Right Declination Astrophotography

Focus Convergence -Focusmax

After creating the  V-curves in FocusMax and want to enable the focus convergence., but first we need to calculate the Critical Focus Zone (CFZ) and translate that into focuser steps.

CFZ for a scope can be worked out with this fomula:

CFZ(in Microns) = 2.2 x (Scope_Focal_Ratio²)

My Telescope is a Takahahsi 106ed (530/106)2 having a focal ratio of 5 gives me a CFZ of 55 microns.

Or  1250/250  = 5F x 2.2 = CFZ 55

That sounds tiny in everyday speak, good thing that I have an awesome high resolution stepper motor focuser.

According to FLI  focuser manual, the total focusing range is a relatively small (but more than adequate) 0.35 inches , the Atlas manages to make an incredible 105,000 precise steps over that range – resulting in each step being only 3.33 millionths of an inch, or 0.085 microns.
 .

	Focus Convergine Calculation
	FLI - Distance Steps	105000
	CFZ mircon	55	0.055 mm
	mm travel	8.89	8890 mircon
	mircons	0.085
	sec trave drawtube	105
	Steps per sec	1000	105 x 1000 steps	105000
	Method:  = CFZ / (Steps  / length of travel distance in Mircon)
	105000 /8890mircon	11.81102	(1 miron=11.81 steps)
	55/11.81 steps	4.66

This leave my CFZ to a 4.66 (5) steps to focus, so the step width in the Focus Convergence setting, Now we need to change the setting in Focusmax focus convergence, start with approximately 1/4th of the CFZ step value:

                                     4.66 x 1/4  =  1.16
                                                                      1 step and 5 samples

Check the focus accuracy if not prefect increase the steps to 1/2th

                                    4.66 x 1/2 = 2.23

                                                                     2 steps and  5 sample