Bartelised Dob Rebuild

[Rod]

I have been re-building my 12 inch F5 Bartels Dob and thought I would share my progress. The first photo below shows the original. I have been using it for about 4 years. It had epoxy gears and nylon allthread for worm drives. The system worked well but I felt I could improve upon it.

The next photos show my rebuild. I hope they are self explanatory but the main features are:



Professionally made worm wheels and gears by Lenord Stage
An aluminium flex rocker made of 'connect it' parts from Bunnings
A square single 'ring' secondary cage again made from connect it parts avalable at Bunnings
Friction clutches on both axes. The flex rocker sits on top of a thin aluminium base with three teflon pads supporting it. The base of the flex rocker is covered with laminate.
A wire spider. A lot of people think they are hard to make. They are not. I show it in the jig so you can see how it is set up.
One piece truss assembly. Ken Beard at the ASV's Instrument Making section uses a similar system which I copied.
Fast slews. Too fast really. I have run it at 6 degrees per second but I actually find that unmanageably fast. I set it to run at around 3.5 degrees per second instead.
Home made mirror

I still have a lot of work to do on it but it is usable now. I get good tracking after a 2 star alignment but some sloppiness in the motor mounts is causing backlash and making the gotots less accurate than I would like.

I will also be adding encoders. These will help address any slip in the clutches.

I would like to lay out the electronics more neatly. At the moment I have just bolted on the original circuit board and laptop mount. I hope to come up with something neater. Any suggestions appreciated!

In the field I control the scope via a serial to wifi adapter. I use skysafari on my iphone to send goto commands. Works great! I also have the DOS version of Guide installed. It is a handy back up and I often use it for the two start alignment.

Rod.

[Rod]

Last pick shows the messy electronics mount mentioned.

[scopemankit]

That is a very nice piece of work!

[tilbrook@rbe.ne]

Hi, Rod.

Very nice scope, how long did it take to build?

I know with my scopes it never ends, refining - tweaking and sometimes improvement!

With the wire spider, do you get any vibration when moving the scope, if so does it settle down quickly?

I've used curved veins in two of my scopes they work well, but a bit of a bugger to centre.

Cheers,

Justin.

[Rod]

Thanks Chris and Justin for your comments.

I agree Justin, my scopes are never really finished either. I am always tinkering with them. But I guess that's part of the fun! The latest re-build I have done over about 6 months working fairy spasmodically. All the electronics came over from the old mount so no need to re-do much there. Originally the mirror took the longest to make. Spent a couple of years on it in the 1990s.

The wire spider does not vibrate. I have been using one for several years. They are very strong. Using a jig makes them easy to centre.

Rod.

[tilbrook@rbe.ne]

Hi, Rod.

One thing I meant to ask, what type of wire do you use in the construction of the spider?
Might try this setup for the 16".

Justin.

[Rod]

Hi Justin,

I use two guitar strings. I think you could also use the wire you hang pictures with but I haven't tried it.

Rod

[alistairsam]

Hi Rod,

I've been thinking of wire spiders as well. how do you know what the max tension is before the string snaps?
is a 3 ended wire spider common/advisable?
when you centre the secondary holder, is it just at the geometrical centre, is the secondary offset at the secondary mirror?
how do you ensure the secondary holder is perfectly aligned to the axis of the ota?

[Rod]

Hi Alistair

There is really no need to overtighten the strings. I tend to tighten them until I here a bright note when I pluck the string.

The problem with a three ended spider in most configurations that it is more susceptible to twisting. Having the veins connect at two opposite points prevents twisting. You can make a three vein spider that won't twist if you use Mel Bartels' design as shown here:

http://www.bbastrodesigns.com/trilateral.html

My first wire spider used Mel's design and it worked very well. The latest one I have built shows less diffraction spikes.

One of the reasons I went with the square upper end and the truss arrangement I did was so that it would be easier to align the optical and mechanical axes of the scope. I felt doing so would improve goto accuracy. The truss tubes only bend in one direction unlike when you use split blocks. The square upper end makes it easier to see if the spider is off centre. The other thing you can do is drop a plum bob down through the centre of the spider hub when the scope is vertical and adjust the truss mounts until the spider is centred. I did this with my previous scope and got good results by adding shims under the split blocks.

Hope that answers your questions. Wire spiders are cheap and really no harder to build. Just make a simple jig to thread the wires. Wiring this one up literally took only minutes. Mel's design takes a little longer (at least for me) but it isn't hard either.

Rod.