Old 18-11-2018, 09:46 PM
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John K
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Collimation fine tuning advice

Hi guys,

I am "tinkering" with my 9.5" CDK type scope to try and get a better understanding on the long term collimation process for this type of scope. Having only owned a Newtonian, what a luxury!

I am only tweaking the secondary at this stage and have tried a version of CCD inspector but was unable to really get measurements from it to make worth while adjustments.

At this stage, I am using Firecapture and and putting an out of focus (inside focus) star to try and get the best looking image in the centre of the camera I have - 1600mm.

My idea as recommended by Stefan who completely rebuilt the scope and the way he likes to do it is to try and find the perfect centre of the imaging train and moving the star around the field until most of the image is totally concentric. This however I am finding hard to achieve and replicate! Another way I to shoot an open cluster like M41 with all of the images out of focus to see where the perfect centre is and then move the secondary accordingly.

I have read the DSI collimation process and the image I am attached I think shows a generally balanced image what slightly out of collimation is some of the corners.

Can someone make some suggestions on the best way to balance the attached image based on the standard 3 screw secondary (centre screw I am ignoring as it will alter the distance to the corrector). One question I have should 3 screws always be adjusted? e.g. if one is turned counter clockwise should the other two be turned the other way? Or can I balance the image by simply tweaking only 2 screws to adjust tilt in one direction?

The airy disk also does seem a bit unusual.

I am not using a Taka Collimator - checking the secondary mark that the scope with a Newtonian type laser is showing that the laser does not point directly at the centre of the secondary - does that make a difference in any way as many manufacturers say it does not??

Thanks in advance for any advice.

Clear skies.

John K.
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Last edited by John K; 19-11-2018 at 08:33 AM.
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Old 18-11-2018, 10:05 PM
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multiweb (Marc)
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Your fov looks well balanced. As far as diffraction rings go I reckon you are too far either side of the focus to make a good reading. You need to get closer to focus to see only a couple of rings. Then you'll be able to see which way to go. I usually go out of focus as the image is less impacted by turbulence then push the star towards the opposite direction where the rings are compressed then recenter and so on. I use all screws but I try to balance as not to over tighten any.
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Old 18-11-2018, 10:41 PM
markas (Mark)
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The process I use for my CDK is first to get the secondary reference marks (two concentric circles on my scope) exactly concentric with the centre of a Tak or cat's eye - easier with the Tak because it has magnification - using only the secondary adjusting screws. I also find that a Howie Glatter laser plus Tublug works well

When that has been achieved, the secondary is collimated.

Then using a defocused star field, I adjust the primary screws only to get the central stars showing exactly concentric donuts, with the extreme corner stars as concentric as possible, with any deviation from concentricity symmetrical about the centre of the fov. This is very critical.

When all that comes together the in-focus image should be optimised across the whole fov.

It is critically important for these designs that the mirror to mirror distance be right on specification. The sweet spot is very tight for the best performance to be attained.

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Old 24-12-2018, 08:56 AM
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John K
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Thanks for the advice everyone.

After 5 weeks of rain here in Melbourne, had another go last night using M41 as the field.

The secondary appears well centered - checked it with a newtonian laser and the laser hits the secondary right in the middle of the donut marker.



What are people's thoughts in terms of the defocused star field attached?

If anything, I am thinking the centre of the field is slightly more above centre/right of the taken image - so would have to move the image slightly down and left, but other then that seems quite symmetrical to me. Any adjustments I am making I am doing using only the secondary at this stage and have not touched the primary at all.

Look forward to advice.

John K.
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Old 24-12-2018, 09:15 AM
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Merlin66 (Ken)
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Al's Collimating Aid is an ideal tool to verify the doughnut is central....
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