An ideal telescope is said to be within the “Critical Focus Zone”, when the imager plane is within a certain range of the true focal plane. The CFZ size depends on the wavelength of the light and the diameter and focal length of the telescope, according to:

CFZ = 4 x lambda x ( f / D )^2.

Typical amateur (higher end) telescope values for planetary imaging are:

lambda = 450 e-9 m (roughly shortest wavelength of visible light, blue, worst case, red = about 650 nm, so focussing red light is less critical)

D = 11 inch = 0.2794 m

f = 5.588 m (assuming a C11 f/10 with 2x Barlow lense, resulting in f/20).

This gives CFZ = 4 x lambda x ( f / D )^2 = 4 x 450e-9 x (20*20) = 720e-6 = 720 microns.

This means that the focuser has to be within +/- 720/2 =360 microns to produce sharp images.

The “faster” a telescope is (larger D/f), the more accurate the focus has to be. An f/3.3 system has to focused (20/3.3)^2 = 37x more accurate than an f/20 system, so within 20 microns instead of 720 microns.

As a reference: a human hair is about 60 microns in diameter.

The Bahtinov grabber has now been expanded with:

- calculation of the CFZ, when you enter your scopes f and D

- calculation of the absolute focus error from the Bahtinov pattern, pixelsize and f/D.

This way the Bahtinov grabber (13397) can tell you whether your system is within critical focus or not: