#21  
Old 26-08-2013, 10:00 PM
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Quote:
Originally Posted by gregbradley View Post
Good guiding and elongated stars seems to be contradictory statements.
The only time I have seen "good guiding" (of course its not good guiding if the result is elongated stars) was when I had differential flexure and I was using a guide scope. So the guide scope was seeing the guide star just jim dandy but the imaging chip was showing drift due to something not being rigid with the guide scope.

If in fact your guiding is good then that only leaves optical movement like the secondary is moving slightly or the primary is distorting slightly.

When you grab these parts are they all rigid? Mirrors, tube, focuser?
See if any move when you push/pull them.

Greg.
Thanks Greg, It all feels tight, but im out now to measure focuser flexure.

Quote:
Originally Posted by Peter.M View Post
If the primary or secondary move slightly then the guider would register this movement also. It would also effect collimation I guess. If it is only in corners of images then you can rule out anything to do with the guider because elongation would be in the center of the frame too. My money is on the secondary or focuser flexing.
Cheers Peter, my money would be on them too, the elongation is all over the image as well.
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  #22  
Old 26-08-2013, 10:14 PM
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If anyone's interested in looking at the full frame fits image here they are.
low south: https://www.dropbox.com/s/lck7g8pelb...h%202%3B41.fit

low north: https://www.dropbox.com/s/qqvwegitpf...h%203%3B24.fit

the image needs stretching out to see the elongation better.

thanks
Josh
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  #23  
Old 27-08-2013, 05:54 AM
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I had a quick look at your full frame images and they have me really confused. To me it looks like differential flexture because the star profiles are the same across the entire frame, I am not familiar with the planewave scopes but whenever I have miscolimation the profiles vary across the frame and usually are better in the center. I would definitely try to take some unguided exposures and check the star profiles obviously guiding with an in guide chip any flex related issues should not be a worry. The only other option is the chip tilting in the focuser at low angles and there I would expect there to be out of focus images on one side and fine profiles on the other.
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  #24  
Old 27-08-2013, 06:07 AM
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Are your filters secure in the filter wheel? If they are not you could get some tilt there.
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  #25  
Old 27-08-2013, 08:47 AM
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Good tip Brett. I remember Mike S posting a long time ago about problems with filters shifting.


Greg.
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  #26  
Old 27-08-2013, 09:06 AM
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One of Martin's tips in his AAIC presentation was to do test images with the filter wheel removed and with it present but through an empty slot. Sounds like this diagnostic technique would be worth trying.
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  #27  
Old 27-08-2013, 10:10 AM
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Quote:
Originally Posted by Peter.M View Post
I had a quick look at your full frame images and they have me really confused. To me it looks like differential flexture because the star profiles are the same across the entire frame, I am not familiar with the planewave scopes but whenever I have miscolimation the profiles vary across the frame and usually are better in the center. I would definitely try to take some unguided exposures and check the star profiles obviously guiding with an in guide chip any flex related issues should not be a worry. The only other option is the chip tilting in the focuser at low angles and there I would expect there to be out of focus images on one side and fine profiles on the other.
thanks Peter. Yes, when the clouds clear my first thing to do will get a 5 sec, 10sec, 15 sec, etc unguided exposure and look at them.

thanks Guys about the filters. i only recently (few weeks) tightened them all up, but i will remove on and image through an empty slot.

Josh
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  #28  
Old 28-08-2013, 12:08 PM
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well, i had clear skies last night to get some more tests. So, first of all are the short exposures to test the tracking. these are croped 200% magnification screen shots. the exposure time is indicated in the title by hovering the mouse over the image. the aberation is there, even in 2 sec exposure, my conclusion is the tracking is not an issue. If anyone wants to see the FITS file for the header info, i can provide you with it.
Attached Thumbnails
Click for full-size image (2 sec low south.JPG)
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Click for full-size image (5 sec low north.JPG)
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Click for full-size image (5 sec low south center of frame.JPG)
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Last edited by Joshua Bunn; 28-08-2013 at 12:51 PM.
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  #29  
Old 28-08-2013, 12:13 PM
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then i tested how the collimation held throughout the night and in the areas where i was getting the elongation.

First there are 2 collimation tests at the zenith some time appart and with much slewing in between. then there are 2 test - one over the south and one over the north. It appears collimation held also. However, if looking closely, there seems to be a minor amount of movement between the 2 zenith tests.
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Click for full-size image (collimation at zenith.JPG)
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Click for full-size image (collimation at zenith 2.JPG)
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Click for full-size image (collimation low north.JPG)
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Click for full-size image (collimation over low south.JPG)
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  #30  
Old 28-08-2013, 12:16 PM
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Next was to rotate the camera in a position where i had the elongation. So this test was taken low in the south. These exposures were also 2 sec.

the description is under the image, and it appears the camera has nothing to do with the elongation as the elongation changes as the camera is rotated (the elongation is still in a line from the zenith to the horizon).
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Click for full-size image (original position before camera rotation.JPG)
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Click for full-size image (after camera 90 deg rotation (no slewing).JPG)
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Click for full-size image (rotate camera back to original position.JPG)
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  #31  
Old 28-08-2013, 12:21 PM
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then i purposly loosened the focuser bearings to see what effect that would have on the elongation. It appeared it had no effect but the image just shifted slightly as expected:the first image is rather loose, the second is tight. these images were taken low in the south. Im not sure how conclusive this test is though.
Im going to keep conversing with planewave about this and send them all this info to see if we can sort this issue out.

thanks
Josh
Oh, how do you add images in the middle of a post, if i knew that, i wouldnt have to post 4 messages
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Click for full-size image (low south tight focuser 30 sec.JPG)
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Last edited by Joshua Bunn; 28-08-2013 at 12:57 PM.
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  #32  
Old 28-08-2013, 04:19 PM
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This is quite a puzzling problem. No doubt when you find the culprit it will be relieving.

Did you try taking a shot with no filter or the filter wheel removed?

With an image showing elongation have you tried taking focus shots and holding the secondary slightly and pushing it one way or the other to see if you can correct the elongation, same with the focuser? It would take 2 people. One looking at the focus images and the other applying slight force to the focuser to try to make it correct back to round or to the secondary to see if it will correct back to round stars and you would have a quick idea where it is coming from or could eliminate an item.

Greg.
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  #33  
Old 28-08-2013, 04:29 PM
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Quote:
Originally Posted by gregbradley View Post
This is quite a puzzling problem. No doubt when you find the culprit it will be relieving.
Greg.
Yes it is, I appreciate your comment Greg.

I didnt get around to taking the camera apart (internal filter wheel). Thats next time.

Yeah, moving the secondary and focuser while getting an image is worth a shot. It would have to be a very quick shot because the person holding the scope will no doubt alter the tracking. But if it can be repeatable by moving the focuser or secondary then wee could be onto something. Ill do it next night out.

thanks
Josh
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  #34  
Old 28-08-2013, 09:48 PM
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I have a small web cam that i can put on the back of the scope. I just need to make an extension tube. That should tell me if the focuser is the culprit because the webcam wont make the focuser flex.

Josh
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  #35  
Old 29-08-2013, 09:18 AM
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Can the guide chip be used to take a long exposure while the main imaging chip is used for guiding? I still think that if the problem is a slow phenomenon - ie - slower than the guide rate, then it must be occurring in the camera (ir at least after the point where the light to the guider is not passing through the same optics as the imaging ray. Anything else would be corrected by the guider. Otherwise, only something not being sampled by the guider ( ie faster than 1Hz if you are guiding at 1sec) could explain the difference between the guiding performance and the image.
Cheers,
Andrew.
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  #36  
Old 29-08-2013, 10:16 AM
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Hi Andrew,

Yes, the guide chip and imager chip can be switched around in their roles so to speak. All the images i posted yesterday were unguided, and the orientation of the elongation changed when i rotated the camera. Are you still of the opinion the problems in the camera?

thankyou
Josh
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Old 29-08-2013, 12:36 PM
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Quote:
Originally Posted by Joshua Bunn View Post
then i tested how the collimation held throughout the night and in the areas where i was getting the elongation.

First there are 2 collimation tests at the zenith some time appart and with much slewing in between. then there are 2 test - one over the south and one over the north. It appears collimation held also. However, if looking closely, there seems to be a minor amount of movement between the 2 zenith tests.
looks to me like something is moving in the scope Joshua, which is surprising given how well they appear to be built. the second zenith image definitely shows some collimation offset - Poisson's spot should be central and it looks like it is down and to the left.
If the primary is stable, is it possible that one of the correcting lenses is slightly loose or is there any movement around that assembly? I read that the CDK design is extremely sensitive to positioning/alignment of the corrector lenses. The secondary is spherical - that component should be fairly tolerant of minor shifts.

Last edited by Shiraz; 29-08-2013 at 01:08 PM.
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  #38  
Old 29-08-2013, 01:02 PM
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looks to me like something is moving in the scope Joshua, which is surprising given how well they appear to be built. the second zenith image definitely shows some collimation offset - Poisson's spot should be central and it looks like it is down and to the left.
If the primary is stable, is it possible that one of the correcting lenses is slightly loose or is there any movement around that assembly? I read that the CDK design is extremely sensitive to positioning/alignment of the corrector lenses. The secondary is spherical, so that should be fairly tolerant of minor shifts.
Thanks Ray,

I would agree with you about the slight movement in the secondary shadow in that image. Yes, the lenses are tight. I made sure the retaining ring was tight less than a week ago and the 1st imaging night after checking them, the elongation was there.I guess it wouldnt hurt to remove them and put them back in just to be sure.

thanks
Josh
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  #39  
Old 29-08-2013, 01:47 PM
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Fair point- but that means your guiding performance must be a red herring. How does the size of the star elongation compare with the logged error in guiding? If they're comparable, then yes, any element in the system could be your problem. If the guide log is telling you that the image should be better than it is, then the problem is restricted to the part of the system where there can be a difference between imaging and guide chip.
I'm sure I'm not adding anything you haven't already figured out here....

Quote:
Originally Posted by Joshua Bunn View Post
Hi Andrew,

Yes, the guide chip and imager chip can be switched around in their roles so to speak. All the images i posted yesterday were unguided, and the orientation of the elongation changed when i rotated the camera. Are you still of the opinion the problems in the camera?

thankyou
Josh
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  #40  
Old 29-08-2013, 02:22 PM
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Andrew,

Bases on the elongation in post 30, ill say the elongation is 2 pixels diagonal. so this is 25 micron for 9 micron pixels. with an image scale of 0.73 arcsec / pix, this gives 2 arcsec of elongation. my guiding is typicaly -/+ 0.3 pix and with an image scale of 1.2 arcsec / pix this guides to 0.72 arcsec.
So, the logged error in guiding is much smaller. but having said that, the guider monitors the star centroid not the whole star profile. these elongated images still have a centroid. this would suggest the guiding is still accurate...??

Is What your suggesting that the main imager chip is insecure? If i get a shot through the guider chip and it showed the same star elongation, then that would suggest its not the camera chip - I can do that.

Josh

Quote:
Originally Posted by alocky View Post
Fair point- but that means your guiding performance must be a red herring. How does the size of the star elongation compare with the logged error in guiding? If they're comparable, then yes, any element in the system could be your problem. If the guide log is telling you that the image should be better than it is, then the problem is restricted to the part of the system where there can be a difference between imaging and guide chip.
I'm sure I'm not adding anything you haven't already figured out here....
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