GSO Dob - Alt Bearing Mount misalignment

[glend (Glen)]

This problem has been driving me crazy for since I got my 12" GSO back in June of this year. It shows up as a slight (3 degree left ) mis-alignment when orienting the scope to a particular azimuth location. I know the base is absolutely level and aligned to true north with my laser designator and good compass. I have an azimuth setting circle on the base and it is correctly aligned to north (0) and south (180) on a hard concrete pad. The ALt bearings (which are fore and aft adjustable on the GSO) are in exactly the same graduation scale location on both sides.

In use I have been compensating by going 3 degrees right on each location to find my target.

I finally realised that the scope tube was not hanging vertically in the box (which was level). The top supports of the box are level however, as are the sides of the box, which I checked when I assembled it. I used my laser designator to draw a vertical line on the tube (using the tube seam as my target). The tube was not vertically aligned, it was leaning slightly to the left at the top ( which is to the west when aligned to north). This means that the Alt bearing adjustment needs to be changed to align to the laser vertical to get back on co-ordinates. So the Alt bearing graduations should not be relied on (matched) to get the tube aligned, you will need to use a beam level or laser level to setup the tube on it's mounts.

This maybe due to a problem with the hole drilling in the tube for the bearing mounts, or mismounting resulting in too much slack in the bearing mount on the tube itself.

If you have a GSO dob and have noticed a slight mis-alignment to your target co-ordinates, it's a good idea to check the tube alignment in the mounts. Of course, that is not mentioned in the assembly instructions that came with the scope, and I will be talking to Andrews Comms about it.

To fix it I put a small car scissor jack under the left side and raised it to meet the bottom tube/mirror support ring, then loosened off the left side bearing mount hex bolts and turned the jack to move the scope to the vertical, then retightened the bearing mount bolts, removed the jack, checked vertical on the tube through rotation and alignment with the laser. Done.

Be interested to know if anyone else has noticed this?