Reproduce the configuration
We inventory diagonals, adapters, reducers, flatteners, filter wheels, off-axis guiders, cameras, and extension tubes in their actual order.
Focus is where the optical system forms its image; back focus describes the physical distance available to place an eyepiece or sensor there. A focuser must reach that plane while carrying the real observing load without slipping or changing alignment.
Evidence checkpoint. Celestron defines back focus as the distance from the end of the drawtube to the focal plane and notes that accessory changes can make that plane physically unreachable. [1]
Measure travel and focal-plane location
We establish available inward and outward travel and compare it with the optical path consumed by the accessory stack.
Load-test the mechanism
The focuser is checked through its travel at realistic orientation and payload for slip, roughness, backlash, bearing preload, and drawtube rotation.
Check tilt and image shift
We distinguish a moving focal plane from a moving image and look for sensor tilt, drawtube sag, SCT mirror shift, and poor adapter registration.
Verify repeatability
A good result returns to focus predictably, holds the load, preserves collimation, and produces consistent field behavior across repeated approaches.
What we separate and measure
Cannot reach focus
The focal plane lies beyond available in- or out-travel.
Focus changes with altitude
Load-dependent slip or mechanical deflection is likely.
One side of the field differs
Tilt or spacing can imitate poor optics.
Focus reversals feel delayed
Backlash, reduction gearing, coupler play, or mirror movement must be isolated.
Sources
Primary agency, manufacturer technical, and original optical-testing references ground the principles described above. Schematics are explanatory and not customer measurement data.