Meridian flip framing

[Carole]

I was imaging almost at Zenith and after half an hour I had the get the mount to do a meridian flip before the camera hit the mount.

On both the original alignment and the one after the flip, I just did a GOTO each side of the meridian (I did not adjust the framing).  But when I examined the subs afterwards, the GOTO was not the same, I have slight frame shift (about 1/8 of the frame vertically) and a bit of rotation.

Is this normal?

I would have thought it should have ended up in the same position but just upside down.

Not really done this before so it's a new one on me.

Result is I can't use the full frame when they are stacked. 

Carole

[mickw]

I would have thought a goto on a star rather than an object (galaxy/nebula) would be more accurate having a smaller area to centre on.

But then again, I've never been involved with the flip.

[MarkS]

I would have thought it should have ended up in the same position but just upside down.

In theory, yes, but it rarely works that way in practice.  You would need to be polar aligned absolutely perfectly on a rock steady mount and must have aligned on multiple stars so that all sources of error ( cone error, flexure, focuser sag) can be corrected for.  Plus complex software to perform those corrections.

Mark

[Mac]

Result is I can't use the full frame when they are stacked. 

Just think of it as built in dither,
Unless your image is completely full frame it shouldn't make that much difference, just crop the final image
to the usable size.

Mac

[Carole]

Thanks Mark makes sense now. 

just crop the final image

Yes that's what I have done, but wanted to understand why it happened.
I now know it's not a good idea to go for an object where I want to use the full frame it's it's close to the Zenith.

Thanks

Carole

[Mac]

I now know it's not a good idea to go for an object where I want to use the full frame it's it's close to the Zenith.

you can, all you have to do is to start the camera from the other side,

just select an object past the meridian and allow the scope to flip and position on the other object,
then manually slew back to the object you want,
as the scope has already done the flip it wont flip back,

Mac

[Carole]

Well yes, what I meant was, when it is close to the Meridian on the eastern side.

Thanks anyway.

Carole

[Mac]

Well yes, what I meant was, when it is close to the Meridian on the eastern side.

I realized that, but if you watch the flip is not exactly when the camera touches the scope, its when the object crosses the meridian, there is probably an hours worth of tracking before
the camera really hits the frame.
so if you start on an object already past the meridian, then the scope would have already flipped and is not going to flip again untill the object re transits,
you can still move the scope back to the original object (manually) on the eastern side, you might find that you might be able to get a couple of hours extra
that way, without the flip.

Mac.

[RobertM]

Don't forget that you'll need the camera sensor aligned exactly either E-W or N-S otherwise you'll loose corners due to misalignment of the frame.

If I know a target is going to transit the meridian then I align the telescope so that it's in the already flipped position then center it.  This only works if the telescope/camera is going to clear the mount in that position.  I find it's only goto's that will flip the mount back from there and guiding works ok.  It also has the advantage that you can keep the same counterweight/scope off balance to ensure better tracking.

Hope that helps
Robert

[Carole]

Thanks

Carole

[MarkS]

Don't forget that you'll need the camera sensor aligned exactly either E-W or N-S otherwise you'll loose corners due to misalignment of the frame.

I disagree.  Whatever the initial orientation of the camera, the meridian flip will rotate it through 180 degrees.  So you won't lose corners.

[Carole]

To be honest, I have no idea how my camera was orientated, I always try to position it so up/down and left/right (even if they are opposite) are at least vertical and horizontal.  So if I have to slew to get an alignment star central, or even to centre the object to be imaged I have some idea which way things will move. 

I did lose some corners because of the slightly misaligned stacking. 

Carole

[RobertM]

Don't forget that you'll need the camera sensor aligned exactly either E-W or N-S otherwise you'll loose corners due to misalignment of the frame.

I disagree.  Whatever the initial orientation of the camera, the meridian flip will rotate it through 180 degrees.  So you won't lose corners.

Ooops, thanks for correcting - you will be right there.  I wonder why I've always though that