Meridian flip for AZ mount?

[BrendanC]

Hi all,

I have a Sky-Watcher AZ Synscan Goto mount - not ideal for AP, I know, but I'm having fun with it so that's ok - and last night, when imaging M101 using ATP, I got an error saying the mount couldn't move. Popped out to see what the problem was, and the scope was pointing directly overhead.

So, I understand the problem ie no more room to move, and I *think* I understand the solution - a meridian flip. Now, ATP can do this automagically, but I'm far from sure this is a suitable manoeuvre, or even a possible one, for an AZ mount.

Can someone advise please? I can think of ways around it this time, for example stopping the plan around the time I think it'll get stuck, then starting a bit later when the object is on the other side (kind of like a manual flip) but I can see this happening in future and if I can/should do a flip, even with an AZ mount, it would be good to know. Oh, and I realise the simple answer is 'try it' but I'd rather get informed opinions before I risk the mount again!

Ta.

Edited March 27, 2020 by BrendanC

[vlaiv]

No, meridian flip is not the solution for this problem.

Some sort of flip is the problem for this, but we can't call it meridian - maybe Zenith flip?

Alt-Az mount tracks in two axis - first is altitude - height above horizon, and second is azimuth - this one is north / south / east / west - direction type motion - along the circle.

To understand why mount can't track past zenith - let's do mental exercise. Imagine that we have an object that we are tracking that is passing thru zenith at one point. Mount starts facing east and as object is rising - so does altitude it goes from 0 to 90 degrees. Azimuth changes just a bit for this time - it stays mainly facing east. In fact if we were doing this experiment on equator on March 20th - it would stay exactly due east all the time without moving at all.

Now as object has reached the zenith it will start going down in altitude - this is fine, we had altitude going form 0 to 90 and then going from 90 down to 0 - this is continuous motion. Issue lies with azimuth - mount was facing due east and moving slowly, but once object reached zenith and started descending - azimuth now needs to be due west - we have discontinuity of 180 degrees - there is half a circle jump in fraction of the second - mount can't do that and for that reason it stops.

There is no automated "azimuth flip", but here is what you can do - when you are about to hit zenith - stop your imaging and your mount. Leave altitude as is but rotate mount by 180 in azimuth - instead of it facing east and "going up" - turn it facing west and tell it to continue tracking - it will now start going down and keep westward orientation.

Not sure if above explanation makes sense - but you can get a feel for what is happening by using your head / eyes. Stand straight and look level at horizon. Now start raising your head to observe higher and higher - at one point you will be looking straight up, now to continue you need to start bending your back which is rather uncomfortable and you can even loose balance and fall back doing that - it is far easier to just turn to the other side using your whole body and then continue by lowering your head / eyes slowly again until you hit horizon on the other side.

[BrendanC]

Makes perfect sense! Very clearly explained, thank you so much.

I totally get what you're saying about the problem. It's like when you're watching a plane overhead and it goes over you until your neck aches. What do you do? You turn around 180 degrees and continue watching it fly away from a more comfortable angle.

I just wondered whether software like ATP could be smart enough to realise it's dealing with an AZ mount and do something clever about it.

The solution is as I thought: just be careful with my timings, make sure any plan I put together stops when I expect to reach zenith, then basically restart a few minutes later, pointing 180 degrees the other way. The only problem with that is I would then need to realign the scope with two stars, I think? Otherwise it wouldn't be tracking properly. Still, if that's what it takes, that's what it takes. The other method is simply to stop the plan when it reaches zenith, then maybe pick a different object that is more convenient, and do two plans in one night, then the next night I do the same, so that I get the same amount of exposures but in two lots, if you see what I mean.

Or, I could just buy an EQ mount! It's on the list, but I just need to wait for the apocalypse to be over before I can think about buying one.

Edited March 27, 2020 by BrendanC

[Louis D]

I'm surprised it can't deal with Dobson's hole at all.  I would have expected it to issue a bunch of rapid AZ motions to eventually get a 180 degree rotation.  Is it because it realizes it can't keep the object centered well enough during the contortions so it gives up entirely?

[vlaiv]

12 minutes ago, Louis D said:

I'm surprised it can't deal with Dobson's hole at all.  I would have expected it to issue a bunch of rapid AZ motions to eventually get a 180 degree rotation.  Is it because it realizes it can't keep the object centered well enough during the contortions so it gives up entirely?

Not only that - what happens if a person is sitting in the chair - observing. It could be very awkward to have mount push the scope against you - potentially knocking you off the chair or something.

Rapid movement is an option - but it should come with some sort of audio warning - "You are about to be flipped! Do you want to continue?"

[Louis D]

24 minutes ago, vlaiv said:

Not only that - what happens if a person is sitting in the chair - observing. It could be very awkward to have mount push the scope against you - potentially knocking you off the chair or something.

Rapid movement is an option - but it should come with some sort of audio warning - "You are about to be flipped! Do you want to continue?"

I hadn't thought of getting hit, or worse, having an eyepiece jammed into your eye socket. 😵 Think about the liability issues for the manufacturer.