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  #101  
Old 05-11-2018, 03:04 PM
Billyboy78 (Bill)
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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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  #102  
Old 05-11-2018, 05:46 PM
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I have actually been using a laser collimator too!
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  #103  
Old 07-11-2018, 08:25 AM
Billyboy78 (Bill)
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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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  #104  
Old 11-11-2018, 04:22 PM
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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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  #105  
Old 16-11-2018, 08:01 PM
Billyboy78 (Bill)
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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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  #106  
Old 16-11-2018, 08:11 PM
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Just curious, is GW's advice directly or some sort of forum?
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  #107  
Old 17-11-2018, 03:49 AM
Billyboy78 (Bill)
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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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  #108  
Old 03-12-2018, 03:58 PM
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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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  #109  
Old 08-12-2018, 04:49 PM
Billyboy78 (Bill)
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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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  #110  
Old 12-12-2018, 04:37 PM
Billyboy78 (Bill)
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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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  #111  
Old 10-02-2019, 10:48 AM
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Any news?
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  #112  
Old 11-02-2019, 07:21 AM
Billyboy78 (Bill)
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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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  #113  
Old 11-02-2019, 08:07 AM
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hope you brought the pitch lap inside!
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  #114  
Old 27-04-2019, 04:47 PM
Billyboy78 (Bill)
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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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  #115  
Old 27-04-2019, 07:33 PM
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Nice! Ronchi pics please!!
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  #116  
Old 28-04-2019, 11:18 AM
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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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  #117  
Old 02-05-2019, 02:50 PM
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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 03:05 PM.
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  #118  
Old 05-05-2019, 02:35 PM
Billyboy78 (Bill)
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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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  #119  
Old 05-05-2019, 03:06 PM
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uh oh!
At least it's F/5 - you should easily be done in that time!
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  #120  
Old 06-05-2019, 07:40 AM
Billyboy78 (Bill)
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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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