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Old 20-01-2013, 01:54 PM
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Joshua Bunn (Joshua)
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assistance required please, collimation, tilt?

Hi all,

Took some exposures with my setup last night, after going through the motions of the initial steps of PA, fitting PEC curve, collimation, then a large tpoint model.

The equipment for the purpose of diagnosis is PME, Planewave cdk 12.5, STL11000.

The exposures i have attached are single 5 min unguided images of m83 and ngc 4706 respectively, both unbinned. The quality has degraded a little due to compression.

Not focusing on tracking, if you look at the top left of the images, there is not soo much elongation but as you go to the bottom right, the stars become elongated. Could this be collimation error or image plain tilt?

Looking at the tilt in X and Y from ccdI, is this enough tilt to cause the issue you see in the exposures at bottom right? I would have thought the tilt measurements would have been much larger looking at the bottom right of the images.

I ran these and other exposures through ccdinspector and i have attached the results. My questions with this are, why would the binned 3x3 exposures exhibit far less field curviture than the unbined images?

Could it be a collimation issue that caused the elongated stars?

What is an exeptable collimation figure to aim for?

Its wierd, cause i get ideal collimation and curviture results when tunning the secondary screws using 30 sec exposures, then when i come to take an image, this happens.

any help appreciated.

thanks, Josh
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  #2  
Old 20-01-2013, 04:15 PM
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Looks like collimation error ( 90% ) to me with a little camera tilt (10%)
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Old 20-01-2013, 06:21 PM
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Joshua Bunn (Joshua)
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Thanks Peter, looks like i have some work to do. I will also try rotating the focuser and camera independently of each other and see where the tilt goes. The tilt should follow or not follow the camera depending on where the source of it is - which on is it?

I Also realize now that these images are not ideal candidates for ccdI.

Josh

Last edited by Joshua Bunn; 20-01-2013 at 09:44 PM.
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Old 26-01-2013, 10:03 PM
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PRejto (Peter)
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I'm no expert at any of this but I will take a stab at answering why you might see less field curvature when binning vs not binning. I think CCDIS measure FWHM over the image and derives a curve based on the changes in FWHM. When binning pixels are summed so I would think that sharpness would diminish in general, and perhaps differences in FWHM would also reduce giving a flatter field. In other words, measuring differences in a blurry image would be more difficult than one sharply in focus, and in the blurry image differences would appear diminished.
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Old 27-01-2013, 06:22 AM
stevous67 (Steve M)
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Hi Josh,

I have the same configuration, and experienced the same issue. Short exposures resulting in better results, with +60 sec exposures showing significantly worse results in CCDINS.

Peter's suggestion may be right, and we may not have the same fault, but I can tell you what you should check first before losing months trying to fix CCD tilt that may not be the problem.

The CDK has beautifully and accurately made parts, and I've measured everything of my CDK in the quest to remove my problem. In my case the whole scope was not assembled correctly, with a bad optical alignment causing the issue. What to first check...

You should check using a collimated laser, that its beam centers on the secondaries centre marker. You should rotate the laser in it's seat to ensure the result. If your laser is out, see that the drawn circle revolves around the centre marker.

If the alignment is precise, tilt may be the issue, but I'll guess this alignment will be out. Considering the design, it may seem that few adjusts are available, and that's true, but you seriously need that dot centered.

From your images, I'll guess your about 5mm to 10mm out. First, you could try adjusting your spider vane lengths. You should only make small adjustments, as large adjustments will make your vanes tilt, and star defraction spikes will deform.

Not to lose time on this not being the issue, I'll stop here. You can PM me and I'll correspond full details here to reshape the CDK.

I have now a very flat CCDINS result, and collimation below 1.

Good luck,

Steve
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Old 27-01-2013, 06:17 PM
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Joshua Bunn (Joshua)
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Thankyou Peter, Appreciate your feedback.

Quote:
Originally Posted by PRejto View Post
I'm no expert at any of this but I will take a stab at answering why you might see less field curvature when binning vs not binning.
Hi Steve, thankyou for the info. I will send you a PM.

Quote:
Originally Posted by stevous67 View Post
Hi Josh,

I have the same configuration, and experienced the same issue. Short exposures resulting in better results, with +60 sec exposures showing significantly worse results in CCDINS.

Peter's suggestion may be right, and we may not have the same fault, but I can tell you what you should check first before losing months trying to fix CCD tilt that may not be the problem.

The CDK has beautifully and accurately made parts, and I've measured everything of my CDK in the quest to remove my problem. In my case the whole scope was not assembled correctly, with a bad optical alignment causing the issue. What to first check...

You should check using a collimated laser, that its beam centers on the secondaries centre marker. You should rotate the laser in it's seat to ensure the result. If your laser is out, see that the drawn circle revolves around the centre marker.

If the alignment is precise, tilt may be the issue, but I'll guess this alignment will be out. Considering the design, it may seem that few adjusts are available, and that's true, but you seriously need that dot centered.

From your images, I'll guess your about 5mm to 10mm out. First, you could try adjusting your spider vane lengths. You should only make small adjustments, as large adjustments will make your vanes tilt, and star defraction spikes will deform.

Not to lose time on this not being the issue, I'll stop here. You can PM me and I'll correspond full details here to reshape the CDK.

I have now a very flat CCDINS result, and collimation below 1.

Good luck,

Steve
Did you contact Planewave about your scope? did you buy the scope second hand?

I measured the distance from the centre bolt on the secondary, to the inside of the outer rim and all 4 measurements are the same, so hopefully this shouldnt be a problem. However, i wonder if that means it will be centred to the primary aswell?

I recollimated, using CCDINS and the results show it alternating between 0.0" and about 1.5" with no or 0.1" tilt in either of the axis. How accurite do you find CCDINS?

In the end, what was the exposure time tou used to get your great collimation results?

Ive had the primary out to clean it and put it back carefully, so i know thats good. Yes, there isnt much that can be adjusted, which is good IMO, so only the secondary really can be moved.

Thankyou
Josh
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Old 27-01-2013, 11:12 PM
stevous67 (Steve M)
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Hi Josh,

Did you contact Planewave about your scope?

No helpful assistance.

did you buy the scope second hand?
NO, New.

I measured the distance from the centre bolt on the secondary, to the inside of the outer rim and all 4 measurements are the same, so hopefully this shouldnt be a problem.

Well, if the front ring is not sitting perpendicular to the dovetail in both axis, then this would be a wrong assumption. In my case, this was part of the problem. The plastic slides between the front ring and the dovetail were installed badly, causing the front ring to tilt to one side.


However, i wonder if that means it will be centred to the primary aswell?

I would not be guessing this, and use a collimated laser to prove as mentioned in my PM.

I recollimated, using CCDINS and the results show it alternating between 0.0" and about 1.5" with no or 0.1" tilt in either of the axis. How accurite do you find CCDINS?

CCDINS is very good for telling your scopes collimation situation. But you need to do +60sec exposures for a real test.

In the end, what was the exposure time tou used to get your great collimation results?

I go up to 180sec unguided to be sure, but at least 60sec.

Ive had the primary out to clean it and put it back carefully, so i know thats good. Yes, there isnt much that can be adjusted, which is good IMO, so only the secondary really can be moved.

No, the whole scope can be losened, aligned, and then tightened again. You may need to do this if you have a bad optical mis-alignment. Do the laser test to do the initial assessment.

Regards,

Steve
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Old 27-01-2013, 11:43 PM
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Joshua Bunn (Joshua)
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Thanks Steve,

Yes, a laser collimator will be on order if i dont get the CCDINS results im looking for.

No, the whole scope can be losened, aligned, and then tightened again. You may need to do this if you have a bad optical mis-alignment. Do the laser test to do the initial assessment.

...
Yes, of course. I totaly forgot about that. There are blue slides for the primary, just like the secondary, I didnt move them though. A laser collimator will tell me whats going on here. If anything hopefully.

Will report when i know the results.

Clear skies
Josh
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