Experiences with High Power Viewing
For planetary observing I purchased a set of high quality eyepieces in focal lengths of 15, 10, and 6mm. I thought this series of eyepieces would give me a good range of magnifications with the telescope I use - a Celestron 8 inch Schmidt-Cassegrain. The magnification of a particular eyepiece is found by dividing the focal length of the eyepiece into the focal length of the telescope. In my case the focal length of the telescope is 2000mm. So my 15mm will give a magnification of 133x (2000 divided by 15). The 10mm will give 200x and the 6mm will give 333x. I have read in numerous articles that the maximum magnification a telescope can give is 50 times the aperture in inches. So, theoretically, my telescope can go up to 400x. In my experience, I do not believe this is accurate. I have found that the highest magnification I can use with this telescope is 200x. The reasons for this have nothing to do with quality of optics, collimation, and seeing conditions.
This past October I had the opportunity to travel with the club to New Mexico for a week of observing. One of the bonuses of observing with others is that you have the chance to swap eyepieces and compare the view though different telescopes. Jupiter and Saturn were popular targets. They were straight overhead and the seeing was very good. I was very happy with the views through the 15mm and 10mm but was disappointed by the 6mm. The view had several bright flares and dark ghost images in the field. I also thought the image was soft at the edge. When I made this comment to the others, Leon Fasano remarked that he owns the same model eyepiece and has not experienced any of the problems I was having. I loaned the set to Greg Lutes and he was very happy with the performance. What was going on here? From everything I had read and other observers' experiences, the 6mm should be giving better images than it was. After thinking about this for a time and doing some research I believe that the problem is in my eye and not the equipment. I came to this conclusion after calculating the size of something called the "exit pupil".
Telescopes have two pupils, an entrance pupil and an exit pupil. The entrance pupil is simply the aperture of the telescope. The exit pupil is an imaginary disk just behind the eyepiece through which the light rays pass on their way to your eye. Its size is the telescope's aperture divided by the magnification. What makes this so important is that the exit pupil of a telescope should not be either too large or too small. If it is too large, your eye can not take in all the light the telescope can give. If it is too small, your eye will have trouble forming a good image. It is commonly noted that the maximum magnification of a telescope is 50 times the aperture in inches. This is a theoretical value based on optics and not your eye. If 50 times the aperture is used, the exit pupil will be .5mm in diameter. Everyone's eyes are different but I suspect that my eye has trouble forming clean images when the exit pupil is under 1mm in diameter. This makes sense after reading an article that stated that the human eye is designed to work best when opened between 1mm and 5mm. I intend to do more experimentation by trying different combinations of telescopes and eyepieces. I would recommend everyone calculate the exit pupil size for their particular telescope and select eyepieces within a practical range for their eyes.Published in the December 2000 issue of the NightTimes