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6" f=6.5 mirror TDE problems

DIY mirror making ATM reflector
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#1 yenice

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Posted 05 September 2019 - 09:23 AM

I have been working on this mirror since some time now. The data of the mirror are as follows:

 

150 mm (6") diameter plate glass, 20 mm thickness, water jet cut, both sides were flat and there was no internal stress in the beginning.

Rough grinding was done with granite tool, 150 mm (6") diameter, 20 mm thickness water jet cut. Both sides were flat, parallel, polished.

After rough grinding was done, the domed part of the tool was used to pour the pitch over it and formed by the mirror.

Optical tests are done with a 5.23 lines per mm (133 lpi) Ronchi Grating. I have a moving light source that moves together with Ronchi Grating

My aim was to have a 200 cm (78.7") ROC, but I have now 194 cm (76.4"), it somehow got shorter in the way a little.

 

I worked recently in 27 degrees Celsius (81 degrees Fahrenheit) constant room temperature.

Cerium Oxide is my polishing compound.

 

You can find my recent progress in the rar file of the following link. It is about 45 Mb in size, 73 files. Ronchi images and my tool.

 

http://www.mediafire...190707.rar/file

 

The order of the files show the progress of the work too.

I take my pictures starting by farthest and going in the direction of the mirror. There may be some cases when I go a little back just a few shots.

 

My lap needs to be poured again, because the pitch is 3.6 mm in thickness on the sides and 4.2 mm thickness near the middle of the lap. But I have also worked with this pitch when it was 8mm thick on the sides.

 

I cold press my lap with mirror and some weight, using a window screen in between for micro channels.

 

I reached 44 total hours of polishing now, using different methods to get rid of the TDE that has developed at the end of 9 hours of polishing. That means I could not get rid of my TDE since 35 hours of polishing with CeO.

 

I have tried the fast methods in the videos of Pinbout too, but could not get better. I have also tried Accented Pressure Technique #5 from the page 21 of "Homebuild Reflector Telescopes" by Sam Brown, TOT but did not get rid of the TDE.

 

My questions are:

1-How bad is my mirror? Could someone please evaluate it?

2-How can I get rid of the TDE?

2-There is also a hill or valley (?) in the right hand side of my mirror, how can I get rid of it, how bad is it?



#2 ed_turco

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Posted 05 September 2019 - 10:46 AM

I think some photographs of the shadows would be a great help.  Also take care to tell whther your knife edge comes in from the left or right.



#3 Pinbout

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Posted 05 September 2019 - 11:10 AM

posting pic on CN

 

https://www.youtube....h?v=SJWsqTOnF5o

 

please post so we can see them... not search in a link

 

 

fixing a tde

 

https://www.youtube....h?v=Q7a7s-BwRPk



#4 ccaissie

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Posted 06 September 2019 - 11:51 AM

So how about another test?  The diffraction from a Ronchi looks like a TDE unless you've understood what's happening.

 

Try Foucault.  Put the knife edge way inside focus and watch the edge of the shadow....it will "hook" if there's TDE.

 

I hope you didn't spend 35 hrs trying to polish out the overlapping diffraction grating images.....or maybe you did, and your TDE is actually nonexistent?

 

https://www.cloudyni...rned-down-edge/    post #5 discusses this with pictures.


Edited by ccaissie, 06 September 2019 - 11:54 AM.


#5 yenice

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Posted 07 September 2019 - 01:46 AM

I think some photographs of the shadows would be a great help.  Also take care to tell whther your knife edge comes in from the left or right.

I do not have KE pictures yet. My KE setup needs to have some improvements before I can take reasonable pictures. Therefore, I only included Ronchi images in my first post.


Edited by yenice, 07 September 2019 - 01:58 AM.


#6 yenice

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Posted 07 September 2019 - 01:58 AM

posting pic on CN

 

https://www.youtube....h?v=SJWsqTOnF5o

 

please post so we can see them... not search in a link

 

 

fixing a tde

 

https://www.youtube....h?v=Q7a7s-BwRPk

I had no problems posting pictures in CN forums in the past. This time I compressed all 73 pictures in a single file so that it could be easier to download and see. They actually show the way my last efforts were going I thought. Do not understand what is needed to be searched.

 

I have already watched your video for fixing a TDE and even made a couple of sessions following your recommendations in your video before posting in this topic. I could not detect any improvement in my TDE profile after these efforts were made.

 

I will post below just a few pictures with the hope that they give an idea of where my mirror surface is.



#7 yenice

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Posted 07 September 2019 - 02:41 AM

IMG_0082.JPG

Outside of ROC

 

IMG_0088.JPG

ROC

 

IMG_0089.JPG

ROC (more near the mirror)

 

IMG_0090.JPG

ROC (even more near the mirror)

 

IMG_0095.JPG

Inside ROC

 

IMG_0107.JPG

My tool (I will pour the pitch again, I know it is very thin now)

 

As I said in my first post, the link there has many more pictures from more sessions, all in consecutive order for anyone who might want to see more detail from my mirror and the tool.



#8 yenice

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Posted 07 September 2019 - 02:50 AM

So how about another test?  The diffraction from a Ronchi looks like a TDE unless you've understood what's happening.

 

Try Foucault.  Put the knife edge way inside focus and watch the edge of the shadow....it will "hook" if there's TDE.

 

I hope you didn't spend 35 hrs trying to polish out the overlapping diffraction grating images.....or maybe you did, and your TDE is actually nonexistent?

 

https://www.cloudyni...rned-down-edge/    post #5 discusses this with pictures.

There are 3 KE tests defined in page 20 of "Homebuild Reflector Telescopes" by Sam Brown, which are marked as A, B and C. I have done A and B of these tests before posting the first here and they showed me there is a TDE. The test marked as C I could not do, due to my setup or due to me lack of KE skills.

 

The link I have read in the past, but read it again with more focus. Maybe you are right, maybe my edge is not so bad, that is why I am asking here others for opinions. So thank you all who has answered me so far.


Edited by yenice, 07 September 2019 - 02:52 AM.


#9 dogbiscuit

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Posted 08 September 2019 - 04:01 AM

There is some turn down but not awful bad.

There is some irregular surface shape, almost certainly from poor maintenance of the lap.

 

Your lap channels should be trimmed better.

Make depth and width of the channels uniform over all of the lap.

The lap is a precision tool to do precision work.  Maintain the precision of the lap as you work the mirror.

Trim it frequently.  Trimming is not difficult if you do it frequently. It will take only a few minutes because there is not much to do.

Just trim where channels are narrowing, and trim to equal depth where the channels are shallow.

 

Here is an example of an excellent lap... except for the chipped facet.

https://www.cloudyni...-sphere/page-10

 

Keep your lap looking like that will make it work much better.

 

One suggestion that in almost all cases improves a tde is using a lap the same diameter as the mirror surface.  If you bevel the edge of the lap to be smaller than the mirror diameter, you will likely have more trouble with tde.  If your lap substrate is larger than the mirror diameter, you can bevel the edge of the lap, but don't make top surface of the lap smaller in diameter than the mirror surface.

 

Suitable pitch hardness for your working temperature is important also.

 

A good lap almost always makes a good edge.  It also helps to make a smooth surface, and gives consistent control in figuring.

Making a high quality lap and maintaining it's quality are major elements of making high quality mirrors.


Edited by dogbiscuit, 08 September 2019 - 04:02 AM.


#10 Pinbout

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Posted 08 September 2019 - 08:17 AM

Since you want a parabola- I would get some correction cause as of those pics your oblate.

 

Parabola:

() inside roc

)( outside roc

 

its easy to turn the edge again while going from oblate to correct prolate.

 

As dog said keep the lap channels open

trim the lap straight down if the disc is same size of glass

 

ToT short fast strokes - test often like every 10mins then as you see things getting better more often

while maintaining the channels

 

it may take an hour or so don’t give up.

 

and you want the channels offset from center- your lap looks symmetrical to the center of the lap. 

not a fan of dividing those 2 rows on the side with channels - otherwise I’d divide the rest of the lap like that. You want the lap even all over but offset from center 


Edited by Pinbout, 08 September 2019 - 08:23 AM.

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#11 yenice

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Posted 09 September 2019 - 10:23 AM

I read your messages and will try to follow your advise as much as possible Dogbiscuit and Pinbout. Thank you both.

 

In the meanwhile, I have a question about my lap trimming (not touched my tool yet, this is about my past efforts and picture will be from the past):

 

IMG_0071.JPG

This picture shows how much I could trim my lap in the past. I can only make V shaped channels. Sometimes the sides chip away, I replace the chipped part back and press a little and it sticks back if the part missing is large, on the other hand, if the chipped part is small and many tiny sections, it gets more difficult to stick them together.

 

When I increase the temperature of the environment to decrease chipping, then the material sticks to utility knife as the knife digs in and the cutting becomes more difficult and the cut gets bumpy, with undulations behind the knife.

 

utilityKnife.jpg

My utility knife looks something like this.

 

I had a silicone mold for pitch channel forming, it gives almost perfect channels, they will be rectangular cross sectioned channels, but after some work is done, I can only do my channels as in the picture above, V shaped. Are these channels as V form all right?

 

Next time no bewel, I will do the top level diameter of the lap the same as the mirror diameter, no problem.



#12 dogbiscuit

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Posted 09 September 2019 - 10:55 AM

V shape is ok.

I make mine v shape.


 

I have a cup of ice water to occasionally dip the channeling blade into so pitch will not stick to it as quickly.

It helps a little.

 

 

I use 2 blades... one to trim channels and one to scrape the pitch off the side of the channeling blade.

Take care to scrape only the side of the blade and not damage the edge.



#13 Pinbout

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Posted 09 September 2019 - 03:10 PM

this is how I do it...

 

https://www.youtube....h?v=_81Mi3FfQo0


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#14 yenice

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Posted 12 September 2019 - 03:06 PM

My lap has 6.25 mm thickness on the sides now.

Channels are 6 mm deep and 5 mm wide.

 

I did not do any press yet, so, the shape of the tool is not ready.

 

Penetration test es per Jean Texereau in 27 degrees centigrade after 5 minutes was 9mm (too soft). I will bring the room temperature to 22 degrees tomorrow and try again.

Attached Thumbnails

  • IMG_0033.JPG
  • IMG_0034.JPG
  • IMG_0035.JPG

Edited by yenice, 12 September 2019 - 03:09 PM.

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#15 yenice

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Posted 11 September 2020 - 01:57 PM

Hi again after a year, I had some personal matters to take care during this time. Now as the summer heat seems like soon will be over, I am trying to pick up my mirror again.

 

I have not done any polishing at all. My pitch on the lap has melted due to heat in the meanwhile. So, I removed it from lap completely and will soon pour it again.

 

I have done a KE test setup as it was required to know the edge of the mirror. Below, are some of the pictures I took with it.

 

It was for me not possible to get the whole mirror surface lit up when the knife is at ROC. Only a middle portion of the mirror would be lit.

 

If I am way inside ROC, whole mirror would be lit.

 

My light source is a 5mm green led,

diffuser on led,

3 mm hole on an aluminum plate 1mm thick

and finally slit made of 2 parallel razor blade parts with about half a mm gap between.

 

 

Knife comes from left, light source moves together with the knife and camera

 

This picture is inside ROC

 

IMG_0109.JPG

 

 

 

 



#16 yenice

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Posted 11 September 2020 - 02:01 PM

Same position inside ROC, knife advanced more.

 

IMG_0111a.jpg

 

Same position inside ROC, knife advanced even more.

 

IMG_0112.JPG

 

 



#17 yenice

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Posted 11 September 2020 - 02:05 PM

Here, in this picture I am at ROC. No matter how I tried, I could not illuminate the full surface of the mirror.

 

IMG_0114.JPG

 

 

 

This picture is outside of ROC

 

IMG_0115.JPG



#18 Don H

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Posted 11 September 2020 - 02:32 PM

When the whole mirror is lit, then you should be at ROC. If it is smaller than the whole primary, you are either inside or out. If you move the light source and it appears to move the same direction on the mirror, you are inside. If it moves opposite, you are outside. Why do you think it is at ROC when it is not fully illuminated? Is in not polished to the edge?



#19 Matthew Paul

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Posted 11 September 2020 - 03:10 PM

The mirror is fully illuminated in this photo, less the portion where the "knife" is. 

 

 

 

attachicon.gifIMG_0109.JPG



#20 Matthew Paul

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Posted 11 September 2020 - 03:16 PM

It does not look like it is fully polished, but it looks like it's fully illuminated. If it is not fully polished, in my experience, it will show a turned edge.


Edited by Matthew Paul, 11 September 2020 - 03:36 PM.


#21 ed_turco

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Posted 17 September 2020 - 12:04 PM

My lap has 6.25 mm thickness on the sides now.

Channels are 6 mm deep and 5 mm wide.

 

I did not do any press yet, so, the shape of the tool is not ready.

 

Penetration test es per Jean Texereau in 27 degrees centigrade after 5 minutes was 9mm (too soft). I will bring the room temperature to 22 degrees tomorrow and try again.

That isn't a pitch lap; it's a masterpiece.   ;)



#22 ccaissie

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Posted 18 September 2020 - 11:49 AM

Here, in this picture I am at ROC. No matter how I tried, I could not illuminate the full surface of the mirror.

 

attachicon.gifIMG_0114.JPG

 

 

 

This picture is outside of ROC

 

attachicon.gifIMG_0115.JPG

Maybe camera isn't able to get close enough to the return beam.  in these pics I see the edge of the mirror's diffraction edge, so there's some sort of interference of the beam returning....

Not a bad turned edge, by the way.  Careful work with accent over the edge will sharpen that.  If it doesn't get worse,and you get a nice figure on the piece, I face grind my mirrors on a flat workpiece with 5 micron to get a nice flat ring, and a "perfect" diffraction edge. I heard Texereau did this.   Some bevel the very edge off.  Cheating, but I've seen some experts' mirrors that have very heavy bevels that were done to kill a TDE.  




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