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Old 09-03-2014, 09:07 AM
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pvelez (Pete)
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Diagnosis

I took some test images from my new dark sky site last night.

Guiding was a bit off - I have some balance to get right as my new rotator has added to the weight of the rig - I will need a new counterweight.

Collimation had looked quite good when adjusting it last week. But on 5 minutes subs (unbinned), I had eggy stars, particularly in the bottom left corner.

So here is an opportunity for armchair critics. Look at the attached material. Do I have a collimation issue, tilt in my imaging train or a combination of the 2?

The aberration image is taken using PI's Aberration Inspector script and shows images at full resolution of the extremities of the image and the centre. The other screenshot is from CCD Inspector.

Any thoughts?

Pete
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Old 09-03-2014, 10:55 AM
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Looks like focuser tilt to me.
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Old 09-03-2014, 12:02 PM
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pvelez (Pete)
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Quote:
Originally Posted by ghsmith45 View Post
Looks like focuser tilt to me.
That was my suspicion too. I hope its not collimation as that is a PITA to get right.

Pete
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Old 09-03-2014, 02:46 PM
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Pete,

I have the same scope as you and same focuser. I highly doubt its focuser tilt if you have the bottom bearing done up quite firm. Also, I doubt its camera tilt to. But im not saying you dont have any either.

In my experience, this is mis-collimation. It took me a while to pin down collimation on this scope, but i have finally done it using a full frame chip (with some great help from Stevous67).

Also, do you see any variation in the orientation of the stars in the corners when you flip the meridian or point to random points in the sky? I'm thinking tube flexure here.

I found that a really reliable way to get very close to collimation is to defocus a star in the centre of the chip and get the rings concentric. do this for 4 position angles of the camera at 90 deg intervals. as my chip is not square, this was important, there was variation as i rotated. but for you, using a square chip, this may not be such an issue.

Josh
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Old 09-03-2014, 03:18 PM
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pvelez (Pete)
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Quote:
Originally Posted by Joshua Bunn View Post
Pete,

I have the same scope as you and same focuser. I highly doubt its focuser tilt if you have the bottom bearing done up quite firm. Also, I doubt its camera tilt to. But im not saying you dont have any either.

In my experience, this is mis-collimation. It took me a while to pin down collimation on this scope, but i have finally done it using a full frame chip (with some great help from Stevous67).

Also, do you see any variation in the orientation of the stars in the corners when you flip the meridian or point to random points in the sky? I'm thinking tube flexure here.

I found that a really reliable way to get very close to collimation is to defocus a star in the centre of the chip and get the rings concentric. do this for 4 position angles of the camera at 90 deg intervals. as my chip is not square, this was important, there was variation as i rotated. but for you, using a square chip, this may not be such an issue.

Josh
Thanks Josh

Can you tell me how your collimation screws are set. I have a 4 screw arrangement with no locking screws. I believe that this is because the scope is an early model as the later versions have 3 screws with an additional 3 locking screws.

Pete
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Old 09-03-2014, 03:23 PM
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Pete,

I have 4 screw arrangement. I removed the thumb screws and replaced them with alen head bolts. I find it much easier to do fine collimation adjustments with them. I have no locking screws either. I wasn't aware there was a 3 screw arrangement.
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