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  #421  
Old 04-08-2021, 10:18 AM
Stefan Buda
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Werner,

You don't need an optical flat for setting your spherometer.
Use your spherometer as a comparator: If you get the same reading on all three surfaces, then you are well within what is achievable with your spherometer. In fact you can grind a good flat using only two discs but you need three later on when doing the figuring.
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  #422  
Old 04-08-2021, 07:16 PM
legswilly (Werner)
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Originally Posted by Stefan Buda View Post
Werner,

In fact you can grind a good flat using only two discs but you need three later on when doing the figuring.

Hi Stefan, What do you mean by figuring? To get a flat, I would proceed with 600 grit, 800 grit and finally cerium oxide to polish it.
Still using the three disks, I will only polish one. I planned to make a pitch lap for the cerium oxide.



Today I ground the three disks with 400 grit and nearly finished. One disk has still an area that shows as not being ground, very small. Once I have this disk showing all ground, I go back to the 600 and 800 grits. Should I go to a finer than the 800 before polishing with cerium oxide?



I am very careful not to chip the disks when changing over in the sequence, still getting chips on the rims. Is it just the glass I am using? It's float glass. Maybe there is better quality float glass. I will use the best disk for the flat for the auto-collimation test rig and cut the remaining disk down to 300mm.
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  #423  
Old 05-08-2021, 07:57 AM
Stefan Buda
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Originally Posted by legswilly View Post
Hi Stefan, What do you mean by figuring? To get a flat, I would proceed with 600 grit, 800 grit and finally cerium oxide to polish it.
Werner,

You need to get to much finer grit than 800 before the polishing with cerium oxide. I would finish the polishing with 9 micron aluminium oxide.

You must understand that no matter how flat you manage to grind your disk, it will not remain the same during polishing. That is why it needs figuring like any other optical surface. For that you will need to polish all three surfaces so that you can compare them during the many figuring stages. Unless you have a reference flat the same size that you are making.
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  #424  
Old 05-08-2021, 07:19 PM
legswilly (Werner)
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Originally Posted by Stefan Buda View Post
Werner,

You need to get to much finer grit than 800 before the polishing with cerium oxide. I would finish the polishing with 9 micron aluminium oxide.

You must understand that no matter how flat you manage to grind your disk, it will not remain the same during polishing. That is why it needs figuring like any other optical surface. For that you will need to polish all three surfaces so that you can compare them during the many figuring stages. Unless you have a reference flat the same size that you are making.

I will ring tomorrow to get #1000 (6.8 - 9.3microns) and #1200 (4.5 - 6.5microns). Have to get it sent up from Sydney. I hope he will post it out to me.
The three disks are 350mm and initially I thought I would make a pitch lap of 75% that is 260mm for the polishing. I did the grinding with a 1/4 stroke. Should I make a 350mm lap, there may not have to be a lot figuring to be done. I can cold press on one while I polish the others. I don't know if at the end of the 1200, there is going to be enough reflection to look at it with a newton interference meter. It works with the un-ground side of the glass. The fringes are all over the place. I got an optical flat, but only an 80mm one, should have bought a bigger one (maybe 100mm), but it would not have made a lot of difference.

At the end of all this, I will get the 1000 and 1200 grit. Should I make the pitch lap 350mm, do I need to make a pitch lap, Or can I use the three to polish against each other?
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  #425  
Old 06-08-2021, 09:17 AM
Rod
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Hi Werner

Welcome to the hobby! Mirror making is engrossing and from a quick read of your previous messages you are obvious enthusiastic.

I gather making flats is your first project and you want a flat to be able to null test a mirror. I think thatís a great idea but making an optical flat as a first project may be a bit of a challenge - partly because not that many amateurs do it and so you donít have a lot of expertise to draw on. Stefan is the obvious exception here and itís great heís following your thread.

I donít want to discourage you but Iíd like to suggest a simpler first project. If you built a long focus smaller mirror say a 6 inch F10 you would learn the basics of mirror making and have a lot of expertise available to you on web sites such as stellafane and through this thread. A 6 inch F 10 can be null tested with a simple ronchi or Foucault test as the mirror can be left spherical so you avoid the difficulties of figuring and testing a parabola which many first timers find challenging. A well made long focus Newtonian is a versatile instrument.

After you have this experience you, youíll be able to tackle more difficult projects. I could supply you with a 6 inch Pyrex blank if you decide to do what I suggest.

Rod
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  #426  
Old 06-08-2021, 09:18 PM
legswilly (Werner)
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@Stefan and Rod
Thank you for your interest and I appreciate your advice.
I finished today with 800 grit grinding and await the 1200 grit.
I calibrated the spherometer with its legs at diameter of 70mm on a glass optical flat.
I did measure how flat my 350mm disks are. At 0, 90, 180 and 240deg and at each coordinate at the centre, 70mm, and 140mm and found two disks consistent at each measurement point to be concave by 0.0003Ē. the third disk is concave by 0.00025 (half a graduation on my spherometer). Which means nothing other than that they are more consistent overall and nowhere near flat-. I am sure the concavity was produced by the way I ground. Tomorrow I will try and increase the stroke and see what happens after one cycle. Maybe next week I will get the 1200 grit and try polishing one disk with cerium oxide. Want to see what it looks like with the interferometer.. I might make a larger spherometer and get it calibrated at the local machine shop. They should have a surface plate.
Thank you Rod for the offer of a blank. At the moment I want to see how far I can go. Like a dog with a bone.
Still need to find someone uphere with a test setup, ronchi and focault. Tried the local Port Macquarie club, but no joy.
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  #427  
Old 07-08-2021, 12:08 PM
Stefan Buda
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Quote:
Originally Posted by Stefan Buda View Post
Werner,

You don't need an optical flat for setting your spherometer.
Use your spherometer as a comparator: If you get the same reading on all three surfaces, then you are well within what is achievable with your spherometer. In fact you can grind a good flat using only two discs but you need three later on when doing the figuring.
Think about it until you understand what I said, otherwise you may be wasting your time.

If you grind two discs against each other, you can get the same reading with the spherometer, on both discs, only if both surfaces are flat. We are talking about matched surfaces. If one is slightly convex the other one will be slightly concave and your spherometer will show the difference. That means you can detect an error that is twice smaller than what you could detect by comparing one disc with an optical flat as reference.

Last edited by Stefan Buda; 07-08-2021 at 12:44 PM.
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  #428  
Old 07-08-2021, 01:13 PM
legswilly (Werner)
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Quote: If you grind two discs against each other, you can get the same reading with the spherometer, on both discs, only if both surfaces are flat. We are talking about matched surfaces. If one is slightly convex the other one will be slightly concave and your spherometer will show the difference. That means twice the the error that you could detect by comparing one disc with an optical flat as reference.End quote


There must be something wrong with my measuring. All three disks are showing to be concave. Two are consistently on all measuring locations 0.0003" concave. The third less at 0.00025". I checked it today again thinking there may be a difference in the way I measured, but it is the same result. grinding one on top of the other would produce concavity/convexity. I checked the specified sequence of which goes on top or bottom and I copied it OK. I kept check on the sequence of grinding by marking it off on a sheet. I counted the strokes for each combination. I washed the disks each time a sequence was finished. As you say, if the measurements are the same for all the disks, which they are more or less, I was splitting .0005" graduations to arrive at 0.0003 and 0.00025, then the disks are flat. That leaves the optical flat. Over ebay, a Russian one, Comes with a certificate, could be the culprit. So I need to find out what is going on. I am making a bigger spherometer, cant do any more anyway

until the grit comes for polishing. I hope I can get it calibrated at the local machine shop.
The grit I was quoted for #1000 and #1200 is silicon carbide. Would I be wrong to ask for aluminium oxide?
Thank you Stefan
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  #429  
Old 07-08-2021, 06:16 PM
Stefan Buda
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Quote:
Originally Posted by legswilly View Post
I hope I can get it calibrated at the local machine shop.
The grit I was quoted for #1000 and #1200 is silicon carbide. Would I be wrong to ask for aluminium oxide?
Thank you Stefan
You still don't get it: You don't need to do any calibration of your spherometer because you are not making a sphere, where you would need to measure the sagitta. You just want to compare two surfaces that have been rubbed together. If you get the same reading, it means they are flat, no matter what the actual reading is.

I have not seen/used silicon carbide finer than #800. All the fine grits that I use are aluminium oxide.

What thickness are your discs? If they are not thicker than 30mm, better not waste your time. Even at 30mm thickness, plate glass of that size would not make a precision flat even in experienced hands.
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  #430  
Old 07-08-2021, 08:39 PM
legswilly (Werner)
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Originally Posted by legswilly View Post
As you say, if the measurements are the same for all the disks, which they are more or less, I was splitting .0005" graduations to arrive at 0.0003 and 0.00025, then the disks are flat.
Thank you Stefan
I got it, thanks to you. I just wanted to find out why the discrepancy between the optical flat and the disks I ground.


Since you are using aluminium oxide, I will ask him to supply me with it instead of the quoted silicon carbide.
The disks I got were square float glass 19mm thick. So they are not precision optical flats, although they should be reasonably flat according to the measurements made. I want to make an auto-collimation test rig, as is on the Stellafane web site. The best one I want to use as the flat mirror with the hole in the centre.
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  #431  
Old 07-08-2021, 10:14 PM
Stefan Buda
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Werner,

Please don't go any further with this idea because it is practically impossible to make a useful flat with the diameter to thickness ratio that you describe.
I understand that you intend to make an optical flat with a D/T=18 - way beyond the realm of amateur telescope makers.

You can epoxy two of the discs together and use it as a grinding tool for making a 14" mirror (but not using the third disc - you would need a thicker blank for that).
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  #432  
Old 07-08-2021, 10:34 PM
legswilly (Werner)
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Quote:
Originally Posted by Stefan Buda View Post
Werner,


I understand that you intend to make an optical flat with a D/T=18 - way beyond the realm of amateur telescope makers.

You can epoxy two of the discs together and use it as a grinding tool for making a 14" mirror (but not using the third disc - you would need a thicker blank for that).

What is meant with D/T=18? I WILL find a site that has translations of commonly used acronyms, but what is it?
I would like to cut the disks to 12" and see if I can produce an F4.5 mirror. Or maybe even a 10" one.
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  #433  
Old 07-08-2021, 10:46 PM
Stefan Buda
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What is meant with D/T=18?.
Diameter divided by Thickness.
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  #434  
Old 08-08-2021, 08:30 AM
legswilly (Werner)
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Thank you
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  #435  
Old 09-08-2021, 07:26 PM
legswilly (Werner)
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Looking at the ground surface

What would one use to look at the ground/polished surface. I would be interested to see it as it gets smoother with each size of grit.Looking at the surface after say the 5micron grind, could one see what imperfections are left?
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  #436  
Old 10-08-2021, 03:21 PM
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mura_gadi (Steve)
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Hello,

Normally you just turn a 25mm plossl around and look at the glass close up in a strong light for the 1st few grades. After that a 10mm reversed around for your seeing glass, but that may just be because most ppl have those ep's.

Its about seeing the same size imperfections across the mirror to know that your done with a grit. Also check to see that the center, middle and edge are all progressing together evenly. You should see no variation in the surface finish from edge to center, neither smoother or rougher in any zones.

I'm still on my first mirror and can only say I can see the difference between 750 and 1100 under a 25mm plossl seeing glass(24-16micron) with fairly old eyes.


Steve
Ps. You generally grind at half the grit size, so, the 750/1100 is going from 12-8micron smoothed surface.

Last edited by mura_gadi; 10-08-2021 at 04:53 PM.
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  #437  
Old 10-08-2021, 07:34 PM
legswilly (Werner)
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Originally Posted by mura_gadi View Post
Hello,

Normally you just turn a 25mm plossl around and look at the glass close up in a strong light for the 1st few grades. After that a 10mm reversed around for your seeing glass, but that may just be because most ppl have those ep's.
I.

What brand eyepieces have you got and what focal lengths. I will buy some, but which is the question. There are so many to choose from.
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  #438  
Old 10-08-2021, 07:56 PM
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Hello,

Sorry I only mentioned the 10/25mm plossl as they come standard with most startup scopes and a lot of ppl have them already. You can find them fairly cheaply and readily normally because of ppl upgrading, or borrowed from a astroclub member etc.

All you need is a strong light and a magnifying glass, jewelers loupe what ever you can get. The eps suggestion was based on an assumption you'd have the plossls.



Steve
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  #439  
Old 10-08-2021, 08:43 PM
legswilly (Werner)
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I just bought a set of eyepieces from Amazon. Really cheap, but all I want to do with them is look at the disk I am grinding at the moment.
So, thank you, I didn't know one can use them like this. I vaguely remember reading about it, but I am getting too old and can't remember. I like this forum, good advice and hints. Once I have my telescope made, I can use them and upgrade to something better.
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  #440  
Old 04-09-2021, 04:41 PM
legswilly (Werner)
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Stuck at polishing

I checked after grinding with 1200 grit and the surface looked OK. Started to polish with cerium oxide. I had some sessions and checked the progress. It looked much clearer and did a test with a mini mag light and came across some very fine scratches. At first I thought it is from the pitch lap I made and I was worried (the first one I made). Then I wanted to see if it is indeed the pitch lap, by doing another 2 sessions of polishing, this time not by hand but on the turntable. There were a few less scratches, with the very faint ones gone. It was not the pitch lap? Tried to polish a few more sessions on the turntable, but the few scratches would not go away, but would not increase in number. I will go back to 800 and 1200 grit and get the scratches out before I start polishing again.
Question, My cerium oxide is of a reddish brown colour. What I see on the net is that people are using a white one. Is my cerium oxide of poor quality? Would it still polish out, but take longer? I bought this from a supplier which we used when I was still working.
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