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Dobsonian base - will this work?


MikeMS
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I have looked at making an equatorial platform for a dob but my latitude of 25 degrees complicates matters.

Thinking about this, I am toying with making basically a dobsonian base with complete holes for the altitude bearings (instead of semi circles) and tipping it all 25 degrees. Do you think this would work?

It would still swivel on the base board, tilted at 25 degrees and the enclosed altitude bearing housing will stop the open truss falling out.

It would then need to be driven in the az only (although it would then be called the RA I guess.

I hope I have described this enough to give the idea.

The bearing in the base board would have to be substanial, as the weight will be offset.

I am grateful in advance for all comments received.

Kind regards,

Mike MS.

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I feel this would not work but cannot confidently say why. My EQP works on the basis of following an appropriate curve which is part of a cross section of a cone. Therefore one side rises and the other drops as it tracks. I am not sure how yours would replicate this unless it tracks in RA and Dec?

Can you not cut a curved north sector? No more difficult than cutting round alt bearings. You just need to get your measurement right.

Edited by Moonshane
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What you're proposing is a fork equatorial mount - like this one:

howie11.jpg

No reason in principle why it shouldn't work, but you're putting the centre of gravity higher, and the current az-turntable won't place nice as it's not designed to take a shear load. But with a redesign of the base why not?

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Aah, hang on - I see the problem. Unlike a dob, this type of mount places the eyepiece all over the place depending on what you're looking at. Unlike a conventional OTA on an EQ mount, there is no option to rotate the tube - so you'd better be a contortionist!

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When I first saw this thread, I thought that's a good idea. However, say you

are using a scope on the equator, to polar align it requires pointing the polar

axis to either pole, ie parallel to the ground. So to do that with a Dob would

mean tipping it so the ground board was vertical !

Mike MS is at latitude 25 degrees, so the ground board would have to be at

65 degrees to the ground.

Or am I missing an essential point here ? Ed.

Edited by NGC 1502
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I think you are right Ed as you'd have to make the mount like shown in the pic and then discard the current dob base. As Gaz says though it would appear that as it tracks in RA, the eyepiece would rotate with it to some awkward angles.

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Dobs are designed to have the gravity always going in the same direction through the bearings -- that's why you can make them relatively simply. Once you start tipping things over, and moving it around, your bearings get more complex as they have to cope with a lot more angles (as you pointed out, saying the baseboard bearing would have to be a lot bigger). Things like Eq platforms work because they never go *that* far from upright.

To make it work properly, you're probably looking at building a new mount, rather than modding the dob mount. For your latitude, have a look at split-ring equatorial mounts. There aren't that many out there, but they are a very pretty looking mount, and quite a sensible choice for low latitudes.

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To All.

Thank you for your considered responses.

I have been talked out of it, but I do like the look of the mount supplied in the photograph and I have found two pillar block bearing out here.

So I am going to make a mount like that shown but make the dec arms of the fork accept the 46 cms semi circle alt bearings on the dob. It will look like two huge lollipops!!

I should have added this is to try and image through it, so I do not need to get to the focuser with my eye. And I don't want to alter the dob bit of the scope as I can then use it away from home.

I will get back to you with the success or otherwise of the project.

Kind regards,

Mike.

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