Drifting and zig zagging Images

[Felipe]

Hey guys,

recently I’ve been trying to get myself started on the this long journey of AP, and as a total beginner I manage myself to get some equipment going, I have an Skywatcher EQM-35 mount and a GSO RC 6”, no guiding system at the moment, I’m still figuring out correction on my polar alignment and doing my homework with all the different setting in the mount, I know that the telescope it is a little bit heavy for the payload capacity for the mount, tho the total weight of the set up with my DSLR cam is 6.4kg, so I’m really pushing the limits with the mount and the telescope.

I got a few photos of the Trifid Nebula the other night and as I go through them I noticed clearly the target drifting away from the FOV, in a linear direction, along with that there are some zig zag in the movement as you may say that the mount is chasing more than tracking the target.

Does anyone might have a suggestion on what am I doing wrong or what can I do to avoid this issue?.

i have a video that can explain better what I’m talking about.

 

thanks for taking the time to read the whole post guys, and any help or suggestion will be much appreciated!.

 

[vlaiv]

I could not play that video but I have strong suspicion I know what is going on.

Something like this:

(if you can't see the animation above - open this link in new tab: http://serve.trimacka.net/astro/Forum/2015-11-21/post_01/RA_vs_DEC.gif )

Linear drift is related to polar alignment - better polar alignment - slower drift.

Zig/Zag movement is related to something called Periodic Error of the mount. Drive train of the mount is not perfect and circular components are not quite circles (but rather egg shaped - on a small scale). That is due to manufacturing tolerances and all - you simply can't make perfect circle.

Two way to deal with periodic error (PE) - first is periodic error correction (PEC) and second of course is guiding. PEC can fix things to varying degree - sometimes providing larger benefits and sometimes only minor - depends on what the error is like. If it is "in sync", or rather all oscillatory errors are harmonics of main worm period - PEC can go a long way. On the other hand if PE consists of unrelated frequencies so baseline frequency is not related to worm period - there could be almost no improvement at all. In reality it is somewhere between the two and more often closer to first case - after all gears are made to power worm and it is likely that their period will be harmonic of worm period - so PEC is something worth doing.

From what I can see, this mount has PEC capability so consult your manual on how to enable it and do the training.

Guiding is self explanatory - and I would advise you to look into it. Maybe off axis guider would be good match for that scope.

 

Edited July 31, 2020 by vlaiv
Something went wrong with the gif

[Felipe]

Hi Vlaiv,

Thanks for the info mate!.

yes, the mount has a PEC training setting that I can play with, The movement in zig zag is basically the same as the gif but very uniform I must say, so yeah, next time I’ll have a play with that.

Polar alignment it is something that I’ve been trying for fix a lot, here in the southern hemisphere (New Zealand) it is tricky to see the SCP so I use the app PS Align pro to roughly aim in the right direction and then do a polar alignment iterating with different stars until I reach 0 arc/sec but I guess there’s nothing like 0 for real.

About the guiding it something I’m having a look at the moment, as the payload capacity of the mount it is already pushed i need to get something very lightweight, so my options are narrow on this side of the planet at the moment.

Thanks Vlaiv, I really appreciate you have taken the time to drop some workd for me!.

 

cheers!.

[Aramcheck]

You could think about getting an iPolar to help with the Polar alignment. I've found it very useful & quick to use:- https://youtu.be/4MXjN34Om_g

 

Cheers
Ivor

[Felipe]

18 hours ago, Aramcheck said:

You could think about getting an iPolar to help with the Polar alignment. I've found it very useful & quick to use:- https://youtu.be/4MXjN34Om_g

 

Cheers
Ivor

Thanks Ivor, 

Do you know if that iPolar would work to check cone error as well?.

[Aramcheck]

54 minutes ago, Felipe said:

Do you know if that iPolar would work to check cone error as well?.

No. It only helps with the Polar Alignment as it sits in the exit aperture of the mount's polarscope, i.e. you're aligning the mount to celestial north, whereas Cone error is caused by misalignment of the telescope axis relative to the mount.

Best check that it would work in Southern hemisphere too!

Cheers
Ivor

[vlaiv]

3 hours ago, Felipe said:

Do you know if that iPolar would work to check cone error as well?.

Cone error is rather easy to check, it takes just a 10-15 minutes.

Take a star that is about to cross Meridian and go to it - take exposure. Wait for it to cross Meridian, do another goto, scope should change side of pier - take another exposure.

Compare star position in two exposures - difference in position is due to cone error.

[Felipe]

12 hours ago, vlaiv said:

Cone error is rather easy to check, it takes just a 10-15 minutes.

Take a star that is about to cross Meridian and go to it - take exposure. Wait for it to cross Meridian, do another goto, scope should change side of pier - take another exposure.

Compare star position in two exposures - difference in position is due to cone error.

Thanks again Vlaiv!, I’ll try that out!.