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Old 08-03-2009, 04:57 PM
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Can anyone explain this to me????

While processing my recent set of images I've come across something I'm having difficulty explaining. When I look at the first, middle and last image in the subs there is a drift in what I believe is RA. How does this happen when the telescope is autoguiding?

Here's my first, middle and last M104 sub from this weekend (total session over about 70 mins apart) showing the issue...
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  #2  
Old 08-03-2009, 05:00 PM
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[1ponders] (Paul)
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Don't quote me on this but I think you will find that it is either inaccurate polar alignment or using a star to guide on that is away from the fov that you are imaging. I think.....
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Old 08-03-2009, 05:59 PM
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I thought the drift would be in DEC if it was a polar alignment issue?
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Old 08-03-2009, 06:07 PM
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Unless your camera is orientated differently to my sbig, that drift would be in Dec for me.
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Old 08-03-2009, 06:31 PM
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I was working off the Astroplanner DSS image - The RA reading increases as I move the mouse over the dust lane from right to left on the image.
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Old 08-03-2009, 06:36 PM
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Don't know what's happening then Peter.
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Old 08-03-2009, 07:03 PM
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The only theory I've come up with is the imaging OTA is flexing differentially to the guide scope as the object rises although I find this hard to believe.
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Old 09-03-2009, 09:18 AM
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Quote:
Originally Posted by peter_4059 View Post
The only theory I've come up with is the imaging OTA is flexing differentially to the guide scope as the object rises although I find this hard to believe.
That might explain why the direction of drift does not appear to be consistent from frame to frame.
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Old 09-03-2009, 11:32 AM
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It is definitely a drift or flex problem, more drift I think. You can see the stars are elongated and moving from top right to bottom left. Is everything bolted down tight and is your balance just heavy in one direction across both mount axes?
All you can do is check and re-check everything.
Nice Sombrero, by the way!!
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Old 09-03-2009, 07:33 PM
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How do you get drift if the autoguiding is working?
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Old 10-03-2009, 07:31 PM
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I don't know what program you're using to autoguide but assuming you stop guiding between your exposures it's enough for the mount to drift until the guider starts autoguiding again for the next exposure. If the drift was happening during exposures and not in between exposures you would get star trails for sure which is not the case in your pictures. In effect you're dithering.
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Old 10-03-2009, 09:02 PM
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Marc,

You might be on to something - I'm using PHD to guide and Envisage to image. I don't stop the guider but perhaps the image downloading is preventing the guiding functionality periodically at the end of each exposure?
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Old 11-03-2009, 10:07 AM
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Well that's it then Nothing wrong with that. You'll get better noise reduction and hot pixel removal by offsetting anyway. You just want to make sure your alignment or balance is not too dodgy as to loose the galaxy between exposures though . The slight elongation of the stars in the same direction as your drift is probably the guider not coping during the exposure with the same thing that causes your mount to shift the whole frame from subs to subs. Definitly not flexure. More like balance. Flexure is "sneaky" it creeps on you...
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Old 11-03-2009, 05:06 PM
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Hi Peter
If you are using PHD you can turn on the fine grid and the RA/DEC real time graph to make sure that you are guiding properly and that the guide star is not moving on the screen. If this is OK then it looks to me like a bad case of flexure. As mentioned previously this may mean that your guide/imaging scopes are not secured properly. Give them a good wiggle and see how they move. Flexure usually occurs in RA when the scopes track across the sky and they lean more or less on their mounts so that the move relative to one another. Although this movement may be very slight it can still result in high pixel drift in RA. I have been plagued by this problem and recently had to build a new platform for my scopes to reduce it. It is worse the longer focal length you use on your imaging scope and the longer images that you take.
If this is your problem you will have to examine how you can stiffen up the mounting of one or both or your scopes to prevent relative movement between them as they track across the sky. Hope this all makes sense.
Cheers Peter
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Old 11-03-2009, 05:53 PM
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Iíve experienced flexure in the focuser as the OTA tracks and the camera weight distribution shifts, as well as flexure that may have been the result of cable drag as the mount was tracking.

Generally, if I am imaging at flís below around 1200mm, flexure tends not to be noticeable but as soon as I pop my head above 1500mm fl, unless I am very, very fastidious about tightening everything down, locking focusers, minimizing cable drag, balancing the rig, etc. then doing it all again, flexure will almost certainly rear its ugly head.

Cheers

Dennis
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Old 11-03-2009, 06:17 PM
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Thanks for the comments. I'm wondering if the Vixen style dovetail between the tube rings is the source of the problem. This would be the weakest link in the rig. I'm also wondering now if the primary mirror could shift slightly as the scope tracks toward the meridian and if so what effect this would have?
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Old 14-03-2009, 12:11 PM
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Hi Peter
I have a Mak 150mm which (as with Maks) is rather heavy and sitting on its ring mounts. These are supported at two feet to match the dovetail mount connection which I guess would be similar to your Vixen. Previously I had my 80mm f6 refractor and Telrad also mounted on the rings. The guide scope (a Mak 100mm) was mounted separately. I had noticeable flexure in the circumstance described by Denis and I suspected that although the Mak 150 mounting rings are sturdy it was best not to load them with other gear (except a small finderscope). I have now constructed two platforms so that the Mak 150 only carries only the finderscope. I found that this arrangement reduced my flexure symptoms from 1.5pixels per minute to 0.6pixels per minute which I can (just) tolerate. Perhaps there is some movement in the Mak mirror (apart from mirror flop!) that is contributing to the remaining flexure. I am now thinking about putting a temporary brace between the Mak imaging and guide scopes to see if this will reduce apparent relative movement between them that may still be the cause of flexure. As all mounts etc are different I guess you just have to look at things like cable drag etc carefullly make changes and measure if there is any improvement. If I had a smaller guidescope I would consider mounting it on the Mak guide rings so the relative movement would then be smaller. Best of luck!
Cheers Peter
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Old 15-03-2009, 11:55 AM
solissydney (Ken)
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Peter.

Are you able to insert an eyepiece with cross hair to check the auto guiding?
That's what I do when I experience guiding problems.
Ken
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Old 15-03-2009, 12:51 PM
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I spent some time checking my alignment last night (firstly with EQalign and then a 10 min drift using K3CCDtools). I also tightened up a few things and ensured there were no cables dragging on the ground.

After all that I gave it another try (with the ED80 rather than the 10" newt so 1/3 the focal length) but did get a much better result with very little drift over a 2x4 minute subs. The next step is to try the 10" again.

Off to Bunnings to get some Velcro today.
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