Here is a question that has baffled me on barlows. All barlows have an advertised magnification. I have an older TV 2.5X model, and it is hard to believe that every eyepiece will yield the same power increase, since designs are different with varying barrel lengths and lens positions. My TV 12mm Nagler T2 pushes its field lens way down into the barlow tube, while high power Plossls are much higher up. Also, some eyepieces have both 1.25" and 2" barrel sizes, which will put the lenses in two different positions to the barlow lens itself.
With my barlow, I am able to achieve a different power by screwing off the lens from the bottom and slipping it into the eyepiece end, putting it much closer to the eyepiece. This accomplishes two things: 1) I get approx. 1.75x from the barlow lens, and 2) the eyepiece/barlow combo then can be focused much closer "in," about 2". This works great on my MN76, which normally needs a lot of out-focus for its sad, floppy focuser. So, is there an accurate way to calculate barlow mags by measuring the distance from the barlow lens to some part of the eyepiece? Please enlighten.
With my barlow, I am able to achieve a different power by screwing off the lens from the bottom and slipping it into the eyepiece end, putting it much closer to the eyepiece. This accomplishes two things: 1) I get approx. 1.75x from the barlow lens, and 2) the eyepiece/barlow combo then can be focused much closer "in," about 2". This works great on my MN76, which normally needs a lot of out-focus for its sad, floppy focuser. So, is there an accurate way to calculate barlow mags by measuring the distance from the barlow lens to some part of the eyepiece? Please enlighten.
