Bent or Folded Refractor, Question about Focal Point and Focal Plane

[RFE3]

I am having trouble wrapping my head completely around this FL amd FP thing...maybe one of you Physics / AP / DIY / Refractor Gurus (come on...you know there's bound to be a few here)  could clarify things for me.

The Idea:

I have an extra SW 100/900 ED Pro lying around that I sometimes use as a second scope for Luminance/Ha subs etc... I was thinking I would cut it down and make a Bent Refractor (or something) out of it to shorten it and bring the weight closer to the mount (EQ6). My confusion arises from the difference between the FL (900mm) and the Focal Plane that my 1000D sensor uses (ca. 27mm Sensor diag.) My understanding is that the full FL is needed for Eyepiece focusing and as I normally have to rack my Focuser inwards to focus my Camera, this point would then be the Focal "Plane" for the Sensor....I think... I made a CAD drawing of the Light Cone in a 100/900 and see that at roughly 650mm the Light Cone has a diameter of about 28mm. Since my Sensor has a diag. measurement of 27mm, could I not theoretically shorten the Tube (and thus the FL) and mount the Camera at the point where the Light Cone fully illuminates the sensor (650mm)? Would the light at this shorter point be too "confused" for imaging? If I could just shorten the Scope to the 650mm length rather than bending it, would make life less complicated.

I can't find any "clear" answer from Dr. Google that doesn't involve math with more letters than my Alphabet Soup has.... Any thoughts?

thanks,

Rick

[Gina]

The focus is at 900mm from the object lens - anywhere else the image is out of focus.  You need to place the camera sensor at the focus.  The only way to produce a shorter telescope while retaining focal length, is to use mirrors to reflect light back and forth and is used in somes other types of telescope.  HTH.

[Peter Drew]

As Gina has said, you need to "fold" the light path with mirrors or prisms to significantly reduce the physical length of a refractor, the 900mm would not change. I have built several folded refractors up to 8" aperture and to be honest I don't think it would be worth going to the trouble unless the objective was at least F15.

[RFE3]

Ok, Thanks... that's what I kind of thought but all of this stuff on Focal Length, Focal Plane, Circles of Confusion and of least Confusion kind of confused me...I know that the Bent or Folded Refractors work better with  a longer FL but they are sooo Cool and I'm fascinated by them. I really wanted to experiment with them and because I have a couple old "Fracs" lying around I thought I'd try it. @Peter Drew That's really Cool that you've built some Folded Fracs, would you mind if I PM'd you some time with a couple of questions about mirror angles and crossing light cones?

My "End Game" is to eventually build a Drum Scope, but I thought I'd practice on folding a straight one first.

[Peter Drew]

No prob, PM away.  

[Dave Lloyd]

An ED100 pro "lying around" is no bad thing.

[RFE3]

You're right...but I got a 150/900 to replace it  so it only gets used occasionally nowadays...