10" F/4.7 Modified Skywatcher Reflector, 38mm Orion Q70, 17mm Modified Ultima LX, 10mm TeleVue Delos, 7mm Pentax XL.
Quote:I have a question concerning the FOV and magnification in all types of eyepieces.Most people, including myself, assume that any given eyepiece will be a certain magnification and FOV in an eyepiece.For example, if I take my Pentax 10mm XW and calculate the magnification and FOV by my focal length in my 12" F/5 GSO Reflector, it will be 152.4x with a field of 27.5' arcminutes. (1524/10mm= 152.4x)I got a different calculation when I did the star-drift method of letting a star very close to the equator drift across the FOV being very careful to make sure that it crossed the center of the EP. With the 10mm Pentax XW, I got 145x with a field of 29' arcminutes The same thing happened with my Meade 14mm series 4000 UWA when I did the same test!!!!I got 114x with a field of 44' arcminutes, but if you do the math without the star drift method, you get 109x/46'arcminutes !!!My question boils down to a few things that could be causing this.The focal length of my telescope is calculated wrong, the eyepiece focal length is wrong, the AFOV is wrong, or the star drift method is not accurate enough!!!!Can anybody please help me here?....am I correct in assuming that the wrong things I mentioned are WHY I am not getting the correct calculations?---------------------------Mark 12" F/5 reflector 30mm 1 rpd (finder EP) 21mm TeleVue plossl(on the way) 14mm Meade series 4000 UWA 10mm Pentax XW 2" GSO Barlow Sony Dig 4.1 Mp Cam
David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right." Hyde Memorial Observatory http://www.hydeobservatory.info Prairie Astronomy Club http://www.prairieastronomyclub.org
Quote:This should be in agreement with the aFOV as measured by the drift method if the focal length of the scope is known.
Quote:Quote:This should be in agreement with the aFOV as measured by the drift method if the focal length of the scope is known.You forgot your "times 57.3" (i.e., 180/pi) to convert from radians to degrees.It will match te field stop formula's result and so will be close, yet subject to sources of variation due to the telescope design (angular magnification distortion and variable focal length in moving mirror catadioptrics). As David notes, this probably amounts to 1% or 2% error.I'd settle for effective field stop diameters for all eyepieces, although this isn't well-defined for eyepieces that lack field stops (as the effective field stop diameter then depends somewhat on the particulars of the light cone).
Quote:>>>I agree that an "effective field stop diameter" is a much better idea than some "nominal Apparent field of view".----The nominal AFOV is really just another way of packaging the field stop diameter so that when you do the TFOV=AFOV/MAG you get the correct answer... Doing the math, the focal length of the eyepiece drops out and one is rleft with TFOV= 57.3 x field stop/focal length scope.jon
Quote:>>>>Thus, at least for this set of eyepieces, the Field Stop Method yields results which are somewhat more consistent and often considerably closer to the actual measured field than those from the older AFOV/Mag formula, although as can be seen above, there are times when the results are comparable. Clear skies to you. ----I think your results are consistent with what I have measured and we are in agreement that measuring the field stop gives the best estimate of the True Field of View.That is actually the motivation behind the "nominal Apparent Field of View." The nAFOV is just a number that results in the best estimation of the TFOV when the first order approximations are used.I defined nAFOV = 57.3 (deg/rad) x (field stop/focal length eyepiece)and that Mag = Focal length Telescope/Focal length eyepieceit means that using the old equations:TFOV = nAFOV/MAG = 57.3 (deg/rad)x (Field Stop/focal length eyepiece)/(Focal length telescope/Focal Length eyepiece) =>TFOV = 57.3 (deg/rad) x (Field stop/Focal length telescope) = nAFOV/MAGThe end result is that the nAFOV is just short hand for the the Field Stop diameter.Jon Isaacs
The end result is that the nAFOV is just short hand for the the Field Stop diameter.