Dob Mount How To

rmcpb (Rob) · #1 25-10-2006, 05:12 PM

Well, I have finally go my act together and started the How-to for the dob base I made.

First, my goals:

Must use current OTA
Must have great movement in both axis
Must have a low centre of gravity
Must have interchangeable side bearings so the cradle can be used in the dob or split ring mount
Must have manual setting circles
Must be easily set up and balanced

rmcpb (Rob) · #2 25-10-2006, 05:33 PM

I pondered over the design of the cradle for a long time and had quite a few goes before the final one.

Basically it is made from double layer 12mm ply giving 24mm thick rings top and bottom.

My OTA has an outside diameter of 230mm so I routed an inner circle of 234mm diameter to allow for the felt insert between the cracle and the OTA.
I started with a square "ring" then took the corners off as I could imagine how often I would hit my knuckles on them. There is no reason they could not be left square on the outside, I happen to like the octagonal look.
The minimum thickness for the rings is 25mm at the sides. This was to give adequate strength and room for the side boards to be attached and away from the OTA.
The side boards are simply 12mm plywood 150mm wide and 180mm high. They were glued and screwed flush with the outer surfaces of the rings.
When the rings were glued to the side boards I ripped the whole setup down the middle with a fine saw giving the two halves.
The two halves were hinged together on one side and the clasps used are simply the suitcase ones available form most hardware stores.
I lined the inside of the rings with self adhesive felt to prevent too much damage to the OTA.
This clamped very well and needed no other adjustments.
The bearings were made out of 12mm ply doubled to give 24mm.
They were made as one unit with a 12" diameter, I worked on the generalisation that the bearing should be 1.5x the diameter of your primary.
The cutouts are just for decoration as were done with a hole saw.
The circle was cut in half with a fiine saw.
The bearing surfaces were finished with some rough finish laminate I "aquired" from a friend. The ends are secured with heavy duty staples.
The bearings were mounted with some knock down furniture fittings. When I get to the split ring mount they will be replaced with T nuts and large knobbed bolts for a simple tool free change.

Enough for now.

Cheers

wavelandscott (Scott) · #3 27-10-2006, 05:45 PM

Looks very nice!

Good work...I hope you enjoy it.

rmcpb (Rob) · #4 28-10-2006, 02:50 PM

I wanted setting circles that did not require me to level, align, level and align, etc. I ended up with simply making a disk of 3 ply the same diameter as the base and cutting the middle out of it leaving a 40mm ring so it sits outside the magic sliders and can be simply aligned to any setting when a known star is centred. On this I marked the 360 degrees of a circle and mounted it on cutouts routed out of the base. I then cut out a part of the upper base to allow the setting circle to be seen and have used a nail as the pointer.

While this works well I will have a look at a digital compas for the next base I am setting up rather than going to all this trouble.

rmcpb (Rob) · #5 28-10-2006, 03:09 PM

The actual base is the same size as the GS one supplied with the scope ie. 50cm diameter. I made two disks this size out of doubled 12mm ply as I wanted the mass to be low and thought that if the base was solid at the bottom then the rest would be easy. Well it worked

I cut the lower disk into a triangular type shape on a whim that I would not bother with again. Now I have to ensure the ends that come in contact with the levelling circle are over the levelling circle feet or it wobbles. If I had left it a circle it would not matter a bit, well you live and learn.

I cut a circle out of the lower base to allow the attachment of the lazy susan bearing (from Carbatec). I also set up three magic sliders with milk jug washers to get the height right to stop the bearing allowing too much freedom. When bolted together this allows smooth movement with some resistance against the laminate surface glued onto the bottom of the upper disk.

The sides were simple and made out of 17mm ply. The cradle is 28.5cm across so I set the inside of the rocker box to 29cm. The side panels were set up to be as wide as possible while still sitting inside the base and looking "right" so they ended up 34cm wide. I routed the bearing surfaces, bearings were 150mm radius, allow for the 6mm teflon so the radius of the cutout was 156mm. I balanced the scope in the cradle, measured how far the scope stuck out from the bottom of the bearings, added 50mm to get the height of the sides to the bottom of the bearing.

These were simply screwed onto the base with some long screws, later I may make it collapsible with the use of piano hinges. The teflon pads were set to 35 degrees each side of the vertical and the scope went on beautifully.

rmcpb (Rob) · #6 28-10-2006, 03:16 PM

As we all know its not absolutely necessary to have a dob level but it sure helps the movement in azimuth and allows the manual setting circles to work correctly.

Its really simple and independent of the base. I made two disks of 12mm ply 60cm outside diameter. One I routed an inner circle out of at 50.2cm, the other 48cm. I glued these together to produce the ledge the base sits in and put some furniture levellers at 120 degrees to allow fine control of the level. The coarse adjustments are done using blocks of wood of differing thicknesses. I glued a circular bubble level on the base near the battery and its really simple to set up.

rmcpb (Rob) · #7 28-10-2006, 03:19 PM

So that is the thought process behind this base and I did not really give too many measurements as the ideas are more important. This base, or parts of it could be applied to any scope so look around and grab the bits from others scopes you like and impliment them into your mods.

The difference in this mount compared to the standard GS mount is amazing and it allows tracking of the moon at over 400x no worries. No sticky movement, no wobbles and steady as a rock.

Keep looking up

mill (Martin) · #8 28-10-2006, 04:14 PM

The idea of a digital compass is just what i am going to do.
I also bought an protractor that will go on the alt bearing.
The protractor is beeing used in the building industry for measuring the angle of beams etc, picked it up at bunnings for about $38

netwolf · #9 31-01-2007, 01:54 PM

Rob would you be able to provide some dimensioned plans that I could pottentially take to someone and have it made for me. Your setup is more ideal for motorising the dob than the standard GSO setup. BTW do you have any simialr plans for your split ring system?

Regards
Fahim

rmcpb (Rob) · #10 31-01-2007, 03:53 PM

Fahim,

I never drew any plans for this mount, that is why I put so many photos up and tried to explain the thought process behind the bits. If you have any specific measurements you want send me a PM and I will get them for you.

As for the split ring system, after much plywood and sawdust I have decided to resurrect my equatorial platform and fine tune it to get the bugs out of it instead.

Cheers

ving (David) · #11 31-01-2007, 04:27 PM

rob, that is fantastic!

if i ever get off my fat patootey i'll consider something similar

GrahamL · #12 03-02-2007, 08:03 AM

Would love to see the new project as it progresses rob as I'd like to have
another try at building my own.