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Old 28-09-2012, 09:54 AM
Carl
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Is this flexure?

Attached is a crop of 7 images stacked (non aligned) that shows a consistant drift. The elongations are consistent across the entire field. they all go the same way, there is no rotation, curvature or distortion to suggest non alignment of optical train. It is interesting to note that the elongation is worse when i am zenith and imaging west.

Polar alignment is as good as i can get, strar remains perfectly centred in crosshair east, west and zenith for over 12minutes. My PHD guiding graph shows that i am accuratley guiding.

I'm guiding with a separate small modified findersope/guidescope (not an OAG) and imaging thru an ed120 refractor.

The elongations are visible in 1 minute and 4 minute exposures.

I cant upload the original RAW files as they are too big for IceInSpace so if you can assist please go to: http://filez.bordermail.com.au/sj8y78t
and download the zip file.

Any assistance will be greatfully appreciated

Regards
Carl Rainer
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  #2  
Old 28-09-2012, 10:19 AM
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Paul Haese
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Carl the way to test for flexure is to take an image at 30 seconds then one at 5 minutes and one at 10 minutes. If you have flexure the stars will progressively get longer and longer. To eliminate drift, you can go over the entire setup looking for it (which can be successful) or you can go the way of an OAG.
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Old 28-09-2012, 10:33 AM
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Hi,
Will be interesting to hear what others have to say,
is the elongation along E-W or in the same direction in the main scope as the stars would drift if you had turned tracking off?
To me, it looks more like tracking errors even though you have stars centred possibly due to the longer focal length of your main scope compared to the finder guider?
I know phd is capable of sub pixel guiding so even if the movement of the star is a lot less through your guidescope/guidecam, can it be significant enough in your main scope?
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Old 28-09-2012, 11:31 AM
Carl
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Thanks for the reply
The elongations are constant east to west. I have rotated my focuser 90 degrees to confirm this plus turned off guiding. the drift seems to constant.

carl



Quote:
Originally Posted by alistairsam View Post
Hi,
Will be interesting to hear what others have to say,
is the elongation along E-W or in the same direction in the main scope as the stars would drift if you had turned tracking off?
To me, it looks more like tracking errors even though you have stars centred possibly due to the longer focal length of your main scope compared to the finder guider?
I know phd is capable of sub pixel guiding so even if the movement of the star is a lot less through your guidescope/guidecam, can it be significant enough in your main scope?
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Old 28-09-2012, 11:33 AM
Carl
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Elongated stars

Thanks Paul
When doing the exposure tests should i do it with the guider off or on?

Regards
Carl


Quote:
Originally Posted by Paul Haese View Post
Carl the way to test for flexure is to take an image at 30 seconds then one at 5 minutes and one at 10 minutes. If you have flexure the stars will progressively get longer and longer. To eliminate drift, you can go over the entire setup looking for it (which can be successful) or you can go the way of an OAG.
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Old 28-09-2012, 01:59 PM
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Quote:
Originally Posted by Carl View Post
Thanks Paul
When doing the exposure tests should i do it with the guider off or on?

Regards
Carl
Guider on.
The purpose is to see if there is any movement between your 2 scopes ie "flexure"
The other way to test is to guide with one scope and then take a time series of relatively short exposures through your imageinig scope. The exposures need to be short enought that poor guiding isn't a factor. 10 secs is probably enough.
Then display the time series one image after another. If there is no flexure then the stars should stay in the same spot on subsequent frames. If after 10 mins or so they have moved considerably then you have flexure.
Try this aiming both east and west. You will find that there is a difference if there is flexure as the flexure will vary depending on the orientation of the scope.
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Old 28-09-2012, 03:38 PM
Carl
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Elongated stars

Hi Terry
With the short exposure test should the images be shot quickly, ie 30 second delay between each pic or should i allow longer?

Regards
Carl


Quote:
Originally Posted by Terry B View Post
Guider on.
The purpose is to see if there is any movement between your 2 scopes ie "flexure"
The other way to test is to guide with one scope and then take a time series of relatively short exposures through your imageinig scope. The exposures need to be short enought that poor guiding isn't a factor. 10 secs is probably enough.
Then display the time series one image after another. If there is no flexure then the stars should stay in the same spot on subsequent frames. If after 10 mins or so they have moved considerably then you have flexure.
Try this aiming both east and west. You will find that there is a difference if there is flexure as the flexure will vary depending on the orientation of the scope.
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Old 28-09-2012, 04:08 PM
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Doesn't matter how long to leave between the shots. What's important is the actual image exposure length.
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