Dob wedges - is motorizing them important?

[Thiink]

With everyone so keen on wedges for dobs I wondered how important it was to motorize them. What advantage does a non-motorized wedge have over the standard rocker box?

I was thinking about having a crack at one of these (http://www.haven.k12.pa.us/orfft/wedge.html), building a new rocker box into it at the same time (leaving the standard one for normal alt/az), but wondered if the effort was worth it if I wasnt going to motorize it.

[ballaratdragons]

Try here for a dob wedge.

http://www.iceinspace.com.au/forum/showpost.php?p=58350&postcount=52

I don't know if they HAVE to be motorised but I will need to find out as I want to eventually get one of Micks trolley wedges. Great for Star camps etc.

[gbeal]

Simon,
not surte about the wedge concept, other than the balance/imbalance aspect.
I tried a motor system for one of my dob bases, and loved it. I thought initially it would make my GEM redundant but logic has prevailed and I will retain the GEM, and am selling the drive system (check the For Sales Forum).
Even if you are a completely visual observer only the ability to push a couple of buttons to track is a boon.
Gary

[davidpretorius]

What is a rocker box?

[ballaratdragons]

The Dob mount Davo. The box the dob rocks in.
:)

[davidpretorius]

I am still in two minds whether to drive the setup as a wedge of alt / az.
Wedge in RA/DEC will mean no field rotation
Wedge in RA/DEC means focusser in funny spots and with no tube rings, well they could be a pain to get to!
The way mick has designed this buggy is that the centre of gravity does not go outside the rear wheels, so it is very stable
Driving = tracking and i am in need of tracking to get longer runs of images for planets.

I am seriously thinking on gary beals motorized kit. It would be great it can work in alt/ az mode and also wewdge ra/dec mode.

[Starkler]

Why would anyone bother with a non motorised wedge ??
Is it so hard to push a dob in two axis instead of one?

I havent heard of many using a wedge for a dob and the issues i can see are many.

1: Flexing of the typical dob base which is designed to handle vertical forces, not sideways also.
2: Base bearings designed to handle vertical forces only.
3: Mods required to capture the alt bearings and also support weight sideways.
4: Focuser ending up in bizarre and inconvenient positions.
5: Needing to finely balance the ota

When all is said and done its functionally the same as a fork mount. If I wanted to go that way I would build the mount from scratch and leave the dob base as is.
In Melbourne the centre axis of the dob pointing at the SCP would have the whole lot tilted to 38.5 degrees from horizontal and the poor dob base wont handle it without a rebuild. The further north you go, the worse it gets.

[ballaratdragons]

You mean South Geoff.

[davidpretorius]

no north, basically at the equator, your wedge would have to be at 90 degrees to be polar aligned. at the south pole, straight up is polar aligned

[ballaratdragons]

At the equator a wedge would be almost level so there is less strain on the balance and bearings. The further South the more the wedge has to lean back causing stresses. The opposite in the Nthn hemisphere. The more north you go there are more stresses on balance and bearings.

[ballaratdragons]

Actually I think the angles we would get on in Australia shouldn't be a problem.

[Starkler]

Ken, Dave is right .

Imagine your at the south pole and the SCP is directly overhead. To track the sky you would simply push the dob on its az axis .

I have seen one page about a dob wedge, the user was in Finland at some place close to the arctic circle so the angle required was quite shallow.

[ballaratdragons]

:doh: Of course!!! The centre bolt is the Polar shaft!!!!

Just ignore my previous comments, I'm having a 'blonde' moment. I was pointing the horizontal surface of the base at the SCP :P

Sorry Davo, Sorry Geoff. Oh well, I was only 90 degrees out. :)

[netwolf]

I think the best way to convert a dob would be to create a new Fork Equatorial rocker base for it with tube rings. Or a split ring equatorial rocker base design. Similar i guess to JMI's NGT and NTT Designs.

Fork
http://www.atmsite.org/contrib/Harbour/Sym.Eq.Mntng.html

Split ring example.
http://www.geocities.com/joe_pearson/splitring_index.html

Or something a bit strange.. three axis rocker base I really like this idea.
http://www.c2optical.com/mount.htm

[davidpretorius]

i have been keeping in touch with Tracey re his tri axis mount (netwolf's third link), he hasn'y got too much further down the track, but he will keep me up to date.

[netwolf]

David, Mel Bartel aslo has mentioned on his site a method described by Richard Berry about using an equatorial table with 3 pistons. I have seen a video clip of this somewhere but at present can not find the link.

I belive Mel has also done some work on cordiante transoformation such that we can map RA/Dec to the three-axis.

Anyways geting back on topic, i belive essentialy the design of the current dob base may need to be rethought to include a wedge or motorisation.

Motorisation may be more easily achived if the rocker base is designed for this function to beging with. That is it is designed to accomadate motor drives and electronics.

Equatorial Tracking will require a wedge or split ring, however Tracey's method described in my third link is one that keeps the dobb look and feel the same. One could also accomadate rings in Tracey's method to allow the EP's to be rotated into a comfortable viewing position. Tracey's method also alows us to keep the visual ease of the dob.

Regards

[rmcpb]

I think the whole rationale behind a wedged dob mount (basically a york/fork mount) is that it can track with simple motorisation for as long as needed and is less prone to vibrations and long settling periods than an equatorial mount.

However, just putting a dob mount on a wedge is not necessarially a good idea. Some problems and remedies I have found:

The centre bolt tends to bind as it is twisted by the top plate moving sideways and jamming it, therefore, need to insert a bearing here.
Better off with actual bearings on the bottom plate instead of sliders as the weight is not even on these surfaces and friction is increased making movement harder.
The top of the standard mount is too shallow to give any security for the scope unless the spring system is used. A top section has to be made to secure the OTA to the base. Better off with custom side bearings and a clamping mechanism to control any movement when tracking and to stop the OTA from coming off!!
The base too big and needs to be made collapsible.
Side bearings too high making mount too high and unstable. Need to put removable counterweights on bottom of OTA and make a cradle to allow the bearings to balance the OTA lower down.
The standard rocker box is well designed to work on nearly level conditions but the very nature of the wedge will place stresses on it that are not what it was designed for. Another reason to build a dedicated base from the ground up.
By making a "collar" locking ring the cradle can be loose enough to allow rotation of the OTA to position the eyepiece in a more convenient position for the observer.

These are a few of my observations for the mount project I am currently doing. But to answer the original question with a question, if its not motorised then why equatorially mount your dob??

[Thiink]

Geoff/Rob: I didnt know, which is why I asked..

Thanks for the replies..

[netwolf]

A German Equatorial mount can allow for manual tracking with fine controls without the need for motors. However with a dob wedge you will need motors to track.

rmcpb, i saw a post i think it was by you on CN forums that mentioned a split ring base for a dob. What are your thoughts on split ring as opposed to the wedge.

[rmcpb]

I had a bit of a go at the split ring before I managed to drop the centre of gravity of the dob to a reasonable level. The ring part of the mount (the large disk) was HUGE and not acceptable. I will revisit this system after the dob wedge just out of interest (I like playing with bit n pieces) as I think the split ring will probably be a better bet as far as collapsability is concerned.

The main advantage of the dob wedge is that if it is made right it can work as a simple dob or be a driven equatorially mounted scope very easily. The hard bit is to get the strength in the rocker box while retaining the collapsability with fast setup and knock down.

Just my sort of project :)

[rmcpb]

Thiink,

Just had some more thoughts on driving a dob wedge and there is no reason why you could not put a manual drive on one like the standard GEMs have. You could make it with a cable handle like the GEMs that is simply turned which in turn moves the base around thus following the stars. Better than push and shove but not as convenient as a motor driven unit.

Cheers

[davidpretorius]

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=6608&stc=1&thumb=1 (http://www.iceinspace.com.au/forum/attachment.php?attachmentid=6608)Th ere is a m10 thread supporting the alt axis in this setup. I have two nylock nuts to keep it locked.

They have to be done up really tight to stop the scope from turning under gravity, but all in all I was very happy with having a very basic ra/dec setup for an our or so. Damn eyepiece got in some funny positions!

For simplicity, i have gone back to using as alt/az whilst development continues.

Wombat has developed a centre bearing for lots of support on the alt axis and i have ordered Gary Beals motorized dob setup to see how it will go?

I like the option of having both methods available to me, and that is my aim. Both options, both motor driven and all on a tranportable base for easy moving!

Project in progress!!

[rmcpb]

Yeh, that is why I amusing a cradle with a locking ring so I can easily turn the scope in the mount but it will stay on target. Hope it all works cose its a lot of work :P