Monday, 11 August 2014

Handy tool for polar-aligning your Equatorial Mount telescope (And you've probably got one in your pocket!)

It's your smartphone.

Pretty well all modern smartphones have a compass and inclinometer built-in - all you need is a suitable app to access the data. I use the "Smart Tools" app on an Android phone, but there are numerous similar apps on most smartphone platforms.

Here's how I do it:

Set up the tripod at the desired location. If your tripod doesn't have a spirit level bubble built in, lay your smartphone on the tripod platen, screen facing up, and use the spirit level app to get the platen as horizontal as practical.

Now install the GEM mount and counterweight, and put the scope on the GEM. Use the altitude dial scale to set the altitude of the GEM RA axis to approximately match your latitude, and point the GEM approximately south. Align the telescope tube to be parallel to the GEM RA axis, so that it is pointing approximately at the south celestial pole (where the GEM is pointing).

I now place my phone on top of the tube, screen facing up, and long sides parallel to the tube axis. My phone has a rubberized case back, and my finder-scope mounting bracket stops the phone from sliding off; you may need to do something to hold your phone safely. (An elastic band wrapped around the tube may be good enough.) 

First, I get the altitude of the GEM set more accurately to my latitude using the spirit level / inclinometer app. The app reads to an accuracy of 0.1 degrees, so you should be able to get better than one degree precision pretty easily. 

Next, I get the RA axis pointed to true south, using the compass app. (My compass app works with the phone held at any angle, and includes an automatic adjustment for the deviation between true north and magnetic north, so I don't need to do any mental gymnastics to get a south alignment of better than 1 degree.)

Voila! A visual polar alignment within about 1 degree or better on both axes in a matter of a minute or so.