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Old 29-05-2015, 07:43 AM
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andyc (Andy)
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Possible tube flex issues?

Hi all,
Much as I'm having a lot of fun with my imaging just now, I'm having a problem I've not encountered before. I think it's tube flex - the guiding is OK, if not absolutely amazing, but over the course of a series of 5-minute exposures, the stars on the actual image drift slowly and fairly consistently across the frame in one direction. They were drifting northeast when the scope was pointed high to the east two nights ago. I use a 150mm Newtonian and Orion mini autoguider.

I've only noticed the issue come prominent recently - and I'm guessing one of two causes: I turned the focuser from being perpendicular to the DEC axis (so more convenient for an eyepiece), round to being parallel with the DEC axis, which balanced the axis much better. I still put a little extra weight on one side of the axis rather than perfect balance.

Or is it because after a break, I'm imaging in colder nights and there's more metal contraction/flex over a night?

The result is that quite a few frames have egg-shaped or downright trailed stars, and though at the turn of the year I was rejecting very few frames indeed from trailing, now I am rejecting 1/4 frames at least, and not ideally happy with some others.

Any suggestions, or tips about tube flex, diagnosing the source, or solutions (assuming that's the cause)? I feel a bit out of my depth working this one out!
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Old 03-06-2015, 05:35 PM
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Slawomir (Suavi)
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Hi Andy,

Sorry to hear about your issues with flex. I think everyone in this hobby at some point has to struggle with this adversary. The good news are that there are myriads of possible reasons why you have flex, so your persistence will be tested and you may learn quite a lot about your equipment.

You may have just noticed flex because you might be imaging a different part of the sky so the telescope's orientation is different.

Flex is usually caused by a number of factors, so the trick is to address potential sources and see if adjustements make any difference.

I guess that the focuser could be the main suspect, as well as mounting of the guider scope. The tube possibly flexes a bit as well, possibly mostly near where the focuser is mounted due to its pull under gravity. I think you will always have some residual flex if using a guidescope...

Have you thought of using a thin OAG instead of a guide-scope? I will dare to say that getting an OAG is the quickest, most effective and sometimes also the cheapest way of getting rid of flex...
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Old 03-06-2015, 06:21 PM
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Quote:
Originally Posted by Slawomir View Post
Hi Andy,

Sorry to hear about your issues with flex. I think everyone in this hobby at some point has to struggle with this adversary. The good news are that there are myriads of possible reasons why you have flex, so your persistence will be tested and you may learn quite a lot about your equipment.

You may have just noticed flex because you might be imaging a different part of the sky so the telescope's orientation is different.

Flex is usually caused by a number of factors, so the trick is to address potential sources and see if adjustements make any difference.

I guess that the focuser could be the main suspect, as well as mounting of the guider scope. The tube possibly flexes a bit as well, possibly mostly near where the focuser is mounted due to its pull under gravity. I think you will always have some residual flex if using a guidescope...

Have you thought of using a thin OAG instead of a guide-scope? I will dare to say that getting an OAG is the quickest, most effective and sometimes also the cheapest way of getting rid of flex...


Yes - an OAG is King when guiding.
I used to have flex problems before I installed an OAG.


cheers
Allan
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Old 03-06-2015, 07:34 PM
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andyc (Andy)
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Hi Slawomir and Allan, thanks for the reply. I've thought about the OAG solution, though with the current focuser it'll have to be pretty thin to fit in there! I've been having a look back through my imaging library at various imaging sequences - and it's an intermittent issue that so far doesn't appear to be strongly connected to particular tube orientations. I might even plot the orientations up to see if there's a spatial pattern! So while that doesn't rule out anything yet, I'm going to closely explore the securing of the camera at the focuser, and the securing of the primary mirror, because some nights it's been not much of an issue at all. Have to eliminate the simple solutions first, and yes I'll learn a lot more in the process...

If it continues a big problem, then maybe OAG is the way to go!
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Old 03-06-2015, 08:00 PM
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Quote:
Originally Posted by andyc View Post
Hi Slawomir and Allan, thanks for the reply. I've thought about the OAG solution, though with the current focuser it'll have to be pretty thin to fit in there! I've been having a look back through my imaging library at various imaging sequences - and it's an intermittent issue that so far doesn't appear to be strongly connected to particular tube orientations. I might even plot the orientations up to see if there's a spatial pattern! So while that doesn't rule out anything yet, I'm going to closely explore the securing of the camera at the focuser, and the securing of the primary mirror, because some nights it's been not much of an issue at all. Have to eliminate the simple solutions first, and yes I'll learn a lot more in the process...

If it continues a big problem, then maybe OAG is the way to go!


The point with an OAG is that if you watch the guide graph
& it's nice & steady you know for sure that the result will have nice pin point stars.
You'll never have to throw away another sub frame.
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Old 04-06-2015, 05:18 AM
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Slawomir (Suavi)
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Quote:
Originally Posted by alpal View Post
The point with an OAG is that if you watch the guide graph
& it's nice & steady you know for sure that the result will have nice pin point stars.
You'll never have to throw away another sub frame.
Unless your polar alignment is inaccurate and then you get field rotation with long subs, in particular when imaging near celestial pole, or there still may be flex between the guide camera attached to the OAG and the main camera... Both happened to me...
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Old 04-06-2015, 05:57 AM
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Quote:
Originally Posted by Slawomir View Post
Unless your polar alignment is inaccurate and then you get field rotation with long subs, in particular when imaging near celestial pole, or there still may be flex between the guide camera attached to the OAG and the main camera... Both happened to me...

That's true -
I replaced the standard screws with Allen head bolts
that I could tighten up properly so my OAG would not move -
see here:

https://www.flickr.com/photos/247194...in/photostream


cheers
Allan
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Old 04-06-2015, 07:55 AM
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It sounds to me like you have differential flexure. An easy way to test for it it to take guide exposures at 10 seconds, 1 minute, 5 minutes and 10 minutes. If the stars get gradually longer in one direction over the ensuing exposures, then you have DF. An OAG will be the easiest solution.
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