I’m hoping to get some feedback regarding field size diameter for my general uses (wide angle viewing, some prime focus). This all started when I investigated a replacement focuser for my Konus 8” F5 reflector. In the absence of a clear sky for exact calculations, it would appear that the focuser needs to be about 3.5” above the tube (and with a 1 ¼” adapter that protrudes a bit over an inch) to achieve focus. Using an excel spreadsheet from Oldham optical, I calculated that the field size is currently in the 7 mm range (using a tube distance from the end of the mirror to the center of the secondary). I have no room to move the primary back, or the secondary forward, so I was considering throwing everything into a sonotube (which I have available in almost exactly the same diameter and wall thickness), and placing the focuser (a new, low profile one) further from the primary. If I could obtain a field diameter of, say, 20mm (or whatever is recommended), does this sound like a good idea, or a waste of effort (ie little apparent improvement)? One last thing, an observation that may contradict my calculations. Prime focus shots of the moon do not appear to suffer from significant light falloff at the edges. If my field diameter was indeed around 7 mm, wouldn’t it be expected to appear vignetted? Thanks for any help!
Sean Prange
Sean Prange
