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First grind question, 6" vintage Pyrex

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#1 Pshuf

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Posted 12 September 2020 - 09:04 AM

Back story: in 1971 I ordered a mirror grinding kit from Edmunds Scientific, using Sam Brown's All About Telescopes as a guide. Started well, formed the curve, but got intimidated and quit after 120 grit. Never threw the stuff away though.

Recently I've picked up where I left off 49 years ago. Still using Sam Brown but also Stellafane's website and some others as guides.

Current dilemma: using feeler gauges, my focal length is about 45. I was shooting for 48, so my sagitta is too deep. BUT the sharpie test shows I've got a hill in the center.

Question: At this point how do I get to a 48" focal length, with good contact and no holes or hills? Thx!

#2 dogbiscuit

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Posted 12 September 2020 - 10:46 AM

Read this thread

https://www.cloudyni...reduce-sagitta/

 

You too could change to 120 grit now.


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#3 davidc135

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Posted 12 September 2020 - 11:40 AM

Yes, it'll be straightforward. CoC strokes ToT will take you there. David


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#4 TOMDEY

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Posted 12 September 2020 - 12:41 PM

At this early stage, the exact radius and sphericity hardly matter. As you get into the finer and finer grits, the surface should become more and more spherical and you can tease the radius closer and closer to your target. I guess you know that you can judge sphericity by the bubble pattern, right? This is most true with the very fine grits. It's OK to be rather blaze at the point you're at now. And if you do a couple/few extra wets, not really any need to microscrutinize for pits or scratches from the last size. The final abrasive or two... nice and methodical and slower, conservative... so you proceed to polishing with a wonderfully smooth, sphere, without a turned edge.

 

[I worked at B&L back when they actually were still an old German optics shop, with all the skills. Amazing how fast and perfect they would turn out surfaces, including final hand figuring. Their greatest skill was knowing which steps you could just blow through without much care, and which steps mattered. For our hobby stuff, most guys tend to over test at the early stages. It's only the very final grind and very final figuring that matter.]    Tom


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#5 ed_turco

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Posted 15 September 2020 - 10:01 AM

Please be aware that Edmund kits came with Alundum (AlO3) in their kits, had done so by getting bad advice when they conceived the idea of making mirror kits.  Carborundum grinds about 3x as fast.  Get some #80 and speed up everything.  I suppose you could use the finer grades of alundum as these are not used for gross changes in sagitta affecting the radius of curvature of anyone's blank --so take heed!


Edited by ed_turco, 15 September 2020 - 10:02 AM.

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#6 Pshuf

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Posted 16 September 2020 - 12:14 PM

thanks for your comments, very helpful!



#7 TOMDEY

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Posted 16 September 2020 - 02:38 PM

Please be aware that Edmund kits came with Alundum (AlO3) in their kits, had done so by getting bad advice when they conceived the idea of making mirror kits.  Carborundum grinds about 3x as fast.  Get some #80 and speed up everything.  I suppose you could use the finer grades of alundum as these are not used for gross changes in sagitta affecting the radius of curvature of anyone's blank --so take heed!

My 6" and 8" Edmund kits used carborundum. Especially on Pyrex... that worked fine! I guess they changed that at some point?   Tom



#8 MitchAlsup

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Posted 16 September 2020 - 05:16 PM

My 6" and 8" Edmund kits used carborundum. Especially on Pyrex... that worked fine! I guess they changed that at some point?   Tom

The Edmund kit I did in the mid 1960's used carborundum until 320 and then switched to alumina.



#9 TOMDEY

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Posted 16 September 2020 - 08:22 PM

The Edmund kit I did in the mid 1960's used carborundum until 320 and then switched to alumina.

That must be it. I also remember that the process from coarse to fine went well. In fact, the Al2O3 left shallower pits... so the final surface wound up smoother for polishing. That was Burgundy Pitch with Red Rouge! I think some beeswax in there too...  Those individually cast and fine-annealed genuine Corning Pyrex blanks were wonderful!  Tom



#10 Sam Danigelis

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Posted 16 September 2020 - 09:18 PM

Pshuf, first, way to go! I had a very similar story to yours, just finished my 6" f/8 mirror a couple years ago, after a 43 year hiatus.

Secondly, I would recommend using a couple other means of verifying your focal length. One way is to make a template curved to your desired radius of curvature (96 inches). The other would be to coat your mirror's surface with oil, or soapy water, and try to catch a focused reflection of a strong light bulb. But I'm sure Sam Brown covers that in his fine book. Keep at it! You will really enjoy using the scope you built.

#11 ckh

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Posted 17 September 2020 - 02:34 PM

Me too with the Edmund kit. Started in 1969 finished over a year ago.



#12 ccaissie

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Posted 18 September 2020 - 11:56 AM

Continue with 120 with Tool on top.  Get it +/- 1"

Use the reflection method with a wet/oiled mirror and the sun, and get it about right.  I've heard rubbing wax on will give enough reflectivity, but not sure how good it is mixed in the slurry.

220 will give you a better reflection test, and adjust +/- 1/2"

Better with finer abrasives



#13 ed_turco

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Posted 22 September 2020 - 12:04 PM

My 6" and 8" Edmund kits used carborundum. Especially on Pyrex... that worked fine! I guess they changed that at some point?   Tom

I bought my Edmund kits in 1957 and 1958.   From your experience, they switched later on.  My warning still applies -- check that #80!


Edited by ed_turco, 22 September 2020 - 12:04 PM.

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