Field Reduction Factors???

[Davekyn]

Field Reduction Factors?:screwy:
I was watching a Canon EOS 40D video tour (http://www.cameralabs.com/reviews/Canon_EOS_40D/video_review.shtml), where the guy explains the benefits of the “Live View” feature with regards to focusing.
http://www.cameralabs.com/reviews/Canon_EOS_40D/live_view_astro-photography.shtml
Basically they are using a “Canon EOS 40D” in Prime Focus mode with a TeleVue Genesis SDF telescope” (Model not specified) with afocal length of 540mm. They go on to mention, that because of the Canon EOS 40Ds field reduction factor…That when the camera is attached, the focal length will now effectively be working at 864mm?
Questions that come to mind:
What is this “Cameras Field Reduction Factor”?
Is it directly related to the change in distance, that the light will now travel down the Tube Assembly, due to pulling out the Diagonal and Eye Piece and then fitting a T-MOUNT and camera?
I can understand that this change in distance will require a readjustment of the focus knob to make up for it, and that “Live View” makes this somewhat easier than the usual method whatever that is; I’m just struggling with the term “Cameras Field Reduction Factor?”

How does one calculate this “Field Reduction” If the telescopes original FL is 540mm and after attaching the camera it becomes 864mm…that’s an increase of 324mm. FORGIVE ME BUT I KIND OF GET LOST HERE…That’s like another 32cm…I can only account for a little extra distance being added with the addition of camera and T-MOUNT…not another 32cm(Although I probably got this whole thing wrong L)

Also…Dare I ask how a 2X Tele-converter works…I do understand that a higher power is needed to view the moon closely to make the field of view smaller, thus this type of telescope needing the extra FL. I intend to use The Orion EON ED80 myself…I just want to get a better grasp of what “field reduction factors there are and how they really work…

No doubt it will eventually sink in…but if anyone would care to reconstruct my scattered thoughts go right ahead…It would be mostly appreciated…Let’s Talk:)

CHEERS
Dave

[Davekyn]

No Probs,
The Lads over at Cloudy Nights, were able to help out on this one. Not that I understood entirely. Somthing about the Term "feild reduction" more having to do with the "Sensor Crop" The effect the size of the ccd sensor has in relation to field of view or somthing.

I dont have it all down pat yet, but I think the messege is to just stick with focal length, pixel size, and chip size; when considering whats important with imaging.

[Dennis]

Hi Dave

Here’s how I understand it. If you enlarge a 35mm film negative (36x24mm) to say, a 10”x8” print, then for the Canon 40D chip (22.2x14.8mm) you will have to enlarge the ccd chip “negative” by a factor of x1.6 to make the same size 10”x8” print.

Cheers

Dennis