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BIG dob ready for first light


mdstuart

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  • 3 weeks later...

Excellent job Mark, it makes a huge difference having some wheels on this size scope. I'm still trying to figure out an elegant solution for mine, but the base is a thin circular cast metal affair so not much area to work with.

You'll be up and running quickly with this set-up 👍 

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So top tip 37. Allow a cm of slack rather than setting the wheels on a level floor. With the carpet or grass the wheels will be slightly too high to easily attach as they are pushed up.. Maybe I can just put the dob on a mat to level things when observing.

Also longer lower angle PTFE bearing material added as v1 was a bit rough and stiff. 

Tip 38 PTFE can be cut using repeated strokes with a Stanley blade and 4mm stuff can be bent by hand..😀

16017329975462105505222.thumb.jpg.007dceab6bea00fccd4dc3df6752baae.jpg

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So first light tonight. The mirror cell is working well holding collimation and no astigmatism present so that's good.

Balance seems about right..

Still getting some rotation on the top uta connectors even with star washers so a bit more work needed on them.

Alt motion still stiff to get going but smooth once in motion even with silicon wax...any ideas anyone?

Much easier to set up and take down than the previous set up so that's good.

IMG_20201006_220854.thumb.jpg.cf7de6a0f21d00cce78e7c2f76a232f0.jpg

Shroud order placed with my wife.

At least I now have a useable set up again. I had a look at Mars, a few galaxies and M13 etc.

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11 hours ago, mdstuart said:

Alt motion still stiff to get going but smooth once in motion even with silicon wax...any ideas anyone?

I'd say reduce the length of the PFTE pads - the more surface area in contact the more stiction. Once smaller, you can also adjust the position of the pads to achieve a nice feel.

Looking good - nice to see the old scope 'reborn'!

Callum

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I gave up on PTFE as prime bearings on large Dobsonians in favour of shielded stainless ballraces.  PTFE is then used solely to dampen the free movement to taste.  This method also easily permits adjustment to the "feel" of the manual movement to be equal on both axes, paramount to smooth operation.    😀

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12 hours ago, mdstuart said:

Alt motion still stiff to get going but smooth once in motion even with silicon wax...any ideas anyone?

Hi Mark, your project is looking great and very inspirational! Couldn’t really tell from the pictures but maybe you mentioned it but what surface have you put on the rockers to run on the ptfe? Is it the “recommended” Formica stuff? Also i’ve read that the spacing of the ptfe pads has an influence on the friction but that might be the moving friction rather than the stiction. (narrower spacing = lower friction) It might be worth trying spacing the pads further apart which should increase the running friction to bring it more into line with the stiction, though it will increase the contact pressure possibly also increasing the stiction 🤔

Ptfe cof actually decreases with pressure to an extent so making the pads smaller might help, though I think the textured formica can bite into the soft ptfe increasing “stiction” if the pressure’s too high. It’s a conundrum!

Mark

Edited by markse68
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Stiction update. Silicon wax helps movement but not stiction.

Halving the length of the PTFE pads did not help so back to full length to keep psi below 15lbs per square inch.

Sailkote is better. Stiction reduced by 50%.

Also. Jubilee clips strategically applied to truss poles avoids top brackets twisting which helps hold collimation better.

Mark

I hope my experience helps others.

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