Dob mount proportions

[Tiki]

Largely inspired by Moonshane's post http://stargazerslounge.com/topic/105895-home-made-dobsonian-base/ I have decided to make a dobsonian mount for my OO 8" f4.5 newt. Smoothness and stability are for what I strive.

Hindered by having never even used a dobsonian mount I have a few design queries:

I would imagine that a larger diameter altitude bearing would be smoothest. Big means more weight, more materials and possibly not so pleasing on the eye. https://stellafane.org/tm/dob/ reckon that it is OK to go up to 1.8x tube diameter. 1.8 x 9"= 16" in my case. If I were to make the altitude bearings any smaller would much smoothness likely to be sacrificed?

Moonshane's  post quoted above suggests 70 deg. as the optimum angle for the altitude bearings to sit. Any reason to do otherwise?

Would a 16" base diameter be too small? too jerky? too anything?

Thanks in advance.

[steppenwolf]

I am currently building a Dobsonian mount for my 10" Newtonian using the Stellafane data as my basic guide with some great input from Shane. I went for 1.7x from tube diameter and the mock-up image on the link above is to exact scale so it will give you a good indication of what the large altitude bearing will look like with your telescope using the Stellafane dimensions. I am not using a box cradle for my optical tube, rather, I am attaching the altitude 'wheels' to the tube rings.

Smoothness comes with large bearings.

[Merlin]

An important point here is that large side bearings ( about the same diameter as the  aperture ) prevent the 'scope from lurching backwards when it approaches the zenith.

[Moonshane]

dimensions of bearings is always something that anyone building their own dob mount stresses over. in the end it's a compromise as with most things.

larger is generally better as it gives more control but as you point out, larger = more weight, more cost, more materials etc.

larger bearings do work more smoothly too.

in two of my scopes, I went for about 1.3x diameter of the mirror but this is because I wanted to keep the bases as small as possible and also wanted to retain aesthetic balance. the operation of both is very smooth and with little if any sag - easily sorted with a box of 2 pence pieces slipped into the tube end.

the things that govern the base dimensions for me at least are:

1) desired size of alt bearings - usually determined after I know the dimensions of the ground board as I like to have them the same size so they move equally (not essential but maybe I am a little OCD)

2) width of tube plus blocks x 2 plus rings x 2 plus material thickness x 2 determines the width of the base which I would usually make square

3) balance point of fully loaded scope

4) desired eyepiece height

5) materials available

6) stability is reduced with a narrower mount

Often I have a certain amount of materials available and this can restrict the sizes I can use without buying more.

You will, no doubt learn as you go and make mistakes but this is all part of the process.

hope this helps and good luck!

[steppenwolf]

You will, no doubt learn as you go and make mistakes but this is all part of the process.

I can confirm this latter point having mounted the azimuth bearing in the centre of the (square) base only to remember that the centre line of the weight of the telescope is offset by the thickness of the front panel! Easily rectified on final assembly by offsetting the (rectangular) rocker box sides and front on the rocker box base.

I cannot tell you how much I have enjoyed building mine - nearly there now, just the two top coats and final assembly to go!

[Tiki]

Thanks for the replies.

dimensions of bearings is always something that anyone building their own dob mount stresses over.

Very true. Would a 3/8" thick altitude bearing be too skinny to be able to present a sufficient surface to the bearing pads?

[Moonshane]

Personally I would use 18mm (3/4 inch) minimum

[Tiki]

Personally I would use 18mm (3/4 inch) minimum

 Is this because of the broader footprint or is it due to other considerations?

[Moonshane]

a bit of both really. the wider teflon pad required for thicker materials will give more support to the bearing and operate more smoothly, the thicker plywood material gives more stability and rigidity which is important.

[quaoar]

Just a suggestion. Have a look at the "Dobstuff" website there's lots of pictures of hybrid dobs Dennis Steele has built over the years. He can also build you a full or part kit to your specs for very reasonable prices and send as a flat pack. He build dobs morning noon and night and is very good with advice. Also lots of good info in Dave Kreiges book "The Dobsonian Telescope".

[Tiki]

Thanks Moonshane and Quaoar .  3/4 ply it is then.

I noticed on the "Dobstuff" site that they fabricate their altitude bearings with a lip that fits inside the rocker. It looks neat.

[Tiki]

Oh, and BTW is it simple enough to mount the tube rings to the altitude 'wheels' so that the wheels sit parallel?

[steppenwolf]

Yes, it is pretty straightforward - mine were smack on. I have standard SW dovetail bars attached to the top and bottom of my tube rings and the 'wheels' bolt through the bars and onto the tube rings using 1/4" - 20 bolts. My wheels are parallel to one another but in my calculations, I left room for two strips of 3mm MDF to act as spacers/pillows between the bars and the wheels with a view to shimming these if necessary to ensure a parallel finish but in the end, no shimming was required.

[quaoar]

Oh, and BTW is it simple enough to mount the tube rings to the altitude 'wheels' so that the wheels sit parallel?

On the Dobstuff website if you go to kits and parts and then kit assembly suggestions there is a description and photos of the process.

[8kids]

Bolt the bearings straight onto the tube rings. I had to shim mine to get them parallel, just the thickness of a plastic milk bottle.

[steppenwolf]

Bolt the bearings straight onto the tube rings.

This is, of course, a perfectly satisfactory way of mounting the bearings but the advantage of my method (using the dovetail bars) is that you dramatically increase the contact area of metal onto the wood of the 'wheel'. The tiny disadvantage of my method is that you will end up with a slightly wider rocker box (30mm) to accommodate the 15mm depth of the dovetail bars.

[Tiki]

Plenty to think about still, although I am now in a position to start a bit of wood butchery.

The "Dobstuff" site has some useful piccys. http://www.dobstuff.com/assemble.htm

I noticed that "Dobstuff" use a relatively coarse laminate/large Teflon bearing for azimuth. I was intending to use smaller (1"ish) square pads and some very matt formica. Any thoughts?

[swamp thing]

It's not so much Matt formica you want as a dimpled finish. The dimples help keep stiction away.

[8kids]

Plenty to think about still, although I am now in a position to start a bit of wood butchery.

The "Dobstuff" site has some useful piccys. http://www.dobstuff.com/assemble.htm

I noticed that "Dobstuff" use a relatively coarse laminate/large Teflon bearing for azimuth. I was intending to use smaller (1"ish) square pads and some very matt formica. Any thoughts?

I recently bought a textured formica edging strip from B&Q.

[swamp thing]

That sounds like what I used Gary. It was just the right width for my alt bearings. Luck huh?

[Tiki]

I recently bought a textured formica edging strip from B&Q.

Me too. Mine is a bit wide so i'll have to trim it down.

I am still undecided on azimuth bearing material but I won't need to decide just yet...