My focus curve

[yomamma]

I have a FLT132 refractor and my focus curves for this scope always want to skew higher and faster once they pass the best focus point.

I will set the params at -220, 250, 9, 40 and it will take 6 or 7 steps to get to focus then the last 2 or 3 after the focus point going up will have much wider gaps between the numbers than the points in the initial downslope. I may start the slope at 4.5 going down but it will end at 9 or 10 on the other side. This causes the focus curve to have the “leaning bowl” look like a large bowl leaning on its right side.

It does produce a good focus curve but wondering if this is normal or should I change something?

[admin]

Hi,

I have seen the same sort of asymmetric focus curve on some telescopes (for instance my Classical Cassegrain), and I don't think it is a problem as long as the lowest point in the curve is being picked up by the autofocus algorithm.

I don't know exactly why it happens that the stars get worse quicker on one side of focus than the other, but you could imagine that for a refractor that the effect might be that different wavelengths defocus at different rates as you move away from best focus (even though the APO design tries to bring all the wavelengths to focus at the same point to avoid chromatic aberration).

cheers,

Robin