|
Barlows like Compressors have a defined position
I'm embarrassed! I should have known better! After all these years I'm still stupid!
Like any supplementary lens in the system, the Barlow is designed to give the "design magnification" at a precise back focal distance. Any deviation from this "sweet spot" will affect the amplification factor and possibly the optical performance.
The error I made was to use a TV Big Barlow on an ED80 to "try" and achieve f15 ( ie x2) for the PST etalon....The distance from the barlow lens to the normal eyepiece position is 60+50=110mm (60 to the shoulder, 50mm for eyepiece insertion); when put in my situation I had:
110 +50 (PST to 2" adaptor) + 60 (diagonal +BF) = 220mm. What this means, is that the amplification ratio was more like 5-10!!!
( As the position of the barlow lens approachs its focal length inside the prime focus, the new focus tends to infinity ie parallel beam!)
I had tooooo much back focus on the Barlow.
It made me think about other applications where a camera adaptor + 55mm camera body may be fitted to a Barlow.... the amplification factor will NOT be the same as design ie possibly x4 rather than x2.
|