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  #441  
Old 04-09-2021, 04:36 PM
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mura_gadi (Steve)
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Hello,

Not sure about the reddish side, should be a pink or salmon at best. I had a quick look at CN and got the post below.

I have read that cerium oxide is used for industrial use in polishing glass, rouge is preferred for large precision instruments.

Throw some of your cerium oxide in a glass with water, it should settle cleanly in the water, you should not have a greasy surface with scum on top.

Can I ask how your water is out there? You might have tap water that is very hard or has impurities in it. You should be able to use the tap water for warming and cleaning the glass. But after drying and cleaning ready for a session, maybe try warmed bottle water with the rouge.



Steve
Ps. Try the jeweller's supply shops for rouge, or any lapping supplies companies if you want to try something fancier.

Microgrits are great, very good for the min/max grain sizes, highly recommended.

(not sure of naming companies on IIS but "Gemcuts" and "Aussiesapphire" are two that might be able to help you with rouge. For lapping suppliers try for zirconium oxide if you can find any, even better than rouge.)

https://www.cloudynights.com/topic/6...-pyrex-mirror/

Last edited by mura_gadi; 04-09-2021 at 04:52 PM.
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  #442  
Old 04-09-2021, 06:57 PM
legswilly (Werner)
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Quote:
Originally Posted by mura_gadi View Post
Hello,

Can I ask how your water is out there? You might have tap water that is very hard or has impurities in it. You should be able to use the tap water for warming and cleaning the glass. But after drying and cleaning ready for a session, maybe try warmed bottle water with the rouge.

https://www.cloudynights.com/topic/6...-pyrex-mirror/

Thank you, did not think of the water supply. I was using tank water. Could have had some bits in it. Change to town water and wash down after each session. A session lasts four lots of 15 min polishing with 20 min cooling of the motor. I will change the motor setup, so it won't heat up and I can polish/grind as long as I want.



Looked at the CN link and it gave me some idea on how long it takes to polish and I am nowhere near the times they say. Thank you for that. Might ask the cerium supplier the particle size of it.
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  #443  
Old 04-09-2021, 07:30 PM
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Originally Posted by legswilly View Post
Might ask the cerium supplier the particle size of it.
Hello,

With grits they tend to have all the smaller grits in them till you get to the next grit grade. Ie 120 grit will have grits from 115-175 or so, when the 180grit kicks in.

The smaller grits within the range tend to act like mud, interfere with the grinding action of the normal sized grits and aren't good for great finishes.

That's were the microgrits and such shine. You have a very small range of grit sizes within the grade. The zirconium is 1-3um for example, so, ask about the particle size range within the grit/oxide more than the particle size itself.


Steve
PS. Remember to keep your channels open on the lap, and try micro-faceting the lap as well. As per Mel Bartels site - https://www.bbastrodesigns.com/JoyOf...Polishing.html - MOT polishing tends to be better as the channels and groves can hold the larger particles away from the mirror surface leaving only the embedded rouge to work on the lap surface.

TBH I would think you would be better going manual at 1200 grit. Use the 1200 to learn the "I" stroke that you will need later for the polish to parabola. The "I" stroke will setup the mirror surface better for polishing and you'll have better control if you practice the stroke first. But I am a newb and my knowledge is only what I can remember from bits of reading.

Last edited by mura_gadi; 05-09-2021 at 03:15 AM.
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  #444  
Old 05-09-2021, 09:14 PM
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My cerium oxide is a pinkish colour. Polishes very well. Scratches at the polishing stage can be left over from previous grits or from dirt and impurities in the air. Such as dust falling from a shelf above or blowing in. Rinse every thing thoroughly before starting a session and keep your lap in a sealed box. I have had a scratch when I was a little careless about dust and rinsing off . Improved after better attention.
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  #445  
Old 06-09-2021, 03:08 PM
legswilly (Werner)
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. Improved after better attention.

I will need to be more careful with the washing/cleaning after each grit. Made myself a wash stand covered in black plastic just outside the shed entrance. After my session today, I found some small scratches and a few little pits. Tomorrow I will clean everything before I start another session. Keep on going with the 800 grit until I get it right.
Another problem I have is the foucault/ronchi test. I built the autocollimation test stand as in the movie on the Stellafane site. Until I have a mirror, which I am making now, I thought I would try and do the ronchi test. They say it is easy, for me it is not obvious.I got some slides with ronchi grids, But the grids are just a small area, maybe 10mm square and one can't shine the light source through them and at the same time look through the grid. Made myself a ronchi grid out of fishing line to shine the light through and at the same time look through it. Cant see any pattern.
I bought a cheap mirror from China solely for the purpose of learning how to test a mirror. It is a 200mm mirror and the specs say it is 750mm focal length. Today I measured the saggita as 0.1325" and that works out to 742.8mm. So I am within the range of the tester set up to 1500mm. What do I do wrong?
Wish I had a telescope/mirror grinder here.
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  #446  
Old 06-09-2021, 04:09 PM
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Hello,

Keeping things clean is a bugger, change all your clothes, change the rags you use, shower, check your fingers nails... list is nearly bloody endless (if your machining - all your pillars and seals/gaskets/grooves etc.). I try and keep anything I touch to one side, then I know it needs to be cleaned. Saves me missing stuff.

What thickness is the fishing line? You only want about 3-4 lines a mm after you have unthreaded the 2nd cord. I have some .105mm line on the way, it's thinner than needed, a mistake from me. You'll end up with a little under 125 lines per inch with that so should still be useable. I can send you a couple of meters after it arrives if you want. (6weeks +??? no idea with OzPost going postal)

Your using a LED diffused light(if its not, just rub it with some very fine wet and dry sand paper)? Strong 30,000mcd rating etc? You'll not get much feed back on the Ronchi till you at least get a flash polish on the face I think...

I could be wrong there, hopefully someone will say at what grit you can expect to see the bands on a blank from a Ronchi test.



Steve
Ps. Another way to test your grits is to try a elutriation process, very effective and fairly easy.

Last edited by mura_gadi; 06-09-2021 at 04:56 PM.
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  #447  
Old 06-09-2021, 05:55 PM
legswilly (Werner)
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Quote:
Originally Posted by mura_gadi View Post
Your using a LED diffused light(if its not, just rub it with some very fine wet and dry sand paper)? Strong 30,000mcd rating etc? You'll not get much feed back on the Ronchi till you at least get a flash polish on the face I think...

Ps. Another way to test your grits is to try a elutriation process, very effective and fairly easy.

I am using a Krypton bulb as it is shown on the autocollimation test movie from Stellafane. It is not diffused. It is a very bright light and I have to dim it to look at the 8" mirror, which is aluminised. What I am not clear about is, do you shine the light through the screen AND look through the screen, or do you shine the light through the screen and look at the mirror but not through the screen, or do you shine the light on the mirror and look at the mirror through the screen. Different sites say different things.

Thank you for the offer of some fishing line. I did it with a 10lb line, which worked out to 80 lines per inch, but what I would like to do is go on a web site like Microsoft or GIMP and see if someone would know enough to make a file with a grid that can be printed on a laser. That would be ideal, I could get it printed at a print shop on transparent film. I came across a site that had a pdf file full of 2 inch 100 grids, but on trying to print it was not good. Surely someone would know enough to produce a file like that. The Stellafane site also has one that gave me no joy.
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  #448  
Old 06-09-2021, 06:25 PM
Rod
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Hi Werner

There are two ways commonly used to set up the ronchi test:

1) use a very compact light source and shine it through one end of the ronchi screen while you observe the reflected image from the mirror through the other.

I think a krypton bulb will be too big. The stellafane video on autocollimation is dated 2009. These days most people get a bright small white LED.

2) The other way is how it was always done on the past and may suit you if your grating is small. Create a small slit - eg with two razor blades. I prefer to use the blades from pencil sharpeners. They are small, only sharp on one side and have a hole for a screw.place the LED behind the slit. If your mirror is of moderate focal length, a bit of separation between the slit and grating won’t hurt. You need to have the grating angled the same way as the grating - ie as vertical as possible. In the other method, this happens automatically.

I usually place a torch in front of the slit to help me roughly line things up.

Rod.
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  #449  
Old 06-09-2021, 06:33 PM
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Hello,

Printing at 600dpi should not be an issue, I tried stellafane's pdf at Office Works and got 8 copies done, all were crap. They printed them on the smooth side not the rough side!

Mind you they need to change printer settings from paper to transparency and run a cleaner sheet through the machine first. That may not have been done either from the look, but I wasn't going back again.

If you know anyone who has a 600dpi printer, they can print the Stellafane pdf etc for you. Just buy good quality transparencies, use the cleaner sheet that comes with it first and check which side you print to.


Steve
I gave up on the printing for now and moved to the fishing line, if you want edmunds optics in Oz sells the 5lpmm Ronchi - 2"x2" for $165 1x1 for $120.

Ps - You can use a monocular or such to look at the screen as long as it focuses close enough. That may help you with your eye spacing.
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  #450  
Old 07-09-2021, 09:40 AM
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Clean everything rinsing under fresh water every time. Clean clothes . Clean hands. Clean work space , no air movement.

The ronchi screen downloaded from stellafane printed on a acetate on a laser printer works. Easiest way to start. Use the Bartels light box or a tiny tube on a stand with an led in it at centre . The auto collimation test is far too complex for the first stage of polishing.

Quote:
Originally Posted by legswilly View Post
I will need to be more careful with the washing/cleaning after each grit. Made myself a wash stand covered in black plastic just outside the shed entrance. After my session today, I found some small scratches and a few little pits. Tomorrow I will clean everything before I start another session. Keep on going with the 800 grit until I get it right.
Another problem I have is the foucault/ronchi test. I built the autocollimation test stand as in the movie on the Stellafane site. Until I have a mirror, which I am making now, I thought I would try and do the ronchi test. They say it is easy, for me it is not obvious.I got some slides with ronchi grids, But the grids are just a small area, maybe 10mm square and one can't shine the light source through them and at the same time look through the grid. Made myself a ronchi grid out of fishing line to shine the light through and at the same time look through it. Cant see any pattern.
I bought a cheap mirror from China solely for the purpose of learning how to test a mirror. It is a 200mm mirror and the specs say it is 750mm focal length. Today I measured the saggita as 0.1325" and that works out to 742.8mm. So I am within the range of the tester set up to 1500mm. What do I do wrong?
Wish I had a telescope/mirror grinder here.
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  #451  
Old 07-09-2021, 06:15 PM
legswilly (Werner)
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Quote:
Originally Posted by Sunfish View Post
Clean everything rinsing under fresh water every time. Clean clothes . Clean hands. Clean work space , no air movement.


The ronchi screen downloaded from stellafane printed on a acetate on a laser printer works. Easiest way to start. Use the Bartels light box or a tiny tube on a stand with an led in it at centre . The auto collimation test is far too complex for the first stage of polishing.
I printed the stellafane ronchi screen and this time it looked OK after I shrunk it to 60% AND printed it at he highest quality setting on the laser. Did not change the setting the first time and it looked bad. Now I am looking for a transparency film to print it on. Rang Officeworks, but they could not help me.
Will have a look at the Bartels light box and see if I can make out how to build it. There are a lot of examples of test stands, but the very detail where the screen is placed, where the light is placed and where and what to look through escapes me. Sometimes the simplest thing won't get into my head.
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  #452  
Old 07-09-2021, 06:39 PM
legswilly (Werner)
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Originally Posted by Sunfish View Post
Easiest way to start. Use the Bartels light box or a tiny tube on a stand with an led in it at centre . The auto collimation test is far too complex for the first stage of polishing.
Had a look at the Bartels light box and it could not be simpler. Next I want the grid.
Thank you for the answer.
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  #453  
Old 08-09-2021, 11:02 AM
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Home made Ronchi screen

Back in the very 1960's (1961-62 from memory) being unable to source a Ronchi screen (and of course before all the modern tech [printers and such]) i built my own Ronchi screen using 2 x 100 threads per inch brass rods and the fine copper windings from an electrical coil salvaged from an old radio, phone or such. Took awhile to do "the wrap around" of about 2" or a thereabouts, but the end result was worth the effort. Was wide enough to receive return beam through the screen back to the eye. Cant remember what I used as a light source.

Stephen.
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  #454  
Old 10-09-2021, 09:29 PM
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Here is my first attempt using the printed 50x50 screen on film on a glass plate and a repurposed kids wood project box and a light bulb , like the Bartels box. Rough but worked.

The next iteration was the LED in a tube and glass ronchi so the eye can be closer to the light source and a view looking through the screen above the light source . On the table with the mirror at centre of curvature in a black box , looking though the screen which is on an adjustable music stand in either case.

Very sensitive to exact position so the stand needs to be movable and adjustable.


Quote:
Originally Posted by legswilly View Post
Had a look at the Bartels light box and it could not be simpler. Next I want the grid.
Thank you for the answer.
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  #455  
Old 11-09-2021, 09:32 PM
legswilly (Werner)
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Originally Posted by Sunfish View Post
Here is my first attempt using the printed 50x50 screen on film on a glass plate and a repurposed kids wood project box and a light bulb , like the Bartels box. Rough but worked.

The next iteration was the LED in a tube and glass ronchi so the eye can be closer to the light source and a view looking through the screen above the light source . On the table with the mirror at centre of curvature in a black box , looking though the screen which is on an adjustable music stand in either case.

Very sensitive to exact position so the stand needs to be movable and adjustable.
This is great and I can see how it is working. I asked at Officeworks to photocopy the screen that I printed on paper, but it came out with a lot of moire. Forgot my USB drive with the file on it, but they said they can print it on their printer on transparencies. So I will go on Monday and get it done. Part of the copy of my file was OK and I cut a 0.5" strip and wanted to try it. Had too high a voltage on the krypton bulb and blew it. So I will get some LED bulbs, 33000 mcd. I dont know if it is bright enough. What is a Cree LED as opposed to a LED? It comes in 45000 mcd. Thank you for posting the photos, that is great.
Have cleaned up and started to grind again. The motor on the turntable is conking out more often now, overheating and I ordered a new one with a gearbox and a variable drive.
What resolution are the photos you posted?
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  #456  
Old 14-09-2021, 08:58 PM
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Not sure about the specs of the led . A standard led from Jaycar . I had a red one lying around. Does not get hot and runs off a penlight battery pack. Does not matter much how bright with the ronchi.

Quote:
Originally Posted by legswilly View Post
This is great and I can see how it is working. I asked at Officeworks to photocopy the screen that I printed on paper, but it came out with a lot of moire. Forgot my USB drive with the file on it, but they said they can print it on their printer on transparencies. So I will go on Monday and get it done. Part of the copy of my file was OK and I cut a 0.5" strip and wanted to try it. Had too high a voltage on the krypton bulb and blew it. So I will get some LED bulbs, 33000 mcd. I dont know if it is bright enough. What is a Cree LED as opposed to a LED? It comes in 45000 mcd. Thank you for posting the photos, that is great.
Have cleaned up and started to grind again. The motor on the turntable is conking out more often now, overheating and I ordered a new one with a gearbox and a variable drive.
What resolution are the photos you posted?
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  #457  
Old 15-09-2021, 03:43 AM
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Hello,

I have just made my Ronchi tester, but not the stand. I used a LED from a cheap $10 headlamp from Jaycar, the printed Stellafane pdf transparency and fixed it inside a 35mm slide. Worked really well, unbelievable the detail on the blanks surface...

The one thing I found tricky was the alignment, I used a A3 sheet behind the tester to locate the returning beam. Once you have the returning beam it is a lot easier to locate your RoC and to align the tester/blank so the returning beam lands on the correct part of the transparency/Ronchi bands.

What ever the design just make sure you can get your eye in really close to the screen. That is probably the single most important thing to get right.


Steve
Ps. My LED is really bright, I was thinking of using a milk bottle cutout as a diffuser. The board is about 8mm thick, and the bulb is inset to about 3mm away from the screen. I was having trouble with light leaks, the insert was the easiest way for me to cancel them and it got the light source close to the screen.

I found that if you move the stand forward after getting the bands right, you can lose the bands and get a Foucault like test image from the blank as well.

You can get a LED head lamp torch for around $15 posted on ebay, that will get you the battery pack and LED light and a switch if you want it. I had already bought the headlamp for the hands free red light feature and had removed the white bulb. The battery pack was $2 something from jaycar.

Get your eye position as close as possible
Get the light as close as possible to the grid
Get the return beam as close as possible to the source
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Last edited by mura_gadi; 15-09-2021 at 10:13 AM.
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  #458  
Old 15-09-2021, 06:39 AM
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A similar setup.

I suppose that is what is good about a two wire led inside a small tube. The light can only be thrown in one obvious direction creating a circle of light and does not need to be blinding bright.

My tube is a drilled out threaded rod given to me by another ATM and can be threaded in to a plate or bolted on.

Quote:
Originally Posted by mura_gadi View Post
Hello,

I have just made my Ronchi tester, but not the stand. I used a LED from a cheap $10 headlamp from Jaycar, the printed Stellafane pdf transparency and fixed it inside a 35mm slide. Worked really well, unbelievable the detail on the blanks surface...

The one thing I found tricky was the alignment, I used a A3 sheet behind the tester to locate the returning beam. Once you have the returning beam it is a lot easier to locate your RoC and to align the tester/blank so the returning beam lands on the correct part of the transparency/Ronchi bands.

What ever the design just make sure you can get your eye in really close to the screen. That is probably the single most important thing to get right.


Steve
Ps. My LED is really bright, I was thinking of using a milk bottle cutout as a diffuser. The board is about 8mm thick, and the bulb is inset to about 3mm away from the screen. I was having trouble with light leaks, the insert was the easiest way for me to cancel them and it got the light source close to the screen.

I found that if you move the stand forward after getting the bands right, you can lose the bands and get a Foucault like test image from the blank as well.
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  #459  
Old 15-09-2021, 09:36 AM
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Hello,

I was worried about the optical quality of the transparency in such a sensitive test. The second 35mm slide will use .105mm fishing line for 120+ lines per inch for zero interference from the substrata.

Sunfish, I see your Ronchi slide looks to be very thick though. I can see why the slides are so expensive if they are on optical glass that thick.


Steve
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  #460  
Old 15-09-2021, 11:16 AM
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The ronchi is very old and was given to me, the problem is the small size.

The 50x50 laser printer transparency from stellafane works fine but is better clipped onto a piece of etched glass to keep flat. I should try one on sticky-back transparency.

Quote:
Originally Posted by mura_gadi View Post
Hello,

I was worried about the optical quality of the transparency in such a sensitive test. The second 35mm slide will use .105mm fishing line for 120+ lines per inch for zero interference from the substrata.

Sunfish, I see your Ronchi slide looks to be very thick though. I can see why the slides are so expensive if they are on optical glass that thick.


Steve
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