Picking the right guide star and guider alignment

[multiweb (Marc)]

Well, you learn something new everynight... Tonight was a bit of a revelation for me. Maybe I was having a "blonde moment" but bear with me on this one. You'll probably laugh your a** off anyway.

I've been using the Hyperstar for a little while now. I have had some little issues to iron out on the way but I kind of got mirror flop sorted out, focus, squaring etc... Tonight everything looked pretty tight and I'm shooting above scorpius in Ha. Subs look real good but for one bloody corner. Always the same corner ... and I'm shooting pretty high so it's not any flexure or flop, noway.

Anyway you all heard about my legendary polar alignment so I always get a tad of field rotation. Looking at the subs it looked like the center of rotation was the bottom left of the field and of course the top right corner was the problem So it hit me (finally). Moved the guider scope towards the left of the field and voila! round stars again... DOH!

[Hagar (Doug)]

I gather what you are saying is the guide scope should be centered on the target or at least very close to the center of the image and the guide star should be as close as possible to the center of the field as is possible to avoid some out of synch between the guide star and the imaging camera.
This would make sense as the polar aligment is centered on the main imaging scope/mount and probably should have the same point of reference/rotation. This would account for the good results gained by using an OAG/ internal guide chip.

Food for thought. This could be part of my problems while trying to stack LRGB images. I might have to break out the OAG that I hate with a vengence or put the guide scope rings back on instead of having the guide scope bolted down to the scope rings.

Thanks Marc.

[multiweb (Marc)]

Originally Posted by Hagar

I gather what you are saying is the guide scope should be centered on the target or at least very close to the center of the image and the guide star should be as close as possible to the center of the field as is possible

Correct. Unless your PA is spot on guiding on a star further away from the center of your image will make everything waaay worse. Like a moment of inertia with a big long stick
I'll post two shots to see how dramatic the difference is. Same guiding, same conditions, just picked a different star closer to the center.

[Paul K]

Hi Marc, i would be interested in seeing the difference between those images...very interesting..

Paul

[multiweb (Marc)]

Here it is - top right corner with maximum field rotation - crop 1:1, 1x5min sub

[Paul K]

proof of the pudding........

[Moon (James)]

Marc,

Great example there. This can bite you any time and it's quite subtle compared to normal tracking issues that show up in a PHD graph. Blinking a few hours of subs is also a good way to spot it - but of course this only happens when it's too late to do anything about it.

This would account for the good results gained by using an OAG/ internal guide chip.

But you are still guiding off a star that is outside the FOV of the main image, and I have seen very small amounts of rotation is this situation too. With a separate guide scope you can pick a star dead smack in the center of your image if you like.

In reality if your PA is ok, then rotation is normally a minor issue for us in the southern hemisphere because most of us know how to do a drift alignment or equivalent. It's more an issue up north where they often use a polar scope / polaris and get it in the rough ballpark.
James

[AdrianF (Adrian)]

Thanks for that that will explain my imaging problems. I always use the brightest star in the guide scope and this isnt necessarily in the centre of the scope. But I do align the guide scope and the main scope on the same object.

Adrian

[Moon (James)]

Adrian,
Blink 1 hour of images and check. If you are taking 5 min subs, the rotation is usually so small it is undetectable in a single image. Over 1 hour, you should be able to see it. You can also measure it in DeepSkyStacker.
Remember you will always get some amount of rotation eventually, selecting a good star just minimises the impact. The key is a make sure your polar alignment is good enough to get through one sub without too much rotation.
James