Eyepiece Projection Factor

Hi there

I tried this question on the 'eyepiece' forum and didn't get any response - maybe the experts are over here.

I want to try eyepiece projection planetary webcam imaging (try saying that quickly!), and am interested in the Baader Hyperion eyepieces (fixed fl) and the T2 system as a possible solution, but I want to pick an eyepiece with the fl for a sensible projection factor/effective fl. I'm happy with the math (efl=f1[x/f2-1]), where f1 is the primary fl, f2 is the eyepiece fl and x is the projection distance - but I guess x needs to be measured from the eyepiece field stop. Does anyone know where that is (as a distance from the front element) on a Baader Hyperion (any fl). If I had the eyepiece, I could work it out by imaging a target of known size then seeing what image size I had, but I haven't bought it yet and want to pick an appropriate fl to get f/20 to f/40 ish with a sensible range of projection distances (ie set of T2 extension tubes).

Grateful for any ideas - the Baader website has a 'stylised' eyepiece cross section diagram but no scale, and I can't identify the field stop either, and I tried emailing Baader without response (I did email in English due to lack of German...)

Gerry

Hi Gerry:

Here's my two cents - for what it's worth. Now that barlows are of such high quality (3 or 4 lens air-spaced APO's) most planetary imagers have abandoned the old positive eyepiece projection method for the more convenient and simple to use negative barlow system, some guys will stack as many as three of these things together. I just built an EP projection unit and my tests show no increase in resolution over the barlow method. I wish you good luck if you choose the EP route! cheers, Dan

PS: I think the 13mm or 17mm Hyperion would be the best choice, from what I've read the shorter focal lengths are (for some reason) lower quality.