Goto for the 16'' Tridob

[tnott]

As you may have seen, my scope does not have any central pivot points on which to mount encoders. So in order to install an Argo Navis/ Servocat Jnr. combo (designed for more conventional scopes) I've had to think hard. For the Azimuth I have retrofitted a central pivot point on the flex rocker and ground ring with leftover ply. angle brackets and some mental torture. Weight added is minimal and all the electronic gear will be tucked away underneath.

Finally got it all together today and it works. Just need to paint.

The Altitude will use 2 tangent arms in the same configuration as the Obsession UC to create the pivot point there. Will post when completed.

[tnott]

BTW- here is a photo showing a bit more how the scope will look when the Goto is added. No room for a stalk so the Argo is mounted on a truss pole. Handpad for Servocat is attached via velcro to the top end for easy access. The AZ. motor drives on the upper of the ground ring and the ALT. cable drive runs on the central back fin to avoid any torsional forces. Mechanically, everything moves beautifully.

[tnott]

Well I have finally finished a successful install of a Goto system on the 16'' Tridob. It took a lot more effort than I originally thought but it is finally finished. For the Altitude, I created a central pivot using two tangent arms. One is fixed on the scope with two wingnuts holding it rigidly in place, the other is slotted and slides over a little spacer on the flex-rocker. The two halves stay permanently connected in the centre and it only takes a few seconds to attach the setup onto the scope when assembling.

So I think I have succeeded in my aim to add Goto without much extra setup time or affecting the look of the scope. Most of the electrics etc. are neatly tucked away under the scope. Power comes up through the centre pivot so there are no tangled cables.

Now I look forward to using all the features the Argo Navis/Servocat combo can provide!:)

[Starkler]

Well done!
The lack of attachment options for dsc encoders on ultralight dob designs has always been a major disincentive to me, but if you have solved the problem thats great!
Bear in mind that you need to find the rotation centres quite accurately as the bearing inside the optical encoders wont like a large telescope acting as a lever against them :scared:

[CoombellKid]

It's the main reason I'm in two minds about this design. I've been watching
as I'm about to start my 16" project. And are not total sure about the
design. I do like the minimalist compact nature of your scope and how this
will be favourable to me. but I think you have achieved the one and only
real way to attach the Argo Navis encoders.

I'll be interested how it all works for you. Also your thoughts on the servo-cat
jnr

regards,CS

[tnott]

Yes the attachment points do need to be at the exact center, which I have been careful to make sure I've done as accurately as possible.

I was lucky that I still had a cutout piece from the laser cutting that fitted exactly in the middle of the ground ring. I marked the exact center then drilled a hole. Then, I flipped the scope over, connected the upper parts attached to the flex rocker and spun it around with the drill bit in the hole, leaving a mark at the point of rotation. For the Alt I just printed cad diagrams of the relevant parts and used this to find the exact center. Since all the parts were laser cut in the first place it all fitted perfectly, but I checked by clamping the bits together first before drilling holes in the side fin.



The advantage of the Stellarcat mount hardware is that the encoder tangent arms come with a slot in them which fits exactly over a post. So they cannot rotate but they will not allow any pressure to be put on the encoders if you were very slightly out with your measurements.

[tnott]

BTW- For my next 22'' scope I will have a center pivot point on the Az. from the beginning to save time. The Alt will have the same double tangent arm as it is a easy, collapsible solution for horseshoe-shaped bearings that works.

The main reason the 16" Tridob had no pivot point on the Az to start with is that it reversed the normal design of having a rigid mirror box and flexible ground ring, having instead a rigid ground ring and a tiny square flex-rocker. Also, the scope swings through the center of the ring a bit to lower the eyepiece height.

Its a great design that works well (and collapses into a small package) when finished, but the 22" will be simpler and less time consuming to actually construct.......






I hope.

[CoombellKid]

Thanks for the feedback tnott,

I see they use the base ring design with the standard Kriege and Berry
style bearings.

I think what will determine the final design for me, is when I have all the
hardware here and I can start taking some weight measurements. So
I can start calculating senarios.

regards,CS

[CoombellKid]

With any movement in the sloted tagent arm, wouldn't you start to introduce
alignment errors to the Argo Navis?

regards,CS

[tnott]

The slot is an exact fit over the post so it cannot move from side to side (ie. radially). It will move a tiny bit from center to the outside if the encoder is not exactly centered on the point of rotation (which of course is the ideal) making sure no pressure is put the encoder.

The powered ground board kit makes no difference to the balance as it is all in the rocker box and ground board. But yes it is important to know the weight of the optics and all the other bits and pieces. This determines the size of the side bearings in an ultralight design. You want the center of rotation of the bearings to be at the scope's COG. Also, these larger bearings can handle more variation in changes of weight in the top end. In the Obsession type you just pick a bearing size and fine tune the balance by moving the mirror cell up and down, then trimming off the bottom of the mirror box to suit.

Fun fiddling with ideas isn't it.:D

[CoombellKid]

Yup I know the powered ground board has no connetion to balance, this
is the second dob I'm building. I was refering to the hardware on the
secondary cage and mirror box. I'm still in two minds about whether to
buy an off the self solution or just design one myself, as Gary mentioned
you can poke power through his pivot bolt, making another contact track
shouldn't be all that difficult, and I'll have my electronics mate wire up
a power rail for me.

Yes it is fun to play with designs and a real joy to build your own scope.
Even if you have your optics made by someone else.

Btw, top job with the tri-dob:thumbsup:

regards,CS

[CoombellKid]

Just another quick Q regarding your tri-dob. How did you work out
the Alt bearing size? having a small mirror box, one which sit inside
the bearing, I'm assuming bearing size along with SC weight comes
into play here.

regards,CS

[tnott]

I just added up or estimated the components then used the weight-distance maths from p59 of the Kreige /Berry book. The larger the bearings the greater the distance stuff at the bottom is from the point of rotation and the closer stuff at the top is to the same.

In other words, if the calculations show the scope is top heavy, then just increase the diameter of the bearings and try again.:thumbsup: