I tried to build a "trackball telescope" some time back... using my 10" mirror.
I used a 16" diameter acrylic sphere which I bought from True Value Hardware. The problems were:
1) a 16" sphere is too small for a 10" mirror; the mirror sits too high in the sphere so a ton of counterweights are needed on the bottom of the sphere; this results in the mirror "box" having an eye-popping weight (I put in 15 lb of weights, plus the 10 lb of the mirror)
2) acrylic is too weak; the sphere is supported on 3 teflon bearings, but with the great weight of the sphere and the small surface area of the teflon pads, the sphere "dented." I tried again by reinforcing the interior of the acrylic sphere with fibreglass tape + epoxy (a very messy operation) but the bottom line, 16" sphere is too small...
I've noticed that one can order 18" to 24" diameter polycarbonate spheres from places in the US. These are for outdoor lighting fixtures. Does anyone know if there are dealers for these in SG?
source for polycarbonate spheres in SG?
- orly_andico
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While a spherical dob has some nice features, there are some inherent design issues for you to consider if you want to DIY:
1. It is not easy to get or make a sphere that is structurally sound, not just at the base supports, but also at the truss end supports.
2. It is huge to use any sphere larger than 16", which will have an issue fitting into a standard car. You can make a smaller mirrorbox for a 10" mirror.
3. A spherical dob will always be bottom-heavy, ie, you will need to put counterweights (battery etc) below the primary mirror. It cannot be efficient in design otherwise. A spherical dob usually will be heavier than a properly designed standard dob without counterweights at the bottom.
4. A spherical dob will always have balance/equilibrium issues: once you decide to use a heavier eyepiece, or bino-viewer, or camera etc, it will be off-balance, and it is not easy to remove the glued primary mirror and add weights.
5. it will be slightly trickier to resolve thermal equilibrium issues, since you need to cut holes and/or add fans into an enclosed sphere.
6. Compared with a mirrorbox dob, you have to be careful not to 'dent' the sphere in the long run.
Just a few thoughts for you to consider. There is a reason why around 99% of DIY dobs are not spherical dobs.
1. It is not easy to get or make a sphere that is structurally sound, not just at the base supports, but also at the truss end supports.
2. It is huge to use any sphere larger than 16", which will have an issue fitting into a standard car. You can make a smaller mirrorbox for a 10" mirror.
3. A spherical dob will always be bottom-heavy, ie, you will need to put counterweights (battery etc) below the primary mirror. It cannot be efficient in design otherwise. A spherical dob usually will be heavier than a properly designed standard dob without counterweights at the bottom.
4. A spherical dob will always have balance/equilibrium issues: once you decide to use a heavier eyepiece, or bino-viewer, or camera etc, it will be off-balance, and it is not easy to remove the glued primary mirror and add weights.
5. it will be slightly trickier to resolve thermal equilibrium issues, since you need to cut holes and/or add fans into an enclosed sphere.
6. Compared with a mirrorbox dob, you have to be careful not to 'dent' the sphere in the long run.
Just a few thoughts for you to consider. There is a reason why around 99% of DIY dobs are not spherical dobs.
Richard Low
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- orly_andico
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hi Richard,
agree with all your posts. I actually completed a spherical dob but encountered many of the problems you mention: difficulty of collimation, very heavy base (15 lb barbell weights inside), denting of the sphere...
but the thing is my carpentry skills are bad. so using a sphere has benefits in that it's complete.
agree with all your posts. I actually completed a spherical dob but encountered many of the problems you mention: difficulty of collimation, very heavy base (15 lb barbell weights inside), denting of the sphere...
but the thing is my carpentry skills are bad. so using a sphere has benefits in that it's complete.