Powermate & Barlow Back Focus Distance
When a Powermate or Barlow is introduced into a telescope system, the native focal plane position of the telescope is projected out to a new position. The location of that new focal plane varies depending on the proximity of the Powermate or Barlow lens to the original telescope focal plane. However, to make measurement easy, the tables below reference physical surfaces on the Powermates and Barlows relative to the original and new focal planes.

The standard 1¼” eyepiece mentioned in the table notes is a Tele Vue 1¼” eyepiece that belongs to Parfocal Group “B” on our Tele Vue Eyepiece Specifications chart (column “G“).

Diagram Notes
A - DISTANCE TO OFFSET LOWER SHOULDER FROM TELESCOPE ORIGINAL FOCAL PLANE. POSITIVE WHEN LOWER SHOULDER IS ABOVE FOCAL PLANE AND NEGATIVE WHEN BELOW.

B - DISTANCE FROM FINAL FOCAL PLANE TO UPPER SHOULDER. POSITIVE WHEN UPPER SHOULDER IS ABOVE FINAL FOCAL PLANE AND NEGATIVE WHEN BELOW.

THERE ARE NO SET VALUES FOR A OR B. CHANGING ONE AFFECTS THE OTHER. THE TABLE SHOWS A LIST OF A and B COMBINATIONS THAT WORK WELL TOGETHER, ALONG WITH THEIR RESULTING MAGNIFICATIONS. THESE CAN BE USED TO POSITION AN AMPLIFIER AND CAMERA ON A TELESCOPE AND HAVE THE CAMERA BE VERY CLOSE TO FOCUS, AND THEN FINALLY TUNING FOCUS USING THE TELESCOPE FOCUSER. THE USER CAN CHOOSE THE A and B COMBINATION THAT WORKS BEST FOR THEIR SETUP.

SMALL TABLE NOTES
* THIS A and B COMBINATION AND NOMINAL MAGNIFICATION OCCURS WHEN A TELE VUE STANDARD 1.25 INCH EYEPIECE IS PLACED IN THE 1.25 INCH BARLOW OR ORIGINAL 2 - 1.25 INCH ADAPTER IN THE CASE OF THE BIG BARLOW.