ICEINSPACE
Moon Phase
CURRENT MOON
Waxing Gibbous 84.8%
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05-11-2018, 04:04 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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(20") My yahooing was a little premature. I had a better look today with a ke tester and it turns out I have a central hill. No biggy, I'm not quite finished polishing so I'll try moving the tool in a smidgin to see if it will fix itself.
I bought a laser collimator yesterday to use later but good for testing the haze on the mirror. I would have said the mirror done but laser says no.
It's a bit ironic that I was following Gordon Waite's tutorial and I note he also had a central hill prior to parabolizing.
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05-11-2018, 06:46 PM
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Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
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I have actually been using a laser collimator too!
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07-11-2018, 09:25 AM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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I need to upgrade my turntable. I no sooner get the lap warmed up and working properly when the motor gets too hot and I've got to quit the session. The session only lasts about 10 minutes when it really needs an hour or more. If you recall I've got 2 el cheapo drill presses joined together but they are not designed for continuous running, I need a 1/2 horsepower.
Even though I'm nearly finished polishing I've still got the secondary to polish.
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11-11-2018, 05:22 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Picked up a motor at the markets this morning, now I need a pulley.
I've been working on the UTA, fashioning the 6 x 25mm ally tubes up to the upper cage. I didn't want to buy 6 x 3m tubes because of the expense, I opted for 3 tubes and cut them in half, which left me with a longer than usual upper cage. I think I said before I've got coarse adjustment on the mirror cell and the secondary has got fine adjustment on the outer edge of the spider, so the entire spider, secondary backing and secondary all move as one. Apparently the mirror spacing is quite critical so I want to try and get it right the first time. (Yes, pics!)
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16-11-2018, 09:01 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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(20") New motor up and running, did a solid 1/2 hr polishing, then did a comprehensive test. Haze test is good, Ronchi test is good, KE slight raised hump in the centre and a little zoney. GW says to smooth it with a 12" lap, so that's what I shall do.
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16-11-2018, 09:11 PM
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Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
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Just curious, is GW's advice directly or some sort of forum?
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17-11-2018, 04:49 AM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Gordon Waite has a whole bunch of tutorials on Utube. His company, Gordon Waite Research, manufactures a Renegade 20" Dobsonian, and he lives in New Jersey. He personally gave me advice about 12 yrs ago when I was making my 16".
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03-12-2018, 04:58 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Had a bit of a rest from pushing glass and did a bit of work on the uta and made up the secondary holder. Its just a 8" circle of ply held flat by 3 lengths of 25 x 1mm aluminium spider. The spider is attached to 3 dovetail slides and there is a length of 3/16" threaded rod through the middle for fine adjustment.
In the meantime I started reading ATM book 1 for the 3rd time. You know how I said the mirror was done, well after reading ATM for a bit, maybe its not so done after all. So, I did 2 x 1 hour sessions at 10rpm and no weight and the surface improved no end. I may do another hour or two then tackle the slight hump in the middle.
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08-12-2018, 05:49 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Did a bit of work on the central hill with a 4" tool. It's very time consuming when you polish for 5 minutes, then let the mirror cool down for 30 minutes, then test for a bit, then cold press for another 30 minutes, then the rellies come and want to know what I'm doing. At the end of the day the hill is diminishing, ready for a new 12" lap to try and smooth the surface.
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12-12-2018, 05:37 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Dumb! Did the aforesaid 3 hours smoothing on the 20" without rotating the mirror on the turntable. Even when testing I would pick it up, place it on the test frame, plonk it back on the turntable and carry on never assuming for a minute I was doing a bad thing, until it was too late. Now I have polished in some astigmatism! The book blames the turntable. I'll have to strengthen it a bit and put in a 4th jockey wheel. Probably the 380mm x 40mm concrete tool didn't help. It's already 3 layers of 12mm marine ply so you would think it was ok.
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10-02-2019, 11:48 AM
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Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
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Any news?
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11-02-2019, 08:21 AM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Hi Logan.
Nah, but I do think about it occasionally though. Its been too hot and humid to play in my tin shed. But thanks for asking, I'll keep you informed on any progress.
Bill
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11-02-2019, 09:07 AM
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Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
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hope you brought the pitch lap inside!
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27-04-2019, 05:47 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Back at it. Whipped up a new 6" lap for the 8" secondary and spent 2 hours polishing. KE looked good.
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27-04-2019, 08:33 PM
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Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
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Nice! Ronchi pics please!!
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28-04-2019, 12:18 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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I don't have much faith in Ronchigrams. When I was measuring the 20" the ronchi looked clean as a whistle but the ke showed a distinct raised centre.
Polishing is done on the secondary, now to figure an ellipse.
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02-05-2019, 03:50 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Ellipse done. I used a 4" star lap and worked on the centre zone for about 10 minutes. Knocked up a bench tester to test the 2 focal points and after a bit of messing around got it to work. I had always thought that the secondary radius of curvature and the shorter P focal point were one and the same, and constructed the upper cage to suit. Turns out to be about 100mm shorter, didn't like the cage anyway.
Working to clean up the primary before I can star test.
Last edited by Billyboy78; 02-05-2019 at 04:05 PM.
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05-05-2019, 03:35 PM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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Pressure is on. My daughter has just bought a house with a granny flat and wants us to move in with them. This means I have 6 weeks to get the scope up and running 'cos I don't want to have to dismantle the turntable and test equipment and reassemble it in the new house.
I'm using a 12" star lap atm but there is not much movement yet.
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05-05-2019, 04:06 PM
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Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
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uh oh!
At least it's F/5 - you should easily be done in that time!
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06-05-2019, 08:40 AM
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Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
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I wish..The 20" primary is F/3.39 and the 8" secondary is F/2.46.
Still battling the astigmatism I polished in. KE says it is around 1mm deep, so nearly there ( ie, 1mm variance in fl). The edge correction has got to be zero so I want to get it as good as possible. Centre correction btw is 18.76mm
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