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Flattener causing massive astigmatism at advertised backfocus! Help please!

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So I got my first light with a second hand 130mm f6.6 triplet (SET optics, equivalent to the discontinued TS Photoline model) and a Photoline 0.79 reducer/flattener.

The previous owner had no issues with this flattener but for me it's producing suuuuper long astigmatism (the type pointing inward to field center) and I already have a bit more than the 55mm backfocus it requests on the product page and adding a few mm more didn't solve it, maybe it made it better but I was in a bit of a rush to best the risk of clouds and didn't save the pictures, so need to wait til next clear to show comparisons.

Meanwhile without a flattener the scope... Had pretty good performance across my apsc sensor. Though the platesolver suggests it actually has a FL of only 808 and not 860 which is weird, and I probably need to solve some focuser tilt as there is mild on-axis coma and one side has asitgmatism and the other has blobby stars.

What reasons might this reducer not be performing at its rated backfocus? The focuser tilt? Not optimised for my bigger sensor? Just plain needs more backfocus despite advertisement?

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I think you can get an effect similar to astigmatism if the sensor is too close.  You might get away with just increasing the distance from how you describe it.

From memory the actual distance required is more of a guideline.


Edited by Ratlet
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