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Lockie

The 8.75" f/6.7 Mirror Grind

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6 hours ago, Rusted said:

It's probably a bit late now, but I'd humbly suggest you are far more generous with the grinding/smoothing powder than I ever was.

Probably by a factor of three to four or even much more. The pros I watched, with their decades of experience, used very little abrasive indeed.

The more powder you use, the quicker it turns to mud as it grinds away at itself instead of rolling and crushing the glass surfaces.

It is the micro-fracturing of the glass surfaces by crushing which wears it away. It's nothing like using sandpaper!

Smaller quantities of grinding/smoothing powder sound and feel much sharper, louder, metallic and cut faster, but with a much shorter wet.

Mud "floats" the mirror or tool away from the opposite surface and may increase or reduce stiction.

Making it harder to control the blank and follow the desired stroke or even to tell when the wet has finished.

Hi Rusted, thanks, yeah that does make sense. Like you say a bit late now, but I can always try using less grit with the next mirror/lens I grind....I'm hoping this wont be my last :) 

 

6 hours ago, Rusted said:

Have you tried the dry glass pencil test for matching spherical surfaces?

 Yes, I did this test after rough grinding and all looked good thankfully. 

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2 hours ago, Astrobits said:

When you make up the test apparatus is up to you. Personally, I don't test until I am satisfied that all the pits have been polished out

I've followed your advice so far and it has done me proud, so I'm happy to carry on doing so and not test until all the pits have been polished out.

I was just thinking that if you use the same lap for both the polishing and figuring I'd have a gap between the two where I needed to construct test equipment which would take time. I understand from yourself that once the lap is made it can be a bit time critical in that it's needs to be kept wet and pressed to the mirror when not in use. If I built the test apparatus next it would mean I could go straight from the mirror being fully polished to the figuring with no pause to construct test kit. 

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2 hours ago, Astrobits said:

The first test that I do when starting figuring is to examine a pinhole image with the 10x loupe in and out of focus. This is a very quick check for astigmatism.

I hadn't heard about doing this before, thanks, I'll Google that one for more info.

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Posted (edited)
3 hours ago, Astrobits said:

Thereafter I use a knife edge for the normal Foucault set up. Moving the knife edge either side of focus gives the same info to me as a single edge of a Ronchi grating hence I have never owned or used a Ronchi grating. When I am getting nearer to the parabola I include a null test ( Dall Null ) which I find is much easier than using Foucault or Ronchi tests. There is info on using a the basic Foucault on the internet to finish mirrors.

Nigel

Foucault sounds good to me, I'll dig a bit deeper on the net, thanks.

I'm still waiting on the book I ordered, and there's probably quite a bit in there on this subject also, so hoping it will arrive soon.

Edited by Lockie

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Posted (edited)
On 13/02/2018 at 14:06, FLO said:

Have spoken with James (he handles our orders for products from the US). It looks like it might be another 2-3 weeks. Sorry :sad2:

Steve 

Any news 6 weeks on, Steve? 

EDIT: I received a response from Martin and I understand you're not getting any joy from your book suppliers :( Martin will get James to credit me as it looks like it might be an indefinite wait. 

Edited by Lockie

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Anyway, onwards and upwards! I've started to round some bits together in order to build the Foucault tester.

Firstly, this 6"x4" XY axis micrometer bed arrived today courtesy of Fleabay. I'll use this for making small adjustments intra and extra focus during testing. The remaining components will be mounting on it :) 

 

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