Quote:
Originally Posted by erick
Hi Duncan
You are right, They have no trouble pointing to the zenith or close. The problem is moving them in the azimuth direction when you have them pointing there! When the scope is at low elevation, you just push away from you or pull towards you to move in azimuth. When it is pointing almost vertically, you lose your mechanical advantage of the OTA as a "lever" in the azimuth direction. Tracking an object at the zenith, as the earth turns beneath your feet, can be difficult at first.
This is called the infamous "dob hole".
Three solutions:-
1. Lazy way - Go and have a long cup of coffee until the object of interest moves 10-15 deg past the zenith
2. Unsophisticated way - Prop up one side of the ground board with, say, a brick. Now the real zenith is no longer in the scope's "dob hole". Remove the brick when done with the zenith.
3. Sophisticated way - Moving in the azimuth direction means rotating the scope around its vertical axis. When the OTA is almost vertical, using it as a lever doesn't work any more - you cannot push/pull, you have to rotate it. Keep your left hand on the top of the OTA to move in altitude direction and reach down with you right hand and grab one edge of the base and rotate the base to the left or right to get the azimuth movement. I find that on an 8" GSO with a smooth movement (Ebony Star laminate fitted) there was plenty of mechanical advantage at the edge of the base to move the scope smoothly in azimuth. On the 12", if I wanted to keep my eye at the eyepiece, I think I struggle to reach the base - but I had other bits sticking up that were connected to the base that I could grab hold of and use to rotate the base.
Yours in "dob-hood"
Eric  |
Ahhh, Thanks erick.
After giving it some thought, I assumed it might have something to do with the maneuverability of the tube in all different directions when it's upright. Thanks for shedding light on that, I was fearing it couldn't point upright for a reason like it's mount only allowed it to swing 80 degrees, not the full 90.
Thanks again.