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Back-focus with Z4

Sharpstar Z4 Flattener Coma
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Andrew Rambaut avatar
Andrew Rambaut avatar
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?
Engaging
Nick Grundy avatar
Nick Grundy avatar
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)
Andrew Rambaut avatar
Andrew Rambaut avatar
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.
Andrew Rambaut avatar
Andrew Rambaut avatar
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).
Leah Deckard Hallett avatar
Leah Deckard Hallett avatar
@Andrew Rambaut 

If you figure it out, let me know.
Andrew Rambaut avatar
Andrew Rambaut avatar
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
Leah Deckard Hallett avatar
Leah Deckard Hallett avatar
I have a tilt plate on the ASI2600 also.
Nick Grundy avatar
Nick Grundy avatar
I assume you already assessed tilt via astap? Can you share a screenshot of that?
Well Written Respectful
Leah Deckard Hallett avatar
Leah Deckard Hallett avatar
Astap would give me different values for each filter.