Chris White- Overcast Observatory:
Bill,
You'll probably need to do a little trial and error. The EAF has a pretty course 4um step size, but as you've said, this doesnt translate to focuser movement especially with how it moves a lens style focuser.
I'm not sure why you actually need this number, but in practicality couldn't you just run a capture sequence where you move the focuser only a fee steps in between captures.
By doing a high resolution curve you could see exactly how many steps correspond with the CFZ. Keep in mind that corners will feel the impact of field curvature and/or defocus much "sooner" than the center.
Can you measure the thread pitch for the focuser? I don't know how easy it is to take a look on that scope. These petzval like designs though can be problematic for lens collimation, so be careful messing with anything.
Why exactly do you need to know this?
Thanks Chris. I think you are the only reply that understood where I was going with this. What I was looking for is the
physical amount that each motor tic/step moves the focal plane. Maybe I should have been clearer. With focusers that move visibly this can be easily measured (and is usually a published spec by by the focuser maker) but not with, as you say, this "lens style" focuser.
The purpose of this is to do what I do on all of my systems and that is to establish a good number for "focus when the temp changes by (amount) ". Decades of imaging have shown me that this is the most reliable and efficient way to make sure you are pretty much always well focused (barring any other mechanical things - but that is a different issue). You can use change of FWHM/HFD but that "closes the door after the horse is out" so is not ideal (but can be used in addition as a failsafe).
One can calculate the CFZ for the system that is not a problem, but that will be in microns.. The data on how much the focus shifts with temperature that one can get in a number of ways will be in focuser movement units (aka motor "tics/steps"). One needs to be able to convert that to microns to know how many microns focus on that optical system shifts with temp. Then one can finally set the "focus when the temp changes by (amount) " that you wish to use so that temp shift does not put you outside the CFZ.
I suspect I may have to do this empirically with this system, as you suggest, but it would be much easier and faster if I had that number.
In addition, this number can be useful (but not essential) when running aberration tip/tilt adjustments in NINA.
