Here is the first draft for the layout of the OTA and the following is a bunch of photos and links showing progress (or lack there of!) of our telescope.
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December 2004 The blank shown is moulded Suprax from Schott and is the same material used in my 10inch f5. It is a "thin" blank having a diameter to thickness ratio of 8:1 means it thermally stabilizes in 30 minutes unassisted by fans or the scope can be used straight away if I have my tube ventilation system going. The material takes a nice black polish and the figure was very controllable. The concentric rings shown through the glass from the underside will be the first to go. |
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16th January 2005 Two hours with 60 grit carbo and the ribs are almost gone and then another hour with 100 grit left me with the mirror back flat to within a thou concave. I'll grind the back further up to 400 grit as I do the optical face but that can wait. Click here (0.6 Mb) to download a short movie of my grinding machine called Donkey hard at work. Hint, run the movie in loop mode to study the action to the best effect. Next, hogg the optical face to a 5.2mm sagitta. |
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30th January 2005 About 8 hours on and off to grind the sagitta to 5.2mm as can be seen in the reading of my spherometer. The only incident of note was when I left Donkey working while I got on with some wood work. Well, pretty soon I forgot all about Donkey and the tool dried out and glued itself to the mirror due to all that nice glass and carbo mud. It was the screaming clutch that was the alarm and no amount of pushing and cursing would shift the tool. |
 
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I used my trusty 120mm steel tool that has now hogged three 10inch mirrors and is still going strong. Next up, a smoothing tool made by casting some dental plaster on the mirror's curve. After curing I leave it sitting on 3 nails to dry for a week before sealing and fitting of the glass tiles using liquid nails. |
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February and March 2005 The goal of smooth grinding is to ensure the convex curve on the tool blends exactly with the concave curve on the mirror. An hour with 100 grit carbo followed by another hour with 200 grit carbo did the trick. Fine grinding is just a process of working through the abrasives, each removing the pits left by the previous grit and ends with 2 micron grit. |
April 2005 -Polishing
Another dental plaster tool cast on the mirror and coated with a 6mm thick layer of resin based pitch. The photo below shows various stages of the polishing tool.
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May 2005 -Chasing the Paraboloid This is the Foucault shadowgram of the mirror as it came off donkey. If the mirror was f6 or f7 then this is would be close to a good paraboloid with the knife at the 70% zone. However, this is an f3 curve and I have a long way to go to correct spherical aberration. Look in the centre and you may notice a "flat" area with a small pimple right in the centre. Both were removed during the first 10 minute parabolising session. I have done 1 hour of parabolising with the polishing tool and have reached about 20% correction but can't go further and I will have to resort to smaller tools. |
| Last updated - 13th May 2005 | Return to Dave's HomePage | email Dave (d a v e g @ t p g . c o m . a u) |