Tracking certain parts of the sky.

[Larry 1969]

This post is about a lesson I may have learned in AP but could apply to visual as well.
Last weekend I took over 100 15 sec exposures of M51 with my tracking dob. DSS would only stack 15 of them and when I zoomed in, the stars were oblong......
I've had surprisingly good success imaging M57 and M27 previously and that got me thinking.......
M51 is in the NW sky as opposed to M27 and M57 which were in the east. I'm thinking my mount only had to rise to track those two whereas in the north my target was rotating.
Does this make any sense to any of you? Should I focus on targets in the east?

Thanks!

Larry

[JayTee]

Hi Larry,

The number of words to describe and answer your question are voluminous. So here is a great article on field rotation.

https://calgary.rasc.ca/field_rotation.htm

Hope this fills in the gaps of knowledge,
JT

[Larry 1969]

Hi Larry,

The number of words to describe and answer your question are voluminous. So here is a great article on field rotation.

https://calgary.rasc.ca/field_rotation.htm

Hope this fills in the gaps of knowledge,
JT

Field rotation! That must be it!
Thanks for the read!

Larry

[KingClinton]

With a Alt/Az mount field rotation is a huge problem, the only real way to mitigate it somewhat is with a wedge, impractical for your dob.

The closer you shoot to the poles(NCP/SCP) the worse it is.
You will also find you need to reduce your exposure times closer to the poles.
Directly in the East is much better and you can stretch those exposure times a bit more.

[Larry 1969]

So I managed to shoot M81 and M82 before it actually turned out decent.
Is it possible that it was so close too the pole that the rotation didn't cause star trailing?
DSS stacked 81 out of 150 (pretty good for me Lol) and I managed to get this out of it.
I think I will continue to pick objects in the eastern sky for now.
Thanks!

Larry

[helicon]

Nice shot - it looks somewhat like these two galaxies appear at the eyepiece. And you may have caught the faint glimmer of NGC 3077 down below M81, hard to tell exactly.

[Larry 1969]

Nice shot - it looks somewhat like these two galaxies appear at the eyepiece. And you may have caught the faint glimmer of NGC 3077 down below M81, hard to tell exactly.

Thanks helicon!
My processing skills are poor to say the least and I remember when I first got those two in the eyepiece! I was STOKED!
My point is (I think) M 81-82 is closer to the pole than M51 right???? Is there some point where you get so close to the pole field rotation has a lesser effect?
My attempt at M51 was dismal. DSS would only stack 15 images of 103 and even those had oblong stars. I know there is too much noise and that's a topic for the AP forum.
I'd like to keep this to tracking here in the beginner forum. Here's my M51......These actually stacked..... Trailing stars and all.....
Just wondering why this turned out so much worse than M81-82????

[KathyNS]

Is there some point where you get so close to the pole field rotation has a lesser effect?

I don't think so. Field rotation is maximum at the zenith, is less along the meridian, and is least at the east and west horizons. The pole, of course, is on the meridian. The closer you get to it, the closer you get to that steady 24 hr/day rotation.

[Larry 1969]

Thanks KathyNS!
I must have just got lucky with M81-82.
I'll stick with easier objects in the east for now I suppose.
If the AP bug bites I'll just have to pony up for the proper equipment.

Larry