Wednesday, August 7, 2013

Polar Alignment Woes

More struggles!

Last night I did some polar alignment by using the drift method and the DSLR with timed exposures.  By the combination of these 2 minute exposures and a quick preview in Maxim with watching the adjustments in the finderscope by eye, I can readily see the corrections made.

Found something very interesting.

Last night, I left the alignment very good when the scope was pointed to the west.

Tonight, I checked the east and was surprised that it was out of alignment!  I adjusted the ALT and then checked in the west.

It was now out of alignment!

Puzzled, I thought about how this could be happening.  The error seems to be from the RA axis sagging depending on which side the scope is positioned.  I tighted the bolts holding the DEC axis in place -- they needed a touching up -- and tightened the legs, which also had slightly loosened.

The symptom was that when the scope was pointing east, the RA axis was pointed too high.  It was reverse (apparently by the same amount) when pointing the other direction.

I'm wondering about the condition of the ground.  Maybe when I come back from my trip I'll put some concrete pads under the legs.  I'll need to flatten the ground underneath the pads and pound the soil somehow to make it more firm.

Update:
I was thinking some more about this problem and I considered if the ground was sagging, then the RA axis would be too low on either side.  This is not the case.  It appears to be too high when looking east and too low when looking west.

The current culprit may be the RA Spacer.  When I put that on, I didn't take special care to center it.  The level of centering required is beyond my ability to hand-hold the parts.  I'd need a mandrel to slide  down the shaft and hold it centered.   That might end up being the final solution.  In the meantime, I'll have to make do with trying to center the deflection.

To this end, I flipped the RA upside down (no weight and no telescope) and then loosened all the bolts holding the spacer.  This allowed the whole device to float - and I could see that there was some movement.  While it was hanging upside down, I gently supported the system and tightened the bolts.

If I were to ever get my hands on a centering mandrel of the sort I'd need, I would do the same procedure.  I'd need to remove the DEC axis and the polar scope to have a clear shot through the mount.

Second Update:

I worked on the spacer again, this time using a metal straight edge to ensure near-identical space on either side of the axis.  Not able to do this for a vertical centering, though.

The more I think about this offset issue, the less I believe that it's an issue.  As a straight offset, the off-centered position just acts the same as a guidescope does vs a main scope.  They still rotate about the same axis.

More things to check:
What is the situation with AZIMUTH?  Does it also vary?
Rotate the RA by 90 degrees by removing the bolts and spinning the RA axis.  If the error shifts, then it's an RA axis issue.
 It may still be an issue with the ground settling under the tripod legs.  I do know that the ground will shift as I walk around the scope.  Putting the legs on concrete pads may not do anything.  It's still easy to check.

I did check whether the weight under each tripod leg changes as the scope changes position.  The shift in the center of gravity didn't seem to matter and using a scale shows it.  The weight under one leg was 41.5 lbs with a variance of .5 lbs when the scope was slewing.  Not much of a difference and it didn't seem to corroborate the deflection.

I did notice that one leg was sitting partly on grass and the red mesh material rather than sitting directly on the ground.  Putting the tripod on a concrete pad will be a good idea.