Star trailing whilst guiding... Confused

[LunarLight]

Hey guys

My venture in to auto guiding has thus far been a frustrating experience. The help I have been given on here has been very helpful so thanks to all those who have helped so far.

I have spent the last 3 cloudless nights figuring out why stars shoot off the side of my synguider only to realise it's because I left the controller for the neq6 on speed 5. Lol I fixed this last night woohoo and set my guider going for and additional 11x 8 min photos

Only to find that the stars were trailed as if it were unguided. Can anyone think of a cause for star trailing whilst the guider is working?

Thanks

mark

[steppenwolf]

Could be several things:-

1. aggressiveness too high

2. aggressiveness too low

3. flexure between the guide scope and the imaging scope (even a loose imaging camera or movement in the focus tube)

[peter shah]

there a a few things that can cause trailed stars when guiding....do you have an example to show?

aggressiveness and flexure steve has already mentioned, poor seeing, bad polar alignment, bad pec training, off balance or over loaded mount, are a few other things to look at.

but i would say that flex is one of the most common causes

[LunarLight]

thanks for the advice guys I think it might be a flex problem. I am using standard tube rings for my guide scope and I had to change the orientation from the main scope to find a guide star in my light polluted skies. It may have been a little loose. I am currently awaiting guide rings so hopefully that will help.

I cant up load a picture just yet as I am at uni and dont have access to it but the trails are not straight, they wiggle a little so I suspect it might be flexure. The seeing wasnt great either but I shouldnt have thought that would have mattered much,

I will upload a picture later guys, thanks for the advice

[LunarLight]

hey guys

here is the image that was requested. It is unedited complete with light pollution and all :-D

any suggestions on why this is would be helpful

here is a link to a larger version: http://img220.imageshack.us/img220/4602/bodespart2032converted.jpg

[steppenwolf]

Judging by the positions of M81 and M82, the movement is in RA only. Have you got any more images of the same objects taken either before or after this one? I'd be interested to see if there is any 'creep' from frame to frame.

[LunarLight]

that was the first image of the session, all the others are exactly the same. You are right there is creep in the frames. When I first stacked them they formed a star trail image.

upon a re-stack I had to crop large sections due to poor overlap of images. (I no longer have those images)

do you think it could be balancing issue? or maybe the RA is moving to fast?

here is the last image in the set, they move slightly between the two: http://img217.imageshack.us/img217/4843/bodespart2043converted.jpg

[ollypenrice]

The creep, I guess, will be down to polar alignment?

The error in RA doesn't look like backlash because you don't have two groups of hits as genrated by a mount swapping sides on which to mesh.

This is certainly one for Steve!

Olly

[steppenwolf]

I agree with Olly that this doesn't look like a backlash error for the reason stated. Polar alignment may have a part to play in this but I don't think so as I would expect to see some DEC slip too which on the face of it isn't present.

I suspect that this is something like a loose focuser, maybe mirror movement or using a nosepiece to attach the camera to the focus tube with only one retaining screw. As the mount rotates, there is a weight shift causing flexure.

[simon hicks]

If the guide star is up in the top right hand part of the image (where the stars are much better) I would suspect image rotation due to polar alignment. Were you guiding on the bright star just above and to the right of the upper galaxy?

The stars look as if they are rotating about that point.

[LunarLight]

Ollypenrice + Steppanwolf = thanks for the advice guys, I will make sure next time to double check my polar alignment and to double check my camera and guide equipment is secure.

how do I check if a mirror is loose? when I collimated my scope the screws didnt feel loose.

simon hicks - my guide star was to the left of the image. I am limited in choosing guide stars as I dont have guide rings yet and I am using a set of standard rings thus I have a limited range of scope movement right now...if that makes sense (maybe that explains the trials I dunno)

[swag72]

Sorry to butt in here, that's an interesting comment made about where the guidestar is and possible rotation. I am still suffering from some rotation, so next time I'm out I'll check the tightness of everything. That leads me then to the question of whether it is better to guide on stars in one part of the image than another? Does it help to minimise rotation if the guide star is in different positions?

[simon hicks]

If the guide star was to the left of the image then that defintely rules my explanation out.

[ollypenrice]

I don't think the location of your guide star is at all important (within reason!) On the SBIG system, for instance, it is always off-image. With an OAG it is always on-image. It really doesn't matter.

Olly

[simon hicks]

Olly, if you are guiding well, but have polar misalignment then your guide star will be perfectly small/round/tight and the other stars will rotate around it...the further away then the larger the trail. Especially a problem with large FOV and near the zenith, and/or if the guide star is some distance from the centre of the image. By looking at the curvature of the trails you can locate where they are rotating around, and that should be a tight star (or region of the image).