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Old 08-03-2011, 04:12 PM
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John K
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Is this differential flexing?

Hi guys,

I am trying to do some deep sky astrophotgraphy with my 12.5" f/5 reflector and a new auto guider attached to my scope with a small guidescope but I am still getting lots of trailing.

Part of one of the images is attached - 5 minutes on Eta Carina

It's an auto-guided exposure at a focal 1600mm focal length. There are three distinct blobs of the star in the trailing which suggests the autoguider was guiding ok but then lost the star.

Does it look like flexing? or poor polar alignment?

A bit like a detective story I guess.

John K.
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Old 08-03-2011, 04:27 PM
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multiweb (Marc)
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Looks like you got three stars which makes me think the scope jumped rather than trail. What program were you using to autoguide and what were the conditions? Windy? I don't think it's flexure at all.
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Old 08-03-2011, 04:31 PM
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Is that a single frame or a stacked image?
James

Edit: re-read your post - seems its a single 5 min sub.
Do the other subs look the same?
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Old 08-03-2011, 04:34 PM
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Hi guys,

Quote:
Originally Posted by multiweb View Post
Looks like you got three stars which makes me think the scope jumped rather than trail. What program were you using to autoguide and what were the conditions? Windy? I don't think it's flexure at all.
It was very still, not windy. Using a new SBIG SG-4 guider

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Is that a single frame or a stacked image?
James
Single 5 minute frame
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Old 08-03-2011, 04:46 PM
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Quote:
Originally Posted by Moon View Post
Is that a single frame or a stacked image?
James

Edit: re-read your post - seems its a single 5 min sub.
Do the other subs look the same?
Here is one more this time 5 mins on M42. Still trailing but no blobs
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Old 08-03-2011, 05:03 PM
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Merlin66 (Ken)
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You can check out the usual suspects..
From your image what was the direction of the trail? In RA or DEC??
You can use your planetarium program to determine the Alt/Az at the time you took the image and see if the track follows Newton's gravity.
It could be cable snagging?
Set up the guide on an artificial star - if you don't want to waste observing time and apply hand pressure to the guide scope, the focuser, the extension tubes and the camera and watch for any movement in the guide star image..
(GSE is good for this sort of Stuff!)
http://www.iceinspace.com.au/forum/s...ad.php?t=56349
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Old 08-03-2011, 05:04 PM
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spearo (Frank)
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John,
Its always a bit difficult especially in the beginning.

I think, as you said, its a bit like detective work.

In this case the process is one of elimination.

To get close to identifying the cause you must be able to rule out certain things.

As a start I would spend quite a bit of time on balancing the equipment and then spend a lot of time doing drift alignment so that you get to a point where a star will move back and forth without drift for at least 5 minutes.

Once balancing and drift alignment are very good, then autoguide to se what hapens

its thgen a process of checking and tweaking autoguiding settings etc. towards an optimal image with decreasing traillling.

these are the first investigative steps I'd recommend.

As it is the images you post show trailling that could be due to a) poor balancing (ensure balanced with a very slight imbalance on the rising end- eg counterweight shaft weighs a bit more if imaging East and the counterweight bar is progressively lifting up over time-this ensures consistent pressure on the worm))
b) poor polar alignment (hence the need for drift alignment) or
c) tracking settings requiring adjustments (no point doing this until the other two are resolved)
hope this helps
frank
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Old 08-03-2011, 06:58 PM
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John,
For a 12.5" f/5 and 5 mins subs, I think you are on the right track suspecting flex of some sort.
Next time you set up your rig, check everything is locked down tight and take a picture of the setup. If you post it here you can get a few suggestions how to reduce the flex (it can never be eliminated 100% with a separate guide scope)
In the mean time, double check your polar alignment and work out how long your subs can be before you get trailing stars.
If you want to know if it's flex or rotation, stack all your images without aligning them. Have a look at the attachment - it's 2 hours worth of field rotation on a self guided STL camera on a poorly polar aligned mount. Notice how the stars rotate around the top of the image? That's where the guide chip is located. Flex would be more like straight lines.
James
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Old 09-03-2011, 01:12 PM
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If these are 5 minute subs there is not enough time for flexure to be an issue. Either there is something wrong with the guide setup, something wrong with your drive(s) or something loose on your scope. With my crappy Meade LX200GPS mount I often get trailing on the first image in a series. Sometime the scope just moves off (probably a small amount of lag in the worm). However, 2nd and subsequent images in that series are fine.

Cheers
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