Janusz

Hello All, I am wrestling with how to properly use my 2xBarlow for planetary imaging. My refractor has a 420mm focal length and focal plane falls 140mm behind the focuser draw tube end. How do I know that? I subtract OTA length (as measured 280mm) from the Focal Length. That is where the camera (be it CMOS or DSLR) sensor should be placed. My CMOS came with just enough spacers to make up the 55mm and all I need is another 85mm. So I add 2x30mm extension rings and extend focuser's draw out tube by the remaining 25mm. This gives me almost equal travel of the focuser in and out. (It works the same with a reducer. I multiply telescope focal length by reducer number (0.8 in my case) and repeat the above, (the reducer is providing 30mm of space by itself.) So far this is simple geometry. Just to dispel any notion that 55mm is somehow a magic number. After this lengthy introduction, enter the 2xBarlow. My first question - Barlow as I understand it, is placed between an eyepiece and the telescope to double total power (magnification) without causing eye strain associated with high magnification eyepieces. So do I need an eyepiece in my imaging train? Or Barlow should work on the telescope's prime focus alone? Or one could have it either way? Interestingly, I do not see my Barlow's focal length listed in its Specs, nor on others either. Does it mean, it behaves like a reducer except it elongates the FL? Then in my case it would mean 840mm? Do I need an extra 420mm of extension tubes? Sounds ridiculous. Assuming no eyepiece is involved, my first instinct would be to place Barlow's lens glass at the prime focus plane of the telescope (140mm) from the end of the OTA. Then position camera's sensor at the focal length of the Barlow's glass. With no FL given is one destined to hunt for it moving the camera back and forth? Or, maybe try to focus a light source through it and measure the distance? There should be a better way, I would think. Please help in as plain language as possible , since this is so new to me. Hope to find something that works before December 21st! Thank you. Janusz