Quote:
Originally Posted by alistairsam
thanks. when choosing reference stars, if the alignment is done with just sigma octanis, will that be reasonably adequate? do its co-ordinates ever change?
else, with two stars, when aligned to the second star, do you adjust the scales again, and then go back to star 1 and adjust again?
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The altitude and azimuth co-ordinates of objects in the sky constantly change as the sky rotates so you need to look these up for the exact time you are aligning the scope. Against the imaginary "celestial sphere" nearby stars do change but only very slowly and this is called proper motion. Google it.
Choose 2 bright stars as far apart in the sky as practical. You don't have to worry too much about proper motion for the purpose of alignment. You're only looking for accuracy no better thanabout 1-3 degrees - enough to get you in the ballpark with a low power eyepiece - the moon is about half a degree and comfortably fits in such an eyepiece in an 8 or 10 inch Dob. Your accuracy should improve with a little bit of practice but it's not going to get much better than a degree or so - standard Dob mounts and home made analogue circles just aren't made to high enough precision and will typically still have some play in them. So unless you're picking stars with very high proper motion and your software can't calculate it, entering your GPS coordinates and time accurately into good sky chart or sky simulator software and looking up altitude and azimuth should be fine. (If you were pointing a large professional instrument for photographic or instrument work, you'd need to be much more precise)