Tonight's newbie error - back-to-front scopes

[rkc]

It looked like it was going to be a good clear night, so I took the time to set up the AZ-EQ6 (in AZ mode) with two scopes - ED80 and 200P - to compare their relative merits for observing planets (Saturn, specifically).

By the time everything was set up, clouds were getting in the way, and the only star I could see in the twilight was Arcturus. I decided to go ahead and attempt alignment anyway, just for practice... So, I centered Arcturus just fine for the first start, then let it slew to where it thought Vega was for the second start alignment.

It didn't go exactly where I expected it to - but I couldn't see Vega to be sure that it was wrong, so I just pretended I had centered Vega too for a very approximate alignment.

Did a goto back to Arcturus - which took me reasonably close - then spotted that Saturn was out of the clouds. So asked the scope to go to Saturn, and was rather surprised to see it heading up rather than down.

Well, I knew my alignment had been very rough and ready, so I just moved the scope over to Saturn manually, and settled down to watch it. But it would not stay in the eyepiece - I think that not only was the goto going on the wrong direction, but so was the tracking.

So I scratched my head for a while, and concluded that the scopes were mounted pointing the wrong way - I turned everything off, turned them round, and repeated the alignment, and all was much better. But I can't quite get my head around why the way the scopes are mounted would make a difference (since spinning the alt axis 180 degrees would have the same effect)

Does any of that make any sense? Is this related to the 'start off pointing north' advice I have seen?

[Carl Reade]

Was the scope setup on polaris before star alignment? might be a silly question.

[Carl Reade]

The mount is a logic computer it depends on the information you give it. The mount does not know where on the planet it is, what time it is or have you aligned it to Polaris and leveled it. So it relies on the information you give it. If you pretend to locate a star it will then pretend to locate it.

Hopefully this is useful Cheers Carl.

[rkc]

I only pretended to do the second star alignment, because I could only see one star at the time....

I think I understand the issue better with some thought.

In AltAz mode, after the first star is aligned, and assuming that the mount is level, the scope computer does actually know (in theory) pretty much all it needs to know. It knows where it is on the Az axis, and it (almost - see below) knows where it is on the Alt axis. That's why it can slew to the right general location for the second star, and usually gets it pretty close. The second star alignment then allows it to compensate for being slightly off-level.

However, and this is where the 'almost' comes in, if your scope is pointing the other way than the mount assumes, then after the first star alignment it will be 180 degrees out in where it thinks the Az axis is (or, equivalently, it will not know which direction on the Az axis is up and which is down). In this instance, the slew to the second star will not be very accurate at all.

It may well be that had I at that point properly aligned the second star, the mount computer is smart enough to say 'aha, now I know which way is up' and everything would work just fine. But life would be easier for both me and the scope computer if I just put the scope on the mount so that 'up' raises the front of the telescope rather than the back, in the first place.

[ronl]

Is it set for northern or southern hemisphere if it is in southern that would

be why it went the wrong way

[rkc]

Is it set for northern or southern hemisphere if it is in southern that would

be why it went the wrong way

No, it was just that I had the scope mounted so that the front of the scope was at the back of the mount....