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Construction of my 13" Ball Scope...

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#1 astrobug

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Posted 23 July 2014 - 12:57 PM

Hi all. I posted a while back asking about sources for a 24" diameter sphere to use for my ball scope project. Well, I bought the sphere back in March, and am finally going full steam on the build. So I wanted to share my progress to date.

This is what it looks like mostly assembled (still need to mount the optics, do some painting, make the strings, and build a proper base, and eventually motorize with a 'trackball' base):

Posted Image

More photos of the re-build are here:
Magic Infinity Ball Build


The primary is 330mm diameter, ~30mm edge thickness, and ~1500mm focal length (still in the wrapper from the coater). It will be housed in a 6-point cell (formerly from a 12.5" Portaball) inside of a 24" OD polycarbonate sphere from formedplastics.com. The sphere had some 'soft' spots where a firm thumb-press could visibly indent the surface, so it has been reinforced on the inside with a single layer of fiberglass/epoxy. The wood rings inside the sphere were epoxied, then fiberglassed into place as well. The ring at the opening of the sphere stiffens the whole structure greatly. If this weren't going to be on a trackball mount, I probably wouldn't have bothered with the fiberglassing (it usually a PITA, and the curves/angles inside the sphere don't help at all :p ).

There are 3 tripod legs that will hold the upper ring in position and collapse down into the ball for transport/storage. They flex too much to act as the only support for the top ring, so their main function is to tension the 12 strings that keeps the bottom and top rings aligned. This will be a 'stacked' string scope (i.e. 6 strings above the sphere, and another 6 inside of it). 3 turnbuckles going through the middle ring will tension the 3 sets of 4 strings that go between the poles.

On the top end, a curved 3-vane spider holds the secondary mirror. I am especially proud of the secondary mount that I designed for this. It has the simplest possible adjustments: rotation & tilt only. Rotation is the typical threaded rod through a threaded hole in the spider hub + locking nut. Tilt is accomplished by a single knob counteracted by a compression spring. Building was trivially easy, and collimation of the secondary should be greatly simplified. Collimating the primary will be simpler as well; just two knobs at the sphere's opening that can be reached while looking through the eyepiece. Here is closeup of the secondary mount:

Posted Image


Inspiration (i.e. ideas I've stolen) include the following:
Mag 1 Instruments Portaball Telescope
Teleport Telescopes
Dan Gray's String Scopes
Don Peckham's 'Stacked' String Scope
David Nemo's 8 inch String Scope
Jerry Oltion's Trackball Telescope
The Magic 8 Ball toy

I will hopefully RTV (silicone) the optics to their holders tonight or tomorrow and should have a pseudo 'first light' on the refigured mirror this weekend. This will give me the precise position of focal plane, strut/string length, how much counterweight, etc. I hope to have everything but the trackball base done in time for the Oregon Star Party. Of course, as of right now, OSP would stand for the Oregon Smoke Party, considering all the surrounding forest fires.

Anyhow, I welcome questions, comments, & suggestions. I will be busy building it after work in the evenings, so it may be a few days/weeks before I can return to this thread.

-Brett

#2 Herr Ointment

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Posted 23 July 2014 - 03:27 PM

The 8 Ball Sez....YES!

#3 jonathanCR

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Posted 23 July 2014 - 03:57 PM

We definitely need more pictures of the spider and the tilt mechanism. Are you using teflon or nylon as the hub?

#4 gpelf

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Posted 23 July 2014 - 04:32 PM

Looks Great, may I ask what the sphere is/was?
Greg

#5 Pierre Lemay

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Posted 23 July 2014 - 04:50 PM

Congratulations on your design choices. A 24 inch diameter ball will provide you enough radius to mount your Mirror without sacrificing to much on the UTA weight budget. My calculations indicate you will need approximately 14 pounds of weight behind the 12.5 pound Mirror if you limit yourself to 1.25 inch, 82 deg AFOV type eyepieces. You have to try and limit the weight of the UTA to 3 pounds to attain this. Not easy but important and doable. If your UTA is heavier than 3 pounds, you will need even more counterweights behind the mirror, which is not a good thing due to the heat accumulation which would be released during the night and because of the stress on the skin of the ball which could induce local bending and vibration. Another advantage of limiting the UTA to 3 pounds is the operating weight of your optical tube assembly will be around 32 pounds, nice and light.

Because of this you will not have the weight budget for an optical finder, but a red dot is generaly sufficient anyway. I also recommend you get a 1.25 inch helical crayford to keep the UTA weight down, while providing very smooth focusing. At f/4.5 you will not need a coma corrector, particularly if you mount the ball scope on a drive platform like Jerry's design (single axis platform with single drive roller) or mine (dual axis platform with double drive rollers).

I'm looking forward to seing your project unfold.

#6 astrobug

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Posted 24 July 2014 - 11:38 AM

Jonathan-

I'm using a piece of 2" OD x ~3/4" Delrin as the hub. It's white because that's what the plastic shop had on hand. There are 3 holes tapped around the circumference to hold the spider vanes, one hole tapped in the middle to accept the seondary support, and one tapped opposite the focuser for the tilt screw. There are two flat -bottomed holes on the lower face, one for the spring and another for the acorn nut. The spring is held captive by one of the spider vane attachment screws. I'll get better pictures of the hub portion tonight. This is what the secondary holder looks like on its own:

Posted Image

The wood part pivots on the aluminum rod that has been tapped to accept the threaded rod. They are permanently joined with red loctite.

#7 astrobug

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Posted 24 July 2014 - 11:51 AM

Greg-

The sphere is a lighting globe, 24" OD, polycarbonate. I got it here.

It is not constant in thickness (average is maybe 4-5mm), so some spots were much more indentable (with heavy thumb pressure) than others, so I decided to reinforce by fiberglassing--much less impressionable now. Also, there is a little nub on the very bottom (at the center of the infinity sign). This appears to be a vestige of the molding process. If it causes 'bumping' when it contacts the bearing surfaces, I will either rout/sand this down a little or drill it out.

#8 astrobug

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Posted 24 July 2014 - 03:18 PM

Pierre-

Thanks for the words of encouragement and balance estimates. I went with the 24" instead of 22" OD as scale drawings showed that this would lower the mirror by about 1", which makes a big difference in the balance calculus.

I'm not exactly sure which glass the primary is made from--it doesn't look like any typical plate glass or Pyrex, and it even stumped Steve Swayze. (FWIW, I bought the mirror blank from Surplus Shed in ~1998. Edge thickness is actually 29mm.) In any case, the mirror in its old cell weighed 15 pounds. The new cell is heavier, so maybe 16 pounds total now. There are some screw holes tapped off-center in the mirror cell, so I may attach the counterweight directly to the cell.

The old structure had a very light UTA with homebuilt integrated focuser (total weight, including 2" 30mm TMB Paragon EP, 2" 2X Barlow, and Rigel finder, was just 3 pounds!) But this UTA didn't handle my newer 100 degree eyepieces (9mm and 14mm ES) very well, which helped motivate the rebuild.

The new UTA will be heavier as I bought a used Feathertouch focuser to handle the monster eyepieces. I will keep using the Rigel finder, and I am skipping the coma corrector as you (and my wallet/eyes) suggest. After I determine the exact focus position/strut length, I will take it inside and weigh the fully-loaded UTA with the scope horizontal (as the excess torque rolls the ball on the wood floor). Then I will know exactly how much counterweight I'll need. If it's going to take a ridiculous amount, I may swap out for a 2" helical Crayford and (begrudgingly) 82 deg eyepieces.

The previous iteration of this telescope weighed 44 lbs total, including the mount. If the new version weighs less than this, I'll be happy. But if it ends up heavier, so be it. Heck, it's a ball, so if I can't carry it, I'll just roll it between car and observing field! :p

As for thermal management, I want to use it for a while before deciding on a strategy. The counterweight can be insulated so it dumps more heat into the ball than the mirror. The mirror cell is also milled to accept a 4" fan, but then I would need battery, wiring, and a way to supply outside air. Maybe I'll just cover the whole thing in a cooler bag and 'pre-chill' it like the big observatories do! :grin:

Finally, as to the drive system, I was planning to use implement Jerry's single motor setup. But I'd love to see more details/description on your dual-motor/dual-axis drive system, as it sounds like it would be a better solution for photography. I saw some photos of your 20" hemisphere scope on the Stellafane website and all I can say is wow--fantastic scope and fantastic machining work!

#9 Pierre Lemay

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Posted 26 July 2014 - 05:12 AM

I think I miscalculated your tube balance the first time. Let’s do this again. Your mirror and cell weigh 16 pounds. Assuming this mass is located -10 inches below the sphere’s CG, that’s -160 in.lbs (I use negative distance numbers below CG and positive above). The focal length of your mirror is 59 inches (13x4.5). Assuming focal plane located 3.5 inches above edge rays and a primary radius of 6.5 inches, this places the UTA mass at 49 inches above mirror surface. Since mirror surface is about 9 inches below CG of sphere, the UTA is thus located 40 inches above sphere’s CG. If the UTA weighs 3 pounds, it’s moment is 120 in.lbs. Adding to this the contributing moment from the struts, wires and their hardware (about 2 pounds at 20 inches above CG = 120 in.lbs) you get +160 in.lbs. If these conditions apply, optical tube is balanced along it’s length without a need for additional counterweights. In this condition OTA weighs 16+3+2 pounds (21 pounds)+ whatever the sphere weighs (10 pounds?) so, close to 31 pounds. Fantastic!

I seem to recall a Feathertouch focuser weighs 1.5 pounds. I don’t know how heavy the ES 9mm and 14mm 100 AFOV eyepieces are but my 20mm ES weighs 2.5 pounds. So just the focuser and eyepiece would weigh 4 pounds, to which you would probably have to add 2 pounds for diagonal and support, Rigel and UTA ring to hold it all. Six pounds, 3 more than you have now. Since each pound at the UTA means about four below the mirror, you would need 12 pounds of counterweight, bringing your telescope weight up to 46 pounds. Still pretty good for a scope this size. However, keep in mind that when you swap a 2 pound+ eyepiece you will have a lot of difficulty maintaining aim while you insert a higher power eyepiece. This is why a made the two eyepiece turret on my 20 inch.

One thing you must take into account and compensate for with heavier eyepieces and eyepiece holder, however, is tube balance around the optical axis. From experience I’ve found on my dual drive platform I need to keep tube imbalance (whether around optical axis or length wise) under 25 in.lbs. I would assume Jerry’s single drive roller would be a bit more sensitive than this (due to the two idler rollers that reduce friction in two planes VS only one idler on my platform design) and the fixed pad, non driven support (ex Portaball) would be the least sensitive, even when mounted on a Poncet.

For thermal management I had considered at one point mounting a Pelletier cooler as part of a cover for the hemisphere, keeping the mirror inside cool while driving to the observing site. If it was cloudy, I reasoned, I could store beer inside the ball and at least have something to do while waiting for the clouds to go away. :)

In September I will do a write up of some kind on CN to provide a full description of my 20 inch ball scope. I’m just waiting to finish and test improvements I’ve been working on since Stellafane 2013 to make the telescope more robust and fun to use.

#10 astrobug

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Posted 05 August 2014 - 05:47 PM

Pierre-

 

Thanks again for the detailed calculations.  I think your guesstimates are pretty much right on my exact numbers.  Here are the component weights (& approximate distances from sphere center) as measured with an accurate electronic balance:

 

mirror & cell:  15.65 lbs, -8.5"

sphere: 11.18 lbs, -2.65"

tripod sections above sphere: 1.46 lbs, 29.5"

UTA/focuser/finder/light shield: 4.25 lbs, 43"

 

total weight so far (before eyepiece & counterweight) is just 32.5 lbs.

 

Without any eyepiece, the top end 'weighs' 1.46 lbs while rotating about the sphere on a hard surface.  The Feathertouch focuser weighs exactly 1.50 lbs.  So it balances pretty much perfectly before including the focuser and eyepiece.  I am thinking I will add only enough 'permanent' counterweight to offset half the weight of my heaviest eyepiece (14mm ES100, 1.84 lbs).  This will mean it's 0.92 lbs x 39" = 35.9 in-lbs. nose-heavy with this eyepiece installed, nose-light by an equal amount with no eyepiece installed, and pretty much dead on with my other eyepieces.  My balance calculation spreadsheet tells me that the fixed counterweight will be 9.44 lbs if I do this.  If I get a lot of 'creeping' when I'm observing near the horizon with this eyepiece, I could add to the permanent counterweight and/or retrofit some sort of sliding/removable counterweight system. Or I could just use a lighter eyepiece in this situation.

 

In testing it out on my temporary base (bucket rim or a small section of sonotube), the scope has to be pointed below 45 degrees before the imbalance causes it to move spontaneously.  It will have less friction in the base when all is said and done, but I think the unmotorized version of this scope will work just fine.  When I go to motorize, I may need to choose bearing materials carefully to maximize friction (without stalling/stressing the motors) and/or fine-tune the balance accordingly.

 

You are correct in that axial balance will be important.  Jerry describes a neat trick for finding out how far to offset the rear counterweight on his website.  I am using this method.  As a double-check, UTA balance should be axially symmetrical except for the focuser/eyepiece, so a line from the center of mass of focuser/eyepiece which goes through the sphere center should point to the same spot.

 

I think you are on to something with the Pelletier beer/mirror cooler.  I swear that every time I forget the beer on an extended astronomy campout, it clouds up.  I don't make that mistake anymore  :).

 

Finally, here are some photos of the progess so far (strings and optics installed temporarily):

 

14652588979_5c0e284839_z.jpg

 

14652527860_8c7e8b154d_z.jpg

 

Next steps: disassemble, paint UTA, install counterweight, loctite (blue) the turnbuckles, cut the tripod legs down a bit for more compact stowage, build the permanent (non-motorized) base and make a more durable cover (currently using a flimsy potted plant coaster).



#11 amicus sidera

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Posted 06 August 2014 - 03:19 PM

Very, very nice scope! Have you considered using an extra-large stainless steel dog bowl as the base? Lots of folks who own Astroscans use smaller ones, often with the rim covered in a strip of self-adhesive felt to protect the ball's finish.


Edited by amicus sidera, 06 August 2014 - 03:19 PM.


#12 Pierre Lemay

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Posted 07 August 2014 - 12:58 PM

Brett,

Looking very good indeed. If you ever find you need to shave off a few grams from the UTA may I make three suggestions:

-You could drill holes around the periphery of the wooden ring that supports the UTA.

-You could drill holes through the three curved spider legs (that aluminum looks fairly thick. Is it 1/8 inch ?)

-Most important: if the threaded rod and coupling nut that supports the diagonal are made of steel you might consider replacing both of them with aluminum threaded rod and coupling nut. Both are available from McMaster-Carr.  Those two items are the farthest from the sphere CG and are negatively contributing a lot to any top heavy tube imbalance you might end up with.

 

I spent many evenings this past winter shaving off small slices of UTA weight here and there on my 20 inch.  Sometimes it seemed insignificant but it added up. As much as possible I tried to use aluminum nuts and bolts instead of stainless steel and in some instances I even used nylon where strength was not important and, fiberglass bolts (yeah!, they sell those) where strength was important, but I needed electrical insulation (spider legs conducting directly to the Kendrick diagonal heater). My goal was to cut 1.5 pounds, I ended up dropping almost two pounds.  That might not seem much but each pound lost in the UTA meant 4 times less behind the mirror (so 2x4=8 pounds + the 2 pounds from the UTA=10 pounds lighter for the OTA).

 

I must try to go to Oregon sometime.  You ATM's over there are making some fine and original instruments.

 

Pierre



#13 jonathanCR

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Posted 08 August 2014 - 11:10 AM

Nice, really nice. Amazing job. 

 

I was wondering if you have a link for the lower truss clamps that you are using. They look strange to me. Are they used for connecting two poles? Is that how they work?



#14 astrobug

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Posted 08 August 2014 - 03:27 PM

Have you considered using an extra-large stainless steel dog bowl as the base?

 

 

Thanks for the kind words.  I like the dog bowl idea--especially the (truncated) conical shape, which would make it extra stable.  But I don't know that I'll be able to find one large enough for a 24" sphere.  The contact points for the sphere need to be on a radius of ~12-14".  (I have been building/testing it either on a 10" diameter x 3" scrap of sonotube or a 5 gallon bucket with ~12" diameter rim.  The 10" radius is definitely too small to support this adequately.)

 

I would also like to be able to fit the base inside the sphere for transport.  So on this count, a triangle or Y shape allows for larger support radius (opening of the sphere is just 13.5")  My potentially brilliant/idiotic thought was to use a billiards triangle (i.e. the thingy used to rack a game of 8-ball) with support posts added at each vertex.  I've bought one, but haven't started modifying it yet.  If it ends up being too flimsy, I will probably build something similar with thicker/wider material.

 

For the bearing surfaces, I will try what I've already got on hand first (teflon & felt).


Edited by astrobug, 08 August 2014 - 03:27 PM.


#15 astrobug

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Posted 08 August 2014 - 04:05 PM

Pierre-

 

Good advice, as always.  I know that every ounce saved on the upper end is multiplied 5x overall.  But I do need some counterweight for radial balance, and even with 9.44 lbs of counterweight, it will end up pretty much the exact same weight as the old structure  (44 lbs, with heaviest eyepiece), so looks like I will meet the 'not any heavier' design goal.  On the old (single-ring) UTA, I drilled holes to make it as light as possible.  But the focuser portion had noticeable flex, even before the monster eyepieces.  Plus it took more time to finish the wood around the holes.  This time around, I have gone with a solid single ring and reinforced it with a section of 3/4" plywood in the focuser/finder area  (In hindsight, I could have drilled some hidden holes most of the way through and had the same stiffness/external appearance).  You are correct about the spider vanes being 1/8" thick aluminum (3/4" W), perhaps overkill, but again, I was trying to make the secondary support rock solid. The highest priority design goals on this rebuild (in no particular order) are: short setup time, ability to handle monster eyepieces and high magnification, and tracking.

 

I am a big fan of McMaster-Carr and their overwhelming variety of materials, but so far I haven't had to place a new order with them for anything on this build.  

 

Yes, Oregon/Pacific Northwest is a fertile ground for ATMing, but certainly not the only one.  The usual joke around here is 'what else can astronomers do for the 9 months of the year when it's cloudy and rainy?'  Of course, this winter, I was just collecting the parts needed (detached, unheated garage is cold to work in...).  I keep telling myself I'm going to make it to Stellafane one of these days.  I actually went to college in upstate New York, so I wasn't too far away, but I always came back here for the summers.



#16 astrobug

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Posted 08 August 2014 - 04:14 PM

Jonathan-

 

The 'truss clamps' are actually the leg locks from a Manfrotto 293 tripod (http://www.manfrotto...ipod-3-sections).  I detached the legs and inverted them, so they collapse into the sphere.  (I have now cut down each of the sections to minimize closed height, too.)  In hindsight, a nice carbon-fiber tripod with larger diameter leg sections might have been sturdy enough that I might not have needed the strings.  But this was much cheaper, and the leg locks on this won't slip under the tension of the strings, unlike most of the carbon fiber 'pods I've seen.  

 

It also occurs to me that I never got back to you with the additional photos/details of the secondary support/adjustment you wanted.  Here you go:

 

The underside of the hub looks like this without the secondary holder:
14743489275_7f375e200b_z.jpg
The center of the hub is tapped to accept the threaded rod.  The mounted secondary screws in until the secondary's center aligns with the focuser axis.  After fine-tuning the rotation & locking the threads from above, a push-push adjustment (in line with the focuser) fine-tunes the tilt.  I am using a compression spring and screw (through a tapped hole in the hub).

Here is one more view, using a mock-up (pre-made furniture connector) to show the 'T' arrangement without the  wood block/secondary in the way:

14743192932_54b16a0606_z.jpg



#17 dbpeckham

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Posted 10 August 2014 - 02:00 PM

Hi Brett,

 

I enjoyed seeing your telescope at yesterday's telescope workshop.

 

For the ground "board" I suggest that you use tennis balls and a billiard rack.  You could put short dowels in the corners of the rack with a tennis ball on the top and a foot on the bottom.

 

Don




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