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DIY SW Explorer 200P Dobsonian convertion


FranTeryda

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Hi! I really want to get into watching the night sky. After a looking at the scopes that are available in my country, I chose to buy a SW Explorer 200P (haven't yet) and build a homemade dobsonian base. 

I really like the aesthetics of the Orion Sky-quest dobsonian bases, so I draw a really similar design to fit the SW scope I want to buy. But while drawing it, I had tons of questions and I hope you can help me. 

First of all, I just could get some aproximate measures of the parts of the scope out of the internet. If any of you has one of these SW reflectors, I would really appreciate if you can take some measurements for me to make the design more precise.  Specially I really need the thickness and size of the holes of the rings.

About the bearing surfaces, I can't get anywhere the Teflon pads you usually use. What can I use to replace them? Also, what about the radius of the altitude bearings, Is it important? 

Finally, which should be the height of a dobsonian mount? I don't want to be bent over to be able to see comfortably, nor I want it to be  too tall. Is it supposed to be used on a stool or something?

I will really appreciate any other tip you can give me. I have never had a telescope, neither have I seen a dob mount. So I can be making a big mistake trying to convert this OTA to use with a dobsonian mount. 

Anyway thank you very much. Cheers from Argentina

Almost forgot to add some pics of the 3D design I made! 

Screenshot (516).png

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I would try to make the trunnions as large as possible like the Bresser Dob's.  It really improves the altitude motion a great deal.  Also, try to mount the scope dead center to minimize the swing from horizon to zenith and to get the best possible balance.  This may mean adding weights somewhere for balance (likely at the top end).  There's not much reason if any to cantilever the mount backward as shown in your diagram.  Straight up and down is perfect.

Nylon chair slides can also work great for the bearing surface.  Teflon can sometimes be too slippery.  A matte to satin pebbly surface for the bearings is ideal.  Smooth glossy is actually too "sticky" in use.  It doesn't want to let go easily to track an object.  Some smooth satin surfaces will also work, depending on the material.  Think about a chair sliding on a floor with those glides on the bottom of each leg.  You can actually try out different combinations with this method.

The Dobsonian design at 8" f/5 excels when used while sitting.  You want to be bending over the telescope when viewing about 25 degrees above the horizon, typically about as low as you'll want to go.  You want to be sitting straight up when viewing at (or near) the zenith.  This is why minimizing the swing is important.  Mounting it too far back causes a large low to high swing.  That 1000mm focal length will be just about perfect if you can mount the scope's trunnion's center point about 27 to 28 inches about the ground in my experience.  I'm 5' 8", so your comfortable mount height might be higher or lower depending on your height.

Here are some examples of what I'm describing:

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12 hours ago, Louis D said:

I would try to make the trunnions as large as possible like the Bresser Dob's.  It really improves the altitude motion a great deal.  Also, try to mount the scope dead center to minimize the swing from horizon to zenith and to get the best possible balance.  This may mean adding weights somewhere for balance (likely at the top end).  There's not much reason if any to cantilever the mount backward as shown in your diagram.  Straight up and down is perfect.

Nylon chair slides can also work great for the bearing surface.  Teflon can sometimes be too slippery.  A matte to satin pebbly surface for the bearings is ideal.  Smooth glossy is actually too "sticky" in use.  It doesn't want to let go easily to track an object.  Some smooth satin surfaces will also work, depending on the material.  Think about a chair sliding on a floor with those glides on the bottom of each leg.  You can actually try out different combinations with this method.

The Dobsonian design at 8" f/5 excels when used while sitting.  You want to be bending over the telescope when viewing about 25 degrees above the horizon, typically about as low as you'll want to go.  You want to be sitting straight up when viewing at (or near) the zenith.  This is why minimizing the swing is important.  Mounting it too far back causes a large low to high swing.  That 1000mm focal length will be just about perfect if you can mount the scope's trunnion's center point about 27 to 28 inches about the ground in my experience.  I'm 5' 8", so your comfortable mount height might be higher or lower depending on your height.

Here are some examples of what I'm describing:

spacer.png

spacer.png

spacer.png

Thank you so much! I’ll take into account everything you suggested, specially the height of the thing. It was undersized. Using your suggested measurements and comparing them to the height of my eye while seating, I believe it is spot on. I’ll make sure to post some updates during the build. Thanks!

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Hello FranTeryda,

You've been given a lot of good advice already. When you buy The SW Explorer 200P are you buying the telescope only or the telescope plus the equatorial mount?  Using the rings to attach the scope to altitude bearings is a nice feature.  I made a new dobsonian mount for my Zhumell Z10 based on the "instructions for building a Dob" that   Louis D provided.  That's a very well known site that's helped a lot of people.  I made the adjustable cradle th site describes. Once finished, the new larger bearings where much better than the original.  Your ring system would take the place of cradle.   Give the site a good read it will explain why large bearings are better than small, proper bearing placement, how to balance the scope and basically everything you need to know.    Using your ring system and large altitude bearings your design will look very close to the Bresser design that Louis D also provided.   Which looks like your original design but modified for larger bearings.

AstroSystems have PTFE bearing kits (https://www.astrosystems.biz/pivot.htm) can ship over seas.   Click on the Secure Order link the to see if that will work for you.  Other alternatives are  HDPE (High Density Polyethylene)
and there's fiber glass material used in boat repair.  Experiment with other materials.   Lack of teflon is not  a show stopper.   

The height of the rocker box depends on the balance point of the telescope.  The bottom of the telescope needs to be able to clear the base when it's vertical.  Add a little  more room so you can re-balance the telescope if you
add extra's such as RACI finder, telrad, heavy EP etc.

Getting an adjustable chair when observing is a good idea.  It's more comfortable and you can see more details  than when hovering over the eyepiece


Phil

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On 14/06/2020 at 11:37, pjsmith_6198 said:

The height of the rocker box depends on the balance point of the telescope.

However, back heavy scopes end up with short height rocker boxes, an extremely large swing in altitude height, super awkward usage at low altitudes, and a particular sensitivity to heavy eyepieces due to the long moment arm magnifying their effect on balance.  I find it much better to weight the top end to counteract such back-heaviness to allow for a balance point more in the middle of the tube and thus a taller rocker box.  If the weight is on the opposite side from the focuser, it also makes the scope less sensitive to not being completely level in azimuth when used with heavy eyepieces.

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On 14/06/2020 at 13:37, pjsmith_6198 said:

Hello FranTeryda,

You've been given a lot of good advice already. When you buy The SW Explorer 200P are you buying the telescope only or the telescope plus the equatorial mount?  Using the rings to attach the scope to altitude bearings is a nice feature.  I made a new dobsonian mount for my Zhumell Z10 based on the "instructions for building a Dob" that   Louis D provided.  That's a very well known site that's helped a lot of people.  I made the adjustable cradle th site describes. Once finished, the new larger bearings where much better than the original.  Your ring system would take the place of cradle.   Give the site a good read it will explain why large bearings are better than small, proper bearing placement, how to balance the scope and basically everything you need to know.    Using your ring system and large altitude bearings your design will look very close to the Bresser design that Louis D also provided.   Which looks like your original design but modified for larger bearings.

AstroSystems have PTFE bearing kits (https://www.astrosystems.biz/pivot.htm) can ship over seas.   Click on the Secure Order link the to see if that will work for you.  Other alternatives are  HDPE (High Density Polyethylene)
and there's fiber glass material used in boat repair.  Experiment with other materials.   Lack of teflon is not  a show stopper.   

The height of the rocker box depends on the balance point of the telescope.  The bottom of the telescope needs to be able to clear the base when it's vertical.  Add a little  more room so you can re-balance the telescope if you
add extra's such as RACI finder, telrad, heavy EP etc.

Getting an adjustable chair when observing is a good idea.  It's more comfortable and you can see more details  than when hovering over the eyepiece


Phil

I bought just the tube without mount (It's being shipped 😀), and thankfully  it comes with the rings. This will make my job much easier and it will look nicer in my opinion. 

I've been reading that guide and it is extremely helpful. After your advice I modified the diameter of the bearings, it's larger now. It even looks better. 

Regarding the bearing surfaces, thankfully I asked a friend of mine if he knew where I could buy some Teflon to build it. It turns out he had a piece of 5 mm sheet of Teflon lying around so I wont have to experiment or order pieces online to get the bearing surfaces right.

I did not take into account the fact that i might have to shift the scope to keep it balanced, so I modified the design and added a little bit more of "legroom".

Anyway, thank you very much for your replies, your time and your help. It means a lot for me.

Cheers,

Fran

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  • 2 weeks later...

The mount is done! Many things have changed.

I found in a local marketplace someone who sold a dob mount for the sw250p for REALLY cheap (around 40usd). After asking for some measurements it was perfect for me!

As my scope had the tube rings included, I just had to adapt them to he mount. I designed the 2 parts cut them out of plywood, and painted them.

20200627_174810.thumb.jpg.a82c461776004ec35bae59145baae9e3.jpg

 

20200701_103837.thumb.jpg.2598e02611049a8fba15214fefd10062.jpg

 

Here comes the interesting part: the altitude side bearings. As I couldnt get ebonystar or formica anywhere, I designed and 3D printed 2 PLA sleeves that cover the bearing surface. I printed a set of flat ones and a set of rough ones using the fuzzy skin setting in cura to mimic the ebony star texture. The first flat ones suffered from stiction, but the rough ones worked amazingly great. I can make any tiny movement I want extremely easily on the altitude axis.

20200622_121325.thumb.jpg.875a1ab0ee8b8493299816ce6a431292.jpg

Now, I have to improve the stock azimuth axis, It is a little difficult to turn it.

After all, I am pretty happy with the results. I believe the scope looks much better than the typical white sw dob

20200701_180415.thumb.jpg.eaec8e5153b76f01741a414a6955b022.jpg

Thank you for your help. Of course it will be rainy all week, so I'll have to wait to try it out

Bye!

Fran

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There are bunch of threads out there on Synta base mods.  There's the milk jug washer mod that goes around the central bolt.  I would probably replace the factory bearings and bearing surface with whatever you used on the altitude bearings.

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