Inventor of the Herschel Wedgie. Watch your back.
Astronomy - The World's Second-Oldest Profession.
"You don't know Swift from Astrola." -C.H.-
Mr. Stacy From: Seattle, WA Proud to be a Cloudy Nights Member since 9/15/02
* Celestron NS11 GPS - Stellarvue Raptor 90mm APO* Vixen Ultima 8X56 - Fujinon 10X50 FMT-SX - Nikon 7X50 ProStar - Vanguard Endeavor ED 8X42 - Nikon Action Ex. 7X35More cowbell please!
Milton Wilcox R.I.P
Quote:--- And the greatest benefit of all this is that the reduced effective aperture proportionally increases effective focal ratio, reducing astigmatism and other aberrations and producing much "sharper", "tighter" images than if the system were operating at full aperture.Kenny
Refractors Reflectors Two Cats A few eyepieces
Happy owner of--
A Mag 1, 12.5 inch Porta Ball
A Dual Axis Equatorial Platform
A PST Double Stack
Quote:The beauty of this is that it is easily testable. Install a temporary central obstruction, leaving only a thin annulus around the circumference of the objective. Swap out eyepieces until you end up with one that produces an exit pupil that forms an image.
Retired airline pilot.Enjoy building grinding and polishing mirrors and showing the wonders of the universe to others at star parties
Quote:Placing an iris at the focus serves only to change the field size. It would be exactly like having an adjustable field stop in the eyepiece.The ideal place for an iris is at the entrance pupil, or the objective. But then it has to be as large as the objective. One could place the iris somewhere between the objective and the focus. But then when stopping down the light cone, it becomes the entrance pupil. The closer to the focus this iris lies, the worse the vignetting it introduces.
Quote:Jon,As I stated, for the reduced iris condition, as the small iris moves laterally across the larger exit pupil, the reduced aperture at the objective correspondingly sweeps across the objective. And so, yes, the full aperture *can* be utilized, but *never at any one time.* If the iris is 1/2 the exit pupil, never more than 1/2 the objective contributes to image formation. As the iris wanders, that 1/2 objective diameter reduced aperture region also wanders about the objective.I guess I'll have to prepare a diagram like that in the bottom panel, but for the case where the iris is abutting the edge of the exit pupil (as opposed to being in the center.) Then one would see that the reduced aperture at the objective is still just as small, but offset so as to abut the objective edge.This in no way means that the objective is ever working at full aperture. Just because one has the freedom to let the iris wander, and thereby utilize different parts of the objective at different times, is not the same as utilizing the full objective at one instant. And so at any one time the instrument is always working at reduced aperture.
Quote: There is nothing hard to understand about where the pupils end up - because it is just a ray trace.
Quote:If that were the case, this thread would not exist.
Quote:I think it should be pretty obvious to anyone, using simple descriptions and few details, that viewing through an eyepiece with an iris smaller than the exit pupil, will result in a loss of throughput - and an effective loss of aperture.
Quote: Separately - the questions about coupling a camera to a telescope afocally cannot be discussed properly without explicitly dealing with the location of the camera pupil relative to the eyepiece. Since that topic is intertwined this the one regarding the more basic aperture reduction caused by a small human pupil - I think it makes sense not to oversimplify the discussion.
Quote:Hi Mitch,How does the lit retina create the background glow? How bad is it? I have not noticed before but sounds interesting and good to know for eyepiece selection.