Guiding after meridian flip

[Carole]

Just had a thought.

When I was at Rother Valley last I went to bed for 1 1/2 hours while my M27 was imaging, and when I came back the mount had done a Meridian flip, or a flip of some sort.  No problem from the stacking point of view, just flipped them over, but it has just occured to me, what happens to the guiding when this happens?  Obviously while the flip is happening it must go a bit bananas, but I can't quite get my head around whether after the flip, the guide camera ends up being on the same star as before and just carries on guiding again.

I never gave it a second thought at the time, but it's just crossed my mind.  Obviously as I was in bed, I was unware of this happening until I came out again a number of subs later.  It certainly appeared as though it had just carried on as normal. 

Carole

[Mike]

The guide star would be lost as unless it was dead centre it will now be in a different position.

[MarkS]

An automatic flip has never happened to me - I've only ever done it manually - maybe the EQ6 doesn't do it automatically.

The guide camera will not end up pointing at the same star in general - in fact it will be lucky to find a star at all, without user intervention.  PhD requires you to select a star manually - you'll be lucky if a star appears in it's small search rectangle.

Mark

[Rocket Pooch]

Hi All,

Mark, the EQ6 will do a flip but only after you stop guiding and if you re-point to the object, this is really cool because as Carole found out its a paint when it just does it.

Carole, you will probably need to re-centre the object find a guid star and re-calibrate the guide star when this happens.

Chris

[Carole]

OOOer, in that case I had better examine my flipped images, as I just stacked them all together, never having thought twice about it, that is assuming I can identify which ones they are and I haven't flipped them back permanently.

This may account for Chris's comment on the final image that there were guiding issues, although I would have thought the guiding would be terrible on the flipped batch.  Will report back when I've re-checked the subs (if possible).

Well that's stuffed going to bed in the middle of a long run of subs.

Thanks

Carole

[Carole]

Well I've examined the subs before and after flip, and they seem to be spot on covering exactly the same amount of sky, one being portrait and the other being landscape, and the guding looks just the same on the flipped lot.  I've examined all the corners and they are spot on the same. 

So I guess I was just lucky but something to think about for another time.

Thanks every-one.

Carole

[MarkS]

Eh?

If one was portrait and the other landscape, this was definitely not caused by a meridan flip.

Instead, someone has manually turned the camera through 90 degrees.

Mark

[Rick]

I'd figured north=up had become south=up, or something like that?

[MarkS]

I'd figured north=up had become south=up, or something like that?

Exactly. 

During a Meridian flip the RA and Dec axes are both "spun" through 180deg.  These two orthogonal 180deg "flips" has the same effect as a rotation through 180deg i.e. from the camera's point of view the whole universe has just rotated through 180deg. Nothing changes by 90deg.

[Rocket Pooch]

you have a point there

[Carole]

Well what is it called then when the mount turns around all on it's own and ends up looking at the same object without any help at all from me?

This has happened on 2 or 3 occasions and although I didn't witness it happen at Rother Valley as I was in bed, I actually saw it happen earlier in the year, and wondered what on Earth was going on.  Then I just assumed this was what people called a meridian flip, although I must admit at the time I witnessed it the object wasn't at the meridian.

Anyway, this is what the subs looked like on my PC when I returned to it.

first 17 looked like this (these are 5 min single subs)


Remaining 7 looked like this (these may not be to scale in transfer from webspace)


Carole

[Rick]

What sort of camera were you using? That doesn't look like a meridian flip. It looks like the camera has an orientation sensor, and changed its mind about whether the images were landscape or portrait part way through the run...

[Carole]

Canon 450D

It still doesn't explain what the "flip" was that I witnessed earlier in the year. 

On reflection I think this might have been a different scenario and the images weren't in fact an exact replica, but were still landscape and portrait. 

Sorry, looks like this wasn't a meridian flip after all.

Carole

[Mac]

you camera has a built in sensor to detect whether your camera is horizontal or vertical, as your mount tracks across the sky, the orientation of your camera changes, at some point your camera decides when the image is horizontal or vertical and just sets a value on the image, all you need to do is to turn off auto rotate on the camera.

As for meridain flip, that is what you saw, but is unrelated to the images turning 90 deg.

you can prove this by taking one image at home, and then turning your camera 90 and taking the same image.
When you view the images, the camera / software will auto rotate the images.


Mac

[Carole]

Thanks Mac, that's got it all sorted now and explains why the guiding carried on OK.

Sorry to confuse every-one, I had no idea the camera would do that.

Carole