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Turbulence in C6 tube?

Celestron C6 Acquisition Solar System Moon Help Needed
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Jim Smith avatar
Jim Smith avatar
First Quarter Moon C6

Towards the bottom of my image, the lunar details look less sharp than elsewhere. I thought it might be that I had not allowed the C6 tube to reach ambient temperature.

I was wondering if more experienced members might think the same. Perhaps there is a better explanation...
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John Hayes avatar
John Hayes avatar
The moon only subtends a half degree so the light at each point in the image passes through almost the same volume inside the tube.  No matter where it occurs in the tube, turbulence will affect sharpness nearly equally over the whole image.  What I see in your image looks more like it might be caused by a sensor that is tilted with respect to the image plane.  How is the camera mounted and what optics are in the light path?

John
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Jim Smith avatar
Jim Smith avatar
John Hayes:
How is the camera mounted and what optics are in the light path?

The Nikon D5500 camera attaches to a Nikon "T" thread adapter. That screws on to a Celestron SCT visual back with "T" thread. It seems to all be firm and secure.
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John Hayes avatar
John Hayes avatar
Ok, thanks for that description.   I'll assume that you aren't doing eyepiece projection and that you are imaging at the prime (Cassegrain) focus of the C6.  Your image shows a lot of blur across the image so something is out of whack.  Have you ever put a high powered eyepiece on the scope to look at the optical alignment?  At the most basic level, a defocused star in the center of the field should show a nice symmetric donut.  That's probably the next thing to check.

John
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Jim Smith avatar
Jim Smith avatar
John Hayes:
Ok, thanks for that description.   I'll assume that you aren't doing eyepiece projection and that you are imaging at the prime (Cassegrain) focus of the C6.  Your image shows a lot of blur across the image so something is out of whack.  Have you ever put a high powered eyepiece on the scope to look at the optical alignment?  At the most basic level, a defocused star in the center of the field should show a nice symmetric donut.  That's probably the next thing to check.

John

Thank you, John,

Yes, I'm using prime focus. I did a collimation before I took that moon image and I did need to adjust the screws to get the symmetrical doughnut. There may be clear skies tomorrow evening so I will check collimation again and take some shots of the full moon. I might also try rotating the camera through 180˚  and take some more shots to see if that gives any clues.
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John Hayes avatar
John Hayes avatar
Yes!  Rotating the camera is the next thing to try.  Good luck.  Hopefully you can chase down the cause of the problem.

John
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Jim Smith avatar
Jim Smith avatar
I got the chance to take a few quick snaps last night but the results were inconclusive. I rotated the camera between taking the two shots sampled below. I did not change focus between shots. So, one crop is from the top of the field of view, the other is from the bottom and rotated. It's hard to tell but I can't really see a distinct difference in focus.




I think I need a higher moon on a night with steadier air to do meaningful tests. One day perhaps...
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John Hayes avatar
John Hayes avatar
I think that the best procedure is to focus, take an exposure and then rotate the camera 90 degrees.  Then refocus and take the second exposure.  You want to see if the image remains in focus across the field and to see if the defocused part of the image appears to remain stationary or to rotate.  Achieving precise focus is critical.  How are you focusing?

John
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Jim Smith avatar
Jim Smith avatar
I focus with the standard C6 focussing knob.
John Hayes avatar
John Hayes avatar
What I'm asking is how you determine when you are at the best focus?  Simply looking through a viewfinder is not a very precise way to focus.   I won't list them all but there are a lot of ways to achieve precise focus.  Using a Bahtinov mask would greatly reduce uncertainty in achieving precise focus.  A split prism might be okay as well.  So, how do you know that your focus is precisely set?

John
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Jim Smith avatar
Jim Smith avatar
I zoom in on live view on my camera and focus until it is sharp. I usually find that the tiniest rotation of the focusser in either direction will reduce the sharpness. I think the main limiting factor is my ability to control the focus using the knob. I am going to fix something on the focussing knob to increase its diameter.

But surely, if I am slightly off focus in one part of the field of view then I should be similarly out of focus elsewhere? In my original image I was happy with the focus at the top. It doesn't seem to me that my rather basic focussing method would be the cause of my original problem.
John Hayes avatar
John Hayes avatar
I know that seems like a reasonable way to focus, but focusing using live view is actually not very precise.  Using a B-mask on a nearby star is far more precise so I suggest considering trying that approach to see how you like it.  Having a very repeatable way to focus helps to reduce uncertainty as you evaluate progress toward a solution.

I completely agree that the whole field should be in focus at the same time and that's the core problem that you first posted.  The only two ways that I can think of that will cause that problem are:  1) The sensor is mechanical tilted with respect to the image plane and 2) The scope is optically aligned to produce an off-axis,  "tilted" image plane.  Putting aside some "exotic" optical problem, those are pretty much the only two options.  So, I think that you need to carefully investigate the optical alignment and make sure that nothing in your camera mounting hardware could be cockeyed.

Good luck with it!

John
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