Dobsonian Trackers

[jleach29]

Hello to all, it has been awhile since I have been in the lounge.

I am currently waiting for my new low profile focuser for my Dob to arrive in the mail, and as I always do while waiting for new toys to arrive, I have started cleaning and tinkering with my scopes to pass the time. I have been looking at dobsonian trackers that they have online, and they seem over complicated, could I not achieve decent tracking capabilities by simply adding a wedge under the base of my dob to achieve polar alignment and to spin the base in a way that would not cause much vibration once my target is sighted to follow the object. When I put it together in my mind I can't see why it would not work. Any thoughts or ideas are welcome.

[Moonshane]

I am going to be making a couple of equatorial platforms for my dobs in the new week or two. Just waiting for the motors to arrive from Ebay (Hong Kong I think).

Total cost should be about £50 per platform.

The problem with a wedge vs a platform is that you'd need a wedge at your latitude which would not be workable with your dob - I'd need a 53 degree angle which would just mean the dob would slide off. With an EQ platform you are using a section of a cone and this the platform is broadly flat/level and just tilts by a few degrees over the hour approx of tracking.

I'll be starting a thread when I start work on the eq platforms but I suspect they will be quite easy to make over a couple of weekends.

[jleach29]

I was going to make it so that the wedge actually attach to the dob base, and make the wedge weighted so there is not a catastrophic tip over incident, so with those problem not an issue can anyone see why it would not work. I will keep an eye out for your thread when you get started.

[Moonshane]

I think in principle it would work. How are you going to turn the base at the correct rate? I am using a small DC motor and a threaded rod with a dew controller for fine tuning of speed. It will be broadly based on this one http://www.mdpub.com/scopeworks/platform.html

[FraserClarke]

I think you'll run into a lot of problems unfortunately.

Dob bearings are not designed to work at an angle. They are designed to work with the weight going directly down through the structure - as soon as you change this by tipping the mount, you'll run into probelms most likely. Dobs typically have very little lateral constraint (usually a pin in the middle of the ground board), because they have very little/no lateral force. If you tip the structure over at 40 odd degrees, you're now asking that central pin to take a lot of weight, which it isn't designed to do.

You probably also have problems with the altitude bearings (at least repositioning the pads), though it's not quite clear how the starhopper alt bearings work. At the very least, there is probably little/no lateral constraint to stop the OTA sliding 'across' the altitude bearings as you slew the telescope over in hour angle (again, a dob never has to deal with this situation, so probably isn't design to handle it).

What you are proposing is also called a polar disc mount. Exactly the same principle, but the bearings are designed to deal with the tilted loads.

[jleach29]

I was thinking of something low tech since I always loved using my manual barndoor mount, I have several ideas but nothing set in stone yet. I was thinking of a hand crank that they use on rope pulley systems with the circular center piece that the rope would normally move across pressed against the upper base of the dob and turning the crank, and using a guide scope to keep it on target. But like a said nothing is set in stone, will probably walk around the hardware store till I see something that will work.

[Moonshane]

sounds familiar!

[jleach29]

TeaDwarf, I do see what your saying, and if it seems to be over stressing the center bolt I would abandon the idea immediately, I think I will have to play around with my dob when I get home from my busy day at work (surfing the forums on here haha), but I do not think it would be to hard to relieve the stress on the center bolt.

[Moonshane]

true, you could make some curved 'bearings' at the lower end of the azimuth board but it all seems more complicated than an eq platform to me.