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# Bearing angle relative to mirror box?

5 replies to this topic

### #1 bbasiaga

bbasiaga

Apollo

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Posted 08 April 2021 - 10:29 AM

I beleive the rule of thumb is that the bearings on a dob should be 30degrees offset from the top of the mirror box.  This means the top of the half circle would be located on a line that started in the center of the top of the mirror box, and rose at a 30 degree angle.  Just wondering if anyone has another way of thinking about it?

Obviously want to make sure you can go to horizon (or close to),but anything else to consider?

-Brian

### #2 KBHornblower

KBHornblower

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Posted 08 April 2021 - 02:07 PM

I beleive the rule of thumb is that the bearings on a dob should be 30degrees offset from the top of the mirror box.  This means the top of the half circle would be located on a line that started in the center of the top of the mirror box, and rose at a 30 degree angle.  Just wondering if anyone has another way of thinking about it?

Obviously want to make sure you can go to horizon (or close to),but anything else to consider?

-Brian

I cannot visualize what you are describing here.  Can you attach a sketch?

### #3 Venator

Venator

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Posted 08 April 2021 - 03:05 PM

What drives the angle in relation to the top of the mirror box that your bearing can get cut down to is the angle of your bearing pads on the rocker box. if your alt bearing pads on the rocker box are 60* apart, your bearing cutoff can be somewhere around 30*. If the pads are 90* apart, your bearings would have to have around a 45* cut in them, etc. You just don't want your bearings to be too short that when pointed at the horizon, the front of the bearing could slip off the forward teflon pad. In summary, figure out what angle you want your teflon pads at, then give yourself a few degrees of bonus off that for your bearings so you don't slip off them while pointed low.

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### #4 MitchAlsup

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Posted 08 April 2021 - 05:05 PM

Here is one altitude bearing drawn at 30º.

a) notice there is a lot of back travel past zenith

b) notice there is no travel forward of the horizon

c) you want a better balance between these two.

For these reasons, I used 45º on this design.

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### #5 bbasiaga

bbasiaga

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Posted 08 April 2021 - 08:31 PM

Here is one altitude bearing drawn at 30º.

a) notice there is a lot of back travel past zenith

b) notice there is no travel forward of the horizon

c) you want a better balance between these two.

For these reasons, I used 45º on this design.

Thanks for the photos.  I had a clear visual in my head, but didn't describe it well.  Your images describe what I'm working through as well.  I drew up 45 degrees and it looked too much to me, but I need to take a better look.

I suppose the goal is to get some flexibility on teflon pad spacing so you can adjust the movement of the scope some, and make sure that you can get to horizon easily, as Venator and yourself suggest.

-Brian

### #6 Pinbout

Pinbout

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Posted 08 April 2021 - 11:04 PM

45* is correct

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