Q: Meridian flip

[Andy Resnick]

TL;DR Summary

Somewhat confused by what is happening

This time of year, a lot of what I want to photograph crosses the meridian during observation time. I am using a Losmandy German equitorial mount and I understand the mechanical clearance issues when tracking through the meridian (my mount can handle a few degrees on either side of the meridian before something hits something else), but I am confused about a couple of things that may be specific to my mount and GoTo system:

1) Even though I carefully balance the mount prior to imaging, the motors seem to work harder (and even stall sometimes) as the meridian is approached. I suspect this is simple geometry (gravity + applied torque), but in case I am doing something wrong... is that normal?

2) My mount GoTo software divides the sky into hemispheres (East-West), and when I execute a meridian flip, the software switches to the other hemisphere and does not keep the multi-star alignment parameters. That is to say, when I execute a meridian flip the GoTo software acts as though I did not perform a multi-star alignment step. Is that a Losmandy thing, or is that common to GoTo platforms? If that is common... why?

Thanks in advance!

 

[Tom.G]

TL;DR Summary: Somewhat confused by what is happening

1) Even though I carefully balance the mount prior to imaging, the motors seem to work harder (and even stall sometimes) as the meridian is approached. I suspect this is simple geometry (gravity + applied torque), but in case I am doing something wrong... is that normal?

That sounds like you are doing a static balance of the scope. That is: applying the correct torque balance with the scope in one position and expecting the balance to be correct in another position.

You are getting the foot-pounds of balancing torque correct by using more/less weight and less/more distance from the pivot.

Either that, or the line thru the scope center of gravity to the counter-balance does not go thru the pivot point.

A sketch of the pivot points of the mount, with the scope attached would help, including the CG of the scope and where the counter-balance is mounted. Just a simple line drawing would be fine.

Cheers,
Tom

 

[glappkaeft]

... when I execute a meridian flip the GoTo software acts as though I did not perform a multi-star alignment step. Is that a Losmandy thing, or is that common to GoTo platforms? If that is common... why?

Never heard any other mount do that so if it not limited to Losmandy mounts then at least not a common limitation.

 

[Andy Resnick]

A sketch of the pivot points of the mount, with the scope attached would help, including the CG of the scope and where the counter-balance is mounted. Just a simple line drawing would be fine.

Here's my attempt at a drawing (not my scope, not my photo), and I apologize in advance for all the notations:

You are correct that I "static balance" the load: with the mount powered off and clutches fully loosened, I first rotate the RA axis so that the scope and counterweight are level with each other and rotate in DEC until the scope is pointing parallel to the ground. I then adjust the scope position by sliding the telescope (adjustment '1') and the counterweight (adjustment '2') until neither the DEC nor RA axes rotate on their own and consider everything mechanically balanced.

After static balancing, I observe the following (again, mount powered off, clutches loose):
1) If I then rotate the scope only in DEC so that it is pointing towards the sky, when I let go it does not remain vertical but rotates (only in DEC) about 10-20 degrees off-vertical.
2) If I instead "meridian flip" (rotate in RA so that the scope and counterweight switch sides, keeping the telescope horizontal), the scope slightly rotates in DEC.

To be sure, there is that 3rd possible adjustment, in case there is a lateral displacement between the center of gravity of the telescope and counterweight axis. I suspect there is a slight lateral displacement because the camera body is not centered on the optical axis (and the two observations above), but there's nothing I can really adjust for that, other than adding a translation stage to the telescope mounting plate.

Does this help better understand my setup? Thanks again!

 

[Baluncore]

I assume, the RA axis is set very close to parallel with the Earth's axis, and tracks sidereal rotation. The Dec axis is then set, giving the object declination, north or south. Any errors in those settings must be removed by multi-star calibration.

When an equatorial mount is used for photography, the image does not rotate while the object is tracked, and the image is accumulated.

With a misalignment of the mount and latitude setting, there may well be changes in tracking velocity necessary as the object passes close to the meridian. You can think of it like passing a gimbals-lock condition. That should NOT be a problem with your equatorial mount. It can be a major problem, for example, with an Az-El mount when tracking an object passing directly overhead, even when ignoring image rotation.

But what is up, or which way is up in the image? There are two possible conventions.
When facing the Eastern horizon, the sky is up, the ground is down. When facing the western horizon, the same is true. But when and where does the polarity of the RA axis rotate? I guess the convention is set when you perform the multiple star calibration. If you then change your hemisphere convention, the software expects you to recalibrate.

The inability of the software to correct across hemisphere conventions, is a deficit in the software. Maybe it should be computing a 3 Degree of Freedom alignment correction, but it is only computing 2 DoF, as RA and Dec corrections. It does that because it only has two degrees of freedom in the mount, so makes an assumption about the orthogonality of the RA and Dec axes.

 

[Tom.G]

Here are a couple of tutorials for balancing:

See especially post #9 and following:
https://www.cloudynights.com/topic/683275-balancing-a-telescope/#entry9761192

And a 2 minute video:


(above, and many more, found with:
http://www.google.com/search?hl=en&q=how+to+balance+telescope)

Cheers,
Tom

 

[Andy Resnick]

The inability of the software to correct across hemisphere conventions, is a deficit in the software. Maybe it should be computing a 3 Degree of Freedom alignment correction, but it is only computing 2 DoF, as RA and Dec corrections. It does that because it only has two degrees of freedom in the mount, so makes an assumption about the orthogonality of the RA and Dec axes.

This makes sense- thanks!