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Ceph and Cass

Seeing or Collimation?

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Image has been sharpened for the sake of emphasis.

I was photographing Saturn a few nights ago using the ASI120mc with a Meade 2x Shorty Barlow and an LX90.

Using ROI, I was shooting at 49 FPS, with a Gamma of 50, Gain at 65 and Debayered the image in PIPP. My issue is I am getting conflicting opinions on what is causing this. My collimation is off, but we are talking "airy disc" stage of off - and once the clouds clear, I will be back at making fine adjustments. Allso never had this kind of fuzzyness when my collimation was way further off whilst imaging Jupiter with my old camera.

But, I am open to the idea that it could simply be collimation.

Now, Saturn is low and made most of it's journey over a street lamp (the awful yellow type), it's low in the sky, and the sky never really gets that dark. So is this the effect of seeing?

Or is it something else? Any thoughts, ideas, comments, etc - very welcome!

Also for reference, and perhaps your own amusement, I have place a compressed (in 7 zip) copy of the original, raw footage.


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It should be possible for you to disentangle seeing and collimation issues with a star test. Use a star (obviously) and a high power.

- Slight de-focus (where you see the first few rings) diagnoses collimation. If everything is concentric you're set. Collimating with a tool is easier than collimating with a star.

- The stability of the rings will convey seeing quality.

- If you keep defocusing until you see a large, uniform, circle with the secondary shadow then you can diagnose telescope cooling issues. Try this shortly after setting up and watch at intervals as the scope cools. The heavily defocused view should look pretty static when it's cooled down.

Your video shows a lot of shaking motion. More than I'd expect. Was it windy? There are also indications of bad seeing (currents flowing across the image). Underneath it all, the planetary contrast looks OK. So I think the motion of the image is the main problem. Was the scope cooled down, BTW?

Were the images, above, taken simultaneously? The blue channel distinctly shows more blur (which looks like it originated in motion) than the red. If the images are simultaneous then you have a non-collimation-related issue because collimation won't do that. Perhaps atmospheric refraction with Saturn low in the sky would (more refraction at shorter wavelengths) do what you show above. Regardless, based on that image, I would stack and show only the red. Too much motion in red and blue.

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All three channels where shot at the same time, I simply split them in Photoshop to attempt to get a grasp on what was happening to my image. I have a feeling that it's a combination of wind and a short USB cable that has introduced "wobble" to the footage. I shall be re-attempting shooting Saturn this Friday if weather predictions hold true. Thank you for such an in depth response - I will be sure to also check cooling, however apart from a thermal spike at the start of set up, the de-focused disc was pretty stable when looking at Polaris.

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When you're looking at the heavily de-focused disk, watch for slow snaking shadows. Those are convection cells caused by the boundary layer of warm air on the mirror. At least that's what I see in a Newt. I've never tried this with an STC and it may well look different.

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