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Build complete, DIY low-profile 12" dob

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#1 Grays

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Posted 15 March 2013 - 09:16 AM

Gallery:
http://imgur.com/a/IGk5O

Reddit r/DIY thread:
http://www.reddit.co...lt_a_telesco...

SketchUp Model screenshot:
http://imgur.com/XEGOBdb

I geared the gallery and DIY post toward the general public, but I'm interested in the opinions of other ATMs. What do you think? :)

#2 Pinbout

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Posted 15 March 2013 - 09:38 AM

great cell, but I would turn the collimation bolts around so they pop thru the aperature plate on the mirror box. you won't believe how nice it is NOT to have to reach around the mirror box to collimate, OMG the difference is night and day. Why didn't my momma tell me about front collimation bolts when I was young! :grin:

#3 gmussman

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Posted 15 March 2013 - 11:21 AM

Um, wow, nice sketchup plans! Really beautiful scope.

I'm curious how you attached your mirror to your cell. Is it glued at the rubber pads?

And how did you balance it? Did you have to counterweight?

#4 RossSackett

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Posted 15 March 2013 - 11:23 AM

Nice build, Grays.

Danny, I totally agree on front-box collimation. I put extenders on mine so I didn't even need to bend over to collimate. With a Cheshire and autocollimator I can get it dead on in just a few seconds. I wish my SCT was 1/10th as easy, and then I only need to tweak the secondary.

Ross

#5 Grays

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Posted 15 March 2013 - 05:18 PM

Thanks for the positive feedback!

I would turn the collimation bolts around so they pop thru the aperature plate on the mirror box.


I considered that, but opted not to for three reasons: I don't mind getting behind the scope to collimate, I *really* liked the feel of the 4" bolts I had on hand already, and I didn't want the hardware to be visible. You'll notice that there are no nails or screw heads exposed anywhere on the box or almost anywhere on the base...this was by design. I wanted it to be aesthetically pleasing as a finished piece, and the compromise of not having collimation handles sticking up wasn't a difficult trade-off.

I take your point, though, and appreciate the suggestion. :)

I'm curious how you attached your mirror to your cell. Is it glued at the rubber pads?


No, they just sit in place. I'm having difficulty coming up with a way to secure the mirror on the edge bearings because of the limited space above the nut, but with the bracket on top, the mirror won't tilt forward. It can still potentially kick out from the bottom, but until I come up with something , there isn't much I can do on that one.

And how did you balance it? Did you have to counterweight?


After my disastrous first scope that came out extremely bottom-heavy, I decided to perfectly balance the entire optical assembly before designing the bearings and base.

I designed and built the secondary cage, mirror cell, mirror box, and truss structure completely without even drilling holes for the bearings. I put it on a board and rolled it across a dowel rod until I found the exact balance point. I calculated the balance of the optical assembly without the board, then calculated the effect the bearings would have on the balance. From that, I had a center point.

Only after I had the center point did I design and build the bearings and base assembly. The scope is perfectly balanced with a lightweight eyepiece.

#6 herrointment

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Posted 16 March 2013 - 01:02 AM

Very nicely done!

Cool router compass!

#7 Mirzam

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Posted 16 March 2013 - 05:05 AM

Nice job and an interesting design!

I'm wondering about the flex rocker style azimuth bearing assembly. Does this serve to lower the overall profile or is it mainly to reduce weight? I guess that if the mirror box can actually be recessed into the void created by the large central hole in the base ring then the overall height and the balance point can be lower? An alternative from the weight reduction standpoint would be to route out the center of a conventional design rocker box leaving one or more struts to hold the center bolt in position (as described in Highe's new book).

Also, a weak link in these designs is the focuser mounting board, which is prone to flexure. I'm wondering how you solved this problem? For my own builds I tend to use heavy L brackets to hold the board, mainly use lightweight eyepieces, and dynamically collimate by slightly weighting the laser to simulate the weight of an eyepiece in the focuser.

JimC

#8 Ed D

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Posted 16 March 2013 - 08:44 AM

WOW, what a beautiful scope. :bow: I really like the way you designed and made the base. :waytogo:

Ed D

#9 Grays

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Posted 16 March 2013 - 12:50 PM

Very nicely done!

Cool router compass!


Thank you! I did a small gallery on the router compass a while back, if you're interested in making one:

http://imgur.com/a/oKtdl

I'm wondering about the flex rocker style azimuth bearing assembly. Does this serve to lower the overall profile or is it mainly to reduce weight? I guess that if the mirror box can actually be recessed into the void created by the large central hole in the base ring then the overall height and the balance point can be lower?


Unfortunately, no. The box is slightly wider than the void + skate bearing, so if it gets any lower it bumps against the bearings. I actually had to get bolts 1/2" shorter so they would stop hitting the box.

The purpose is twofold: one, it looks nifty. ;) Two, my first scope build has a center bolt bearing and I can't stand it, I wanted to try something more stable and less finicky. I had seen some similar roller builds and opted to try it, both to reduce weight and to make something interesting.

These bearings perform beautifully; the fit is perfect, I actually have to loosen the bolts just slightly for it to roll smoothly, so it doesn't bounce around at all on the track. The 3/4" ring is more than sufficient to support the 40 lb. optical assembly without flexing (and for heavier scopes later, I can always double the ring thickness if needed). It was such a massive improvement over a single bolt that I doubt I will ever do a single-bolt azimuth bearing again.

Also, a weak link in these designs is the focuser mounting board, which is prone to flexure. I'm wondering how you solved this problem?


While I am having some difficulties, it isn't from the focuser board. It's a lightweight helical focuser on a board supported by two stubby blocks mounted securely on the ring; the whole thing barely budges at all. By popping the spider up on L-brackets, it allowed the focuser to mount much closer to the ring, reducing the distance for the flexure you describe.

The problem I'm having is the focuser itself; I don't think I like the helical design very much. There's enough slack in the focuser threads that the eyepiece bounces around very noticeably, far more than any flexure in the mount. It's a minor annoyance, but I probably won't use a helical on the next build.

Really, the main problem I'm having is with my truss design. After an extended observing session last night, the more I am finding that it's not keeping as rigid as I had originally thought it was. The entire idea was experimental, and while it still performs adequately, lesson learned: no more string trusses as the *only* structure for rigidity. Straight, clamped-down trusses supplemented by strings might hold shape better, but the only thing my poles do is create compression force against the string tension, and the strings just aren't enough to hold stability. It performs adequately, but only just. A six-truss or eight-truss design would have worked better.

WOW, what a beautiful scope. I really like the way you designed and made the base.


Thanks!

#10 m. allan noah

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Posted 16 March 2013 - 03:22 PM

Really, the main problem I'm having is with my truss design. After an extended observing session last night, the more I am finding that it's not keeping as rigid as I had originally thought it was. The entire idea was experimental, and while it still performs adequately, lesson learned: no more string trusses as the *only* structure for rigidity. Straight, clamped-down trusses supplemented by strings might hold shape better, but the only thing my poles do is create compression force against the string tension, and the strings just aren't enough to hold stability. It performs adequately, but only just. A six-truss or eight-truss design would have worked better.


I think your big issues there might be 1. your choice of string material, 2. your choice of lower string mounting points (as required to keep the strings out of the light path with three struts) and 3. the small diameter of your struts.

I assume you have read this:

http://dbpeckham.com...StringScope.htm

allan

#11 Pinbout

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Posted 16 March 2013 - 03:40 PM

Really, the main problem I'm having is with my truss design. After an extended observing session last night, the more I am finding that it's not keeping as rigid as I had originally thought it was.



you can still switch truss design :grin:

#12 GilATM

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Posted 17 March 2013 - 12:35 AM

I like the scope! I also like the compass - I'll have to make one!

Gil
Ventura

#13 jpcannavo

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Posted 17 March 2013 - 06:02 AM

I considered that, but opted not to for three reasons: I don't mind getting behind the scope to collimate, I *really* liked the feel of the 4" bolts I had on hand already, and I didn't want the hardware to be visible. You'll notice that there are no nails or screw heads exposed anywhere on the box or almost anywhere on the base...this was by design. I wanted it to be aesthetically pleasing as a finished piece, and the compromise of not having collimation handles sticking up wasn't a difficult trade-off.


Wonderful craftsmanship! I hope you work out the rigidity issues.

Could not help but commenting though, a front collimation mechanism can be very uncluttered and aesthetic. You could almost miss it in this picture:
Front Collimation

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#14 Grays

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Posted 17 March 2013 - 01:09 PM

I think your big issues there might be 1. your choice of string material, 2. your choice of lower string mounting points (as required to keep the strings out of the light path with three struts) and 3. the small diameter of your struts.


The string material is 1/16" wire, I didn't think it would flex at all. If that is a source of the problem, I'm quite surprised. I had just assumed that it would keep its form.

As for the mounting points, I'm not sure what you mean by my choice. The mounting points defined the properties of the box; the width and length of the mirror box are what they are specifically to allow for equidistant mounting points. They are in a precise equilateral triangle centered on the mirror. Could you elaborate on what you mean?

As to the diameter of the struts, that's possible, but it may also be the material and may be the fact that they are not "clamped" in place. I had thought it would not be necessary because the strings were creating the rigidity, but the trusses are just creating compression force, and that's it.

I assume you have read this:

http://dbpeckham.com...StringScope.htm


What...how did I miss this?! Unbelievable, this is perfect information. I researched string trusses as I was planning the build, I have no idea how I never saw this website. I was mostly going off of pictures of other designs and trying to figure out what might work.

There are many flaws in the design that I am only now realizing...I wish I had seen this sooner. The feedback in this thread has been valuable, and I anticipate I will be rebuilding this scope at a later date (or at least modifying my current build).

Could not help but commenting though, a front collimation mechanism can be very uncluttered and aesthetic. You could almost miss it in this picture:
Front Collimation


Interesting. I hadn't considered a removable collimation rod. I may try that on an upcoming build of a 16" mirror sitting on my shelf.

Thanks again to everyone offering positive feedback and suggestions. I'm nowhere near finished building, especially once I complete my CNC machine. I have two more scope projects on the immediate horizon (rebuild of the original 12.5" and a new build of the 16"), so I may not return to fix this 12" for a while, but when I do, I'll be taking the advice on this thread. I appreciate the input!

#15 droid

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Posted 18 March 2013 - 03:00 AM

Grays; beautiful scope, well done






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