#1  
Old 29-05-2015, 07:43 AM
andyc's Avatar
andyc (Andy)
Registered User

andyc is offline
 
Join Date: Dec 2011
Location: Sydney
Posts: 1,003
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!
Reply With Quote
  #2  
Old 03-06-2015, 05:35 PM
Slawomir's Avatar
Slawomir (Suavi)
Registered User

Slawomir is offline
 
Join Date: Sep 2014
Location: North Queensland
Posts: 3,240
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...
Reply With Quote
  #3  
Old 03-06-2015, 06:21 PM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 3,602
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
Reply With Quote
  #4  
Old 03-06-2015, 07:34 PM
andyc's Avatar
andyc (Andy)
Registered User

andyc is offline
 
Join Date: Dec 2011
Location: Sydney
Posts: 1,003
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!
Reply With Quote
  #5  
Old 03-06-2015, 08:00 PM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 3,602
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.
Reply With Quote
  #6  
Old 04-06-2015, 05:18 AM
Slawomir's Avatar
Slawomir (Suavi)
Registered User

Slawomir is offline
 
Join Date: Sep 2014
Location: North Queensland
Posts: 3,240
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...
Reply With Quote
  #7  
Old 04-06-2015, 05:57 AM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 3,602
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
Reply With Quote
  #8  
Old 04-06-2015, 07:55 AM
Paul Haese's Avatar
Paul Haese
Registered User

Paul Haese is offline
 
Join Date: Jan 2009
Location: Adelaide
Posts: 9,937
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.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 06:22 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
Testar
Advertisement