HFR is not at minimum when at focus?

Sequence Generator Auto Focus
1

I have a rather strange situation wherein when I am in focus the HFR is not at minimum. Rather the HFR on either side of focus is smaller than when it is at focus. As a result, my focus curve is W-shaped. This really creates lots of problem with the autofocus routine, as it sets the focus point to the place where the HFR is smallest, but this is not the focus! I am not really sure what to do! Has anyone encountered similar problem? My setup is a Takahashi e130D newtonian (130 mm f/3.3 FL = 430mm) with a standard crop size DSLR (about 4.xx micron, pixel scale = 2.x arc sec / px).

2

Posting the auto focus graph would be helpful so we can get an idea of the range.

Thanks,
Jared

3

I had the same problem with my 127 mm ES and a Rigel nsteper. I down loaded the latest drivers but have not been out to try it. At one point after doing the SGP manuals calculations for figuring out the steps to use fo the graph I was anywhere from 25 too 245 steps. I am hoping the new drivers will be what I need.

4

I will try to get a focus graph next time I have the same DSLR set up. I have switched to a mono CCD and the problem went away. I suspect a combination of DSLR Bayer matrix, moderate undersampling at 2”/pix and the diffraction spikes from the newt which become more obvious at focus may have contributed. I am happy though it is not a problem with my CCD as it is my main camera.