What causes this?

[Babalyon 5]

Howdy. Hope your all enjoying some clear skies. I was wondering if any of you could identify what the problem may be by my posted picture. Visually, I cant fault this scope, great views, awesome detail, not 100% star collimated but , heck, its not too bad, and being a fast focal ratio, I keep it well tuned. It is my LXD55 SN10 tube. Is this vignetting? I cant seem to get a good round star in any camera, and sometimes they can look more like a dash than a dot.
Thanks.

[iceman]

How long was your exposure?
Are you guiding?
How do you know you're polar aligned?

[Babalyon 5]

I didnt check the fits header, however, I think it is two or four minutes.
Yes, I was guiding, honest I was, Sir!!:) Not sure if through Envisage or PHD with GPUSB.
Um, when I Line up the dots in my polar scope to the Trapezium and I have no drift, that should be Polar aligned, right?:D

[Alchemy]

clearly a tracking problem, icemans last two questions will reveal the answer.

[iceman]

How long do you check for drift for? Have you tried drift aligning?

Are you using a wedge or in alt/az?

[Alchemy]

hmm ok so you are guiding..... my next guess if youre aligned and tracking with phd or similar is......

flexure between guide scope and imaging scope

[Alchemy]

ooks like a 3 way conversation :lol:

can you put up a pic of your gear as you use it, that might help clarify a point or two

[Babalyon 5]

Umm..... I dont have any photos of my full setup, its always too dark to get a good shot, so I,ll post what Ive got. When I set this up , I drifted for about a week, you know, over several nights. I can land a star on the chip or observe visually for a very long time with little or no correction.
Oooops, there is a piccy or two of near full setup.
I must admit that Im very happy with the Polar alignment and drift alignment, I dont think that this is the issue. It may be flexure, but how do you sus it out, and still think it may be something else altogether.

[Zuts]

Hi,

This really looks like poor polar alignment and or tracking error to me as all the stars are extended in the one direction. Another possibility is that your pier is not stable and when you walk close to it you cause it to move and then settle ever so slightly.

I am not sure why or how you can align your polar scope with the trapezium as this would imply that your polar axis is in the competely wrong direction. It should be pointing at the SCP.

Vignetting is drop off in light levels at the corners of the image and that is not your problem.

Maybe you could post your drift alignment procedure?

Paul

Edit, I guess you meant the trapezium in the polar scope and not the trapezium in M42, sorry :)

[Babalyon 5]

Honestly, guys, thanks for your help, but you are all missing the point or I wasn't clear enough! Even when I just bring up the image on the screen in a live view, there is a distinct elongation of the stars on the screen. Whether that be a 1 second view or 2miute view, I see that they are elongated, to the point that they can seem as a dash not a dot.
And yes, I have a Trapezium in my polar scope that is next to the SCP to line up for initial setup. I then drift my scope via every drift alignment document on the web(and they all say the same thing), including the one/s here. I then mark and lock everything I can into place and frequently recheck polar alignment visually and drift as I have to pull it down and set it up every time(RoR Observatory coming soon)
I frequently check the backlash in the drive train and recalibrate the motors and train the drives, but this has nothing to do with what I see on my screen. If the stars were round on my screen and mis-shapen in the image, then I would think of other things.
Thanks.:)

[Terry B]

If the image is elongated even with very short exposures then the problem is the optics. Either the collimation is out or terrible coma.

[iceman]

ah I thought were talking about the tracking. Can you post a 30sec image? Or even a 5s image?

Have you checked for pinched optics of the mirror? (clips too tight?

[Kokatha man]

This is where I shoot back at you Paul!:rofl:;):)

Can I ask the simple question - what do the star images look like with your eye to the scope Babylon? This should give some indication of whether the optics are causing the problem: especially if, as you're saying, that live views of 1 sec are still showing this on screen.

Cheers, Darryl.

[RB]

Looks like collimation is out and also your focus is a little soft.

[Zuts]

Ouch, right between the eyes on this one :)

[Alchemy]

looks like we all grabbed the wrong end of the stick......... ah well blame him :lol:

as previous sugestion check visually the collimation and recollimate , then the camera mounting , if your tracking is good then thats about all it can be.

be nice to us were only trying to help:D

[Hagar]

Something else you could look at is your guide rate. A lot of problems arrise when the guide rate is set to 1XSidreal or faster. The problem could be your PEC. With guide rates set high the small movements of the guider will often initiate PEC correction so your guiding is a push pull arrangement. By reducing the guide rate to 0.5 or better still 0.25 sidreal may very well fix a guiding problem.
Remember GUIDE rate not TRACKING rate.
Good luck but it certainly looks like a guiding problem.

[spearo]

Well,
my 2 cents worth:
I had similar problems recently.
I think the focus is a bit off on the image.
Double check the drift align for 5 min at least but i suspect that's not the issue.
I suspect it may have to do with guiding if it looks good visually as you say.:
ensure the rising side of the scope is offbalanced a bit (possibly a bit more than it curently is to ensure good sitting of the gears.

check guiderate as mentionned earlier., play with aggressiveness rates and lenght of pulse sent to the mount as well as frequency of feeddback to the mount.


that fixed it for me
cheers
frank

[Babalyon 5]

What is the guide rate? Why & how is this different from the tracking rate. So, if The mounts tracking rate is sidereal, how does the guide rate work? Very curious on this as Hagar has suggested "push pull" and some times I wonder about this as I watch the guide star ??:shrug:


PS: LOL, focus is off for sure!!:lol:

[Babalyon 5]

Kokatha, I think the views are awesome. I am constantly amazed at how great deep sky objects look in this scope and on nights of steady air, Jupiter and Saturn have taken our breathe away. I never seem to see any unusual shaped stars or planets and I can always achieve a good focus(electric focuser). It is comparable to my LX90, the LX90 is just a bit sharper on the planets.:)

[Omaroo]

Babalyon (Babylon?) - Uhmm.. no. I still reckon you're not polar aligned properly. If you were attempting to base your whole polar alignment purely by pointing your polar scope at the "Trapezium", (which I interpret you mean as Octans, at the SCP) and thinking that if you couldn't "see" it move for a while that you might be polar aligned, you need to do more. The polar scope will land you in the rough vicinity, but really isn't for getting accurate drift alignment from. You need to be well off-centre to do that - not pointing at the pole.

You need to drift align in azimuth by using a star way away from the SCP, on or near the meridian, near zenith, or straight overhead. Then you could try to drift align in elevation using a star to the east near the horizon - or if that's not easily visible then to the west. You'll need an illuminated reticle for this and use your scope's optics to detect the drift.

Just looking at Sigma Octans through you polar scope, seeing it spin slowly and assuming that if it doesn't shift too much then you're aligned won't get you aligned for anything other than 30-second (maybe) photographs. You won't get adequate magnification to see the trend - which is why you need to use the other stars and a 12mm reticle.

Your focus is out a little too. Good luck with the alignment.

[Kokatha man]

That would certainly lay to rest the collimation/optics faults possibility. And in live view this elongation is still evident - struth Babalyon, I cant for the life of me work this out! Not being an imager (yet.....) it seems mighty strange that visually the images are fine but on the screen they're not, with tracking seemingly immaterial judging by your comments re live view.....

[Zuts]

Well,

I gotta ask. Is the camera OK, does it still take decent images? Maybe the chip is not at right angles to the image plane, maybe the nosepiece is not flat?

Paul

[Kokatha man]

That and even stranger speculations were on my mind Paul....!