Posted 01 May 2020 - 12:57 AM

Thank you,
I’ll start with 30 minutes MOT doing 3/4D « zigzag » strokes and post pictures afterwards.

#42 chantepierre

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Posted 01 May 2020 - 10:15 AM

Here's the result of 30 minutes doing 5/6D zigzags MOT, going from left to right with 9 to 10 strokes (I sometimes skipped one).

After each zigzag, mirror is rotated 1/12 turn counterclockwise. After each turn of the mirror, turn the tool 1/7 turn clockwise.

 

It seems that my grating is really not that precise and gets worse with manipulation. Luckily I have a full A4 sheet of it so I'll cut new slices next time.

It's the 85LPI of this file : http://ronchiscreens...nThreeSizes.pdf

 

823CoiN.jpg

 

FT8yGDq.jpg

 

It seems that I need to fix the hardware problem before going further into work : right now, I won't be able to take any significant measurement.

The stand lacks tilt and the tester lacks YZ movement.

 

Have a nice day (it's labor day here),

chantepierre

Attached Thumbnails

  • Reference.png

Edited by chantepierre, 02 May 2020 - 08:52 AM.


#43 chantepierre

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Posted 02 May 2020 - 08:52 AM

Hello,

I thought the previous session was a bit aggressive and that the pictures were really bad : today I pressed longer, worked 5/6D W MOT for 13 minutes (half yesterday's session).

 

Regarding the pictures, I was using the pinhole to take ronchis. Today I dimmed it with masking tape and pictures are way clearer.

 

Here's an annotated picture. Starting point is what I thought was ROC, but I was quite a few millimeters offset.

Is that a TDE showing up ?

 

MKvbkpk.png

 

I plan to continue working short sessions of wide zigzag strokes, unless that would make things worse.

 

Here's Mel Bartels's simulator with correct parameters and units are mm, with distances from ROC acheivable with my current equipment :

 

WiLxN9c.png

 

Have a nice day,

chantepierre



#44 Pinbout

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Posted 02 May 2020 - 10:03 AM

lower left shows a kink where its lacking parabolizing on the outer zones...

 

tot, center tool on .7 zone and tanget strokes for 5min and test.

 

draw a circle on the back of the mirror with a sharpie at the .7 zone.

 

or

 

use the edge of the tool, tot, and scrub, short zigzags, from inside the kink to the edge of the mirror... 5mins and test

 

at least theres no clocking of your lines going thru the caustic, means no significant astig.


Edited by Pinbout, 02 May 2020 - 10:06 AM.

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#45 chantepierre

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Posted 02 May 2020 - 11:35 AM

Hello Pinbout, thank you for your answer.

Here's a picture. I went for your first solution : drawn a circle at the 70% zone then worked 5 mins TOT with tangent strokes.

The bottom row is simply roughly the same as the top one but I rotated the mirror about 45-60 degrees on the stand.

 

lFfp8Q8.jpg

 

I'm not able to see the effect of those five minutes on this picture, but I was wary and did not apply much pressure on the tool.

good evening,

chantepierre



#46 Pinbout

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Posted 02 May 2020 - 01:15 PM

 

but I was wary and did not apply much pressure on the tool.

good instinct

 

all those pictures are from inside CoC... show 3 lines outside CoC ... duh...lol.gif

 

most people use outside RoC to read how the mirror is progressing. tongue2.gif


Edited by Pinbout, 02 May 2020 - 01:16 PM.


#47 chantepierre

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Posted 03 May 2020 - 04:53 AM

Whoops...

 

Lap and contact after last session and leaving MOT overnight (I found contact janky yesterday)

x9kKFUo.jpg

 

Ronchigrams of last session outside ROC (little centering problem on this series of pictures) :

ZeqF2tG.jpgIt seems the break between parabolized/not parabolized is smoothing. Instinctively I'd do another 5 minutes of what you advised me.

 

Lap trimmed, leaving MOT until next session (I lost less chunks than before ;-) ) :

 

CkhQvJf.jpg

 

Have a nice day !



#48 Pinbout

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Posted 03 May 2020 - 05:50 AM

Whoops...

 

Lap and contact after last session and leaving MOT overnight (I found contact janky yesterday)

x9kKFUo.jpg

 

Ronchigrams of last session outside ROC (little centering problem on this series of pictures) :

ZeqF2tG.jpgIt seems the break between parabolized/not parabolized is smoothing. Instinctively I'd do another 5 minutes of what you advised me.

 

Lap trimmed, leaving MOT until next session (I lost less chunks than before ;-) ) :

 

CkhQvJf.jpg

 

Have a nice day !

Looks like your going in the right direction

 

the reason for using outside RoC is... what you see is what you get - a hill Is a hill type of thing. Inside everything is opposite 



#49 davidc135

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Posted 03 May 2020 - 07:37 AM

If the polishing slips and grabs one tip is to brass wire brush the lap before a session. It usually helps promote smooth resistance. On top of good contacting, of course.  David



#50 chantepierre

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Posted 03 May 2020 - 08:15 AM

New session of 5 minutes TOT tangent at 70%. Contact was excellent relative to yesterday (leaving MOT overnight then reversing one hour before working made it feel perfect).

 

It seems the lap and contact are everything and I'm just here to push along.

 

nwfrtnD.jpg

 

It seems smoother and I'm tempted to go back to 15 minutes 5/6 D zigzag strokes MOT to add parabolization, but I'm not sure.

I feel or see a slight change at the edge that I'm unsure of.


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