A work in progress: magnetically aligning the LX90 (or any other telescope) tripod to True South.
Sat, 11. April 2015
Telescope Polar Alignment Using a Compass
Aligning a telescope in azimuth to True South (in the Southern Hemisphere) ought to be simple but is not as straightfrorward as it appears. Magnetic compasses shake and are affected by nearby metallic objects. Small diameter compasses are not very accurate and the pointers and scales often lack sufficient contrast, whilst the graduations are often indistinct.
My attempts to align magnetically recently have been three or four degrees off, necessitating more time spent aligning after dark, when it would have been better spent improving the accuracy before sunset. At least errors of this magnitude can be corrected after dark using the azimuth adjuster on the LX90, which has a total adjustment range of ten degrees.
The initial magnetic alignment for the LX90 must be within five degrees to avoid having to adjust the tripod legs after dark. With a required magnetic deviation offset of 12.25 degrees, (the current approximate deviation between True South and Magnetic South at my two observing locations**) this means that it must be no less than 7.25 degrees and no more than 17.25 degrees. With practice, the error ought to be well inside this range and accurate to a degree or so.
The standard compass on my Meade LX90 telescope wedge (below) mount does not work in its normal position on the mount, although it seems to perform ok when taken out and used away from the mount. Even if it did work, the pointer does not extend to the scale and so measuring the deviation offset is difficult.
The azimuth adjuster has a range of plus or minus five degrees.The standard Meade compass should heve been pointing towards approximately 140 but it wasn't moving!
Up until now I have been using a compass app on my Ipad by placing it on the little round centre piece directly below the mount. The advantage of using the app is that it allows you to choose True South rather than Magnetic South, so the arrows can be lined up without compensating further for the deviation.
I then checked again using a hand-held magnetic compass. It also got me reasonably close but was not as accurate as I would like. This was partly because of the difficulty in viewing it in that position and partly because of a reliance on making a judgment of direction.
This week I decided to try a new approach by placing a pre-cut timber plank on the ground, aligning it East-West and then butting the two tripod legs up against it, so that the scope was pointing towards True South.
The board must be offset from the magnetic coordinates, using the compass, by about 12.25 degrees.
Here is an image taken of two magnetic compasses on the plank:
Accurately aligning with a small compass can be tricky.
Then with the compass app on my Ipad:
Aligning with an Ipad app ought to eliminate some innacuracies.
The Ipad app has the advantage of selecting True South (rather than Magnetic South) or eliminating the need to compensate for the magnetic deviation - but it has the disadvantage of difficult to read pointers:
This Ipad app does not have the best design features - but it seems to work as well as a classic magnetic compass does and the Ipad has the benefit of being rectangular.
So, a little bit of extra care when setting up can save a lot of time when fine aligning - but I am still looking for a perfect compass!
** Visit GeoScience Australia to get the current deviation at any site. My home deviation (2015) is 12.36 degrees. The MAS observing site at Stargard is 12.12 degrees. Those decimal points are very hard to judge on a magnetic compass and I simply try for 12.25 degrees.