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  #41  
Old 28-07-2018, 11:26 AM
Billyboy78 (Bill)
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.25 mm to go.! I was still using the 9" tool but I want to grind the last bit with a bigger 15" tool but I couldn't make it until I had a spherical centre to use as a template. So, cement tool has been cast but I have to twiddle my thumbs waiting for it to cure before I can seal it with resin and attach the tiles. I have found resin to be cheaper than epoxy and it cures hard as a rock.
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  #42  
Old 04-08-2018, 05:35 PM
Billyboy78 (Bill)
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Sag reached, beautiful sphere, #80 grit done. move onto #220 tomorrow. Fixed post grinding has to be the secret to a smooth figure.
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  #43  
Old 04-08-2018, 06:00 PM
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Lognic04 (Logan)
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Nice!
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  #44  
Old 04-08-2018, 06:04 PM
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Quote:
Originally Posted by Billyboy78 View Post
Sag reached, beautiful sphere, #80 grit done. move onto #220 tomorrow. Fixed post grinding has to be the secret to a smooth figure.
Where are the pictures Billy . Sounds like great progress...
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  #45  
Old 05-08-2018, 06:44 PM
Billyboy78 (Bill)
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#220 done. Did a FL test using my slitted led light source. Turns out we are a bit short, 1600mm instead of 1785mm target. F 3.2 but I'm not fussed, I don't want to mess up this figure
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  #46  
Old 05-08-2018, 07:13 PM
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Awesome! Pics?
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  #47  
Old 10-08-2018, 09:07 AM
Billyboy78 (Bill)
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Halfway through #400. Life keeps getting in the way. I'm limited to 15 minute wets due to the drill press motor getting too hot. I've got a desk fan blowing on it and I call a stop when I can't hold my hand on the casing. I'm using GW's technique of recharging on the go.
Confounded scratches, got a small one now and by the size of it it can only be the heavy concrete tool. Spent an hour yesterday bevelling everything down.

Pic's soon.
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  #48  
Old 10-08-2018, 09:47 AM
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Nice!
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  #49  
Old 12-08-2018, 09:30 AM
Billyboy78 (Bill)
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Started on #600 but after a short session I found a couple of pits I hadn't seen before. Went back to #400, then back to #220. Now I'm back to #600 all looking good. I'm using an old film camera 50mm lens as a magnifier, I got no idea the power of it compared to a loupe but it is awesome.
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  #50  
Old 25-08-2018, 03:37 PM
Billyboy78 (Bill)
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Feel free to say "I told you so" but I mistakenly presumed all the grits for optical and lapidary came out of the same factory. Wrong! The locally purchased grits from #40 - #400 worked great, it was the #600 and #1200 that were contaminated. Yes, I'm an idiot but I had to know. 40 yrs ago they said not to use them, but I thought things may have changed over time. Patiently waiting for some good stuff to arrive. They are only fine scratches that come out with 10 minutes of #600.

The delay has given me time to work on the mirror cell. I put the mirror into the cell for the first time and the 18 points worked great. Rather than expensive aluminium triangles, I used 1" x 1/8" metal strap welded into a T with the same dimensions as the PLOP design. The bit I worked on were the edge supports designed by Mike Lockwood using a wiffletree arrangement with tiny roller bearings contacting the mirror. I'm going to use the cell for any testing with the ke, ie sit the cell below the level of the ke so the mirror is resting on the 18 supports.
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  #51  
Old 25-08-2018, 04:49 PM
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Pictures Billy......... where are the pictures????????
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  #52  
Old 25-08-2018, 04:50 PM
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Pics please!!!!
Dang, that is annoying
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  #53  
Old 27-08-2018, 07:51 PM
Billyboy78 (Bill)
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Very much work in progress, and its not very pretty.

The mirror cell is attached to the frame by threaded rod used for coarse collimation, the secondary will have very fine tpi adjusting screws. The light path is directed down through those large bearings to an eyepiece. The FL of the mirror really determines the sequence of events in construction.

The other pic shows the grinding with a fixed post.
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  #54  
Old 29-08-2018, 02:41 AM
Billyboy78 (Bill)
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What, no comments? Is it 'whose going to tell him its never going to work' or are you in awe of my brilliance ? I'm happy with the progress so far.
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  #55  
Old 30-08-2018, 01:32 PM
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Don't get frustrated, mate. This section of the forum is probably not as frequented as others. Please keep posting your progress and pics and the comments will come.

I've been following this thread out of interest, and can certainly say that I'm always impressed with the skill and patience of you ATM guys. Well done.
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  #56  
Old 30-08-2018, 05:58 PM
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+1 Always fascinated by anything to do with the black art of grinding glass.
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  #57  
Old 30-08-2018, 09:46 PM
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Iíve been lurking in the background also reading this post in sheer awe....true grit !
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  #58  
Old 30-08-2018, 10:59 PM
croweater (Richard)
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What, no comments? Is it 'whose going to tell him its never going to work' or are you in awe of my brilliance ? I'm happy with the progress so far.
Very impressed mate Please keep some pics coming. I'd say plenty of people watching. Cheers, Richard
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  #59  
Old 31-08-2018, 06:03 AM
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Quote:
Originally Posted by Billyboy78 View Post
Very much work in progress, and its not very pretty.

The mirror cell is attached to the frame by threaded rod used for coarse collimation, the secondary will have very fine tpi adjusting screws. The light path is directed down through those large bearings to an eyepiece. The FL of the mirror really determines the sequence of events in construction.

The other pic shows the grinding with a fixed post.
That's better impressive project
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  #60  
Old 16-09-2018, 04:48 PM
Billyboy78 (Bill)
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So, here I was today twiddling my thumbs waiting for the fine grits and wondering how I overshot on the sag so much. It was down to below 3 and still had the fine grits to go I was starting to worry. It was then that I noticed the bar attached to the unistrut looked a bit askew, so I measured it and found it at least an inch too close to the centre of the mirror. As I was grinding it must have worked itself closer and closer without me noticing, thus grinding chunks out of the centre. After a bit of a think I decided if I was going to change to chordal strokes it would be advantageous to go the whole hog and aim for f3.5, or f4 even, as I have plenty of meat on the edge and I have read lots of bad things about f3 mirrors. After a couple of sessions with #80 the sag has started moving backwards .010mm. At least I'm keeping to form and making everything twice!
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