ETX 2" Eyepiee Solution......
Posted 09 December 2003 - 11:05 PM
Posted 10 December 2003 - 06:15 AM
"Tell Wyman and Warphavoc that Siebert said the only people who swear this won't work are people who haven't used it."
Posted 10 December 2003 - 11:53 AM
If you want it, buy it. Remember that I have no financial interest in your decision. I'm not trying to sell you something. I didn't make the laws of physics, I just have to deal with them. My Mak and my Erfle vignette. Why? Because the scope can't provide a fully-illuminated field of view to the eyepiece. Siebert says that the ETX has a hard time providing a fully illuminated field of view to a 32mm and 40mm 1 1/4" Plossl. If that's the case (and I've heard it from other people as well so I've no reason to doubt him), how can it provide a fully-illuminated field of view to a larger, wider-field eyepiece? In a word, it can't. It will vignette. The laws of physics and optics dictate this. Will you have some sort of image to the edge? Maybe. Will it be fully-illuminated? No.
Try this, if you want. Take your lowest power, longest focal length eyepiece. Make a small aperture mask for it out of cardboard or card stock. Use the same ratios we are using here, so make it about 1/3rd smaller than the field-stop in the eyepiece (if it is a 32mm or 40mm, it will have the inside of the barrel as the stop, so use about 18mm/0.75". Tape the cardboard aperture mask on the bottom of the eyepiece to represent the constriction created by the ETX's exit aperture that will be in front of Siebert's adapter and the Siebert eyepiece's field-stop. Put the eyepiece in your scope and look through it. Does it vignette? Is does for me. Why? Because it does not matter how big the field lens is if the light path is constricted. My eyepiece starts to vignette 2/3rds of the way to the edge. Do I have an image to the edge? No, but I have an image about 95% of the way there (sounds familiar). Why? Because the cardboard aperture mask restricts the fully-illuminated field of view to 2/3rds of the eyepiece's field-stop diameter, almost the exact same ratio as the 32mm constriction in the ETX to the 45mm field stop in a low-power, wide-field eyepiece. If you don't have a fully-illuminated image circle, you will have vignetting. Maybe more, maybe less, but you will have it.
I've demonstrated logically, mathematically, and experimentally that this won't work as Siebert claims (100% illumination).
Yes, I believe it will not work because I verified mathematically, logically, and now experimentally that it will not work as advertised. Sorry, but that's the way it is. The question then becomes, can you live with the degree of vignetting this will produce?
Posted 10 December 2003 - 11:54 AM
Posted 10 December 2003 - 12:05 PM
Fact: Nobody makes a 40mm 1 1/4" eyepiece with a larger apparent field of view than 43 degrees. Not Tele Vue, not Takahashi, not Vixen, not Meade, not Celestron, not Clave, nobody. Why? You can't.
Fact: It doesn't matter how large the barrel diameter is after the 32mm constriction, you can't get a fully illuminated field of view larger than 32mm with a 32mm constriction.
Oh, by the way Siebert, vendors CANNOT post here, they can only post in the Vendors/Experts forum. If you want to discuss this there, please start a thread there. If you want me to meet you there, send me a PM. I'll meet you there, if you want. Otherwise, delete your post as you are in violation of the forum rules.
Posted 10 December 2003 - 12:34 PM