Star Adventurer Polar Scope Alignment

Having checked the tracking accuracy of my Star Adventurer already, I wanted to make sure the polar scope was correctly aligned before trusting it to location the pole using Polaris. There are a couple of things to worry about for this: first that the polar scope is concentric with the RA axis and second that it is in the correct rotational alignment with the setting circles.

The first should be easy to test but can be a bit of a pain to sort out. I set the mount up during the day and adjusted it so the cross of the reticle was over a distant object. Rotating the RA axis should leave the cross over the same spot. When I first tried this I took some photos at different positions:

There’s clearly a slight “wander” there, but I wasn’t happy that it was due to misalignment of the polar scope. I took off the counterweight bar and repeated the test, this time finding that the cross did indeed stay in the same position. Clearly the weight of the counterweight bar does induce some flex in the mount. I think that means that balance is going to be quite important when it comes to putting actual kit on the mount.

The good news of course is that no adjustment is required there so I don’t have to play with the three adjustment grub screws at the eyepiece end of the polar scope.

That done, the next step is to check the rotational alignment of the polar scope. Years ago I read that Polaris transits the northern celestial pole at midnight on 1st November every year (at the prime meridian). So if the index ring is aligned with a zero offset on the inner scale on the date ring, then the RA axis rotated so the rings have 1st November aligned with midnight then the polar scope should have midnight and 6 o’clock in a vertical line.

I’ve never questioned this initial piece of information, so I thought I should just confirm it this time around. It doesn’t seem to match the results from Stellarium. Here is the position of Polaris at midnight on 1st November this year:

At transit it should be on the orange 0 degrees line. Clicking on Polaris or using “Find” to locate it gives a lot of information in the top left of the screen including the “hour angle”, which should be 00h00m00.0s at transit. In fact it’s 15 minutes out. Quite possibly that’s within the margin of error of the mount design, but I thought I might as well get it right, so advancing a few days I found that midnight on 5th November is very close:

And again checking the hour angle it’s 17 seconds out which I think I can live with 🙂

So back at the mount I set the index ring to zero offset, rotated the RA axis so the rings read midnight on November 5th and looked through the polar scope expecting 12 to be at the top and 6 vertically underneath it.

No such luck. I reckon it’s between ten and fifteen degrees out 🙁

The next step then is to find some convenient vertical edge to line the polar scope up against. Surrounded by the natural world as my house is, there aren’t too many handy verticals. Even the walls of my house are not vertical. Fortunately I found a door that was sufficiently far away to focus on and rotated the RA axis to get the polar scope to coincide with that:

Without moving that I then turned the date ring to read midnight on 5th November:

And finally loosened the locking grub screw on the index ring and moved the index to be at the zero degree meridian offset mark:

(Apologies for the quality of these last two photos. I turned off the flash as it was making the dials almost impossible to read.)

Locking the grub screw up again the job is complete.

As I’m a few degrees west of Greenwich I shan’t use the index ring at this setting when I actually want to use the mount. I’ll rotate the date right a little to give the right offset. Then rotate the entire RA axis to show the correct time and date. That should account for the fact that being a bit rural and backward, the stars are ten minutes late getting here 🙂

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