Hi Z4 users,
With all these Askar/Sharpstar flat-field scopes, the manufacturer says that back-focus doesn't matter but I am getting some distorted stars in the corners in a radial pattern that looks a bit like when you don't have precise back-focus on a telescope where it does matter. It isn't very strong - the stars just look a bit smudged.
At the moment I am using a 55mm back focus from the 48mm flange on the telescope to the sensor of my camera. Has anyone experimented with this or is using a back-focus distance that works well for them?
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Other than finding the focus point, I haven't noticed this at all. (on my current Z4 or previous PHQ's)
it is possible that one of the internal elements?
I just checked some of my subs and the corners look round. (using APS-C sensor)
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Thanks. I just looked at previous shoot with the Z4 and I don't see the same issue so must have been something specific to my set up that night (I had an Askar flatfield before with no issue). The only thing I think was different was the filter in the filter holder. I am not too sure how that would produce a radial distortion (rather than just halos or bloat). Maybe there is some slight tilt on the camera that needs correcting.
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It looks like the issue I am having is with tilt - analysing it with FWHMEccentricity script shows that two opposite corners have the greatest eccentricity. And to confirm the back-focus - Sharpstar shows the focal plain is 77mm back when the focuser is fully retracted. So a 55mm optical length should be fine (i.e., with the focuser extended by 22mm).
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@Andrew RambautIf you figure it out, let me know.
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My camera (ASI2600) has a tilt adjustment on it but I don’t fancy the trial and error it will take on a dark night. I am going to check that all the threads were tight.
Andrew
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I have a tilt plate on the ASI2600 also.
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I assume you already assessed tilt via astap? Can you share a screenshot of that?
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Astap would give me different values for each filter.
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