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Old 22-11-2012, 02:22 PM
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pmrid (Peter)
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SBS system producing image shifting

I've been wanting to get more use out of my Titan mount and so have installed a SBS system on it with a BabyQ on one side and a Vixen R200SS fast Newt on the other. The BabyQ has a SXVR camera on it which means it uses an Ex-View guidecam which integrates with the main camera. On the Vixen, I have just put up an old QHY8. The planned outcome should have been that I can image through both scope/camera systems while guiding for both using the Ex-View camera on the BabyQ.

Previously, the BabyQ on its own tracked and imaged beautifully - no odd things happening. But the Vixen - 8" F4 is decidedly not. Here is an overlay of every 3rd or 4th image out of a sequence - 10 minute subs. There is a real progressive image shift that can't be explained away by the obvious. The guiding remains tight as a fishes' but the Vixen - which is on the other side of the SBS is shifting.

As a comparison, the BabyQ side of the SBS does not seem to exhibit any jump/shift behaviour. I also attach an overlay of images taken before I set up the SBS and another overlay taken with the SBS setup simultaneously with the Vixen image attached. To my eye, there does not seem to be any or any significant shifting in the BabyQ images befor-and-after. But with the Vixen there definitely is.


Is this curable or does it spell the end of my grand plan to bang away with 2 scopes simultaneously?

Peter
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Last edited by pmrid; 22-11-2012 at 03:22 PM.
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  #2  
Old 22-11-2012, 04:12 PM
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Merlin66 (Ken)
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Hmmm
Certainly looks like something is moving!!
Can you confirm the direction of the movement relative to the scopes and the SBS....
There's usually not a lot of movement in the saddle clamps...could it be some movement in the R200? Mirror movement?
What about a couple of photos of the setup -may give some clues....
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Old 22-11-2012, 04:41 PM
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Nico13 (Ken)
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Hi Peter,
Looks a bit odd in those images alright, I run a SBS system and although not imaging from both at the same time I can swap my camera from the refractor to the SCT and continue on without issue.
I have an Orion ST80 and SX Superstar for guiding and it's piggy backed onto the ED80 all running on an NEQ6 Pro.
So yes a couple of pictures of your setup may shed some light.
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Old 22-11-2012, 05:27 PM
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Quote:
Originally Posted by Merlin66 View Post
Hmmm
Certainly looks like something is moving!!
Can you confirm the direction of the movement relative to the scopes and the SBS....
There's usually not a lot of movement in the saddle clamps...could it be some movement in the R200? Mirror movement?
What about a couple of photos of the setup -may give some clues....
It can't be a movement issue - simply because the movement is progressive, frame by frame, in the one doirection and by more or less equal increments. If it were some loose clamp or whaver, the movement would more likely be a one-off or in any case the pattern of movements would not be linear but random. My guess is that it is a rotation of the Vixen around an axis set by the BabyQ - the BabyQ fixes on the guide star and the other scope pivots around that. My gut feeling anyway.
I will grab some shots of the setup in the morning.
Peter
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Old 22-11-2012, 05:50 PM
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Peter I'm not sure where I heard/read it, but I remember something about if the two imaging systems are not orthogonal then you well get frame shift. Particularly if they are also non-orthogonal to the mount. I experienced the same as you at one stage and just about went balmy trying to eliminate it. I also seem to remember something about even slight polar misalignment can exacerbate the problem as well.
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Old 22-11-2012, 06:05 PM
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Pics of the setup.
Peter
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Old 22-11-2012, 06:46 PM
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Isn't that just flex. Aren't you basically guiding with the tak and imaging with the vixen on a sbs bar.
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Old 22-11-2012, 06:47 PM
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Quote:
Originally Posted by Tandum View Post
Isn't that just flex. Aren't you basically guiding with the tak and imaging with the vixen on a sbs bar.
Yep. That's basically it - but FLEX WHAT!!!!
Peter
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Old 22-11-2012, 06:50 PM
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If both systems are not each exactly 90 deg to the RA axis then they are non-orthogonal. This means that each imaging system will scribe a slightly different arc through each exposure even though the guiding system is working fine. I'm no mathematician but i remember doing some work with Gary Kopf at Astrofest one year trialing an AN update and it was one of the things we were working on modelling. I'm sure he would know more about it (as I'm sure plenty of others here would as well) so maybe drop him a pm?
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Old 22-11-2012, 07:20 PM
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Peter
I've never seen a successful SBS setup that didn't have some amount of flexture in it. It will be an interesting couple of months spent by you trying to track down where it's occurring. Several of my mates had SBS as did I and gave up due to flex.
Hope you have better luck but I'm sure my comments will draw a few acerbic retorts.
Allan
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Old 22-11-2012, 07:21 PM
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So if I understand what Paul is saying, if you use a separate guide scope you will always get a flex like result if the scopes aren't orthogonal?

If it's just flex the amount of movement seems to change depending on altitude.
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Old 22-11-2012, 07:30 PM
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No, not necessarily Robin. I don't understand the maths or the way it all works, but I've seen it often enough and tried long enough to remove flexure to accept it. The movement is very regular over frames, unlike flexure, though over a few hours there may also be a rotational component in it as well.
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Old 22-11-2012, 07:48 PM
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Quote:
Originally Posted by [1ponders] View Post
No, not necessarily Robin. I don't understand the maths or the way it all works, but I've seen it often enough and tried long enough to remove flexure to accept it. The movement is very regular over frames, unlike flexure, though over a few hours there may also be a rotational component in it as well.
Well, Paul's explanation makes sense. I haven't got the patience to mess around with getting the 2 scopes orthogonal so the answer is simple - ditch it ango back to what works.
Peter
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Old 22-11-2012, 08:05 PM
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Quote:
Originally Posted by [1ponders] View Post
If both systems are not each exactly 90 deg to the RA axis then they are non-orthogonal. This means that each imaging system will scribe a slightly different arc through each exposure even though the guiding system is working fine. I'm no mathematician but i remember doing some work with Gary Kopf at Astrofest one year trialing an AN update and it was one of the things we were working on modelling. I'm sure he would know more about it (as I'm sure plenty of others here would as well) so maybe drop him a pm?
thanks paul i think you have fixed mine too!
i tried to get the same image through my lodestar as i do on my exposure in my 60da
at the same time i attempted native f10 imaging and i got elongated stars so i immediately thought it was because my lodestar was at 500mm and my SCT was at 2800mm
i inadvertantly made the orthogonality (great word) worse
makes perfect sense.... thanks paul i will try and align them better now
pat
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Old 23-11-2012, 08:58 AM
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I think what Paul is talking about is field rotation. Field rotation is a result of imperfect polar alignment. It is exacerbated when there is an angular difference between the guide star and the centre of the imaging field (pretty much guaranteed if you are using separate guide and imaging scopes on a SBS system without tweaking them carefully to make them orthogonal).

The gory details are explained here for anybody who wants to dig further:
http://celestialwonders.com/articles...ntAccuracy.pdf

For those who just want an executive summary, the things that make field rotation worse are:
  • less accurate polar alignment
  • larger guide angle
  • larger FOV
  • smaller image scale (determined by focal length and pixel size)
  • longer exposure times
  • higher declination (field rotation is worst near the poles)
Peter: I reckon you probably have a mix of field rotation and flexure.

Cheers,
Rick.
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Old 23-11-2012, 11:02 AM
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Peter: I reckon you probably have a mix of field rotation and flexure.



Cheers,
Rick.
Phew, thanks Doc. I thought I had the %$^&

Peter
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Old 23-11-2012, 11:34 AM
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Peter,
IMHO you need to analyse the images - check and confirm the direction (relative to the scopes and the SBS) and magnitude of the drift, before assuming a worse case scenario.
In a SBS the "cone" error is just an up and down adjustment of either the tube rings or the saddle(s) - yes this can cause field rotation but no worse than you'd probably get using a piggyback system - what's the difference??
Do you have any prior images with the Vixen that were guided with a piggyback system? (ie not OAG etc) Do they show any relative movement?

Once you know how much movement is involved and the direction, you can home in a bit easier on possible causes.......
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Old 23-11-2012, 04:20 PM
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Thanks Ken,
I take your point. It's going to one of those jobs that I have to approach with unaccustomed patience - meaning "not now". I don't have any comparison images with the Vixen - it's a recent addition to the stable (more blasted impulse buying) and this has been it's baptism of fire so to speak.
I'll plug away with it and see what I can figure out.
Peter
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Old 23-11-2012, 06:38 PM
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Sbs

In my experience, sbs is far more difficult to get right than over under. If over under is possible, steer down that route.
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Old 24-11-2012, 08:22 AM
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I remember a post by Roland Christen about piggybacked guide scopes that may be relevant. He advised that the piggyack guide scope had to be connected to the imaging scope and not the rings.

I guess it eliminates some flex as the main tube flexes slightly perhaps so would the imaging scope.

So what about this:

1. Measure the distance between bottom and top of the 2 scopes and get it as exact as you can.

2. Add another bar across both of the scopes and attach the guide scope/guide camera there.

See if that helps.

With the Vixen R200S the question would also be whether the focuser is free of flex (Newts have heavy gear hanging off at 90 degrees to the tube and is asking for flex). And also whether there is any mirror shift.

I would have though mirror shift would be more erratic but that's just a guess. So its probably as discussed differential flex between the 2 scopes (how good are those Vixen mounting rings?) and non squareness.

Best guiding practice today is off axis guiders or high quality encoders on the RA axis. If you were really keen then high quality encoders may pay dividends as its not dependent on a flex prone guide scope.

Greg.
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