Pretoria Centre of the Astronomical Society of Southern Africa
Danie's 10 inch Dobsonian
I took over 4 years on this 10" mirror - and finally it is finished! It was very much an experimental journey, trying out various different techniques: Hogging out with a 6" tool; grinding with a full-size tile-tool (on cast concrete); polishing with a subdiameter lap (8" lap on cast concrete). After picking up problems at the polishing phase, I decided to make the scope shorter and deepend the saggita to 3.97 to create a F5.3 mirror. I picked up sctratches and had to go back to 320 grit to remove these. Finally, I started "polishing" with Aluminium oxide on a 8" lap at the 600 grit stage to avoid the scratches (which worked extremely well!) and figuring with a lap on an old 6" tool; going back to the 8" lap. Worked some more with the 8" lap, finally ending up with an overcorrected mirror = the centre is too deep. Reverts back to the 6" lap, polishing on the 70% zone and finally, after some 35 minutes more polishing with this lap, takes it to Johan again for resting. Johan checks the mirror again and declares it fit for aluminising - what a relief after so many years. The telescope was finished in the meantime and I could assemble it with the un-aluminised mirror installed. Checks it the evening of 30 January 2012 on the first quarter Moon - looks perfect, although the collimation is not up to scratch. Mirror is finally tested again in the completed telescope and brought closer to collimation. We focus on the Moon (brilliantly sharp detail!), Jupiter and Sirius. It is good enough for aluminising.
10 inch Mirror log
| Grit | Amount (containers) |
Date | Stroke | Remarks |
| 80 | 1 | 14 Apr 2007 | COC; TOT | With old 6" glass tool |
| 80 | 1 | 27 Apr 2007 | COC; TOT | With old 6" glass tool |
| 80 | 1.5 | 28 Apr 2007 | COC; TOT; wide 3W | With old 6" glass tool; Sagitta 1.76mm |
| 80 | 0.5 | 4 May 2007 | COC; TOT; wide 3W | With old 6" glass tool; Sagitta 1.92mm |
| 80 | 1 | 5 May 2007 | COC; TOT; wide 3W | With old 6" glass tool; Sagitta 2.08mm |
| 27 May 2007 | Johan Smit cast full-size concrete/tile tool | |||
| 80 | 2 | 2 Jun 2007 | COC; TOT; wide 3W | Sagitta 2.76mm; with full-size tool |
| 80 | 1 | 9 Jun 2007 | wide 3W | Concrete/cement tool almost in contact |
| 120 | 1 | 23 Jun 2007 | TOT; wide 3W | Tool in contact; Sagitta 2.65mm |
| 120 | 0.5 | 30 Jun 2007 | TOT; wide 3W | |
| 120 | 0.5 | 14 Jul 2007 | TOT; wide 3W | Pits removed |
| 320 | 0.5 | 14 Jul 2007 | TOT; wide 3W | Sagitta 2.63mm |
| 18 Jul 2007 | Johan Smit cast full-size polishing tool | |||
| 320 | 0.5 | 28 Lul 2007 | TOT; wide 3W | Pits removed |
| 400 | 1 | 9 Aug 2007 | TOT; wide 3W | Pits removed |
| 9 Aug 2007 | Bevel mirror and tile edges | |||
| 500 | 1 | 10 Aug 2007 | TOT; wide 3W | Pits removed |
| 600 | 1 | 10 Aug 2007 | TOT; wide 3W | Pits removed; Sagitta: 2.63mm |
| 11 Aug 2007 | Bevel and seal polishing tool | |||
| 800 | 1 | 15 Aug 2007 | TOT; wide 3W | Pick up scratch - go back to 500 |
| 500 to 800 |
1 | 19 Aug 2007 | MOT; wide 3W | Scratch again = back to 400 |
| 400 | 1 | 6 Sep 2007 | MOT; wide 3W | Bad contact at first - grid to good contact |
| 500 to 800 |
8 Sep 2007 | MOT; wide 3W | Good contact - pick up scratch again! | |
| 1000 | 1 | 10 Sep 2007 | MOT; wide 3W | Fine grind to smooth |
| 1200 | 1 | 12 Sep 2007 | MOT; wide 3W | Fine grind to smooth |
| 1200+ | 1 | 14 Sep 2007 | MOT; wide 3W | To final fine grinding |
| 1 Jul 2008 | Cast pitch lap on full-size concrete tool | |||
| CeO2 | 1 | 19 Jul 2008 | TOT | Bad contact |
| CeO2 | 20 Jul 2008 | TOT | Hot press; 2 hours polishing | |
| CeO2 | 1 Aug 2008 | TOT | 1 Hour; contact now good | |
| CeO2 | 2 Aug 2008 | TOT | 1 Hour | |
| CeO2 | 8 Aug 2008 | TOT | 1 Hour; last 5 mm around edge of mirror not polishing out | |
| 20 Sep 2008 | Cast new pitch lap | |||
| CeO2 | 1 Nov 2008 | TOT | 1 hour; ring around edge getting smaller | |
| CeO2 | 8 Nov 2008 | TOT | Ring reduced to 3 mm | |
| Decide to reduce F-ratio - re-bevel mirror | ||||
| 320 | 1 | 2 May 2009 | TOT; wide 3W | About 1.5 hours grinding |
| 320 | 3 May 2009 | TOT; wide 3W | About 1.5 hours grinding - f5.1 reached (with Johan's measuring method) | |
| 400 | 1 | 3 May 2009 | TOT; wide 3W | About 1.5 hours |
| 17 May 2009 | TOT; wide 3W | Measure with Johan's sferometer | ||
| 400 | 24 May 2009 | TOT; wide 3W | 1 hour | |
| 24 May 2009 | Cast 8" concrete tool - seal and paint tool - focus length 1350mmMOT; wide 3W - cast pitch lap on tool | |||
| 600 | 1 | 16 Jun 2009 | TOT; wide 3W | "Polish" with 600 grit; 1 hour |
| Jul 2009 | Cast new pitch lap on 8" tool | |||
| Al2O3 | 1 | Jul - Aug 2009 |
TOT; wide 3W | Total of about 3 hours polishing Managed to test with faucault tester - turn-down edge & high centre |
| Aug 2009 | Cast new pitch lap | |||
| CeO2 | 1 | 6 Mar 2010 | TOT; wide 3W | Start polishing with cerium - soon mirror is nearly spherical |
| CeO2 | 7 Mar 2010 | TOT; wide 3W | Polish for 30 minutes | |
| CeO2 | 13 Mar 2010 | TOT; wide 3W | Polish for 30 minutes | |
| CeO2 | 27 Apr 2010 | TOT; wide 3W | 45 minutes | |
| CeO2 | 15 Aug 2010 | TOT; wide 3W | 45 minutes | |
| CeO2 | 17 Oct 2010CeO2 | TOT; wide 3W | 1 hour | |
| CeO2 | 23 Oct 2010 | TOT; wide 3W | 45 minutes | |
| CeO2 | 24 Oct 2010 | TOT; wide 3W | 45 minutes | |
| CeO2 | 12 Jan 2011 | TOT; wide 3W | Test - over corrected! | |
| 22 Jan 2011 | Cast new lap on 6" glass tool | |||
| CeO2 | 2 Feb 2011 | TOT; short on 70% diameter | About 2 rounds around mirror | |
| 5 Feb 2011 | Test; can see "flats" of 5 minute polishing - go back to 8" pitch lap | |||
| CeO2 | 16 Feb 2011 | TOT; short on 70% diameter; 5 min | Test; little more even, but still shape is not right | |
| CeO2 | 26 Feb 2011 | TOT; short on 70% diameter; 30 min | Test; little better; advised to continue to do short z strokes on 70%; edge turn down too sharply | |
| CeO2 | 26 Mar 2011 | TOT; short on 70% diameter; 20 min | With 6" lap | |
| CeO2 | 12 Jan 2012 | TOT; short 3W on 70% diameter; 30 min to improve overcorrection | With 6" lap | |
| CeO2 | 22 Jan 2012 | Johan tests mirror (test good) | ||
| 1 Feb 2012 | Completes telescope - test on Moon, Jupiter and Sirius - Looks perfect | |||
| 4 Feb 2012 | Mirror handed in for aluminising |
Mirror diameter: 254 mm; Sagitta: 2.97 mm; F-ratio: 5.3; Focus length: 1350 mm
Abbreviations: COC= Centre-over-centre; 3W= 1 third W-stroke; MOT= Mirror on top; TOT= Tool on top