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Nstiesi's 4" f/10 Newt

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#1 nstiesi

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Posted 16 July 2019 - 11:47 AM

My 8" Celestron to DIY dob transformation is heading for the home stretch, and I just can't help start thinking about my next project, which is mostly meant for my son. He's 6 and not the most patient, but he does get a kick out of using my powerseeker 70az on the moon. I am hoping this gets him hooked and can help me teach him some calm and patience.

I got a 6", thick walled cardboard shipping tube from work and couldn't help but see mirrors inside of it. I rescued from the trash and ordered a 4" spherical primary from Sheldon Faworski, and and eBay .866" secondary.

I am taking this build just a tad further on the DIY end as this time I will be building my own spider and focuser, whereas I simply adapted these existing components on my 8". (Who knows, maybe my 3rd project I will try and make a mirror.....)

I am starting out with the focuser. Not breaking ground here. I read Pierre Lemay's page on DIY helical focusers, then put my own spin on it.

The draw tube will be pvc. The base will be 3d printed in pla (at least to start, but maybe permanently), and the bearing posts turned on the lathe from 1/4-20 bolts or rod. A few details not pictured (#4 screws to retain the bearings, but allow removal, the pressure set screw), but you get the idea. The posts are set at 2.5 degrees. I am hoping my printer can accurately enough produce that slant on the holes so the tap can follow. Hopefully I can work on this a bit tonight!

Thanks!

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Edited by nstiesi, 16 July 2019 - 11:57 AM.

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#2 nstiesi

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Posted 17 July 2019 - 06:57 AM

Made a bit of progress on the 1st prototype.  I had to get creative with the PVC since I couldnt find a pipe that had enough ID and OD stock to clean up.  My solution was to turn the OD of a 1" standard coupler to fit inside of 1-1/2" pipe, cement them, then cement a 1" standard pipe into the coupler.  I faced off both sides, turned the OD to size, bored the inside for an eyepiece and away we go.

 

I also printed the base.  It is going to need some work because in tapping the 1/4-20 holes, I relied on the tap to follow the hole to remain at 2.5° to the base, but the hole was so shallow this did not happen.  I used a soldering iron to heat a screw and re-set the tilt, but Im not sure how accurate the result was.  It "looks" about right.  I may 3d print a bushing with a 2.5° skew on one side to go between the bearing and the base to set this relationship; there is enough play in the fit of the tapped hole I think.

 

I just need to make the bearing posts then I can see what I've got.

 

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Edited by nstiesi, 17 July 2019 - 07:01 AM.

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#3 bcarter1234

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Posted 17 July 2019 - 07:25 AM

nstiesi,

 

It might not help but if you need PVC that has a useful ID for 1.25" eyepieces look at 1-1/4" Schedule 80 pipe. It is rated for higher pressure so the wall thickness is greater. I picked up a 10' stick at Lowes for about $10. You should measure the ID before buying as there is variation. Some pieces will be just under 1.25" ID which is perfect as you can then bore them to a nice slip fit. So far I've been threading them at 14TPI and making mating female pieces for focusers. I like the fine adjustment they provide but would like to try the style you are using at some point.

 

Take care,

Brent



#4 nstiesi

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Posted 17 July 2019 - 09:28 AM

Thank for the tip! I was trying to use sch 80 but couldn't find the sizes I needed at my local big boxes.

The nice part is that I had all the pieces I ended up using on hand. So far this focuser is costing me $0.00.

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Edited by nstiesi, 17 July 2019 - 09:28 AM.


#5 ed_turco

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Posted 17 July 2019 - 10:39 AM

Your son is one lucky kid :)


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#6 nstiesi

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Posted 17 July 2019 - 09:51 PM

Printed a few more parts and put it together. It really works quite nicely. Very smooth.

To Mr. Lemay: what an elegant and effective concept. I appreciate you sharing your design with the community.

The presence of the bearings under the base will make the tube cutout a little tricky, but it should be manageable.

I wish I could have used smaller bearings, like in the cad model....but the math using the smaller bearings I had on hand would have required the draw tube wall to be unacceptably thin.

I may print another base with the bottom bosses integrated, I need to install a thumb screw in the draw tube, get/cut screws to appropriate length, and I need to come up with stops so the tube can't be rotated out. Other than that, this went well and I should be free to move on!0548c02b79ccfc7051dcf3ac2119b36b.jpgb14413d0424187d8d73166e3bfcb6cbc.jpg

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Edited by nstiesi, 18 July 2019 - 06:29 AM.

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#7 nstiesi

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Posted 19 July 2019 - 08:26 AM

Some pencil sharpening is needed on this one.  After making some improvements last night, I realized that the bearings on the bottom extend too far into the tube, and will severely limit in-focus before the draw tube gets into the light path.  Tonight (hopefully) will be another design tweak to get bottom bearings to sit slightly above the focuser base.  I am aiming for a minimum of 1.5" total travel, with the bottom of the draw tube at least 3/16" away from the light path when fully racked in.  I estimate that the bottom bearing must be approximately 1/4" above the focuser base...I will be able to calculate the exact number tonight when I throw some calipers on the thing and can look at my CAD models.


Edited by nstiesi, 19 July 2019 - 08:27 AM.


#8 nstiesi

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Posted 21 July 2019 - 07:20 PM

After getting the bearings on the outside of the OTA, I ended up with rigidity issues resulting from moments imposed by the tensioning system.

After a few more design iterations, this incarnation sorted everything out. Only thing I may do is continue to optimize sizes of components in order to keep the draw tube from getting too far into the OTA.

It ended up a little chunky, but it is rigid, and pla is light so it's not bad. It's size was apps dictated by the size of the free bearing used, so I can't complain about that!66b263cdd14c231182b198ed68b51189.jpge2036132abbbc702395a95b84b007147.jpg

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#9 nstiesi

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Posted 23 July 2019 - 09:31 PM

3d printed spider. The vanes are only 1/32" thick, which is not much for PLA. not awe inspiring in resisting torsion, but it should see much torsion in use. Due to the height of the vanes, it seems pretty stiff in supporting the moments causes by the secondary. Its an experiment to see what I can get away with in a small scope, but maybe it'll work.a09e4dd7b66db142aae7a5e2641c4465.jpg105cd49fef4f1d528424bd73f2207925.jpg

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#10 nstiesi

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Posted 25 July 2019 - 11:02 PM

My impatience and enthusiasm is turning this into a nearly fully 3d printed scope....not that there is anything wrong with that?

Anyway I did a one piece mirror cell. It may need some improvement in the name of corner radio where the clips meet the base to guard against stress cracking....but there really isn't much bending force there. Just enough to keep the mirror centered. Well see how this whole thing holds up.693dc9f2d382a86ff206f84e248619b1.jpg3311f562ca112232228f0111afaff793.jpg

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#11 mcmaker

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Posted 26 July 2019 - 03:06 AM

Awesome! Looking forward to see completed telescope :D

#12 luxo II

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Posted 26 July 2019 - 03:36 AM

Regarding the mirror cell, I'd file the ends of the retaining clips back so they don't stick into the optical path but leave just enough to stop it falling out.



#13 John Miele

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Posted 27 July 2019 - 09:34 AM

I think with your low weight optics you just might get away with it!

I think the plastic might contract a lot in the cold. Make sure you don't pinch the primary!

#14 nstiesi

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Posted 27 July 2019 - 09:43 AM

I think with your low weight optics you just might get away with it!

I think the plastic might contract a lot in the cold. Make sure you don't pinch the primary!

That's the hope!

Also, I'm in Florida so I shoudnt have to worry about the cold. I'm more concerned about the stinkin humidity making the plastic brittle.

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#15 nstiesi

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Posted 31 July 2019 - 10:18 PM

Patterned cloth applied to cardboard shipping tube and soaked with two coats (so far) of shellac.

Also hit the inside with flat black Rust-Oleum.

Hopefully my bubble smoothing efforts hold overnight as it dries. Tomorrow I may lightly scuff it with sand paper and do a final coat, assuming it sets up strong enough.ac95ac79180266565d5f62715fe5f3b4.jpg

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#16 ccaissie

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Posted 01 August 2019 - 03:35 AM

Cool tube.

I once used paisley print on my old Valiant fender.  Way to go... 



#17 nstiesi

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Posted 01 August 2019 - 10:14 PM

So the shellac actually hardened quite nicely. The fabric certainly isn't going anywhere. The ends are a little sketchy, but the end rings should take care of that.

I don't have any pics yet, but the tube was too short for this mirror. The mirror cell is actually stood off the end by about 4 inches based on my calculations. Functionally, my attempt at a Star test revealed that my cell is about a half inch too far back. I will have to print new standoffs when I get more filament tomorrow.

Also, as predicted the little fingers were too brittle and snapped when I tried to put the mirror back in. I'm gonna thicken them up, add fillets, and back off on the interference. If it doesn't work still, I may change to a more traditional design.

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#18 nstiesi

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Posted 03 August 2019 - 10:50 PM

I had to print a new mirror cell since the tabs on the original broke. I thickened then and added some generous radii at the bases of the tabs.

I also printed the front end ring....again. I shouldnt have printed the first one before adding the fabric, which ruined the fit.d61240d2b3b189a8a42428eb5a09339c.jpgf31336c74dcc092a10b89a413d98fa40.jpg

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#19 nstiesi

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Posted 04 August 2019 - 07:42 PM

Shortened the extension and remounted the mirror.508873518b345e22f49470511a595a3c.jpgca0d3f7c6e7125a1d19c7aebfa16e781.jpg

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#20 Adun

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Posted Yesterday, 10:10 PM

This scope is looking great!

 

I'm thinking I could us some "calm and patience"




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