Thanks for the response Earl. I think imaging at such low f ratios definitely brings its challenges !
I think my steps are 237 per mm
OK, step size about 4.2 microns then. A bit large for that f ratio. Could be part of the problem.
Hi,
Have a FSQ160 and FR so the f/3.6 ratio, using a QSI 6120 and the lakeside and AF is pretty much perfect.
Use 2x2 binning, step size of 17 and 9 steps and pretty much a text book curve each focus run.
I went through and each filter pretty quickly to set the AF points, only need a 15sec exp for the NB and 3sec for the LRGB.
All were crossed checked with bâmask after and were within 10micros of the focus point according to bahtinov grabber.
All the above said that was only one night but last night went through the same procdure but with the scope and natcive f/5 and the results were the same though now using a step size of 19.
Just to add to the thread after some findings: I calculated the offsets for my Astrodon 3nm filters on my Tak FSQ106 and found them to be nearly parfocal â I was able to get nice v-curves on the narrowband with 50s exposures (I think 60 sec with SII). The calculated focus point for the NB filters was within 1-4 steps of the Lum filter. This is with a microtouch focuser, where I have a step size of 5.
Upon inspection this seems to be within CFZ, perhaps near the edge of it. Just wanted to post a follow up.
Interesting. Mine are not.
Ha to S2 - 13.2 micron difference - pretty close
Ha to O3 - 105 micron difference - not so close.
Makes sense as the Ha and S2 are pretty close in frequency compared to the O3.
I should note that this is with the older Fluorite version of the FSQ.
BTW, see my post on step size terminology. This can be confusing as there are two meanings.
Hello, a question 
does the offsets can be input as relative steps or absolute ?
(ie : -36 steps for Ha , +20steps for S2 relatvive to L or 25020 for ha and 25040 for s2 etcâŚ)
Agree on the step size. Honestly I never figured out the exact distance moved per step as long as the v curve was good. However, unless someone has an identical setup, step size is only relative and wouldnât really be applicable to another system without knowing how far the focuser is moving per step.
That said, Ha and SII were -1 steps compared to Lum, where OIII was -3. If I can calculate the step size travel distance, I will be able to compare my findings to yours. Iâm not sure if AF with long exposures on NB is better or worse than any other method for determining offsets. It is certainly easier when unattended.
Easiest way to calculate is to use a machinistâs caliper and go to an even number (say 20,000 steps), measure the distance between the focuser body and camera/collar/whatever. Then go to 10,000 steps or whatever and do the same measurement. Take the difference and it is then easy to calculate the physical step size.
A caliper is also a great way to check for and/or quantify focuser backlash. I have always checked that. Interestingly, when I was using a Robofocus there was a fair amount of backlash. With the Starlight Feathertouch focuser it is so small that it is below the appx .0005 inch that my dial gauge can measure.