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  #161  
Old 20-11-2018, 06:27 PM
Stefan Buda
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So first lens goes in, then spacer, second lens and last ring to secure? Is that how it works? Does the orientation or order of the lenses matter?
Yes, that's how it goes. Not only orientation and order matter but also spacing and centration. The key is the spacer, and it must be made to very accurate dimensions, both of it's diameters and the thickness. The lenses are a bit smaller in diameter than the housing but the spacer is not.


Logan,

I'm very happy for you. When you become rich and famous, you can buy me a beer. Providing you reach adulthood by then.
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  #162  
Old 20-11-2018, 07:29 PM
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Thanks for the explanation and cross section, Stefan.
Do you provide these little gaps so the aluminium casing doesn't press on the glass with temperature changes?
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  #163  
Old 20-11-2018, 08:44 PM
Stefan Buda
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Thanks for the explanation and cross section, Stefan.
Do you provide these little gaps so the aluminium casing doesn't press on the glass with temperature changes?
That is one reason. Another is to give the lenses the freedom to self centre themselves. For that to work, with such small gaps, the lenses must have very small wedge errors.
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  #164  
Old 21-11-2018, 12:43 AM
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really enjoying reading these updates and looking at your fine work, Stefan.
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  #165  
Old 21-11-2018, 06:50 AM
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That is one reason. Another is to give the lenses the freedom to self centre themselves. For that to work, with such small gaps, the lenses must have very small wedge errors.
Just one more question(s) Stefan, then I'll leave you alone
Is the rotational aspect important when stacking small lenses like it is for bigger surfaces like mirrors in a folded light path and what is achieved by rotating optical components on their axis? I read it can add up or cancel errors. Is that all it is and does it make a big difference in your opinion or it doesn't really matter? Is that something you test when you do the final assembly with your mirrors?
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  #166  
Old 22-11-2018, 09:12 PM
Stefan Buda
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Just one more question(s) Stefan, then I'll leave you alone
Is the rotational aspect important when stacking small lenses like it is for bigger surfaces like mirrors in a folded light path and what is achieved by rotating optical components on their axis? I read it can add up or cancel errors. Is that all it is and does it make a big difference in your opinion or it doesn't really matter? Is that something you test when you do the final assembly with your mirrors?
In an optical train every element has a tilt error (and other errors too), simply because nothing can be made absolutely perfect, and tilt errors cause astigmatism, therefore by rotating various elements relative to each other, it is possible, sometimes, to minimise astigmatism. But that only works if the effects of individual errors have similar values. For example, tilt errors in a field corrector may be able to compensate to some extent the residual astigmatism of a primary or secondary mirror in a cassegrain system.

To answer your question: It all depends on how accurate the optical components have been manufactured and mounted. It is possible to keep the errors small enough so that rotating elements won't make any difference. That is what I aim for. After I assembled my field corrector, I screwed it into a jig on the lathe and looked at the reflections of a laser beam while it was spinning. The reflected spots, projected onto a surface some distance away, showed little wobble, so I'm confident that it will be ok.
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  #167  
Old 28-11-2018, 08:18 AM
Stefan Buda
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Don't know how I managed to do ATM without 3D printing in the past.
The primary baffle is done and it would have been a pain of a job without 3D printing/carbon filled filament. The whole thing, including the aluminium ring, weighs only 75g.
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  #168  
Old 28-11-2018, 10:17 AM
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Beautiful job, as always - you'd swear looking at it that it was just a sandblast anodise.
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  #169  
Old 29-11-2018, 07:57 AM
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Thanks Lewis, I like the stuff but it is a bit slow to print - those two cylinders took about 7hrs each. This filament is very abrasive and I should invest into a ruby nozzle as the SS one I'm using is not a good heat conductor.
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  #170  
Old 29-11-2018, 08:21 AM
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Do you think it would be possible to 3D print an OTA structure? Given that it took 7 hours to print those bits it would take a week or 3 to print an OTA! I’m curious as to whether it would have the same structural strength as the way you’ve made the carbon fibre OTA?
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  #171  
Old 29-11-2018, 11:48 AM
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First, most 3D printers won't be able to print something as large as an OTA. Second, carbon charged plastic will be nowhere near as stiff/strong as proper skinned tube. Carbon fibers in a cloth (that is used for lamination) are contiguous and very long; it's their extraordinary resistance to stretching that gives stiffness and thermal stability. Carbon in 3D printed items is a mass of very short and small fibers that are not connected in any way. Better than ordinary PLA or ABS, sure, but you will never see any bows, arrows, fishing rods, skis, surfboards or telescope OTAs (anything that requires stiffness or thermal stability) 3D printed.
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  #172  
Old 29-11-2018, 12:00 PM
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Had a feeling that would be the answer
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  #173  
Old 29-11-2018, 12:59 PM
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Originally Posted by Atmos View Post
Do you think it would be possible to 3D print an OTA structure? Given that it took 7 hours to print those bits it would take a week or 3 to print an OTA! Iím curious as to whether it would have the same structural strength as the way youíve made the carbon fibre OTA?
Bratislav already answered your question so I'll just add a couple more comments.

Carbon filled filaments have a loading of max 20% fibre - a lot less than laminated CF.

"Delta" 3D printers would be perfect for the job as they can print very tall objects.

Once graphene becomes cheap, it may be possible to produce printer filaments based on super plastics, such as PEEK, that could possibly have the necessary characteristics for structural parts.

Already there are 3D printers that can imbed contiguous fibres in the print, but the cost is prohibitive for OTAs.
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  #174  
Old 02-12-2018, 08:57 AM
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At long last I got the two backplates engraved so next week I can take them to the anodizers. I used a friend's CNC mill and I gave serial number 000 to the prototype.
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  #175  
Old 02-12-2018, 10:15 AM
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At long last I got the two backplates engraved so next week I can take them to the anodizers. I used a friend's CNC mill and I gave serial number 000 to the prototype.
Looks great. First of many?
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  #176  
Old 02-12-2018, 04:16 PM
Stefan Buda
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Looks great. First of many?
First of two, for now. We'll see.
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  #177  
Old 08-12-2018, 06:19 PM
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As I was making the hub for holding the primary mirror, I realized that I can do a test of the finished mirror for eccentricity and co-axiality before taking it off the lathe.
It spun without any perceptible wobble so I'm happy with it.
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  #178  
Old 08-12-2018, 06:24 PM
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Originally Posted by Stefan Buda View Post
As I was making the hub for holding the primary mirror, I realized that I can do a test of the finished mirror for eccentricity and co-axiality before taking it off the lathe.
It spun without any perceptible wobble so I'm happy with it.
How's the glass mounted? Is there an aluminium bush running through it with a retaining ring. Do you use cork as spacers?
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  #179  
Old 08-12-2018, 06:46 PM
Stefan Buda
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Yes, there is a flanged bush going through the central hole and there is another flange at the front. The front flange has a thread for attaching the baffle. The mirror's location is defined by its back face and its internal diameter. The front flange also has a silicone rubber O-ring that pushes the mirror gently into contact with the bare aluminium flange on its back side. I may add a very thin shock absorbing layer, but definitely not something as rough and agricultural as cork.
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  #180  
Old 08-12-2018, 10:46 PM
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Quote:
Originally Posted by Stefan Buda View Post
As I was making the hub for holding the primary mirror, I realized that I can do a test of the finished mirror for eccentricity and co-axiality before taking it off the lathe.
It spun without any perceptible wobble so I'm happy with it.
So glad the chuck didn't accidentally come loose.........

Mike
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